Difference between revisions of "FG: Magnetotail Dipolarization and Its Effects on the Inner Magnetosphere"

From gem
Jump to navigation Jump to search
 
(38 intermediate revisions by the same user not shown)
Line 26: Line 26:
 
For full FG proposal [https://drive.google.com/open?id=1U1XrvDQl24NPw8O82Kz2M7r0ZcsnzmRk CLICK HERE].
 
For full FG proposal [https://drive.google.com/open?id=1U1XrvDQl24NPw8O82Kz2M7r0ZcsnzmRk CLICK HERE].
  
 +
 +
== GEM 2023 ==
 +
 +
===Monday 13:30-15:00 PDT Joint Session with MESO: Room A===
 +
 +
<b>The Substorm Current Wedge Paradigm</b>
 +
 +
Scene setting talks on the Substorm Current Wedge Paradigm.
 +
<ol><li> Bob McPherron (UCLA)
 +
<li> Larry Kepko (NASA/GSFC)
 +
<li> Jesper Gjerloev (JHU/APL)
 +
</ol>
 +
 +
===Monday 15:30-17:00 PDT Joint Session with MESO: Room A===
 +
 +
<b>The Substorm Current Wedge Paradigm</b>
 +
 +
Discussion with a panel of experts.
 +
<ol><li> Joachim Birn (Space Science Institute)
 +
<li> Slava Merkin (JHU/APL)
 +
<li> Shin Ohtani (JHU/APL)
 +
<li> Toshi Nishimura (Boston University)
 +
<li> Karl Laundal (University of Bergen)
 +
</ol>
 +
 +
===Tuesday 13:30-15:00 PDT Joint Session with RB: Room A===
 +
We will discuss the following:
 +
<ol><li> Where are we in terms of quantifying the contribution of mesoscale injections/bubbles to the ring current and/or radiation belt?
 +
<li> What can we accomplish with current assets?
 +
<li> Do we need new missions to answer the question (and if so what is needed)?
 +
</ol>
 +
 +
Confirmed discussion leaders:
 +
<ol><li> Matina Gkioulidou Observing the global geospace at mesoscale resolution
 +
<li> Anthony Sciola - The contribution of plasma sheet bubbles to stormtime ring current buildup and evolution of the energy composition
 +
<li> Sina Sadeghzadeh - RCM Modeling of Bubble Injections into the Inner Magnetosphere: Spectral Properties of Plasma Sheet particles
 +
<li> Anton Artemyev - ELFIN+injections: Relativistic electron precipitation driven by plasma injections
 +
</ol>
 +
 +
===Tuesday 15:30-17:00 PDT: Room Coa===
 +
 +
We will review the FG’s original goals/questions and then ask, “Where were we and how far have we come?” There will be a panel discussion concluding with community discussion.
 +
 +
Confirmed Panelists:
 +
<ol><li>Andrei Runov
 +
<li>Joachim Birn
 +
<li>Anton Artemyev
 +
<li>Grant Stephens
 +
<li>Shin Ohtani
 +
</li>
 +
</ol>
 +
 +
===Wednesday 10:30-12:00 PDT Joint Session with CPMP and RB: Room A===
 +
 +
Scene setting talk by George Clark about the vision for CPMP with regards to particle energization, radiation belts, etc.
 +
 +
Panel-led Discussion
 +
 +
===Wednesday 13:30-15:00 PDT: Room Coa===
 +
 +
We pose the question, “Where are we going? What are the open questions we should carry into new FGs?”
 +
 +
Contributed Speakers:
 +
<ol><li>Toshi Nishimura: Connection between substorm onset and expansion phase activity: How near-Earth instability transitions to expansion-phase BBFs/DFs
 +
<li>Harry Arnold: Grey Box Modeling: Empirical Resistivity Maps and Thin Current Sheets
 +
<li>Xiantong Wang: BBF simulations using the two-way coupled MHD-PIC code
 +
<li>Konstantinos Horaites (Minna Palmroth): Magnetotail plasmoid eruption: Interplay of instabilities and reconnection
 +
<li>Sanjay Chepuri: Testing Adiabatic Models of Energetic Electron Acceleration at Dipolarization Fronts
 +
<li>Anthony Rogers: Utilizing GPS as an asset to study injections
 +
<li>Yangyang Shen: Contribution of Kinetic Alfven Waves to Energetic Electron Precipitation from the Nightside Transition Region during a Substorm
 +
</ol>
 +
 +
===Thursday 13:30-15:00 PDT Joint Session with MESO and MPEC: Room A===
 +
 +
We will have a panel-led discussion on the following questions:
 +
<ol><li> What can ground-based observations tell us about the mesoscale phenomena occurring in space?
 +
<li> How can we leverage the existing ground-based networks and operational spacecraft data we have for multi-scale coupling research without having the propelling effect of a big NASA mission (like THEMIS or GDC)?
 +
<li> How do model results of mesoscale dynamics translate to ground-based observations in the nightside? Do the model results present similar ground-based signatures in terms of scale size, temporal evolution, and overall dynamics?
 +
</ol>
 +
 +
After the panel provides their thoughts, we will open up the discussion to the room. Please join in! We wish to run it "workshop style", so please bring a slide if you want to help with the discussion, or just your thoughts.
 +
 +
Panelists:
 +
<ol><li> Kareem Sorathia (Modeling)
 +
<li> Bashi Ferdousi (Modeling)
 +
<li> Toshi Nishimura (Data)
 +
<li> Sneha Yadav (Data)
 +
<li> Emma Spanswick (Data)
 +
</ol>
  
 
== mini-GEM 2022 ==
 
== mini-GEM 2022 ==
Line 55: Line 144:
 
*Sasha Ukhorskiy
 
*Sasha Ukhorskiy
  
 +
<b>Discussion Summary</b>
 +
*Adam Michael - Meso to Micro: Cross-Scale Modeling of Bursty Bulk Flows in the Inner Magnetosphere. Adam began by asking the question how do bursty, mesoscale flows in the magnetotail, alter the microscale wave particle interactions in the inner magnetosphere. He highlighted past results published by Wiltberger et al. (2015; https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2015JA021080) and Sorathia et al. (2021; https://www.frontiersin.org/articles/10.3389/fspas.2021.761875/full) detailing the ability of global magnetosphere models to produce similar statistical behavior of BBFs as observations. He showed that the reduced density and increased magnetic field within BBFs enable electrons that otherwise would not resonate with chorus waves, to be scattered and energized and for lower energies (10s of keV) BBFs can increase the bounce averaged quasi-linear diffusion rates to be on the order of a few minutes, which can potentially lead to localized, bursty precipitation features.
 +
*Sasha Ukhorskiy – Cross-Scale Nature of Magnetotail Dynamics. Sasha discussed that understanding causal relationships among intrinsically coupled multi-scale manifestations of magnetotail dynamics has been elusive due to the sparse nature of in situ measurements available so far. He showed numerical test particle simulations that detail how magnetic islands can stably trap particles and transport them from the tail to the inner magnetosphere, across >10 Re leading to energization to MeV energies of the core radiation belt population. The mesoscale flows also drive interchanging regions of parallel and perpendicular anisotropies and he discussed the potential connection to wave activity observed along dipolarization fronts by MMS and the observed correlation of electron microbursts and pulsating aurorae which might be attributed to both being produced by whistler waves generated in BBFs/DFs. He argues that understanding microscale/kinetic effects of mesoscale dynamics, such as the possible connection of microbursts and pulsating aurora with streamers/BBFs requires a system view of the M-I system and could potentially be addressed by a distributed LEO observatory equipped with auroral imagers, magnetometers, and energetic particle sensors.
  
 
== GEM 2022 ==
 
== GEM 2022 ==
Line 72: Line 164:
 
Upload Talks here: [https://vgem.org/groups/DIP https://vgem.org/groups/DIP]
 
Upload Talks here: [https://vgem.org/groups/DIP https://vgem.org/groups/DIP]
  
 +
Discussion Summary:
 +
*Toshi Nishimura (work presented by Christine Gabrielse): Kinetic Plasma Structures Associated with Substorm Auroral Beads by Space-Ground Coordinated Observations
 +
**Addressing, “What is the role of reconnection and/or other plasma instabilities in producing elementary magnetotail dipolarizations?”
 +
***From an auroral imaging point of view, substorm onset instability occurs first and then substorm dipolarizations occur. A movie was shown that showed the waves along the onset arc start first and then auroral streamers come next.
 +
***(1) Growth-phase streamers do occur before substorm onset. Their flows and dipolarizations are weaker than expansion-phase, intense streamers/flows/dipolarizations. The two types of events should be considered separately.
 +
***(2) This auroral observation doesn't rule out the possibility that a dipolarization occurs before reconnection. Near-Earth Neutral Line (NENL) activation could occur soon before or after each streamer/dipolarization. It's difficult to evaluate this causality from observation.
 +
***During the growth phase, however, a PBI is followed by a streamer, suggesting that reconnection occurs first.
 +
**Addressing, “How do the physical properties of the dipolarizations depend on their spatial location and the time of their appearance relative to the Dungey cycle?
 +
**Toshi thinks dipolarizations that form closer to the Earth penetrate deeper into the inner magnetosphere due to its low initial entropy and less closed magnetic fluxes. Dipolarizations forming farther away have higher entropy and are decelerated more by the dipolar field. Very narrow channels may dissipate quickly, but both narrow and wide channels can penetrate deep.
 +
 +
A discussion about auroral beads followed. It was expressed that not many in the audience understand what auroral beads are. What do we know about these beads? Are they really associated with substorm onset? What’s the physics?
 +
*One difficulty in answering this is the difference across community members on what defines the substorm onset.
 +
*Another difficulty is mapping: When parallel electric fields are involved (as with dynamic, discrete aurora), we can no longer map along a modeled magnetic field line to know where that auroral signature comes from in space.
 +
*It was mentioned that an electric field in space is related to the beads, but without in situ measurements.
 +
*Incoherent scatter radars have showed that there is an electric field and oscillating electrical field associated with the beads. So you can infer the electric field pattern from the SuperDARN data with the beads.
 +
 +
*Sneha Babu: Auroral beads, substorm onset mechanisms with a focus on the current disruption paradigm and ballooning instability as a trigger
 +
**Focused on the current disruption paradigm where she sees that ballooning instability could trigger the onset of substorm.
 +
**For the ballooning instability, the threshold is mildly parallel anisotropy, so even if you have mild parallel anisotropy, it can trigger ballooning instability.
 +
**Audience member asked, “Any idea why there is a dropout of the perpendicular particles to create the parallel anisotropy?
 +
***Two combined effects:
 +
****Drift shell splitting: there can be more parallel particles in the polar region.
 +
****Conservation of the first and the second adiabatic invariants. You see more parallel particles when the magnetic field line stretches. The 90 degree pitch angle particles try to follow a constant field strength, but low pitch angle particles follow these stretched lines.
 +
**Audience member asked, “What is the scale size of the ballooning instability? Like we are talking about something that covers the entire nightside magnetotail? Is it localized?”
 +
***It’s localized. The ballooning instability is because of the curvature of the magnetic field line, because it’s stretching, so it’s localized.
 +
**An audience member asked if the parallel anisotropy might relate to the beads.
 +
***Case study showed beads in the image when ballooning instability occurred.
 +
 +
*Shin Ohtani (work presented by Jesper Gjerloev): New Insights into the Substorm Initiation Sequence from the Spatio-temporal Development of Auroral Electrojets
 +
**Jesper asked the audience, “When is the Bz signature a dipolarization and when is it just a wiggle?”
 +
**Andrei Runov: Dipolarization is larger scale and “sustained”, dipolarization fronts and dipolarizing flux bundles are smaller and separate and more short-lived. We are still addressing their relationship to one another. We must consider the pressure build-up ahead of dipolarizing flux bundles (DFBs) as part of what helps sustaining the larger dipolarization.
 +
**Editor’s note: See Paper that was published after this summarizing dipolarization definitions: https://www.frontiersin.org/articles/10.3389/fspas.2023.1151339/full
 +
**Jesper shared opinion that the definition of a substorm used to be focused on a larger scale thing, but nowadays some people define smaller scale, short-lived events as substorms, so the definition is more hand-wavy. We need to clean up our language.
 +
***For example, is a single BBF a substorm? Jesper says no, but feels some people do define it that way.
 +
**More discussion continued about definitions and phenomenology.
 +
**An audience member pointed out that a substorm is not a clean “domino effect” in which one phenomenon starts off the process that leads to the next, to the next, and so forth. A substorm is a system response with lots of phenomena that go into making up a substorm, that all have to work together in order to make the large-scale current system and the auroral expansion. So a little wiggle may not be a substorm, but it may help comprise a substorm. This is perhaps an unsatisfying answer for why there is a sense of confusion surrounding the substorm.
 +
**Christine concluded mentioning that a paper was going to be written up that summarized a lot of definitions that were set when the DIP FG began in 2017. That paper is linked above.
 +
 +
*Jinxing Li: The response of ionospheric currents to different types of magnetospheric fast flow bursts
 +
**An audience member asked why the substorm related fast flows can penetrate into the inner magnetosphere?
 +
***Jinxing’s answer was that he thinks the substorm onset related fast flows have a larger magnetic flux transport, so they can bring more energies into the inner magnetosphere. He also pointed out the low entropy bubble model.
 +
***Andrei Runov shared that the bubble explanation details that the fast flows are underpopulated flux tubes with smaller entropy, which is mainly dependent on the length of the field line. When it gets closer to Earth, it moves towards lower entropy and will propagate until the flux tube entropy equals the surrounding entropy.
 +
**From a modeling perspective, Xiantong found that in some driving conditions there can be current sheet break ups closer to Earth that have a larger geomagnetic impact. He’s wondering if there’s observational evidence showing that if these fast flows are generated closer to Earth?
 +
***Hard to answer observationally since satellites don’t know when/where the flow originated.
  
 
=== Thursday 10:30-12:00 ===
 
=== Thursday 10:30-12:00 ===
Line 92: Line 228:
  
 
Upload slides here: [https://vgem.org/groups/DIP https://vgem.org/groups/DIP]
 
Upload slides here: [https://vgem.org/groups/DIP https://vgem.org/groups/DIP]
 +
 +
<b>Discussion summary:</b>
 +
Discussion Leaders/Speakers were:
 +
*Christine Gabrielse: An overview of a white paper drafted based on prior GEM debates. This was ultimately published in a longer form here:https://www.frontiersin.org/articles/10.3389/fspas.2023.1151339/full
 +
*Slava Merkin: Slava discussed that both global and regional models robustly suggest that BBFs and injections play an important role in cumulatively building by RC pressure and bringing in mag flux to inner mag. Global dipolarization was result of many local ones (see Merkin et al., 2019 https://doi.org/10.1029/2019JA026872; Birn et al., 2019 https://doi.org/10.1029/2019JA026658). He pointed out that the simulation resolution matters a lot—but that before we try to model non-MHD physics, we first need to resolve mesoscales in the simulations. He discussed current model limitations and the advances that are required in the next 5-10 years (test particle feedback in the transition region, global Hall for real events, Hall MHD and embedded kinetics like PIC, collisionless Hall), and the next 10-30 years (global hybrids that run for longer, artificial intelligence).
 +
*Joachim Birn: Electron acceleration and anisotropies in dipolarization events. Joachim showed that electron anisotropies in dipolarization fronts, if field-aligned, may directly cause precipitation, whereas perpendicular anisotropies will drive waves and Poynting flux. He showed MHD simulations of near-Earth tail reconnection and earthward propagating DFBs, combined with backward tracing test particles. He showed that acceleration takes place during neutral sheet crossings and is mostly parallel, but also perpendicular.
 +
*Andrei Runov: Dipolarizations & Injections: Unfinished Business. Andrei asked the question, are mesoscale structures rapid flux transport (RFTs) associated with global substorm onset? He found no correlation between the DFB observed at THEMIS (~10 RE) and ion injections observed by LANL at GEO, but for large dipolarizations that last for half an hour there is a 30% connection between the two missions. More details in his publication: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020JA028470. Importantly, he posed the following open questions: What is the key ingredient to make a dipolarization effective for ion injection into GEO? DFBs in the near tail show magnetic field variations on short scales (< 100 s) but we do not see this in the ion fluxes at GEO, why? What is the fate of energetic particles that drifted away flank-ward? Do they contribute to the partial ring current? He also supplied what he thinks is required to answer these questions: Observations by a fleet of azimuthally-separated equatorial probes with MAGs and energetic particle detectors in the tail-dipole transition region; conjugate observations by polar-orbiting LEO probes with MAGs and energetic particle detectors for remote sensing, field aligned currents; models resolving energy-dependent drifts. This was especially important in helping form the White Paper for the Decadal Survey.
 +
*Matina Gkioulidou: Mesoscale Processes: The bridge to cross. Matina highlighted that mesoscale processes are the bridge to connect the local to global nature of geospace. She brought up the fact that the role of mesoscale injections on ring current build-up and transport still remains a major open question, as well as the mesoscale contribution to the substorm current wedge. Another open question is how auroral features evolve with respect to plasma sheet structures. She shared the mission concept, PARAGON, which would address all these questions in the magnetosphere and M-I coupling. It requires coordinated measurements from imaging spacecraft (ENAs in the tail and of auroral emissions in the ionosphere) plus in situ measurements from satellites with elliptical orbits in the equatorial plane.
 +
 +
The group discussed these open questions and emphasized the importance of imaging to get a 2D perspective of magnetosphere and ionosphere dynamics.
  
 
===Friday 10:30-12:00 MESO/DIP Joint Session===
 
===Friday 10:30-12:00 MESO/DIP Joint Session===
Line 106: Line 252:
 
* James Weygand: ASI and GOES Observations of Nighttime Magnetic Perturbation Events Observed in Canada
 
* James Weygand: ASI and GOES Observations of Nighttime Magnetic Perturbation Events Observed in Canada
 
* Homayon Aryan: The response of ionospheric currents to different types of magnetospheric fast flow bursts
 
* Homayon Aryan: The response of ionospheric currents to different types of magnetospheric fast flow bursts
 +
 
Upload Talks here: [https://vgem.org/groups/DIP https://vgem.org/groups/DIP]
 
Upload Talks here: [https://vgem.org/groups/DIP https://vgem.org/groups/DIP]
 +
 +
<b>Discussion Summary</b>:<br>
 +
The final session was a joint session with the Mesoscale FG. For the duration of the DIP FG, we have discussed the roles that mesoscale phenomena (e.g. DFBs, BBFs, injections, streamers) play with respect to the global system response (e.g. global dipolarization, MLT wide injections). As our FG winds down, the MESO FG will take the lead on this topic.
 +
 +
*Chih-Ping Wang: RCM simulation of azimuthal expansion of plasma sheet bubble in transition region
 +
**Azimuthal expansion is due to magnetic drift of high energy ions in the bubble.
 +
 +
*Yangyang Shen: Contribution of kinetic Alfvén waves to energetic electron scattering and precipitation from plasma sheet injections
 +
**Energetic electron precipitation during substorm: riometer observation
 +
**Berkey et al 1974
 +
**THEMIS Observations of injection, dipolarization, and KAWs
 +
 +
*Sheng Tian: Coordinated observations on how global-scale dipolarizations couple to the ionosphere and mesoscale dipolarizations
 +
**Coordinated observations on how global-scale dipolarizations couple to the ionosphere and mesoscale dipolarizations
 +
***Azimuthally propagating dipolarization (APD) both eastward and westward.
 +
***Several mesoscale injections during the global expansion
 +
***2 sheet current system
 +
***Current sheet expands in pace with global dipolarization
 +
***Expands due to pileup
 +
***3 events showing azimuthal expansion of dipolarization
 +
***Track individual dipolarizations via multiple spacecraft
 +
***Saw upward and downward current expand similar in latitude
 +
***Global dipolarization couples to ionosphere by a 2 sheet FAC system.
 +
***Bostrom type II model: the global-scale westward current (auroral electrojet) is a Hall current
 +
***2 mesoscale dipolarizations, occurred around the expanding edge of a global dipolarization
 +
***Aurora expands much faster (10 deg per minute) than global dipolarization (2-3 deg per minute).
 +
***Different expansion speeds: aurora vs. dipolarization. Different generation mechanism? Is auroral expansion a wave or material? Maybe due to mesoscale flows at the border instead of the actual dipolarization?
 +
***Audience asked: When you look at auroral expansion vs spacecraft footpoint, do you see activity there that could give you precipitating particles?
 +
****When aurora expands, so does convection speeds.
 +
****Eric Donovan: it’s extremely unlikely that the expansion of the aurora has anything to do with the bulk transport of material. Change in aurora=change in convection, but doesn’t necessitate bulk motion of material in the magnetosphere. Sheng agrees—ionosphere is not a reliable screen of the magnetosphere. 
 +
 +
*Kareem Sorathia: Global modeling of multiscale stormtime magnetosphere-ionosphere coupling
 +
**Multiscale Stormtime MI Coupling
 +
**Case study of the dawnside current wedge-Ohtani ‘21
 +
**Dawnside current wedge (DCW)
 +
***Dawn/dusk SMR (SuperMAG SYM-H index) asymmetry common feature of storm main phase
 +
***Ohtani found connection between dawn-dusk asymmetry and enhancement of dawnside auroral electrojet.
 +
***Do they see in models?
 +
**AMPERE AND TWINS
 +
**Summary
 +
***Reproduce ground phenomenology of dawnside current wedge as seen in Ohtani
 +
***Confirm importance of westward auroral electrojet current closure during stormtime
 +
***Find DCW occurs following the dawnside penetration of BBFs which provides
 +
****Dipolarizing flux
 +
****Ion pressure->changes in R2 FACs
 +
****Energetic electrons->diffuse precipitation->eastward propagating conductance enhancements
 +
 +
*Matt Cooper: Field-aligned thermodynamic features represented in the Middle Energy Inner Magnetosphere (MEIM) Model
 +
**Field aligned thermodynamic features in the middle energy inner magnetosphere (MEIM) Model – a quiet time reference
 +
**Discrete structures in nightside magnetosphere seen during active times
 +
**Structures seen in plasmas satisfying mirror/drift-mirror instability conditions
 +
**Uncertainty in whether structures are active/fossil mirror modes or some other
 +
**Peaked mirror mode structures typically found in unstable plasma regions
 +
**Dipped mirror mode structures typically found in regions where the plasma is stable to the mirror mode
 +
**If formed in transition region, could flow into inner magnetosphere.
 +
**Anisotropy not high enough to be mirror modetearing instability (reference)
 +
**But Cooper found it can happen.
 +
**Nightside mirror/drift mirror modes—dusk to midnight observations?
 +
 +
*James Weygand: ASI and GOES observations of nighttime magnetic perturbation events observed in Canada
 +
 +
*Homayon Aryan: The response of ionospheric currents to different types of magnetospheric fast flow bursts
 +
 +
At the end we did have 10 min to try to discuss the future of the FG as we have one more year left. We tried soliciting questions other than “how does mesoscale phenomena relate to global phenomena”. The question of mapping came up (since we are joint with the Transition Region/Mesoscale group). We talked about how there used to be a mapping FG. Andrei Runov mentioned assimilative mapping—where you use a Tsyganenko model and tweak the input parameters to force the model to fit the few data points you have (Marina Kubushkina did this work a decade or so ago).
 +
 +
==GEM 2021==
 +
During the summer 2021 Virtual GEM workshop, we held two sessions.
 +
*Session 1 had 75 participants online and 7 speakers.
 +
*Session 2 had 66 participants online and also 7 speakers.
 +
Since we had too many requests for speaking slots and the time was limited, we had no time for additional discussion. This was partly intentional since under the umbrella of the Helio2050 discussions, our FG supported the corresponding session held by the MPS RA.
 +
 +
Our first session highlighted new insights on current systems and MI coupling associated with dipolarization structures in Earth’s magnetotail. In particular, attention was given to some of the microscopic to mesoscale currents and Poynting flux associated with dipolarization fronts, energy transfer into the ionosphere, and how the current systems associated with dipolarization fronts close through the ionosphere and contribute to the global scale R1/R2 current systems. New details and insight on the multi-scale (micro to macro) nature of particle acceleration associated with bursty bulk flows and dipolarization fronts was also discussed.
 +
 +
The second session focused on global-scale asymmetries and more on meso-scale structures in the context of MI-coupling and multi-scale, multi-point observations. Finally, data-model comparisons were discussed including new results from a machine learning model that successfully predicts the location of magnetotail X-lines and thin current sheets and statistical results from test-particle simulations in high-resolution MHD fields that successfully reproduced observed statistical characteristics of mesoscale structures and plasma characteristics associated with bursty bulk flows and dipolarization fronts.
 +
2021 has had some challenges, not just with Covid but with extra meetings (e.g., Helio2050) that has left the community a bit tired. We are working on determining the best use of the upcoming virtual or hybrid mini-GEM meeting during the Fall AGU 2021 meeting. It may include time to discuss Decadal Survey white papers and collaboration on that. And it may include a discussion of what the next Challenge Question(s) are.
 +
 +
We do fully intend to continue our Challenge Question format going forward, as this format had resounding support when we asked for feedback from the community.
 +
 +
===Contributed Speakers===
 +
*Jiang Liu. Embedded Region 1 and 2 currents: consequence of enhanced convection in the plasma sheet, newly recognized from LEO observations
 +
*James Weygand. Magnetic Perturbation Events observed in ionospheric current systems including events during substorms and north-south streams
 +
*Hongtao Huang. Understanding the magnetic dip ahead of the dipolarization fronts using PIC simulations: the dependence on the guide field.
 +
*Kun Bai. Ion trapped acceleration at rippled dipolarization fronts
 +
*Louis Richard. Turbulent Jet Fronts and Related Ion Acceleration.
 +
*Sheng Tian. Evidence of Alfvenic Poynting Flux as the Primary Driver of Auroral Motion During a Geomagnetic Substorm.
 +
*Larry Lyons. Two-dimensional Structure of Flow Channels within the inner magnetosphere and Associated Upward Field-Aligned Currents: Model and Observations.
 +
*Chih-Ping Wang. North-south asymmetry of the tail lobe density and magnetic field.
 +
*Amy Keesee. Mesoscale plasma sheet structures observed with energetic neutral atom imaging with TWINS.
 +
*Grant Stephens. Reconstructing the global x-line configuration by data mining spaceborne magnetometer observations.
 +
*Slava Merkin. Mesoscale Electrodynamics and Ring Current Formation.
 +
*Andrew Menz. Investigating Substorm-Related Flows and Thinning Using Multi-Point Spacecraft and All Sky Imager Data.
 +
*Joachim Birn. Dipolarizing flux bundle braking: Energetic ions.
 +
 +
 +
==GEM 2020==
 +
The summer 2020 GEM workshop was our first time running a virtual workshop with fewer sessions. We used the opportunity to regroup, refresh, and refocus. We reviewed previous workshop activity since the focus group’s inception. We also solicited talks that would help the community decide which science questions to focus on next. Those talks were presented in the first session. We compiled the questions and forward-looking perspectives from Session 1 to use as input for discussion in Session 2. In Session 2, we also surveyed the community on what session format they would prefer to use going forward. The overarching opinion was to continue the format we had introduced early on, which was to present a “challenge question” in advance that is addressed by the community during the workshop. This was a successful format in the past that has resulted in papers that would not have been written without the focus group’s structure and guidance. Using the GEM session as a place to connect data analysts and modelers to solve questions was also discussed.
 +
Challenge questions the community discussed as important were as follows:
 +
*PV to the gamma: Not easy to confirm with magnetic models. Heat flux across the field would violate it. Can we observe this? Models can address. Confirmed by 2D but leave out cross-tail drifts. 3D PIC could address. Ionospheric outflow/inflow could violate it too.
 +
*Divergence of heat fluxField aligned currents
 +
*Building up of dipolarization/SCW isn’t over!
 +
*Would be important modeling challenge. RCM? PIC?
 +
*Observations of multiple simultaneous flows? (4-5 streamers) SCW shows build-up from mid-latitude positive bay (Pi2s)
 +
*Lack 2D cross-plasma sheet mission to answer this question. Idea: use NASA program to rideshare/put additional payload on launches to more readily access space.
 +
*How much current can be contributed to total SCW? Not all currents go to ionosphere, must close locally.
 +
*Must be different instability to account for energy budget?
 +
 +
===Accomplishments===
 +
One significant accomplishment is our Focus Group’s ability to listen to the community and provide a Focus Group that fits their needs. We heard there was confusion on terminology, so we began our Focus Group with a panel that discussed terminology. We heard there was lack of understanding on the different types of models, so the following year we had a panel of modelers explain what their models are capable of studying in terms of the physics. Next we heard that trying to debate in real-time was difficult. Although audience members have interesting and valid counter-points to a speaker, without any time to reflect and respond it is difficult to have meaningful discourse. So, we responded by creating the “Challenge Question” format, where a Challenge Question, highlighted by the community, is posed months ahead of time. Community members can address the question by submitting their talk title/opinion about the answer. Focus Group leaders facilitate the debate by coordinating the speakers ahead of time. At the GEM meeting, speakers debate amongst each other, having had adequate time to prepare. Audience participation is very welcome. A very successful example of this is detailed below, and resulted in publications and collaborations that would not have organically formed without the Focus Group’s leadership.
 +
 +
2021 has had some challenges, not just with Covid but with extra meetings (e.g., Helio2050) that has left the community a bit tired. We are working on determining the best use of the virtual summer 2021 GEM meeting. It may include time to discuss Decadal Survey white papers and collaboration on that. It may include addressing a Challenge Question. We are waiting for Helio2050 to conclude to feel out the community’s posture for the GEM workshop in July.
 +
 +
We do fully intend to continue our Challenge Question format, as this format had resounding support when we asked for feedback from the community.
 +
 +
===Notes on community engagement and participation at the GEM 2020 Summer Workshop are listed below:===
 +
====Solicited speakers:====
 +
<b>Session 1:</b>
 +
*Christine Gabrielse Intro and review of FG activities and resulting publications
 +
*Kareem Sorathia The role of mesoscale injections in ring current evolution: Global MHD and test particle simulations
 +
*Slava Merkin Ballooning-interchange Instability at the Inner Edge of the Plasma Sheet as a Driver of Auroral Beads: High-resolution Global MHD Simulations
 +
*Amy Keesee Tying the reconnection region to the dipolarization front and injections
 +
*Xiangning Chu How much of the currents surrounding the DF are connected to the ionosphere, and contributing to a SCW?
 +
*Louis Richard MMS Observations of Short-Period Current Sheet Flapping
 +
*Bob McPherron Solar Wind Coupling and Magnetic Indices
 +
 +
<b>Session 2</b> had no formal presentations, other than the Focus Group leaders facilitating conversation by displaying slides with the compiled questions and ideas from the solicited talks in Session 1. This resulted in a very “workshop style” conversation that gave the community the floor.
 +
*There were ~70 participants in each session
 +
*We used Slack Channel and video chat software. Session 1 set the scene and session 2 was a very interactive session that was focused on listening to the community.
 +
*We try to give early career folks a platform and facilitate the discussion so that people with different backgrounds and personality types can be heard. This was a different year, but when we invite speakers we do try to bring in underrepresented groups.
 +
 +
 +
 +
==GEM 2019==
 +
The Dipolarization Focus Group had three sessions during the summer 2019 GEM Workshop that were categorized by topic. The Focus Group leaders organized a session on Energy Transfer and Dissipation to guide presentations towards answering specific questions:
 +
1. Can we estimate a percentage that energy is dissipated into waves, direct ion heating, etc.?
 +
2. Can we determine if and to what extent a dynamo in the transition region (driven by pressure gradients or vorticity) converts energy dissipated from the tail into field aligned currents to drive dissipation in the ionosphere?
 +
3. What are the ways to estimate these values (simulation or theory or observational)?
 +
It was especially timely to address these questions since the MMS mission had an overlapping Science Working Team meeting, bringing more of our European colleagues to GEM and multiple experts on energy dissipation to the session.
 +
 +
Focus Group leaders also solicited contributed presentations, resulting in two community-driven sessions organized by two prevailing topics. The first topic, Particle Energization and Injections, had an even split between observation and modeling presentations. The second topic, Currents, was a natural follow-on to the session on Energy Transfer and Dissipation.
 +
 +
The following list provides the speaker name and title of presentation from each of the three sessions. Summaries submitted by presenters are included. Note that Focus Group leaders have been collecting publications that are in part thanks to or discussed in this Focus Group on the bottom of this GEM Wiki page.
 +
 +
 +
===Topic 1: Particle Energization/Injection===
 +
*Raluca Ilie - The role of inductive electric fields on particle energization
 +
*Jianghuai Liu - The role of inductive electric fields on particle energization (continued)
 +
*Sam Bingham - Adiabatic Particle Energization using MMS
 +
*Wonde Eshetu - Simulations of electron energization and injection by BBFs using High-Resolution LFM MHD fields
 +
*Christine Gabrielse - Heliophysics System Observatory observations of small and large-scale injections: DFBs vs. large-scale dipolarization
 +
**The injection region's formation, scale size, and propagation direction have been debated throughout the years. How do temporally and spatially small‐scale injections relate to the larger injections historically observed at geosynchronous orbit? How to account for opposing propagation directions—earthward, tailward, and azimuthal—observed by different studies?
 +
**A combination of multisatellite and ground‐based observations were used to knit together a cohesive story explaining injection formation, propagation, and differing spatial scales and timescales.
 +
**A case study was used to put statistics into context.
 +
**Fast earthward flows with embedded small‐scale dipolarizing flux bundles transport both magnetic flux and energetic particles earthward, resulting in minutes‐long injection signatures.
 +
**A large‐scale injection propagates azimuthally and poleward/tailward, observed in situ as enhanced flux and on the ground in the riometer signal. The large‐scale dipolarization propagates in a similar direction and speed as the large‐scale electron injection.
 +
**Small‐scale electron injections result from earthward‐propagating, small‐scale dipolarizing flux bundles, which rapidly contribute to the large‐scale dipolarization.
 +
**Large‐scale dipolarization is the source of the large‐scale electron injection region, such that as dipolarization expands, so does the injection.
 +
**Ion injection region >90-keV in the plasma sheet is better organized by the plasma flow.
 +
*Bob McPherron - MHD simulation of substorm including progressive approach of X-lines, flow channels, and flow penetration to the inner plasma sheet
 +
**An interval of moderate magnetic activity from 0-8 UT on March 14, 2008 has been investigated with a global MHD simulation using high spatial and temporal resolution.
 +
**Observations show several distinct substorms during this interval. One of these with expansion onset at 04:48 UT is also seen in the simulation with onset at 04:44 UT.
 +
**The simulation shows that reconnection is continuously present at multiple sites throughout the interval. During the growth phase, the number of x-lines and their total length increase with time and their locations approaches the Earth. The x-lines create multiple fast flow channels with dipolarization fronts. The total length and area of these channels increase during the growth phase as they penetrate closer to the Earth carrying more magnetic flux.
 +
**The 04:44 UT onset in the simulation was preceded by the growth of an x-line that eventually extended 55 Re from 12 Re premidnight to 50 Re on the dawn side. It produced a narrow flow channel parallel to the x-line that eventually penetrated inside 10 Re rapidly depositing magnetic flux as the expansion phase developed.
 +
**Despite good agreement in expansion onset time the ground and satellite observations suggest a quiet growth phase with a sudden onset of reconnection.
 +
**It may be possible to explain the difference between observations and simulations if all growth phase activity in the simulation map to the ionosphere at very high latitudes.
 +
*Tetsuo Motoba - Azimuthally localized dispersionless injections inside GEO
 +
**Tetsuo Motoba presented a case study of deep energetic particle injections observed by the two Van Allen Probes (RBSP-A and -B) in the premidnight sector.
 +
**Although the spacecraft separation was only ~0.5 Re in the azimuthal direction, the injection signatures were different between the two probes: RBSP-B observed dispersionless electron and ion injections, while RBSP-A observed the corresponding injections but they were characterized by an energy-dispersed flux enhancement and/or by a relatively weak flux enhancement.
 +
**Such different injection signatures are attributed to the presence or absence of a transient, strong dipolarization front (DF). The two closely located RBSP observations suggest that the azimuthal scales of deeply penetrating DF and injection region are highly localized.
 +
*Discussion on Particle Energization/Injection
 +
**Role of different fields?
 +
**Large vs. Small-scale?
 +
**This discussion was inspired by the main points and questions presented.
 +
 +
===Topic 2: Energy Transfer and Dissipation===
 +
*Misha Sitnov - Kinetic dissipation in dipolarization fronts and magnetic reconnection
 +
**Irreversibility of magnetotail dipolarizations is provided both by the collisional dissipation in the ionosphere and by collisional Landau dissipation in the magnetotail.
 +
**Misha Sitnov pointed out that the Joule heating rate, which is a good measure of collisional dissipation and which is widely discussed in MHD models, is not appropriate as a measure of collisionless dissipation in the magnetotail: Values of J*E’ in ion and electron frames practically coincide. Thus, j*E’ cannot be a measure of ion and electron Landau dissipation processes, which are very different.
 +
**Sitnov discussed kinetic analogs of the Joule heating rate, the so-called Pi-D parameters. PIC simulations show that the ion Pi-D peaks of the dipolarization front (DF), while the electron Pi-D peaks behind DF or earthward of the X-line.
 +
**Measurements of the Pi-D parameters, which have become possible due to the MMS mission, remain very challenging: Because of the small probe spacing (~10km), DFs often pass the MMS tetrahedron in times smaller than the plasma instrument cadence. This prevents calculating the spatial derivatives of the bulk flow velocity, the key elements of the new kinetic dissipation parameters.
 +
*Rumi Nakamura - MMS observations of multi-scale field-aligned currents during dipolarization in the near-Earth plasma sheet
 +
**Substorm current wedge contains multi-scale field-aligned currents. 
 +
**Ion-scale process is essential in generating field-aligned currents in near-Earth magnetotail.
 +
**Intense field aligned currents corresponds to generator region in the flow braking region. Two types of generator region observed. (1) Embedded current layer in return flow region of  localized BBF. (2) Electron flow shear region in thin Hall-current layers ahead of BBF.
 +
*Olivier Le Contel - Multiscale kinetic processes associated with fast flows and dipolarization fronts
 +
**Two dipolarisation front events associated with fast plasma flows detected by the MMS mission in last August 2016 have been presented.
 +
**Intense lower-hybrid drift waves associated with parallel electric fields have been identified (frequency, phase speed) at the dipolarization front as well as fast electromagnetic electron holes moving tailward. Possible coupling between the lower-hybrid waves and electron holes was discussed.
 +
*Amy Keesee - Concurrent enhancements in ion temperatures and auroral brightenings seen by TWINS and ASIs
 +
**At the 2018 GEM summer workshop, Amy Keesee showed movies of ion temperature maps during two of the challenge storm intervals.
 +
**Also at 2018 GEM, Toshi Nishimura showed movies of all sky imager auroral maps of the same intervals, and we discovered enhancements in both at the same times.
 +
**Keesee reported on their ongoing collaboration to use these intervals to study the connections from the magnetosphere to the ionosphere.
 +
**Keesee and Nishimura are working on mapping algorithms to identify intervals that have concurrent enhancements when the Van Allen Probes are in a favorable location to study the detailed particle distributions.
 +
**Keesee also discussed the availability of a database of TWINS ion temperature maps being made available at CDAWeb through the support of a H-DEE award.
 +
**They are also developing an automated detection algorithm to identify regions of ion temperature enhancement in that database for further studies.
 +
*Joachim Birn - Energy release and conversion and dynamo action in the tail on the basis of MHD simulation
 +
**Joachim Birn used an MHD simulation of tail reconnection associated with a flow burst and dipolarization to identify energy conversion, dynamo and load, and field-aligned current generator regions.
 +
**Two regions stand out as loads (E.J>0): slow shocks and the dipolarization front, where incoming Poynting flux is converted primarily to enthalpy flux.
 +
**Dynamo actions (E.J<0) are found in the braking region and at higher latitude on the outside of the Region 1 type field-aligned currents, built up by vortical flow in and near the equatorial plane.
 +
*San Lu - Strong energy dissipation at the transition region
 +
**Pritchett and Lu (2018) investigated the response of magnetotail to a longitudinally limited, high-latitude driver using 3-D particle-in-cell simulations.
 +
**After the onset of localized reconnection caused by the external driver, the later response involves sudden disruption of the plasma sheet in the transition region with much stronger energy dissipation and particle energization than that at the reconnection site.
 +
*Shin Ohtani - Dissipation and the ionosphere
 +
**Shin Ohtani discussed the transport of energy from the magnetotail to the ionosphere during substorms by synthesizing the results of previous observational and modeling studies. 
 +
**He concluded that (1) the area around the duskside poleward boundary of the auroral bulge (i.e., auroral surge) is a unique and persistent sink of substorm energy, and it accounts for a few tens of percent of the ionospheric substorm energy dissipation; (2) kinetic energy carried by BBFs is comparable to the energy deposited to the ionosphere in association with auroral streamers, and each BBF accounts for ~1% of the total substorm energy deposition, which may sum up to 10% throughout the expansion phase.
 +
 +
====Discussion on energy dissipation====
 +
1. Can we estimate a percentage that energy is dissipated into waves, direct ion heating, etc.?<br>
 +
2. Can we determine if and to what extent a dynamo in the transition region (driven by pressure gradients or vorticity) converts energy dissipated from the tail into field aligned currents to drive dissipation in the ionosphere?<br>
 +
3. What are the ways to estimate these values (simulation or theory or observational)?<br>
 +
 +
===Topic 3: Currents===
 +
*Misha Sitnov - Dipolarizations and their connection to the ring current buildup and magnetic reconnection
 +
**The new data-mining (DM) technique applied to magnetospheric storms and substorms was presented by Misha Sitnov (in collaboration with Grant Stephens and others).
 +
**The DM reconstruction of the magnetosphere resembles launching swarms of ~50,000 synthetic probes.
 +
**It shows that the response of the inner magnetosphere to magnetotail dipolarizations is very diverse: 1) both the TCS and the ring current increase in the substorm growth phase; 2) the decay of a thin current sheet (TCS) associated with the tail dipolarization on substorm scales (~0.5 hour) is followed by the buildup of a proto-ring current in the inner magnetosphere on the time scales of several hours; 3) the response of the ring current to magnetotail dipolarizations may have both storm and substorm time scales; 4) sometimes magnetotail dipolarizations during substorms do not modify the near-Earth ring current at all.
 +
*Shin Ohtani - Double-wedge current system based on the GOES-RBSP comparison of dipolarization signatures
 +
**Shin Ohtani showed, based on the timing comparison of dipolarization signatures at RBSP and GOES, that the dipolarization region expands earthward. 
 +
**He argued that the result apparently contradicts the conventional substorm current wedge model, which suggests that dipolarizations take place simultaneously everywhere inside the current  wedge. 
 +
**He proposed that the actual substorm current system has a R2-sense current wedge on the earthward side of the (conventional) R1-sense current wedge, and the dipolarization region expands earthward as the R2-sense current wedge moves earthward.
 +
*Yi-Hsin Liu - An explanation of the opposite dawn-dusk asymmetry at magnetotails of Earth vs. Mercury
 +
**PIC simulations reveal that the dawn-ward transport of the normal magnetic flux (Bz) by electrons beneath the ion kinetic scale is a critical feature of current sheets.
 +
**While the normal magnetic field in the tail geometry suppresses reconnection onset, the reconnected magnetic field (i.e., also Bz) enhances reconnection after the x-line develops. These all together will result in the competition of opposite dawn-dusk asymmetries.
 +
**Liu proposed that the vastly different global dawn-dusk scale of the magnetotails at Earth and Mercury will lead to opposite outcomes in this competition of asymmetry. This new finding can be important to the on-going ESA-JAXA mission, BepiColombo.
 +
*Ryan Dewey - Flow braking of dipolarizations in Mercury's magnetotail
 +
**Ryan Dewey presented statistical observations of dipolarizations in Mercury's magnetotail and demonstrated that their associated fast flows typically brake before reaching the nightside surface of the planet.
 +
**Due to the small spatial scales of Mercury's magnetosphere, a small fraction of dipolarizations (~10%) may impact the planet while the majority brake and contribute to flux pileup.
 +
**Whether this pileup is associated with a current wedge system remains to be constrained.
 +
*Xiangning Chu - The generation of STEVE and penetration of fast flows to the plasmapause
 +
 +
 +
 +
==GEM 2018==
 +
===Session 1. ULF waves during particle injections and dipolarizations: Joint with ULF Wave Modeling, Effects, and Applications Focus Group and Substorm FGs===
 +
This session focused on the relationship between particle injections/dipolarizations and ULF waves (e.g., Why are waves driven in only some events? Do waves impact the ring current/radiation belts?). Model and observational results showed that Pi2 wave properties – including the arrival time of Pi2 wave packets at ground stations – are significantly affected by ionospheric conductivity and radial Alfven speed profiles. Incoherent scatter radar observations of large ionospheric electron density and conductivity variations with Pc5 frequency were shown, while SuperDARN radar measurements showed highly localized ionospheric velocity perturbations associated with poloidal ULF waves; more observations are needed to identify the source(s) of the ULF modulation of ionospheric parameters. Numerical simulation (new version of RCM) and theory of buoyancy waves were presented, demonstrating that some nightside Pc5/Pi2 waves may be associated with
 +
the buoyancy mode. Finally, theory of the relationship between ULF waves and substorms was discussed, including Alfvenic interactions that can trigger substoms.
 +
 +
===Sessions 2 and 3. Observations of the challenge events, discussion of steady magnetospheric convection, storm-time substorms, and isolated substorms and their effects on the inner magnetosphere: Joint sessions with the Substorm Focus Group===
 +
The focus of this session was to compare and contrast observations of storm-time substorms, isolated substorms, and steady magnetospheric convection (SMC), and the effects that these tail modes have on the inner magnetosphere. Four events where chosen for initial studies: (1) an SMC event between 2013 August 24-28, storm time substorms on (2) 2016-09-04 ~7:20 UT and (3) 2016-09-27 ~04:30 UT, and (4) an isolated substorm on 2017-02-02 ~4 UT. An overview of the events can be found at goo.gl/zCeiAa.
 +
 +
Ground-based, in situ, and model results were presented including, all sky imagers, riometers, ground-based magnetometers, in situ plasma and wave measurements and global MHD simulations. Christine Gabrielse and Toshi Nishimura presented detailed observations from the THEMIS probes, ASI, and ground-based riometers.
 +
Drew Turner presented observations from MMS and the Van Allen Probes. Amy Keese presented observations from TWINS. Lauren Blum presented EMIC wave observations from the Van Allen probes. Colin Komar present initial global MHD results from the Solar Wind Modeling Framework for each challenge event. Kyle Murphy presented injection signature from the LANL spacecraft and Anna DeJong presented ground-based observations regarding the steady magnetospheric convection event.
 +
 +
One of the major highlights from the session was discussion regarding steady magnetospheric convection: how it was manifested in in situ, geosynchronous, and ground-based data, how steady/stable steady magnetospheric convection needs to be considered as an SMC event, and whether or not SMCs can be accurately defined
 +
without global auroral imaging. Christine Gabrielse showed that during the SMC event, there was almost one-to-one correlation between AE enhancements and riometer observations of precipitating electrons from injection. (This was part of what led to the discussion regarding SMC definition. If AE varied that much, was it really an SMC?) Anna DeJong argued that the event was not truly an SMC for this reason. Toshi Nishimura correlated injections observed at MMS with THEMIS all-sky-imager observations of auroral streamers. Drew Turner also presented initial observations from MMS that elude to direct loss of tail injected plasma to the dawn-flank magnetopause. Lauren Blum showed evidence of EMIC wave activity during storm-time substorms but saw little activity during the SMC and isolated substorms. At geosynchronous Kyle Murphy showed clear differences between the SMC event and storm-time substorms – the SMC event showing little injection activity while the stormtime substorms showed both numerous and intense injections. Future sessions will narrow in on some of these highlights for additional discussion.
 +
 +
 +
 +
===Session 4. Panel Session on the topic of Dipolarization and Global Modeling===
 +
 +
The Dipolarization FG held a panel discussion session to focus on how magnetotail dipolarization is currently captured in global models and how those models need to be developed to better simulate dipolarization and its effects (in the inner magnetosphere and ionosphere) based on what observations are telling us about the nature of the system.  Approximately 70-80 members of the greater GEM community were in attendance.  The panelists included: Katie Garcia-Sage, Colby Lemon, San Lu, Yann Pfau-Kempf, Jimmy Raeder, and Misha Sitnov. Christine Gabrielse chaired the panel and guided the conversation with comments and questions. 
 +
 +
Prior to the panel session, panelists were sent the following three questions to consider and respond to as guidance for the topics that were to be discussed during the session:
 +
1) Given our current modeling capabilities, discuss which kinds of models are best at capturing which aspects of dipolarization events and their effects in the magnetosphere.
 +
2) What determines the dipolarization scale size in different models? (e.g., physical description, boundary conditions, model input parameters, ionosphere conditions, etc.?)
 +
3) The transition region is where both inertia and energy dependent drifts are important. No existing models treat that region correctly.
 +
(a) How do we move forward?
 +
(b) Or, more specifically, address the question of dipolarization front deceleration: (i) How do various models treat dipolarization deceleration as they approach the inner magnetosphere? (ii) What processes are decelerating the fronts in the models? (iii) What inner magnetosphere processes are missing (e.g. plasmasphere, complex ionospheric conductivity models) and does excluding these processes lead to different deceleration predictions?
 +
(c) And/or address: (i) What are the relative roles of ExB, energy-dependent drifts and particle trapping in transport and energization in the transition region? (ii) To what extent are these processes adiabatic for particles of different energies? (iii) What is their overall contribution to the ring current build up?
 +
 +
====Types of Modeling: Which are best for what questions?====
 +
Christine started the panel discussion by reviewing some of the responses she had received from the panelists concerning question 1.  The panel then moved into open discussion on that topic.  There was general consensus that the relevant physics are global in nature, and in particular that the role of the ionosphere and small-scale physics are both relevant and not properly being captured by any of the models. Models must capture both the plasma sheet and dipolar inner magnetosphere correctly plus the feedback loop provided by the non-idealized ionosphere.
 +
 +
San stressed that a combination of models, such as global MHD with embedded PIC and global hybrid models is our best current approach for capturing both global and critical small-scale processes.
 +
 +
Concerning small-scale physics, Misha raised the point that we still don’t have a good sense of where in the tail the reconnection X-line typically forms and whether the models are capturing even that correctly.  He also stressed that with empirical models, such as TS07, we can much more accurately capture individual events. 
 +
 +
Yann introduced the Vlasiator model, and stressed that the location of inner boundary conditions and 2D limitations in the global hybrid model are still a major limitation for accurately capturing magnetotail reconnection, dipolarization, and substorm activity. 
 +
 +
Jimmy focused on the differences between global MHD and other models, stressing that global MHD has a “lack of knobs” that is both limiting in one sense but more trustworthy in another sense.  Jimmy also stressed the importance of the ionosphere and also the transition region in and around GEO, where reconnection fronts (dipolarization fronts and the associated BBFs) start to decelerate and deflect in the inner magnetosphere; he stressed that once these plasma “bubbles” start to slow down and disperse, the fluid picture no longer applies, so it is difficult to say how well MHD model results showing that represent reality.  There was also general consensus that data-model comparisons are very important and we need to continue developing those capabilities and approaches.
 +
 +
From the audience, Andrei Runov asked about the nature of the X-line in the magnetotail: did the panelists think it was a global scale feature?  The panelists consider X-lines in the tail to be fragmented and spread out throughout the tail between around X_GSE of -15 and -20 RE.  Misha Sitnov thinks the typical X-line lies further downtail, more like -30 RE or beyond, and that models that include too much resistivity will get this closer to Earth.  Andrei also stressed the tailward side of the picture, that is, those reconnection jets that are ejected tailward from an active X-line.  From recent ARTEMIS results, the reconnection jets observed at lunar orbit (-60 RE) are still localized in nature, which is further evidence that the X-lines in the magnetotail are also localized.
 +
 +
====Dipolarization Scale Sizes====
 +
Christine next steered the panel to question 2.  Katie stressed that the resolution in global MHD tends to break down in the ionosphere, which might fundamentally limit the scale sizes of features in the magnetosphere.  She also mentioned that ion composition and ionospheric outflow are not well captured in global models currently but might play a key role in scale sizes of magnetotail dipolarization via instability leading to reconnection, reconnection scale sizes, and the global magnetotail properties.  Colby also agreed that the grid resolution in the ionosphere in the RCME model was also a major limiting factor.  Two grid points in the ionosphere in the model map to a very large region of the magnetosphere, meaning that the model might not be able to capture localized features in a stretched magnetotail.  Colby stressed that RCME seems to be doing a good job capturing the Y (i.e. cross-tail, azimuthal) scales of flow channels (BBFs) but is concerned about how well they are capturing the X (downtail) scales. 
 +
 +
From the audience, Shin Ohtani asked about time scales: at 500 km/s velocity, it takes only a few minutes to go from 20 RE to GEO, which is similar to the Alfvén speed travel time from the reconnection site to the ionosphere, so does the ionospheric feedback really matter?  Colby responded that was a good but unresolved question.  Jimmy disagreed, saying the speed is much faster down to the ionosphere.  Bob Lysak mentioned that models often don’t capture the density along field lines correctly, but that with the current best estimates, the travel time for information down to the ionosphere was a few minutes.
 +
 +
That discussion transitioned into the importance of Pi2 waves.  Joachim Birn mentioned that oscillation in the transition/stopping region is on the scales of the ionospheric travel time (PI2 period timescales).  He again stressed the importance of the transition region and how many of our challenges currently fall back into that region around GEO.  Yann showed a movie from Vlasiator, and stressed that with a perfectly conducting “ionosphere” at 5 RE, the speeds were too fast in their simulations.  They were seeing peak flows around 2000 km/s.  He also stressed that with 2D simulations, all of the reconnection in the system was forced into the XZ plane.  The Vlasiator simulations take some time to get reconnection after initialization, and they are actively investigating how the addition of oxygen ions to the plasma sheet will affect that delay time.
 +
 +
Larry Lyons introduced another question of the audience. He asked, “Do dipolarizations only occur in thin current sheets?”  He stressed that with streamers being observed under a variety of different conditions, is a thin current sheet a necessary condition to get dipolarization fronts and BBFs in the plasma sheet?  Misha stressed that the problem is multiscale and that no, a thin current sheet is not a necessary condition.  Tail reconnection, dipolarization fronts, and BBFs may develop in a thick current sheet.  San agreed with Misha’s point and stressed that the thickness of a dipolarization front is determined by ion kinetic physics and that from observations, the width of a front is complicated and might have to do with the scale of the responsible X-line or with the conductance in the ionosphere or both. 
 +
 +
San then showed results from the ANGIE3D global hybrid model.  Slava Merkin asked: what determines the scale size of the X-line?  San didn’t know but stressed that it was not resistivity but likely an inherent property of the X-line itself, perhaps due to a non-uniform magnetotail.  Shin asked why dipolarization fronts moved dawnward, and San replied that it was just a result of ExB drift.  Mostafa asked if there is a correspondence between sizes of X-lines and dipolarization fronts/flows?  Can larger X-lines produce smaller flows or vice versa?  There was some disagreement and discussion between whether or not dipolarizing flux bundles should get smaller as they move inwards.  Joachim brought up that when reconnection starts, an X-line might be extended in the tail due to solar wind driving conditions and the distribution of resistivity in the model, but over time the active X-line narrows to a few RE due to entropy reduction and the system becoming unstable to ballooning.  He discussed how the tearing mode and ballooning mode can either compete or act in concert, and ultimately, that the cross tale scale depends on the region of the outflow where the BBFs go to.
 +
 +
Jimmy brought up an analogy to seismology and terrestrial earthquakes.  He stressed that an earthquake in one place on the planet can trigger another earthquake 1000s of miles away.  He thinks that one active X-line can similarly trigger reconnection elsewhere in the plasma sheet.  The formation of the active X-line changes the entire environment in the tail; it is a disruptive event.  This is of course all driven by changes in the solar wind too, which further complicates the picture.  He pointed to auroral arcs as evidence that there is likely no preferred scale size for X-lines and the dipolarization fronts they spawn.
 +
 +
====The Transition Region: How do we Move Forward?====
 +
Christine next turned the discussion to question 3.  Misha kicked off the discussion on that and stressed that we do have a comprehensive picture of the transition region from a collection of many, many years of observations throughout it.  He argues that with data mining, relying on observations from many, many similar cases, we have full coverage of the region.  From his empirical model, which employs data mining, he finds that the transition region expands downtail from ~-8 to -18 RE during substorm dipolarization.  From here, Andrei asked how Misha defines a substorm, to which Misha replied with the AL index.  This sparked a debate on how to define substorms. 
 +
 +
Katie changed the subject to stress that plasma pressure in the inner magnetosphere has to be captured correctly to properly model the transition region.  This requires that plasma sheet models be coupled to accurate inner magnetosphere models.  She again stressed the important role of the ionosphere, and how that can help dictate how far into the inner magnetosphere a dipolarizing flux bundle can travel and the properties of its rebound and oscillations as it comes to rest there. 
 +
 +
Larry Lyons brought up that we had not discussed the ground based observations point of view.  He asked how we can connect where reconnection is occurring in the models to what we are seeing in the aurora with streamers.  He stressed that in the aurora, much of it is east/west aligned, which corresponds to azimuthal drifts in the inner magnetosphere, and streamers are the only features that can correspond to dipolarizing flux bundles and BBFs.  San agreed and mentioned that localized reconnection and dipolarization fronts may be the consequences of dayside streamers loading small, localized portions of the tail.  Jimmy agreed and stressed that models might be capturing the east/west features but that we just haven’t focused on analyzing them.  Jimmy stressed too that we had to be careful, because there is a filter effect with the ionosphere too.  Not everything seen in the aurora/ionosphere is reflecting what is happening in the magnetosphere.
 +
 +
***From this panel discussion, we established a GEM challenge: modelers are challenged to simulate three different cases: storm-time substorm, isolated substorm, and magnetotail reconnection during steady magnetospheric convection.  From the simulation results, how well can a given model capture the observed similarities and differences between these different cases?  How will models be constrained so that they do not start reconnection prematurely?  This challenge will be further developed and fully defined at the mini-GEM meeting at AGU 2018 and will be conducted in partnership with the focus group on mesoscale aurora, polar cap dynamics, and substorms.***
 +
 +
 +
===Session 5. Contributed Talks===
 +
 +
The Dipolarization FG held a second session immediately following the panel, chaired by Drew Turner, to allow for contributed talks. Also attended by about 70-80 GEM members, the session had ten contributed talks and excellent discussion:
 +
 +
1. Chih-Ping Wang presented on “RCM simulations of entropy reduction caused by plasma bubbles from different MLT locations”. He showed that the earthward transport of the simulated plasma bubble qualitatively explains the two-point THEMIS observation of a BBF event. He showed that the simulated entropy reduction caused by a plasma bubble varies significantly with the bubble’s initial MLT and background convection. A plasma bubble starting at 23 MLT results in an entropy reduction that extends closer to the Earth and azimuthally wider than does a bubble starting at 1 MLT. 
 +
 +
2. Ryan Dewey presented on "Dipolarization effects at Mercury and comparisons to Earth". He used MESSENGER observations at Mercury to identify dipolarizations in Mercury's near magnetotail, and discussed the statistical characteristics of these events. He showed that dipolarization fronts are short-lived (~2 s) enhancements of the northward component of the magnetotail field (~30 nT) and are associated with fast sunward flows, energetic particle acceleration, and thermal plasma heating/depletion. He discussed that these signatures are analogous to those at Earth, however, he showed that dipolarizations are most frequently observed in the post-midnight plasma sheet at Mercury, opposite to that at Earth.
 +
 +
3. Joachim Birn presented results from an analysis of the stopping region of DFBs, both from a fluid and a particle point of view, based on test particles combined with an MHD simulation. The stopping region was characterized by pileup of plasma sheet flux tubes ahead of the DFB, leading to an excess of pressure gradient force. Particle distributions were characterized by perpendicular ion and electron anisotropy with a high-energy electron ring, all originating from the inner plasma sheet particles.
 +
 +
4. Brian Swiger presented a talk entitled, “Do different substorm strengths accelerate keV electrons the same?” He showed that from X=-6 to -25 RE, for all electron energies between ~5-52 keV, the average flux increase was greater for larger AE events.
 +
 +
5. Andrei Runov presented THEMIS and LANL observations in the near-Earth plasma sheet and at GEO, respectively, during events of prolonged, extreme solar wind/IMF driving. Events with IMF Bz <-10 nT during longer than 5 hours were selected. THEMIS measurements indicate that the magnetotail responded by a set of thinning-dipolarization events with a duration of 1 hour, which resemble the sawtooth events. The dipolarizations were accompanied by ion and electron injections in energy ranges ~50 to 500 and ~20 to 200 keV, respectively. Dispersionless and dispersed injections in these energy ranges were also detected by LANL spacecraft at GEO.
 +
 +
6. Sasha Ukhorskiy presented on ion acceleration and transport from the tail to the inner magnetosphere, the effects of trapping, adiabaticity, and the role of charge. (See Ukhorskiy et al., 2017.) Recent analysis showed that the buildup of hot ion population in the inner magnetosphere largely occurs in the form of localized discrete injections associated with sharp dipolarizations of magnetic field, similar to dipolarization fronts in the magnetotail. Because of significant differences between the ambient magnetic field and the dipolarization front properties in the magnetotail and the inner magnetosphere, the physical mechanisms of ion acceleration at dipolarization fronts in these two regions may also be different. He discussed an acceleration mechanism enabled by stable trapping of ions at the azimuthally localized dipolarization fronts, and showed that trapping can provide a robust mechanism of ion energization in the inner magnetosphere even in the absence of large electric fields.
 +
 +
7. Anton Artemyev discussed regimes of ion energization during injections: adiabatic vs. nonadiabatic acceleration. The canonical approach for the guiding center theory was proposed, and using this approach the particle equations of motion were rewritten in the coordinate frame with vanishing inductive electric field (a non-inertial coordinate system).  Using these equations of motion, Anton discussed three regimes of plasma acceleration: the hot plasma in a large background Bz field, the cold plasma in a small background Bz field, and the intermediary plasma/background Bz field. He referenced Zhou et al. [2018] to discuss mass dependence on energization, with more massive particles (e.g., O+) able to gain the most energy. He showed that ions of different charges at ~5-6 keV will gain a similar amount of energy, but that ions with greater positive charge (e.g., O+6 vs. O+) at ~20 keV can gain more energy.
 +
 +
8. Xiangning Chu discussed broadband waves on plasmapause induced by deep penetration of dipolarization front. He showed that most plasmapause observations with broadband waves are centered around pre-midnight, similar to the distribution of flows/dipolarization fronts. He also found parallel electron fluxes around the same time. He found that AE was larger when the waves were observed at the plasmapause than when no waves were observed at the plasmapause.
 +
 +
9. Shin Ohtani presented on “Spatial structure and development of dipolarization in the near-Earth region”.  By statistically comparing the relative timing of dipolarizations at two satellites, he found that the dipolarization region expands earthward as well as away from midnight at r <= 6.6 Re.  The expansion velocity was estimated at several tens of km/s, noticeably slower than outside geosynchronous orbit.  He suggested that this earthward expansion of the dipolarization region can be attributed to a two-wedge current system with a R2-sense wedge moving earthward and a R1-sense wedge staying outside of geosynchronous orbit.
 +
 +
10. Tetsuo Motoba reported on "A near-Earth dipolarization event observed by MMS (r ~13 Re)". In the course of the dipolarization, MMS observed multiple dipolarization fronts (DFs, < 1min), energetic particle injections (> 70 keV), and oscillating flows. The injected energetic ions were field-aligned accelerated with pitch angle asymmetry, while no apparent pitch angle asymmetry was found for the energetic electrons. The MMS-GOES and MMS-ground comparisons revealed good correlation between the dipolarizations at MMS and GOES and between the oscillating flows and low-latitude Pi2 pulsations, respectively.
 +
 +
 +
 +
 +
==GEM 2017==
 +
The “Magnetotail Dipolarizations and their Impact on the Inner Magnetosphere” Focus Group kicked off its inaugural year with two joint sessions (combined with the Midtail and Reconnection Focus Groups, with ~ 70 attendees), two panel-led “controversy” sessions (each with ~35 attendees), and one contributed session (~45 attendees). The over-arching theme of this year’s discussion was defining dipolarization, including how different scale-sizes relate and impact the magnetosphere.
 +
The panel on the “controversy sessions” consisted of R. McPherron, J. Birn, A. Runov, S. Ohtani, M. Sitnov, X. Li, R. Wolf. Through dialogue with each other and the audience, they addressed the following questions:
 +
1. How do you define dipolarization?
 +
2. Is there a difference between small- and large-scale dipolarization?
 +
a. If there is a difference, how do the two types compare/contrast?
 +
b. If there is a difference, do the two types impact the inner magnetosphere differently? (Or similarly?)  Specifically, on injections/particles?
 +
3. How are current models doing at modeling dipolarizations (small and/or large scale)? Should they be modeled differently?
 +
4. What key observations are required to constrain/test current models?
 +
 +
===Definitions and Paradigms===
 +
Bob McPherron began by reminding us that the original definitions (in a 1972 Planetary and Space Science paper, and his 1979 paper) was “a return to dipolar orientation”. Using GEO spacecraft, they saw each onset causes an increase in magnetic field, or “dipolarization”—data that looks very similar to what THEMIS now presents around 10 RE. Baumjohann et al. [1999] later discussed the tailward moving dipolarization front that reaches the near-Earth neutral line distance downtail about 45 minutes after onset. This definition of the “dipolarization front” differs from the “front” discussed in Nakamura et al. [2002], Sitnov et al. [20??], and Runov et al. [2009; 2011], which is the earthward-propagating boundary between the ambient plasma sheet and the hot, tenuous plasma following reconnection.
 +
 +
Andrei Runov expressed some regret at the word-choice, given that the terminology is now a bit confusing (not to mention the fact that a Google search will alter the search term to “depolarization”). To reduce this confusion, he suggested to change our way of thinking regarding the phenomenon. Instead of discussing magnetic field, total magnetic field elevation angle, etc., we should discuss the phenomenon in terms of currents. He pointed out that there are clearly two, distinct current systems. One, the substorm current wedge, is responsible for the global dipolarization. The other, a local current system generated in a high beta regime, supports the “dipolarization front”. This locally generated diamagnetic current flows on the boundary between rarefied, hot plasma coming from reconnection and compressed, colder plasma ahead of the front.
 +
 +
Runov also explained the difference between the “dipolarization front” and the “dipolarizing flux bundle”. The former is the sharp boundary (about one thermal ion gyroradius thin) separating two plasma populations, whereas the latter follows the front, lasting ~40-50 seconds, and is the region where the electric field enhances. Joachim Birn also included the caveat that these events have to be sufficiently fast, agreeing that they last on the order of minutes. Tying in the Baumjohann et al. tailward-propagating front with the transient earthward-propagating front, he expressed that the earthward-propagating dipolarization event piles up in the near-Earth, transition region. He agreed that the region of enhanced Bz behind the front is the dipolarizing flux bundle (DFB), but views the flow channel behind the DFB (where the magnetic field is not enhanced) as separate. Birn explained that there is a “snowplow effect” before the front, observed as in increase in pressure, but behind the front is reduced entropy. He pointed out that most people now see the transient, small-scale dipolarization and the global dipolarization as two different stages of the same thing.
 +
 +
Misha Sitnov shared his observation that we usually pay attention to the final result of the process that occurs within ~9 RE, what he referred to as “substorm scale dipolarizations” lasting ~20-60 minutes. However, he noted, similar structures are seen by MMS at 25 RE. THEMIS has even observed the tailward-retreating front expanding all the way to lunar distances.  Sitnov expressed his opinion that the conversation surrounding “dipolarization” is semantics; meaning, it is simply some way that the field becomes more dipolar. The method could be a front, a DFB, a substorm, or something completely different. Because the inner magnetosphere has such a large background magnetic field, he pointed out that the phenomenon is more pronounced in the particles.
 +
Xinlin Li shared a similar view, pointing out that one can model the dispersionless injection associated with dipolarization in order to infer information about the dipolarization. Models allow for making the dipolarized region narrow or wide in order to fit the dispersion observed in injections.
 +
Shin Ohtani expressed that in the past, dipolarization was a very simple concept that simply explained that the magnetic field went from a more stretched state to a more dipolar state. He explained that using the auroral definition [e.g., Akasofu 1972; Friedrick 2001], tail stretching and ensuing dipolarization was observed as the poleward boundary moving equatorward, then expanding poleward. The magnetic field at the equator increases sharply close to Earth, then gradually farther out.
 +
 +
Ohtani also pointed out the conundrum of the term “dipolarization” in the near-Earth region where the intense ring current contributes to a field that is “more dipolar” than a dipole. In essence, it is strange to call something “dipolarization” when the field becomes stronger than a dipole. Continuing the topic of conundrums, and perhaps similar to points made about semantics, Ohtani expressed that it is difficult to demarcate between scale sizes: there is no clear line between “large” and “small”. On the extreme “large scale”, we have sawtooth events, which are larger than the substorm dipolarization for example. His preference, therefore, is to use “substorm” as part of the definition when discussing dipolarization. The original definition was a substorm-related reconfiguration of the near-Earth magnetic field, and thus a change in tail current which appears in the ionosphere and which forms the substorm current wedge.
 +
Dick Wolf, on the other hand, agreed with Birn’s analysis and distinguished between two stages in the dipolarization process. He pointed out that if the ionosphere is perfectly conducting (such that the field-line feet are fixed), a localized, depleted flux tube will come to rest in a shortened form. It will have a different shape from the background, a downward parallel current on the eastside and upward on the westside. The equatorial motion involves just an induction electric field which doesn’t map to the ionosphere. However, if the collapse is narrow across Y and conductance is finite, then parallel current leads to westward potential electric field in the ionosphere and in the equatorial plane. The ionospheric foot points move equatorward, and the equatorial crossing point moves earthward. The depleted flux bubbles take the same shape as the background. The time-integrated potential electric field is typically at least as big as the time-integrated inductive electric field. The currents map to the sides of the narrow channel in the ionosphere, and an intense potential electric field exists in the channel. This process would not work for a wide injection, as the currents would map to widely separated spots in the ionosphere. This would not result in an intense potential electric field, and therefore no strong flows.
 +
 +
===Entropy and Bubbles===
 +
Matina Gkioulidou brought up the question of entropy and how it plays a role in bubble formation and propagation. Wolf explained that although you cannot measure entropy directly, it can still be the agent behind the physics. Ohtani shared that he was against any definition based on physics (e.g., referring to the small-scale dipolarizations as entropy “bubbles”). In such cases that the physics behind the phenomenon is later discovered to be different, the field would be stuck with an incorrectly named process. Instead, he advocated to defining phenomena based on morphology (what it looks like in the data), after which the physics can be discussed.
 +
 +
Runov explained that he did try to address it with observations. Using Wolf’s formula for the entropy function, he found it significantly dropped behind the front. The physics is there; however, because he could not obtain concrete numbers, the study did not progress past reviewers. The conversation opened up to the idea, though, that there may be away to estimate it using multi-spacecraft data combined with models. The idea is to translate to the language of local forces (i.e., Li et al., 2011). From their work, the DFB was clearly propelled earthward by curvature force, stopping when the gradient of total pressure became comparable to the magnetic tension force. Vassilis Angelopoulos pointed out that the entropy description allows us to estimate a final state given the initial state, but it doesn’t describe the forces (as the force balance does).
 +
 +
Misha Sitnov slightly disagreed, saying that most processes are driven by interchange such that reconnection ends up as the final point, after interchange instability. Mike Wiltberger disagreed, stating that his model shows reconnection occurring first.
 +
 +
===The Relationship between Scale Sizes===
 +
We then shifted the topic of conversation to the relationship between scale sizes: how do they fit together? Do multiple DFBs make the large-scale, global dipolarization?
 +
Shin Ohtani started us off by pointing to Tanskanen et al. [2002] and Akasofu [2013]. From these works, he believes that 10,000 BBFs are required to compile the energy of the substorm—meaning, the large-scale morphology cannot be simply the compilation of multiple BBFs. Angelopoulos questioned whether or not Akasofu included the thermal energy in his calculation, a question that was followed by Joachim Birn who explained that the major energy source is in the thermal speed, not flow speed. In that case, the accumulation of multiple BBFs/DFBs should suffice. McPherron agreed that his favorite idea is a cumulative effect. Sitnov, on the other hand, stated that dipolarization fronts have no relation to substorms, and that substorms have no relation to storms.
 +
 +
Runov emphasized his earlier point that we are dealing with two sub-systems, or two distinct plasma regimes. What happens in the magnetotail regarding dipolarization fronts may or may not be created by reconnection, it doesn’t matter so much as the fact that it is a high beta regime. Time scales are different than in the low beta regime. In the high beta regime, transient structures are supported. These are connected to a local current system, which propagate earthwards, create field-aligned currents, and connect to a low beta region. The major deposit of energy goes to heating of the local, ambient plasma. The VxB channel accelerates particles around it, which builds localized pressure inside, providing field-aligned current and connecting the low beta regime to the high beta regime.
 +
 +
Runov further explained that when these processes power enough to create a sustained system—and the ionosphere is responsive—then global dipolarization happens. If it isn’t powerful enough, or the ionosphere is not responsive, then global dipolarization does not happen. Pointing to Mercury as an example of a planet with transient dipolarizations but no substorms nor sustained dipolarization, he suggested that the low beta regime is not involved because there is no ionosphere to maintain the current system. Meanwhile, at Earth, each DFB twists flux tubes and creates field-aligned current that the ionosphere then maintains even after the DFB is gone. This allows the time response to be much longer than the actual DFB’s lifetime.
 +
 +
Ryan Dewey explained that currents close through Mercury’s conducting core, not an ionosphere. Runov asked him if he knew why dipolarizations occur in Mercury’s post-midnight sector, because at Earth they start in the pre-midnight sector. Dewey explained that the running hypothesis is that the higher concentration of sodium on Mercury’s duskside could affect local reconnection rates, modifying the asymmetry. 
 +
 +
McPherron explained that he used to think the substorm current wedge originated at the X-line. However, no current wedges form on the dayside—even though there is dayside reconnection—which is evidence that flow bursts are an essential feature. He pointed out that in his MHD simulation, he only saw two flow bursts coming in. This opened discussion on the fact that during a substorm, there are 2-3, sometimes up to 6, flow bursts, and that only 2-3 are enough for flux to build up. Runov, in response, underscored that the effect could be cumulative, BUT it has to be more than that. The process must include the currents, which will sustain the build-up.
 +
 +
Because most flows do not make it in to geosynchronous orbit, Ohtani was still uncomfortable with the idea. Citing Pulkkinen [1992] and Kaufmann [1987], he pointed out that most current enhancement occurs within 10 RE. Therefore, the current must somehow intensify just outside GEO…how? McPherron suggested that as the magnetic field strength goes up, the flow velocity decreases to below instrument measurement levels.
 +
 +
Unconvinced, Ohtani pointed to his 2006 paper that demonstrated the magnetic field at geosynchronous orbit can continue to be stretched even though Geotail observed the flow at large distances. He concluded that the magnetic field measured at GEO by GOES is determined by a more global current system. For example, in a psuedobreakup, Geotail observed the dipolarization front and fast earthward flow. Meanwhile, at GOES, the magnetic field became more stretched. Then, after substorm onset, there was dipolarization. He therefore sees localized and large-scale dipolarizations as completely different events that may have no physical connection.
 +
 +
This concluded Session 1.
 +
 +
===Session 2===
 +
We began session 2 by recapping session 1, and answering an audience question about bursty bulk flows (BBFs). Angelopoulos explained that they are fast flows lasting over ten minutes with a series of distinct dipolarizations and dawn-dusk Ey. He also explained that the most efficient flux transport occurs as the DFB (75% within the BBFs). Runov further detailed the BBFs by saying that many observations have shown the cross-scale structure of the DFBs are only one to a few RE wide. In terms of plasma physics, that’s a few tens ion inertial lengths. Along with them, Christine Gabrielse answered a question that yes, electrons could be transported all the way from the reconnection region, although ions have different drift motions. Drew Turner expressed that all of these terms are related to the small-scale.
 +
Using ground magnetograms, McPherron obtained substorm parameters, such as that it can expand East and West. The difficulty is that you must know the onset time in order to do the inversion. Plus, you’re looking at changes on the ground, which is an indirect observation. If the onset is isolated, it is easier to accomplish.
 +
 +
===Entropy Part II===
 +
The discussion about entropy and how models deal with it resurfaced at this point. Sitnov shared his opinion that in a global simulation with a boundary, entropy is stable—it is decreasing with R—which makes it different than reality. Birn explained that when you compress the tail and assume some closed field boundary, you get flux tubes.
 +
 +
Don Mitchell shared that the phenomenology is similar at Saturn, which doesn’t depend on reconnection. The field is stretched by a different mechanism. In that case, whatever preconditions the system is less important than how the system reacts to the configuration.
 +
Sitnov’s thought is that for small-scale fronts and related processes, we need to understand the source region or mechanism (such as reconnection), which can only be simulated with kinetic codes. He stresses that with MMS, it is prime time to understand what causes this ideal process that releases the stresses in the magnetotail. Although he is uncertain that we have a model that can accomplish this, he feels we have enough data to empirically put the substorm sequence together. We are now asking ourselves: What happens to dipolarization fronts when they penetrate the inner magnetosphere? No equilibrium model exists in the inner magnetosphere, so we rely on Wolf’s ring current model and the quasi-static approximation. This works very well in the inner magnetosphere, and MHD works well in the tail; however, we are lacking a robust description for the transition region. One suggested solution is to utilize hybrid models, which can take energy-dependent drifts into account.
 +
 +
It was then pointed out that San Lu is using a hybrid code that is coupled to the transition region using PIC code, which is used to model ~7-10 RE. (However, it doesn’t go farther in than that.) The resolution is as high as the computers can take.
 +
 +
===Looking Forward===
 +
Sitnov then suggested we compare one global, one hybrid, and one PIC model for the same event to see if the resolution is enough. Anton Artemyev shared that hybrid models produce the current sheet better than kinetic models, because kinetic models are stationary. The problem is the boundary conditions: if the initial state is not correct, the model cannot produce the system’s evolution. He also reminded the audience that there is no such thing as a good or bad model…but good or bad questions to ask the model.
 +
Ohtani shared that the question we should ask is whether we can have a substorm without the ionosphere? He reminded us that before the THEMIS era, substorms were called “the two-minute problem” because the resolution of observations allowed for a two minute window of uncertainty. This window of uncertainty is what gave rise to the in-out vs. out-in interpretation of the onset phenomenology. Because of the ionosphere’s importance in this, he asked whether we include the ionospheric effect well enough? What aspect(s) are we missing? How do we associate what we see in the auroral images with phenomena in the tail?
 +
Wolf answered the first question by explaining that we cannot neglect the ionosphere: it’s an active participant in the process. He also agreed with earlier points that no one model can get everything correct. McPherron agreed, pointing out from his model that conductance is essential for substorms: if conductivity is increased, bubbles can make it farther earthward.
 +
 +
Eric Donovan shared that as far as modeling goes, one thing that is troubling is that the simulation movies are so different than what he observes in the ionosphere. His perspective is that the movies (with all the fast flows, bubbles, and DFBs) are what happen AFTER the expansion phase occurs in the ionosphere. Learning how to reconcile what the simulations show with the 2D picture from the ionosphere is something that we should take very seriously. For instance, the simulations look very chaotic. However, in the late growth phase, things are very ordered…auroral arcs are very clear. He therefore does not see how BBFs, flux bundles, DFBs, etc. can be causal for the onset in the inner magnetosphere.
 +
 +
Donovan suggested that we make maps of the magnetospheric models in the ionosphere. What would the diffuse aurora look like in the ionosphere? What would the proton aurora be doing? Then compare with the data. He also suggested that a mission with 50 spacecraft in the nightside transition region between 6-12 RE, similarly distributed, would provide the better fidelity required to explore what is really happening.
 +
 +
Runov shared that what he would like to see is an increased fleet of low-orbiting spacecraft equipped with high energy particle detectors and better magnetometers. These would remove the need to remote sense the magnetic configuration. This could be very powerful, but we would require auroral observations to assist with the models in order to complete a comprehensive picture.
 +
 +
To address the question of, “Is the auroral observation an ionospheric source or a magnetospheric source,” Drew Turner suggested conjugate imagers in the Northern and Southern hemispheres. Donovan followed up by stressing that our field has really undervalued imaging. We are willing to spend millions on satellite missions, but balk at spending money on imagers.
 +
 +
In the spirit of forward-looking ideas, Ohtani shared that it would be great to have an EM imager: low energy, stereo imaging. The ENA image could look at the change of topology with the flux enhancement.
 +
 +
===Contributed Talks===
 +
We had ten contributed talks that discussed dipolarization and its effect on the inner magnetosphere.
 +
<ol>
 +
<li>  Sheng Tian presented on “Poynting flux at the PSBL in conjunction with the ground aurora: dipolarization at L~6”. He created a new mapping perspective using a vertical and a horizontal box, where the vertical box maps to the PSBL while the horizontal box maps to the equator. He showed that the dipolarization front correlates to poynting flux in the PSBL which mapped to the ionosphere where aurora was observed. Enhancement of ion outflow occurred right after the increase in poynting flux.
 +
<li> Grant Stephens presented on “Magnetotail thinning and dipolarization during substorms: Empirical picture”, using an empirical model (TS07D). He replaced uniform equatorial current sheet thickness with multiple current sheets of differing thicknesses to reproduce current sheet thinning in the growth phase. He also utilized a new field aligned current description to reproduce the Harang reversal, which proved to be critical to reproducing the substorm dipolarization. The model is not a statistical average, but a statistical average for specific events.
 +
<li> Katie Garcia-Sage presented on “Global MHD Simulations in the context of Magnetotail Stability Theory”. She showed that a ridge or “hump” in Bz could form downtail, which could be interchange unstable. The ridge corresponds to fast flows at the flanks, and remains stable for a long time before going unstable. She showed that distant reconnection causes flows which break around -20 RE. On average, she sees a nice, smooth entropy profile downtail, but gets a high Cd ridge sitting at the velocity convergence. This builds up in what she calls the “flow braking region”, which is at -25 RE (not at -12 RE where we typically think of flow braking).
 +
<li>  Don Mitchell presented on “Ion injections inside geosynchronous orbit: charge- (not mass) dependent (quasi-) adiabatic acceleration”.  He found that all ion species were being energized by the same process (adiabatically). A 180 keV O ion behaves like a 180 keV H ion. The energy gain of the O6+ particles is six times that of a singly charged ion. 
 +
<li> Kareem Sorathia presented “Ion Transport and Acceleration at Dipolarization Fronts: High-Resolution MHD – Test-Particle Simulations”. Using Mike Wiltberger’s LFM simulation, he followed particles in a convection surge (an increase in earthward flow/azimuthal EY). The inverse magnetic field gradients associated with a localized dipolarization front form magnetic islands that can trap ions in their guiding center trajectories. This trapping enables ions to propagate earthward. When he traced many particles, a core group remained at 90 degrees, even though many were pitch angle scattered. These would be able to continue traveling earthward with the front. Looking at the phase space density evolution, he saw a transition to a kappa distribution.
 +
<li> Tetsuo Motoba presented on “Response of energetic particles to dipolarization with GEO”. He discussed whether large/impulsive dipolarization electric fields are necessary for particle injections. In observations, these fields are azimuthally localized, and range from a few mV/m to tens of mV/m.
 +
<li>  Andrei Runov discussed “Ion distributions within dipolarizing flux bundles (DFBs) in the near-Earth plasma sheet and the tail-dipole transition region”. Using THEMIS event studies, PIC simulations, Test Particle Modeling and, he discussed how ion injections associated with DFBs may provide a free energy source for the EMIC and MS wave excitation in the inner magnetosphere because DFBs may bring 90 degree anisotropic distributions into the inner magnetosphere.
 +
<li> Yiqun Yu discussed “Effects of bursty bulk flows on large-scale current systems”. She coupled MHD with ionosphere and ring current using BATSRUS, RCM, and RIM to plot field aligned current patterns. As BBFs break around -10 RE, vortices emerge in pairs on the edge of the breaking region (type 1) and in the inner magnetosphere (type 2), connecting to the substorm current wedge. BBFs continually impinge on the dipolar region and brake, disturbing the pressure distribution and field aligned currents. A new ring current is created as a result of multiple localized BBFs.
 +
<li>  Xiangning Chu discussed “Magnetotail flux accumulation leading to auroral expansion and substorm current wedge: A case study”. Because pressure gradient and flux tube volume are hard to obtain from in-situ observations, the SCW cannot be obtained from spacecraft. He explained that the substorm current wedge is generated by accumulated flux from the dipolarized magnetic field lines, which causes poleward expansion. Flow braking and diversion can bend field lines and generate field aligned currents.
 +
<li> Eric Donovan presented his view, in response to the earlier discussion, that it is an instability—not flux pile-up—which causes auroral brightening.
 +
</ol>
 +
 +
===Joint Sessions with “Magnetic Reconnection in the Magnetosphere” and “Tail Environment and Dynamics at Lunar Distances” FGs===
 +
The “Magnetic Reconnection in the Magnetosphere” focus group joined with the “Tail Environment and Dynamics at Lunar Distances” and “Magnetotail Dipolarization and Its Effects on the Inner Magnetosphere” FGs on Monday afternoon at GEM this year (06/19/2017). These two joint sessions encouraged cross-focus group interaction, and open ended discussion on the topics including the onset of tail reconnection, the role of cross-tail instabilities, the difference between the tailward and earthward reconnection jets/flux bundles, the interaction of dipolarization fronts with ambient plasmas. There were approximately 70 attendees in these two joint sessions.
 +
 +
Vassilis Angelopoulos kicked off the first session with a tutorial talk. Vassilis provided a broad view of the observation and modeling of the nightside phenomena and substorms. Topics include the ionospheric signature, substorm current wedge (SCW), near-Earth-neutral line, current disruption versus reconnection models, external-driven versus spontaneous onset, dipolarization fronts, bursty-bulk flows (BBFs). In particular, Vassilis challenged global modelers for a quantitative assessment of the rate and intensity of BBFs, which brought up discussion on the time-scale difference of BBFs and SCW. At the end of his talk, Vassilis suggested the idea of employing neural networks, to conjoint statistics of occurrence rates and characteristics from multi-mission datasets.
 +
 +
Misha Sitnov described the internally driven (aka spontaneous) onset of magnetotail reconnection, which is only possible - in the case of electrons magnetized initially by the normal magnetic field - when that field has a region with a tailward gradient. 3D PIC simulations of the corresponding ion tearing instability show that its distinctive features are: 1) spontaneously generated earthward plasma flows that precede the topology change, 2) new Hall pattern, opposite to the classical quadrupole pattern near the X-line; 3) new dissipation region (j*E’>0) at the dipolarization front that may form before the X-line electron dissipation region.”
 +
 +
Heli Hietala presented ARTEMIS two-spacecraft observations of reconnection in the presence of density asymmetry in the lunar distance magnetotail. The observations also indicate the reconnection flow channel had a finite width, of the order of 5 Earth radii.
 +
 +
Andrei Runov discussed kinetic properties of earthward-contracting dipolarizing flux bundles (DFBs) observed by THEMIS in the near-Earth tail and tailward progressing rapid flux transport  (RFTs) enhancements observed by THEMIS in the near-tail and by ARTEMIS at lunar orbit, respectively. The DFBs and RFTs are considered as earthward and tailward ejecta from near-Earth reconnection. It was shown that whereas DFBs interacts with near-tail plasma populations and particles within DFBs gain energy from the increasing magnetic field, the RFT particles do not interact with ambient field and plasma and keep the energy gained during reconnection. The plasma state within RFTs is close to isothermal.
 +
 +
Joachim Birn presented a comparison of ion distributions earthward and tailward of the reconnection site, obtained by a combined MHD/test particle approach. While ions on the earthward side might experience multiple, Fermi or betatron-like, acceleration, leading to multiple beams and ring-like distributions, ions on the tailward side experience only single direct acceleration, adding a beam to an unperturbed population.
 +
 +
The GEM-style forum successfully stimulated active discussions between the presenters and audience, including Bob McPherron, Mostafa El Alaoui, Eric Donovan, Matina Gkioulidou, San Lu, Xiangning Chu, Chih-Ping Wang, Drew Turner, Christine Gabrielse et al.
 +
 +
  
 
== Resulting Papers ==
 
== Resulting Papers ==
Line 137: Line 712:
 
<li> McPherron, R.L., M. El-Alaoui, R.J. Walker, and R. Richard (August 2020), '''Characteristics of Reconnection Sites and Fast Flow Channels in an MHD Simulation''', J. Geophys. Res. - Space Physics, [https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019JA027701 https://doi.org/10.1029/2019JA027701].</li>
 
<li> McPherron, R.L., M. El-Alaoui, R.J. Walker, and R. Richard (August 2020), '''Characteristics of Reconnection Sites and Fast Flow Channels in an MHD Simulation''', J. Geophys. Res. - Space Physics, [https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019JA027701 https://doi.org/10.1029/2019JA027701].</li>
 
<li> Ghaffari, R., Cully, C. M., & Gabrielse, C. (2021). '''Statistical study of whistler-mode waves and expected pitch angle diffusion rates during dispersionless electron injections.''' Geophysical Research Letters, 48, e2021GL094085. https://doi.org/10.1029/2021GL094085</li>
 
<li> Ghaffari, R., Cully, C. M., & Gabrielse, C. (2021). '''Statistical study of whistler-mode waves and expected pitch angle diffusion rates during dispersionless electron injections.''' Geophysical Research Letters, 48, e2021GL094085. https://doi.org/10.1029/2021GL094085</li>
 +
<li> Runov, A., Angelopoulos, V., Henderson, M. G., Gabrielse, C., & Artemyev, A. (2021). '''Magnetotail dipolarizations and ion flux variations during the main phase of magnetic storms.''' Journal of Geophysical Research: Space Physics, 126, e2020JA028470., https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020JA028470
 
<li> Nishimura, Y., Artemyev, A. V., Lyons, L. R., Gabrielse, C., Donovan, E. F., & Angelopoulos, V. (2022). '''Space-ground observations of dynamics of substorm onset beads.''' Journal of Geophysical Research: Space Physics, 127, e2021JA030004. https://doi.org/10.1029/2021JA030004.</li>
 
<li> Nishimura, Y., Artemyev, A. V., Lyons, L. R., Gabrielse, C., Donovan, E. F., & Angelopoulos, V. (2022). '''Space-ground observations of dynamics of substorm onset beads.''' Journal of Geophysical Research: Space Physics, 127, e2021JA030004. https://doi.org/10.1029/2021JA030004.</li>
 
<li> Ohtani, S., Motoba, T., Gjerloev, J. W., Frey, H. U., Mann, I. R., Chi, P. J., & Korth, H. (2022). '''New Insights into the Substorm Initiation Sequence from the Spatio-temporal Development of Auroral Electrojets.''' Journal of Geophysical Research: Space Physics, 127, e2021JA030114. https://doi.org/10.1029/2021JA030114.</li>
 
<li> Ohtani, S., Motoba, T., Gjerloev, J. W., Frey, H. U., Mann, I. R., Chi, P. J., & Korth, H. (2022). '''New Insights into the Substorm Initiation Sequence from the Spatio-temporal Development of Auroral Electrojets.''' Journal of Geophysical Research: Space Physics, 127, e2021JA030114. https://doi.org/10.1029/2021JA030114.</li>
 +
<li> Gabrielse C, Gkioulidou M, Merkin S, Malaspina D, Turner DL, Chen MW, Ohtani S-i, Nishimura Y, Liu J, Birn J, Deng Y, Runov A, McPherron RL, Keesee A, Yin Lui AT, Sheng C, Hudson M, Gallardo-Lacourt B, Angelopoulos V, Lyons L, Wang C-P, Spanswick EL, Donovan E, Kaeppler SR, Sorathia K, Kepko L and Zou S, (2023) '''Mesoscale phenomena and their contribution to the global response: a focus on the magnetotail transition region and magnetosphere-ionosphere coupling.''' Front. Astron. Space Sci. 10:1151339, https://doi.org/10.3389/fspas.2023.1151339

Latest revision as of 16:28, 6 June 2024

Contents

Focus Group Chairs

Christine Gabrielse -- The Aerospace Corporation ----------- contact: christine.gabrielse at aero.org
Matina Gkioulidou --- John Hopkins Applied Physics Lab
Drew Turner ---------- John Hopkins Applied Physics Lab
Slava Merkin --------- John Hopkins Applied Physics Lab
David Malaspina ----- LASP, University of Colorado
Adam Michael -------- John Hopkins Applied Physics Lab

Focus Group Science Topic

The overarching goal of this focus group is to utilize both in situ and ground-based observations alongside state-of-the-art models and theory to better incorporate magnetotail dipolarizations in global stand-alone and coupled magnetospheric models, refining our conceptual models of this phenomenon and examining its impacts on the inner magnetosphere.

In our pursuit of that goal, we plan to work with the community in formulating and investigating science questions that pertain to this focus group topic and its overarching goal, some examples of which include:

  1. What are the mechanisms responsible for both elementary and global magnetotail dipolarizations and are they captured by current state-of-the-art models?
  2. What is the role of reconnection and/or other plasma instabilities in producing elementary magnetotail dipolarizations?
  3. What is the relationship, if any exists, between elementary magnetotail dipolarizations and more global dipolarization during substorms?
  4. What is the role of elementary magnetotail dipolarizations in:
    • enhancements of the ring current?
    • creating the seed electron population for the radiation belts?
    • the generation of different wave modes (e.g., ULF, chorus, hiss, EMIC, equatorial noise, etc.) in the inner magnetosphere?


For full FG proposal CLICK HERE.


GEM 2023

Monday 13:30-15:00 PDT Joint Session with MESO: Room A

The Substorm Current Wedge Paradigm

Scene setting talks on the Substorm Current Wedge Paradigm.

  1. Bob McPherron (UCLA)
  2. Larry Kepko (NASA/GSFC)
  3. Jesper Gjerloev (JHU/APL)

Monday 15:30-17:00 PDT Joint Session with MESO: Room A

The Substorm Current Wedge Paradigm

Discussion with a panel of experts.

  1. Joachim Birn (Space Science Institute)
  2. Slava Merkin (JHU/APL)
  3. Shin Ohtani (JHU/APL)
  4. Toshi Nishimura (Boston University)
  5. Karl Laundal (University of Bergen)

Tuesday 13:30-15:00 PDT Joint Session with RB: Room A

We will discuss the following:

  1. Where are we in terms of quantifying the contribution of mesoscale injections/bubbles to the ring current and/or radiation belt?
  2. What can we accomplish with current assets?
  3. Do we need new missions to answer the question (and if so what is needed)?

Confirmed discussion leaders:

  1. Matina Gkioulidou Observing the global geospace at mesoscale resolution
  2. Anthony Sciola - The contribution of plasma sheet bubbles to stormtime ring current buildup and evolution of the energy composition
  3. Sina Sadeghzadeh - RCM Modeling of Bubble Injections into the Inner Magnetosphere: Spectral Properties of Plasma Sheet particles
  4. Anton Artemyev - ELFIN+injections: Relativistic electron precipitation driven by plasma injections

Tuesday 15:30-17:00 PDT: Room Coa

We will review the FG’s original goals/questions and then ask, “Where were we and how far have we come?” There will be a panel discussion concluding with community discussion.

Confirmed Panelists:

  1. Andrei Runov
  2. Joachim Birn
  3. Anton Artemyev
  4. Grant Stephens
  5. Shin Ohtani

Wednesday 10:30-12:00 PDT Joint Session with CPMP and RB: Room A

Scene setting talk by George Clark about the vision for CPMP with regards to particle energization, radiation belts, etc.

Panel-led Discussion

Wednesday 13:30-15:00 PDT: Room Coa

We pose the question, “Where are we going? What are the open questions we should carry into new FGs?”

Contributed Speakers:

  1. Toshi Nishimura: Connection between substorm onset and expansion phase activity: How near-Earth instability transitions to expansion-phase BBFs/DFs
  2. Harry Arnold: Grey Box Modeling: Empirical Resistivity Maps and Thin Current Sheets
  3. Xiantong Wang: BBF simulations using the two-way coupled MHD-PIC code
  4. Konstantinos Horaites (Minna Palmroth): Magnetotail plasmoid eruption: Interplay of instabilities and reconnection
  5. Sanjay Chepuri: Testing Adiabatic Models of Energetic Electron Acceleration at Dipolarization Fronts
  6. Anthony Rogers: Utilizing GPS as an asset to study injections
  7. Yangyang Shen: Contribution of Kinetic Alfven Waves to Energetic Electron Precipitation from the Nightside Transition Region during a Substorm

Thursday 13:30-15:00 PDT Joint Session with MESO and MPEC: Room A

We will have a panel-led discussion on the following questions:

  1. What can ground-based observations tell us about the mesoscale phenomena occurring in space?
  2. How can we leverage the existing ground-based networks and operational spacecraft data we have for multi-scale coupling research without having the propelling effect of a big NASA mission (like THEMIS or GDC)?
  3. How do model results of mesoscale dynamics translate to ground-based observations in the nightside? Do the model results present similar ground-based signatures in terms of scale size, temporal evolution, and overall dynamics?

After the panel provides their thoughts, we will open up the discussion to the room. Please join in! We wish to run it "workshop style", so please bring a slide if you want to help with the discussion, or just your thoughts.

Panelists:

  1. Kareem Sorathia (Modeling)
  2. Bashi Ferdousi (Modeling)
  3. Toshi Nishimura (Data)
  4. Sneha Yadav (Data)
  5. Emma Spanswick (Data)

mini-GEM 2022

12:00-13:30 CST Joint Session with MESO and RB: Williford C Room

To attend virtually: https://njit.webex.com/meet/gperry

We will run the session like many successful sessions in the past, during which we ask participants to address one or more of the following questions:

How important are mesoscale injections that initiate in the tail plasma sheet to the inner magnetosphere? What’s the role of the transition region in mass and energy transport? How effectively can plasma sheet injections get through the transition region, and/or does the transition region act as a filter? Are the important injections those that are more global in scale? Or, what are we missing in data and/or models to answer these questions?

Confirmed Participants:

  • Ian Mann - The Canadian RADiation Impacts on Climate and Atmospheric Loss Satellite (RADICALS)
  • Amy Keesee - How effectively can plasma sheet injections get through the transition region
  • Christine Gabrielse - Using ground-based observatories to probe the mesoscale contribution
  • Harry Arnold - Meso-Scale Injections from Global Magnetosphere Simulations using Data Mining Derived Resistivity

15:30-17:00 CST: Williford A Room

To attend virtually: https://jhuapl.zoomgov.com/j/1611494286?pwd=MndKdXFuSnV4bnVsckFRSjZyLzA2QT09

Understanding how energy is transferred between scale sizes is an important yet unresolved question in the field. How much energy from large- or meso-scale phenomena is dissipated into smaller scales via turbulence/energy cascade? How much energy is transferred up to larger scales via waves? Topics could include generation of anisotropies, kinetic instabilities, particle scattering at BBFs and DFs, and more.

Confirmed Participants:

  • Adam Michael
  • Sasha Ukhorskiy

Discussion Summary

  • Adam Michael - Meso to Micro: Cross-Scale Modeling of Bursty Bulk Flows in the Inner Magnetosphere. Adam began by asking the question how do bursty, mesoscale flows in the magnetotail, alter the microscale wave particle interactions in the inner magnetosphere. He highlighted past results published by Wiltberger et al. (2015; https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2015JA021080) and Sorathia et al. (2021; https://www.frontiersin.org/articles/10.3389/fspas.2021.761875/full) detailing the ability of global magnetosphere models to produce similar statistical behavior of BBFs as observations. He showed that the reduced density and increased magnetic field within BBFs enable electrons that otherwise would not resonate with chorus waves, to be scattered and energized and for lower energies (10s of keV) BBFs can increase the bounce averaged quasi-linear diffusion rates to be on the order of a few minutes, which can potentially lead to localized, bursty precipitation features.
  • Sasha Ukhorskiy – Cross-Scale Nature of Magnetotail Dynamics. Sasha discussed that understanding causal relationships among intrinsically coupled multi-scale manifestations of magnetotail dynamics has been elusive due to the sparse nature of in situ measurements available so far. He showed numerical test particle simulations that detail how magnetic islands can stably trap particles and transport them from the tail to the inner magnetosphere, across >10 Re leading to energization to MeV energies of the core radiation belt population. The mesoscale flows also drive interchanging regions of parallel and perpendicular anisotropies and he discussed the potential connection to wave activity observed along dipolarization fronts by MMS and the observed correlation of electron microbursts and pulsating aurorae which might be attributed to both being produced by whistler waves generated in BBFs/DFs. He argues that understanding microscale/kinetic effects of mesoscale dynamics, such as the possible connection of microbursts and pulsating aurora with streamers/BBFs requires a system view of the M-I system and could potentially be addressed by a distributed LEO observatory equipped with auroral imagers, magnetometers, and energetic particle sensors.

GEM 2022

Tuesday 10:30-12:00

Models have shown that BBFs/DFBs/Mesoscale Injections are important for both ring current and radiation belt build-up, but it remains difficult to verify with observations and therefore the exact role remains an open question. Model results are inconclusive because existing models do not describe all the relevant domains (e.g., magnetotail, transition region, inner magnetosphere) self-consistently while capturing the required range of spatiotemporal scales. We therefore ask, How is plasma and electromagnetic energy transported through the plasma sheet to the inner magnetosphere at different spatiotemporal scales?

In this session, we will discuss the perennial question, What is the role of reconnection and/or other plasma instabilities in producing elementary magnetotail dipolarizations? How do the physical properties of the dipolarizations depend on their spatial location and the time of their appearance relative to the Dungey cycle?

Discussion Leaders/Speakers

  • Toshi Nishimura (work presented by Christine Gabrielse): Kinetic Plasma Structures Associated with Substorm Auroral Beads by Space-Ground Coordinated Observations
  • Shin Ohtani (work presented by Jesper Gjerloev): New Insights into the Substorm Initiation Sequence from the Spatio-temporal Development of Auroral Electrojets
  • Sneha Babu
  • Jinxing Li: The response of ionospheric currents to different types of magnetospheric fast flow bursts


Upload Talks here: https://vgem.org/groups/DIP

Discussion Summary:

  • Toshi Nishimura (work presented by Christine Gabrielse): Kinetic Plasma Structures Associated with Substorm Auroral Beads by Space-Ground Coordinated Observations
    • Addressing, “What is the role of reconnection and/or other plasma instabilities in producing elementary magnetotail dipolarizations?”
      • From an auroral imaging point of view, substorm onset instability occurs first and then substorm dipolarizations occur. A movie was shown that showed the waves along the onset arc start first and then auroral streamers come next.
      • (1) Growth-phase streamers do occur before substorm onset. Their flows and dipolarizations are weaker than expansion-phase, intense streamers/flows/dipolarizations. The two types of events should be considered separately.
      • (2) This auroral observation doesn't rule out the possibility that a dipolarization occurs before reconnection. Near-Earth Neutral Line (NENL) activation could occur soon before or after each streamer/dipolarization. It's difficult to evaluate this causality from observation.
      • During the growth phase, however, a PBI is followed by a streamer, suggesting that reconnection occurs first.
    • Addressing, “How do the physical properties of the dipolarizations depend on their spatial location and the time of their appearance relative to the Dungey cycle?
    • Toshi thinks dipolarizations that form closer to the Earth penetrate deeper into the inner magnetosphere due to its low initial entropy and less closed magnetic fluxes. Dipolarizations forming farther away have higher entropy and are decelerated more by the dipolar field. Very narrow channels may dissipate quickly, but both narrow and wide channels can penetrate deep.

A discussion about auroral beads followed. It was expressed that not many in the audience understand what auroral beads are. What do we know about these beads? Are they really associated with substorm onset? What’s the physics?

  • One difficulty in answering this is the difference across community members on what defines the substorm onset.
  • Another difficulty is mapping: When parallel electric fields are involved (as with dynamic, discrete aurora), we can no longer map along a modeled magnetic field line to know where that auroral signature comes from in space.
  • It was mentioned that an electric field in space is related to the beads, but without in situ measurements.
  • Incoherent scatter radars have showed that there is an electric field and oscillating electrical field associated with the beads. So you can infer the electric field pattern from the SuperDARN data with the beads.
  • Sneha Babu: Auroral beads, substorm onset mechanisms with a focus on the current disruption paradigm and ballooning instability as a trigger
    • Focused on the current disruption paradigm where she sees that ballooning instability could trigger the onset of substorm.
    • For the ballooning instability, the threshold is mildly parallel anisotropy, so even if you have mild parallel anisotropy, it can trigger ballooning instability.
    • Audience member asked, “Any idea why there is a dropout of the perpendicular particles to create the parallel anisotropy?
      • Two combined effects:
        • Drift shell splitting: there can be more parallel particles in the polar region.
        • Conservation of the first and the second adiabatic invariants. You see more parallel particles when the magnetic field line stretches. The 90 degree pitch angle particles try to follow a constant field strength, but low pitch angle particles follow these stretched lines.
    • Audience member asked, “What is the scale size of the ballooning instability? Like we are talking about something that covers the entire nightside magnetotail? Is it localized?”
      • It’s localized. The ballooning instability is because of the curvature of the magnetic field line, because it’s stretching, so it’s localized.
    • An audience member asked if the parallel anisotropy might relate to the beads.
      • Case study showed beads in the image when ballooning instability occurred.
  • Shin Ohtani (work presented by Jesper Gjerloev): New Insights into the Substorm Initiation Sequence from the Spatio-temporal Development of Auroral Electrojets
    • Jesper asked the audience, “When is the Bz signature a dipolarization and when is it just a wiggle?”
    • Andrei Runov: Dipolarization is larger scale and “sustained”, dipolarization fronts and dipolarizing flux bundles are smaller and separate and more short-lived. We are still addressing their relationship to one another. We must consider the pressure build-up ahead of dipolarizing flux bundles (DFBs) as part of what helps sustaining the larger dipolarization.
    • Editor’s note: See Paper that was published after this summarizing dipolarization definitions: https://www.frontiersin.org/articles/10.3389/fspas.2023.1151339/full
    • Jesper shared opinion that the definition of a substorm used to be focused on a larger scale thing, but nowadays some people define smaller scale, short-lived events as substorms, so the definition is more hand-wavy. We need to clean up our language.
      • For example, is a single BBF a substorm? Jesper says no, but feels some people do define it that way.
    • More discussion continued about definitions and phenomenology.
    • An audience member pointed out that a substorm is not a clean “domino effect” in which one phenomenon starts off the process that leads to the next, to the next, and so forth. A substorm is a system response with lots of phenomena that go into making up a substorm, that all have to work together in order to make the large-scale current system and the auroral expansion. So a little wiggle may not be a substorm, but it may help comprise a substorm. This is perhaps an unsatisfying answer for why there is a sense of confusion surrounding the substorm.
    • Christine concluded mentioning that a paper was going to be written up that summarized a lot of definitions that were set when the DIP FG began in 2017. That paper is linked above.
  • Jinxing Li: The response of ionospheric currents to different types of magnetospheric fast flow bursts
    • An audience member asked why the substorm related fast flows can penetrate into the inner magnetosphere?
      • Jinxing’s answer was that he thinks the substorm onset related fast flows have a larger magnetic flux transport, so they can bring more energies into the inner magnetosphere. He also pointed out the low entropy bubble model.
      • Andrei Runov shared that the bubble explanation details that the fast flows are underpopulated flux tubes with smaller entropy, which is mainly dependent on the length of the field line. When it gets closer to Earth, it moves towards lower entropy and will propagate until the flux tube entropy equals the surrounding entropy.
    • From a modeling perspective, Xiantong found that in some driving conditions there can be current sheet break ups closer to Earth that have a larger geomagnetic impact. He’s wondering if there’s observational evidence showing that if these fast flows are generated closer to Earth?
      • Hard to answer observationally since satellites don’t know when/where the flow originated.

Thursday 10:30-12:00

Models have shown that BBFs/DFBs/Mesoscale Injections are important for both ring current and radiation belt build-up, but it remains difficult to verify with observations and therefore the exact role remains an open question. Model results are inconclusive because existing models do not describe all the relevant domains (e.g., magnetotail, transition region, inner magnetosphere) self-consistently while capturing the required range of spatiotemporal scales. We therefore ask, How is plasma and electromagnetic energy transported through the plasma sheet to the inner magnetosphere at different spatiotemporal scales?

Especially given the upcoming Decadal Survey, we want to use a designated session to discuss the following:

  • What do we currently know from models?
  • What are the limitations of the current models?
  • What modeling advances are required and feasible to resolve these limitations in the near- and long-term future (e.g., 5-10 years vs 10-30 years)?
  • What do we currently know from observations?
  • What can we still learn from existing observations and what are their limitations that inhibit further progress?
  • What new observations are required and feasible to resolve these limitations in the near- and long-term future?

Discussion Leaders/Speakers (Contact the DIP FG leaders if you want to participate as well!)

  • Christine Gabrielse: An overview of a white paper drafted based on prior GEM debates
  • Slava Merkin
  • Joachim Birn: Electron acceleration and anisotropies in dipolarization events
  • Andrei Runov: Dipolarizations & Injections: Unfinished Business


Upload slides here: https://vgem.org/groups/DIP

Discussion summary: Discussion Leaders/Speakers were:

  • Christine Gabrielse: An overview of a white paper drafted based on prior GEM debates. This was ultimately published in a longer form here:https://www.frontiersin.org/articles/10.3389/fspas.2023.1151339/full
  • Slava Merkin: Slava discussed that both global and regional models robustly suggest that BBFs and injections play an important role in cumulatively building by RC pressure and bringing in mag flux to inner mag. Global dipolarization was result of many local ones (see Merkin et al., 2019 https://doi.org/10.1029/2019JA026872; Birn et al., 2019 https://doi.org/10.1029/2019JA026658). He pointed out that the simulation resolution matters a lot—but that before we try to model non-MHD physics, we first need to resolve mesoscales in the simulations. He discussed current model limitations and the advances that are required in the next 5-10 years (test particle feedback in the transition region, global Hall for real events, Hall MHD and embedded kinetics like PIC, collisionless Hall), and the next 10-30 years (global hybrids that run for longer, artificial intelligence).
  • Joachim Birn: Electron acceleration and anisotropies in dipolarization events. Joachim showed that electron anisotropies in dipolarization fronts, if field-aligned, may directly cause precipitation, whereas perpendicular anisotropies will drive waves and Poynting flux. He showed MHD simulations of near-Earth tail reconnection and earthward propagating DFBs, combined with backward tracing test particles. He showed that acceleration takes place during neutral sheet crossings and is mostly parallel, but also perpendicular.
  • Andrei Runov: Dipolarizations & Injections: Unfinished Business. Andrei asked the question, are mesoscale structures rapid flux transport (RFTs) associated with global substorm onset? He found no correlation between the DFB observed at THEMIS (~10 RE) and ion injections observed by LANL at GEO, but for large dipolarizations that last for half an hour there is a 30% connection between the two missions. More details in his publication: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020JA028470. Importantly, he posed the following open questions: What is the key ingredient to make a dipolarization effective for ion injection into GEO? DFBs in the near tail show magnetic field variations on short scales (< 100 s) but we do not see this in the ion fluxes at GEO, why? What is the fate of energetic particles that drifted away flank-ward? Do they contribute to the partial ring current? He also supplied what he thinks is required to answer these questions: Observations by a fleet of azimuthally-separated equatorial probes with MAGs and energetic particle detectors in the tail-dipole transition region; conjugate observations by polar-orbiting LEO probes with MAGs and energetic particle detectors for remote sensing, field aligned currents; models resolving energy-dependent drifts. This was especially important in helping form the White Paper for the Decadal Survey.
  • Matina Gkioulidou: Mesoscale Processes: The bridge to cross. Matina highlighted that mesoscale processes are the bridge to connect the local to global nature of geospace. She brought up the fact that the role of mesoscale injections on ring current build-up and transport still remains a major open question, as well as the mesoscale contribution to the substorm current wedge. Another open question is how auroral features evolve with respect to plasma sheet structures. She shared the mission concept, PARAGON, which would address all these questions in the magnetosphere and M-I coupling. It requires coordinated measurements from imaging spacecraft (ENAs in the tail and of auroral emissions in the ionosphere) plus in situ measurements from satellites with elliptical orbits in the equatorial plane.

The group discussed these open questions and emphasized the importance of imaging to get a 2D perspective of magnetosphere and ionosphere dynamics.

Friday 10:30-12:00 MESO/DIP Joint Session

For the duration of the DIP FG, we have discussed the roles that mesoscale phenomena (e.g. DFBs, BBFs, injections, streamers) play with respect to the global system response (e.g. global dipolarization, MLT wide injections). As our FG winds down, the MESO FG will take the lead on this topic. We therefore are hosting a joint session to address these topics.

We further solicit general MI-Coupling presentations to discuss how the two regions affect each other.

Discussion Leaders/Speakers

  • Chih-Ping Wang: RCM simulation of azimuthal expansion of plasma sheet bubble in transition region
  • Yangyang Shen: Contribution of kinetic Alfvén waves to energetic electron scattering and precipitation from plasma sheet injections
  • Sheng Tian: Coordinated observations on how global-scale dipolarizations couple to the ionosphere and meso-scale dipolarizations
  • Kareem Sorathia: Global modeling of multiscale stormtime magnetosphere-ionosphere coupling
  • Matt Cooper: Field-aligned thermodynamic features represented in the Middle Energy Inner Magnetosphere (MEIM) Model
  • James Weygand: ASI and GOES Observations of Nighttime Magnetic Perturbation Events Observed in Canada
  • Homayon Aryan: The response of ionospheric currents to different types of magnetospheric fast flow bursts

Upload Talks here: https://vgem.org/groups/DIP

Discussion Summary:
The final session was a joint session with the Mesoscale FG. For the duration of the DIP FG, we have discussed the roles that mesoscale phenomena (e.g. DFBs, BBFs, injections, streamers) play with respect to the global system response (e.g. global dipolarization, MLT wide injections). As our FG winds down, the MESO FG will take the lead on this topic.

  • Chih-Ping Wang: RCM simulation of azimuthal expansion of plasma sheet bubble in transition region
    • Azimuthal expansion is due to magnetic drift of high energy ions in the bubble.
  • Yangyang Shen: Contribution of kinetic Alfvén waves to energetic electron scattering and precipitation from plasma sheet injections
    • Energetic electron precipitation during substorm: riometer observation
    • Berkey et al 1974
    • THEMIS Observations of injection, dipolarization, and KAWs
  • Sheng Tian: Coordinated observations on how global-scale dipolarizations couple to the ionosphere and mesoscale dipolarizations
    • Coordinated observations on how global-scale dipolarizations couple to the ionosphere and mesoscale dipolarizations
      • Azimuthally propagating dipolarization (APD) both eastward and westward.
      • Several mesoscale injections during the global expansion
      • 2 sheet current system
      • Current sheet expands in pace with global dipolarization
      • Expands due to pileup
      • 3 events showing azimuthal expansion of dipolarization
      • Track individual dipolarizations via multiple spacecraft
      • Saw upward and downward current expand similar in latitude
      • Global dipolarization couples to ionosphere by a 2 sheet FAC system.
      • Bostrom type II model: the global-scale westward current (auroral electrojet) is a Hall current
      • 2 mesoscale dipolarizations, occurred around the expanding edge of a global dipolarization
      • Aurora expands much faster (10 deg per minute) than global dipolarization (2-3 deg per minute).
      • Different expansion speeds: aurora vs. dipolarization. Different generation mechanism? Is auroral expansion a wave or material? Maybe due to mesoscale flows at the border instead of the actual dipolarization?
      • Audience asked: When you look at auroral expansion vs spacecraft footpoint, do you see activity there that could give you precipitating particles?
        • When aurora expands, so does convection speeds.
        • Eric Donovan: it’s extremely unlikely that the expansion of the aurora has anything to do with the bulk transport of material. Change in aurora=change in convection, but doesn’t necessitate bulk motion of material in the magnetosphere. Sheng agrees—ionosphere is not a reliable screen of the magnetosphere.
  • Kareem Sorathia: Global modeling of multiscale stormtime magnetosphere-ionosphere coupling
    • Multiscale Stormtime MI Coupling
    • Case study of the dawnside current wedge-Ohtani ‘21
    • Dawnside current wedge (DCW)
      • Dawn/dusk SMR (SuperMAG SYM-H index) asymmetry common feature of storm main phase
      • Ohtani found connection between dawn-dusk asymmetry and enhancement of dawnside auroral electrojet.
      • Do they see in models?
    • AMPERE AND TWINS
    • Summary
      • Reproduce ground phenomenology of dawnside current wedge as seen in Ohtani
      • Confirm importance of westward auroral electrojet current closure during stormtime
      • Find DCW occurs following the dawnside penetration of BBFs which provides
        • Dipolarizing flux
        • Ion pressure->changes in R2 FACs
        • Energetic electrons->diffuse precipitation->eastward propagating conductance enhancements
  • Matt Cooper: Field-aligned thermodynamic features represented in the Middle Energy Inner Magnetosphere (MEIM) Model
    • Field aligned thermodynamic features in the middle energy inner magnetosphere (MEIM) Model – a quiet time reference
    • Discrete structures in nightside magnetosphere seen during active times
    • Structures seen in plasmas satisfying mirror/drift-mirror instability conditions
    • Uncertainty in whether structures are active/fossil mirror modes or some other
    • Peaked mirror mode structures typically found in unstable plasma regions
    • Dipped mirror mode structures typically found in regions where the plasma is stable to the mirror mode
    • If formed in transition region, could flow into inner magnetosphere.
    • Anisotropy not high enough to be mirror modetearing instability (reference)
    • But Cooper found it can happen.
    • Nightside mirror/drift mirror modes—dusk to midnight observations?
  • James Weygand: ASI and GOES observations of nighttime magnetic perturbation events observed in Canada
  • Homayon Aryan: The response of ionospheric currents to different types of magnetospheric fast flow bursts

At the end we did have 10 min to try to discuss the future of the FG as we have one more year left. We tried soliciting questions other than “how does mesoscale phenomena relate to global phenomena”. The question of mapping came up (since we are joint with the Transition Region/Mesoscale group). We talked about how there used to be a mapping FG. Andrei Runov mentioned assimilative mapping—where you use a Tsyganenko model and tweak the input parameters to force the model to fit the few data points you have (Marina Kubushkina did this work a decade or so ago).

GEM 2021

During the summer 2021 Virtual GEM workshop, we held two sessions.

  • Session 1 had 75 participants online and 7 speakers.
  • Session 2 had 66 participants online and also 7 speakers.

Since we had too many requests for speaking slots and the time was limited, we had no time for additional discussion. This was partly intentional since under the umbrella of the Helio2050 discussions, our FG supported the corresponding session held by the MPS RA.

Our first session highlighted new insights on current systems and MI coupling associated with dipolarization structures in Earth’s magnetotail. In particular, attention was given to some of the microscopic to mesoscale currents and Poynting flux associated with dipolarization fronts, energy transfer into the ionosphere, and how the current systems associated with dipolarization fronts close through the ionosphere and contribute to the global scale R1/R2 current systems. New details and insight on the multi-scale (micro to macro) nature of particle acceleration associated with bursty bulk flows and dipolarization fronts was also discussed.

The second session focused on global-scale asymmetries and more on meso-scale structures in the context of MI-coupling and multi-scale, multi-point observations. Finally, data-model comparisons were discussed including new results from a machine learning model that successfully predicts the location of magnetotail X-lines and thin current sheets and statistical results from test-particle simulations in high-resolution MHD fields that successfully reproduced observed statistical characteristics of mesoscale structures and plasma characteristics associated with bursty bulk flows and dipolarization fronts. 2021 has had some challenges, not just with Covid but with extra meetings (e.g., Helio2050) that has left the community a bit tired. We are working on determining the best use of the upcoming virtual or hybrid mini-GEM meeting during the Fall AGU 2021 meeting. It may include time to discuss Decadal Survey white papers and collaboration on that. And it may include a discussion of what the next Challenge Question(s) are.

We do fully intend to continue our Challenge Question format going forward, as this format had resounding support when we asked for feedback from the community.

Contributed Speakers

  • Jiang Liu. Embedded Region 1 and 2 currents: consequence of enhanced convection in the plasma sheet, newly recognized from LEO observations
  • James Weygand. Magnetic Perturbation Events observed in ionospheric current systems including events during substorms and north-south streams
  • Hongtao Huang. Understanding the magnetic dip ahead of the dipolarization fronts using PIC simulations: the dependence on the guide field.
  • Kun Bai. Ion trapped acceleration at rippled dipolarization fronts
  • Louis Richard. Turbulent Jet Fronts and Related Ion Acceleration.
  • Sheng Tian. Evidence of Alfvenic Poynting Flux as the Primary Driver of Auroral Motion During a Geomagnetic Substorm.
  • Larry Lyons. Two-dimensional Structure of Flow Channels within the inner magnetosphere and Associated Upward Field-Aligned Currents: Model and Observations.
  • Chih-Ping Wang. North-south asymmetry of the tail lobe density and magnetic field.
  • Amy Keesee. Mesoscale plasma sheet structures observed with energetic neutral atom imaging with TWINS.
  • Grant Stephens. Reconstructing the global x-line configuration by data mining spaceborne magnetometer observations.
  • Slava Merkin. Mesoscale Electrodynamics and Ring Current Formation.
  • Andrew Menz. Investigating Substorm-Related Flows and Thinning Using Multi-Point Spacecraft and All Sky Imager Data.
  • Joachim Birn. Dipolarizing flux bundle braking: Energetic ions.


GEM 2020

The summer 2020 GEM workshop was our first time running a virtual workshop with fewer sessions. We used the opportunity to regroup, refresh, and refocus. We reviewed previous workshop activity since the focus group’s inception. We also solicited talks that would help the community decide which science questions to focus on next. Those talks were presented in the first session. We compiled the questions and forward-looking perspectives from Session 1 to use as input for discussion in Session 2. In Session 2, we also surveyed the community on what session format they would prefer to use going forward. The overarching opinion was to continue the format we had introduced early on, which was to present a “challenge question” in advance that is addressed by the community during the workshop. This was a successful format in the past that has resulted in papers that would not have been written without the focus group’s structure and guidance. Using the GEM session as a place to connect data analysts and modelers to solve questions was also discussed. Challenge questions the community discussed as important were as follows:

  • PV to the gamma: Not easy to confirm with magnetic models. Heat flux across the field would violate it. Can we observe this? Models can address. Confirmed by 2D but leave out cross-tail drifts. 3D PIC could address. Ionospheric outflow/inflow could violate it too.
  • Divergence of heat fluxField aligned currents
  • Building up of dipolarization/SCW isn’t over!
  • Would be important modeling challenge. RCM? PIC?
  • Observations of multiple simultaneous flows? (4-5 streamers) SCW shows build-up from mid-latitude positive bay (Pi2s)
  • Lack 2D cross-plasma sheet mission to answer this question. Idea: use NASA program to rideshare/put additional payload on launches to more readily access space.
  • How much current can be contributed to total SCW? Not all currents go to ionosphere, must close locally.
  • Must be different instability to account for energy budget?

Accomplishments

One significant accomplishment is our Focus Group’s ability to listen to the community and provide a Focus Group that fits their needs. We heard there was confusion on terminology, so we began our Focus Group with a panel that discussed terminology. We heard there was lack of understanding on the different types of models, so the following year we had a panel of modelers explain what their models are capable of studying in terms of the physics. Next we heard that trying to debate in real-time was difficult. Although audience members have interesting and valid counter-points to a speaker, without any time to reflect and respond it is difficult to have meaningful discourse. So, we responded by creating the “Challenge Question” format, where a Challenge Question, highlighted by the community, is posed months ahead of time. Community members can address the question by submitting their talk title/opinion about the answer. Focus Group leaders facilitate the debate by coordinating the speakers ahead of time. At the GEM meeting, speakers debate amongst each other, having had adequate time to prepare. Audience participation is very welcome. A very successful example of this is detailed below, and resulted in publications and collaborations that would not have organically formed without the Focus Group’s leadership.

2021 has had some challenges, not just with Covid but with extra meetings (e.g., Helio2050) that has left the community a bit tired. We are working on determining the best use of the virtual summer 2021 GEM meeting. It may include time to discuss Decadal Survey white papers and collaboration on that. It may include addressing a Challenge Question. We are waiting for Helio2050 to conclude to feel out the community’s posture for the GEM workshop in July.

We do fully intend to continue our Challenge Question format, as this format had resounding support when we asked for feedback from the community.

Notes on community engagement and participation at the GEM 2020 Summer Workshop are listed below:

Solicited speakers:

Session 1:

  • Christine Gabrielse Intro and review of FG activities and resulting publications
  • Kareem Sorathia The role of mesoscale injections in ring current evolution: Global MHD and test particle simulations
  • Slava Merkin Ballooning-interchange Instability at the Inner Edge of the Plasma Sheet as a Driver of Auroral Beads: High-resolution Global MHD Simulations
  • Amy Keesee Tying the reconnection region to the dipolarization front and injections
  • Xiangning Chu How much of the currents surrounding the DF are connected to the ionosphere, and contributing to a SCW?
  • Louis Richard MMS Observations of Short-Period Current Sheet Flapping
  • Bob McPherron Solar Wind Coupling and Magnetic Indices

Session 2 had no formal presentations, other than the Focus Group leaders facilitating conversation by displaying slides with the compiled questions and ideas from the solicited talks in Session 1. This resulted in a very “workshop style” conversation that gave the community the floor.

  • There were ~70 participants in each session
  • We used Slack Channel and video chat software. Session 1 set the scene and session 2 was a very interactive session that was focused on listening to the community.
  • We try to give early career folks a platform and facilitate the discussion so that people with different backgrounds and personality types can be heard. This was a different year, but when we invite speakers we do try to bring in underrepresented groups.


GEM 2019

The Dipolarization Focus Group had three sessions during the summer 2019 GEM Workshop that were categorized by topic. The Focus Group leaders organized a session on Energy Transfer and Dissipation to guide presentations towards answering specific questions: 1. Can we estimate a percentage that energy is dissipated into waves, direct ion heating, etc.? 2. Can we determine if and to what extent a dynamo in the transition region (driven by pressure gradients or vorticity) converts energy dissipated from the tail into field aligned currents to drive dissipation in the ionosphere? 3. What are the ways to estimate these values (simulation or theory or observational)? It was especially timely to address these questions since the MMS mission had an overlapping Science Working Team meeting, bringing more of our European colleagues to GEM and multiple experts on energy dissipation to the session.

Focus Group leaders also solicited contributed presentations, resulting in two community-driven sessions organized by two prevailing topics. The first topic, Particle Energization and Injections, had an even split between observation and modeling presentations. The second topic, Currents, was a natural follow-on to the session on Energy Transfer and Dissipation.

The following list provides the speaker name and title of presentation from each of the three sessions. Summaries submitted by presenters are included. Note that Focus Group leaders have been collecting publications that are in part thanks to or discussed in this Focus Group on the bottom of this GEM Wiki page.


Topic 1: Particle Energization/Injection

  • Raluca Ilie - The role of inductive electric fields on particle energization
  • Jianghuai Liu - The role of inductive electric fields on particle energization (continued)
  • Sam Bingham - Adiabatic Particle Energization using MMS
  • Wonde Eshetu - Simulations of electron energization and injection by BBFs using High-Resolution LFM MHD fields
  • Christine Gabrielse - Heliophysics System Observatory observations of small and large-scale injections: DFBs vs. large-scale dipolarization
    • The injection region's formation, scale size, and propagation direction have been debated throughout the years. How do temporally and spatially small‐scale injections relate to the larger injections historically observed at geosynchronous orbit? How to account for opposing propagation directions—earthward, tailward, and azimuthal—observed by different studies?
    • A combination of multisatellite and ground‐based observations were used to knit together a cohesive story explaining injection formation, propagation, and differing spatial scales and timescales.
    • A case study was used to put statistics into context.
    • Fast earthward flows with embedded small‐scale dipolarizing flux bundles transport both magnetic flux and energetic particles earthward, resulting in minutes‐long injection signatures.
    • A large‐scale injection propagates azimuthally and poleward/tailward, observed in situ as enhanced flux and on the ground in the riometer signal. The large‐scale dipolarization propagates in a similar direction and speed as the large‐scale electron injection.
    • Small‐scale electron injections result from earthward‐propagating, small‐scale dipolarizing flux bundles, which rapidly contribute to the large‐scale dipolarization.
    • Large‐scale dipolarization is the source of the large‐scale electron injection region, such that as dipolarization expands, so does the injection.
    • Ion injection region >90-keV in the plasma sheet is better organized by the plasma flow.
  • Bob McPherron - MHD simulation of substorm including progressive approach of X-lines, flow channels, and flow penetration to the inner plasma sheet
    • An interval of moderate magnetic activity from 0-8 UT on March 14, 2008 has been investigated with a global MHD simulation using high spatial and temporal resolution.
    • Observations show several distinct substorms during this interval. One of these with expansion onset at 04:48 UT is also seen in the simulation with onset at 04:44 UT.
    • The simulation shows that reconnection is continuously present at multiple sites throughout the interval. During the growth phase, the number of x-lines and their total length increase with time and their locations approaches the Earth. The x-lines create multiple fast flow channels with dipolarization fronts. The total length and area of these channels increase during the growth phase as they penetrate closer to the Earth carrying more magnetic flux.
    • The 04:44 UT onset in the simulation was preceded by the growth of an x-line that eventually extended 55 Re from 12 Re premidnight to 50 Re on the dawn side. It produced a narrow flow channel parallel to the x-line that eventually penetrated inside 10 Re rapidly depositing magnetic flux as the expansion phase developed.
    • Despite good agreement in expansion onset time the ground and satellite observations suggest a quiet growth phase with a sudden onset of reconnection.
    • It may be possible to explain the difference between observations and simulations if all growth phase activity in the simulation map to the ionosphere at very high latitudes.
  • Tetsuo Motoba - Azimuthally localized dispersionless injections inside GEO
    • Tetsuo Motoba presented a case study of deep energetic particle injections observed by the two Van Allen Probes (RBSP-A and -B) in the premidnight sector.
    • Although the spacecraft separation was only ~0.5 Re in the azimuthal direction, the injection signatures were different between the two probes: RBSP-B observed dispersionless electron and ion injections, while RBSP-A observed the corresponding injections but they were characterized by an energy-dispersed flux enhancement and/or by a relatively weak flux enhancement.
    • Such different injection signatures are attributed to the presence or absence of a transient, strong dipolarization front (DF). The two closely located RBSP observations suggest that the azimuthal scales of deeply penetrating DF and injection region are highly localized.
  • Discussion on Particle Energization/Injection
    • Role of different fields?
    • Large vs. Small-scale?
    • This discussion was inspired by the main points and questions presented.

Topic 2: Energy Transfer and Dissipation

  • Misha Sitnov - Kinetic dissipation in dipolarization fronts and magnetic reconnection
    • Irreversibility of magnetotail dipolarizations is provided both by the collisional dissipation in the ionosphere and by collisional Landau dissipation in the magnetotail.
    • Misha Sitnov pointed out that the Joule heating rate, which is a good measure of collisional dissipation and which is widely discussed in MHD models, is not appropriate as a measure of collisionless dissipation in the magnetotail: Values of J*E’ in ion and electron frames practically coincide. Thus, j*E’ cannot be a measure of ion and electron Landau dissipation processes, which are very different.
    • Sitnov discussed kinetic analogs of the Joule heating rate, the so-called Pi-D parameters. PIC simulations show that the ion Pi-D peaks of the dipolarization front (DF), while the electron Pi-D peaks behind DF or earthward of the X-line.
    • Measurements of the Pi-D parameters, which have become possible due to the MMS mission, remain very challenging: Because of the small probe spacing (~10km), DFs often pass the MMS tetrahedron in times smaller than the plasma instrument cadence. This prevents calculating the spatial derivatives of the bulk flow velocity, the key elements of the new kinetic dissipation parameters.
  • Rumi Nakamura - MMS observations of multi-scale field-aligned currents during dipolarization in the near-Earth plasma sheet
    • Substorm current wedge contains multi-scale field-aligned currents.
    • Ion-scale process is essential in generating field-aligned currents in near-Earth magnetotail.
    • Intense field aligned currents corresponds to generator region in the flow braking region. Two types of generator region observed. (1) Embedded current layer in return flow region of localized BBF. (2) Electron flow shear region in thin Hall-current layers ahead of BBF.
  • Olivier Le Contel - Multiscale kinetic processes associated with fast flows and dipolarization fronts
    • Two dipolarisation front events associated with fast plasma flows detected by the MMS mission in last August 2016 have been presented.
    • Intense lower-hybrid drift waves associated with parallel electric fields have been identified (frequency, phase speed) at the dipolarization front as well as fast electromagnetic electron holes moving tailward. Possible coupling between the lower-hybrid waves and electron holes was discussed.
  • Amy Keesee - Concurrent enhancements in ion temperatures and auroral brightenings seen by TWINS and ASIs
    • At the 2018 GEM summer workshop, Amy Keesee showed movies of ion temperature maps during two of the challenge storm intervals.
    • Also at 2018 GEM, Toshi Nishimura showed movies of all sky imager auroral maps of the same intervals, and we discovered enhancements in both at the same times.
    • Keesee reported on their ongoing collaboration to use these intervals to study the connections from the magnetosphere to the ionosphere.
    • Keesee and Nishimura are working on mapping algorithms to identify intervals that have concurrent enhancements when the Van Allen Probes are in a favorable location to study the detailed particle distributions.
    • Keesee also discussed the availability of a database of TWINS ion temperature maps being made available at CDAWeb through the support of a H-DEE award.
    • They are also developing an automated detection algorithm to identify regions of ion temperature enhancement in that database for further studies.
  • Joachim Birn - Energy release and conversion and dynamo action in the tail on the basis of MHD simulation
    • Joachim Birn used an MHD simulation of tail reconnection associated with a flow burst and dipolarization to identify energy conversion, dynamo and load, and field-aligned current generator regions.
    • Two regions stand out as loads (E.J>0): slow shocks and the dipolarization front, where incoming Poynting flux is converted primarily to enthalpy flux.
    • Dynamo actions (E.J<0) are found in the braking region and at higher latitude on the outside of the Region 1 type field-aligned currents, built up by vortical flow in and near the equatorial plane.
  • San Lu - Strong energy dissipation at the transition region
    • Pritchett and Lu (2018) investigated the response of magnetotail to a longitudinally limited, high-latitude driver using 3-D particle-in-cell simulations.
    • After the onset of localized reconnection caused by the external driver, the later response involves sudden disruption of the plasma sheet in the transition region with much stronger energy dissipation and particle energization than that at the reconnection site.
  • Shin Ohtani - Dissipation and the ionosphere
    • Shin Ohtani discussed the transport of energy from the magnetotail to the ionosphere during substorms by synthesizing the results of previous observational and modeling studies.
    • He concluded that (1) the area around the duskside poleward boundary of the auroral bulge (i.e., auroral surge) is a unique and persistent sink of substorm energy, and it accounts for a few tens of percent of the ionospheric substorm energy dissipation; (2) kinetic energy carried by BBFs is comparable to the energy deposited to the ionosphere in association with auroral streamers, and each BBF accounts for ~1% of the total substorm energy deposition, which may sum up to 10% throughout the expansion phase.

Discussion on energy dissipation

1. Can we estimate a percentage that energy is dissipated into waves, direct ion heating, etc.?
2. Can we determine if and to what extent a dynamo in the transition region (driven by pressure gradients or vorticity) converts energy dissipated from the tail into field aligned currents to drive dissipation in the ionosphere?
3. What are the ways to estimate these values (simulation or theory or observational)?

Topic 3: Currents

  • Misha Sitnov - Dipolarizations and their connection to the ring current buildup and magnetic reconnection
    • The new data-mining (DM) technique applied to magnetospheric storms and substorms was presented by Misha Sitnov (in collaboration with Grant Stephens and others).
    • The DM reconstruction of the magnetosphere resembles launching swarms of ~50,000 synthetic probes.
    • It shows that the response of the inner magnetosphere to magnetotail dipolarizations is very diverse: 1) both the TCS and the ring current increase in the substorm growth phase; 2) the decay of a thin current sheet (TCS) associated with the tail dipolarization on substorm scales (~0.5 hour) is followed by the buildup of a proto-ring current in the inner magnetosphere on the time scales of several hours; 3) the response of the ring current to magnetotail dipolarizations may have both storm and substorm time scales; 4) sometimes magnetotail dipolarizations during substorms do not modify the near-Earth ring current at all.
  • Shin Ohtani - Double-wedge current system based on the GOES-RBSP comparison of dipolarization signatures
    • Shin Ohtani showed, based on the timing comparison of dipolarization signatures at RBSP and GOES, that the dipolarization region expands earthward.
    • He argued that the result apparently contradicts the conventional substorm current wedge model, which suggests that dipolarizations take place simultaneously everywhere inside the current wedge.
    • He proposed that the actual substorm current system has a R2-sense current wedge on the earthward side of the (conventional) R1-sense current wedge, and the dipolarization region expands earthward as the R2-sense current wedge moves earthward.
  • Yi-Hsin Liu - An explanation of the opposite dawn-dusk asymmetry at magnetotails of Earth vs. Mercury
    • PIC simulations reveal that the dawn-ward transport of the normal magnetic flux (Bz) by electrons beneath the ion kinetic scale is a critical feature of current sheets.
    • While the normal magnetic field in the tail geometry suppresses reconnection onset, the reconnected magnetic field (i.e., also Bz) enhances reconnection after the x-line develops. These all together will result in the competition of opposite dawn-dusk asymmetries.
    • Liu proposed that the vastly different global dawn-dusk scale of the magnetotails at Earth and Mercury will lead to opposite outcomes in this competition of asymmetry. This new finding can be important to the on-going ESA-JAXA mission, BepiColombo.
  • Ryan Dewey - Flow braking of dipolarizations in Mercury's magnetotail
    • Ryan Dewey presented statistical observations of dipolarizations in Mercury's magnetotail and demonstrated that their associated fast flows typically brake before reaching the nightside surface of the planet.
    • Due to the small spatial scales of Mercury's magnetosphere, a small fraction of dipolarizations (~10%) may impact the planet while the majority brake and contribute to flux pileup.
    • Whether this pileup is associated with a current wedge system remains to be constrained.
  • Xiangning Chu - The generation of STEVE and penetration of fast flows to the plasmapause


GEM 2018

Session 1. ULF waves during particle injections and dipolarizations: Joint with ULF Wave Modeling, Effects, and Applications Focus Group and Substorm FGs

This session focused on the relationship between particle injections/dipolarizations and ULF waves (e.g., Why are waves driven in only some events? Do waves impact the ring current/radiation belts?). Model and observational results showed that Pi2 wave properties – including the arrival time of Pi2 wave packets at ground stations – are significantly affected by ionospheric conductivity and radial Alfven speed profiles. Incoherent scatter radar observations of large ionospheric electron density and conductivity variations with Pc5 frequency were shown, while SuperDARN radar measurements showed highly localized ionospheric velocity perturbations associated with poloidal ULF waves; more observations are needed to identify the source(s) of the ULF modulation of ionospheric parameters. Numerical simulation (new version of RCM) and theory of buoyancy waves were presented, demonstrating that some nightside Pc5/Pi2 waves may be associated with the buoyancy mode. Finally, theory of the relationship between ULF waves and substorms was discussed, including Alfvenic interactions that can trigger substoms.

Sessions 2 and 3. Observations of the challenge events, discussion of steady magnetospheric convection, storm-time substorms, and isolated substorms and their effects on the inner magnetosphere: Joint sessions with the Substorm Focus Group

The focus of this session was to compare and contrast observations of storm-time substorms, isolated substorms, and steady magnetospheric convection (SMC), and the effects that these tail modes have on the inner magnetosphere. Four events where chosen for initial studies: (1) an SMC event between 2013 August 24-28, storm time substorms on (2) 2016-09-04 ~7:20 UT and (3) 2016-09-27 ~04:30 UT, and (4) an isolated substorm on 2017-02-02 ~4 UT. An overview of the events can be found at goo.gl/zCeiAa.

Ground-based, in situ, and model results were presented including, all sky imagers, riometers, ground-based magnetometers, in situ plasma and wave measurements and global MHD simulations. Christine Gabrielse and Toshi Nishimura presented detailed observations from the THEMIS probes, ASI, and ground-based riometers. Drew Turner presented observations from MMS and the Van Allen Probes. Amy Keese presented observations from TWINS. Lauren Blum presented EMIC wave observations from the Van Allen probes. Colin Komar present initial global MHD results from the Solar Wind Modeling Framework for each challenge event. Kyle Murphy presented injection signature from the LANL spacecraft and Anna DeJong presented ground-based observations regarding the steady magnetospheric convection event.

One of the major highlights from the session was discussion regarding steady magnetospheric convection: how it was manifested in in situ, geosynchronous, and ground-based data, how steady/stable steady magnetospheric convection needs to be considered as an SMC event, and whether or not SMCs can be accurately defined without global auroral imaging. Christine Gabrielse showed that during the SMC event, there was almost one-to-one correlation between AE enhancements and riometer observations of precipitating electrons from injection. (This was part of what led to the discussion regarding SMC definition. If AE varied that much, was it really an SMC?) Anna DeJong argued that the event was not truly an SMC for this reason. Toshi Nishimura correlated injections observed at MMS with THEMIS all-sky-imager observations of auroral streamers. Drew Turner also presented initial observations from MMS that elude to direct loss of tail injected plasma to the dawn-flank magnetopause. Lauren Blum showed evidence of EMIC wave activity during storm-time substorms but saw little activity during the SMC and isolated substorms. At geosynchronous Kyle Murphy showed clear differences between the SMC event and storm-time substorms – the SMC event showing little injection activity while the stormtime substorms showed both numerous and intense injections. Future sessions will narrow in on some of these highlights for additional discussion.


Session 4. Panel Session on the topic of Dipolarization and Global Modeling

The Dipolarization FG held a panel discussion session to focus on how magnetotail dipolarization is currently captured in global models and how those models need to be developed to better simulate dipolarization and its effects (in the inner magnetosphere and ionosphere) based on what observations are telling us about the nature of the system. Approximately 70-80 members of the greater GEM community were in attendance. The panelists included: Katie Garcia-Sage, Colby Lemon, San Lu, Yann Pfau-Kempf, Jimmy Raeder, and Misha Sitnov. Christine Gabrielse chaired the panel and guided the conversation with comments and questions.

Prior to the panel session, panelists were sent the following three questions to consider and respond to as guidance for the topics that were to be discussed during the session: 1) Given our current modeling capabilities, discuss which kinds of models are best at capturing which aspects of dipolarization events and their effects in the magnetosphere. 2) What determines the dipolarization scale size in different models? (e.g., physical description, boundary conditions, model input parameters, ionosphere conditions, etc.?) 3) The transition region is where both inertia and energy dependent drifts are important. No existing models treat that region correctly. (a) How do we move forward? (b) Or, more specifically, address the question of dipolarization front deceleration: (i) How do various models treat dipolarization deceleration as they approach the inner magnetosphere? (ii) What processes are decelerating the fronts in the models? (iii) What inner magnetosphere processes are missing (e.g. plasmasphere, complex ionospheric conductivity models) and does excluding these processes lead to different deceleration predictions? (c) And/or address: (i) What are the relative roles of ExB, energy-dependent drifts and particle trapping in transport and energization in the transition region? (ii) To what extent are these processes adiabatic for particles of different energies? (iii) What is their overall contribution to the ring current build up?

Types of Modeling: Which are best for what questions?

Christine started the panel discussion by reviewing some of the responses she had received from the panelists concerning question 1. The panel then moved into open discussion on that topic. There was general consensus that the relevant physics are global in nature, and in particular that the role of the ionosphere and small-scale physics are both relevant and not properly being captured by any of the models. Models must capture both the plasma sheet and dipolar inner magnetosphere correctly plus the feedback loop provided by the non-idealized ionosphere.

San stressed that a combination of models, such as global MHD with embedded PIC and global hybrid models is our best current approach for capturing both global and critical small-scale processes.

Concerning small-scale physics, Misha raised the point that we still don’t have a good sense of where in the tail the reconnection X-line typically forms and whether the models are capturing even that correctly. He also stressed that with empirical models, such as TS07, we can much more accurately capture individual events.

Yann introduced the Vlasiator model, and stressed that the location of inner boundary conditions and 2D limitations in the global hybrid model are still a major limitation for accurately capturing magnetotail reconnection, dipolarization, and substorm activity.

Jimmy focused on the differences between global MHD and other models, stressing that global MHD has a “lack of knobs” that is both limiting in one sense but more trustworthy in another sense. Jimmy also stressed the importance of the ionosphere and also the transition region in and around GEO, where reconnection fronts (dipolarization fronts and the associated BBFs) start to decelerate and deflect in the inner magnetosphere; he stressed that once these plasma “bubbles” start to slow down and disperse, the fluid picture no longer applies, so it is difficult to say how well MHD model results showing that represent reality. There was also general consensus that data-model comparisons are very important and we need to continue developing those capabilities and approaches.

From the audience, Andrei Runov asked about the nature of the X-line in the magnetotail: did the panelists think it was a global scale feature? The panelists consider X-lines in the tail to be fragmented and spread out throughout the tail between around X_GSE of -15 and -20 RE. Misha Sitnov thinks the typical X-line lies further downtail, more like -30 RE or beyond, and that models that include too much resistivity will get this closer to Earth. Andrei also stressed the tailward side of the picture, that is, those reconnection jets that are ejected tailward from an active X-line. From recent ARTEMIS results, the reconnection jets observed at lunar orbit (-60 RE) are still localized in nature, which is further evidence that the X-lines in the magnetotail are also localized.

Dipolarization Scale Sizes

Christine next steered the panel to question 2. Katie stressed that the resolution in global MHD tends to break down in the ionosphere, which might fundamentally limit the scale sizes of features in the magnetosphere. She also mentioned that ion composition and ionospheric outflow are not well captured in global models currently but might play a key role in scale sizes of magnetotail dipolarization via instability leading to reconnection, reconnection scale sizes, and the global magnetotail properties. Colby also agreed that the grid resolution in the ionosphere in the RCME model was also a major limiting factor. Two grid points in the ionosphere in the model map to a very large region of the magnetosphere, meaning that the model might not be able to capture localized features in a stretched magnetotail. Colby stressed that RCME seems to be doing a good job capturing the Y (i.e. cross-tail, azimuthal) scales of flow channels (BBFs) but is concerned about how well they are capturing the X (downtail) scales.

From the audience, Shin Ohtani asked about time scales: at 500 km/s velocity, it takes only a few minutes to go from 20 RE to GEO, which is similar to the Alfvén speed travel time from the reconnection site to the ionosphere, so does the ionospheric feedback really matter? Colby responded that was a good but unresolved question. Jimmy disagreed, saying the speed is much faster down to the ionosphere. Bob Lysak mentioned that models often don’t capture the density along field lines correctly, but that with the current best estimates, the travel time for information down to the ionosphere was a few minutes.

That discussion transitioned into the importance of Pi2 waves. Joachim Birn mentioned that oscillation in the transition/stopping region is on the scales of the ionospheric travel time (PI2 period timescales). He again stressed the importance of the transition region and how many of our challenges currently fall back into that region around GEO. Yann showed a movie from Vlasiator, and stressed that with a perfectly conducting “ionosphere” at 5 RE, the speeds were too fast in their simulations. They were seeing peak flows around 2000 km/s. He also stressed that with 2D simulations, all of the reconnection in the system was forced into the XZ plane. The Vlasiator simulations take some time to get reconnection after initialization, and they are actively investigating how the addition of oxygen ions to the plasma sheet will affect that delay time.

Larry Lyons introduced another question of the audience. He asked, “Do dipolarizations only occur in thin current sheets?” He stressed that with streamers being observed under a variety of different conditions, is a thin current sheet a necessary condition to get dipolarization fronts and BBFs in the plasma sheet? Misha stressed that the problem is multiscale and that no, a thin current sheet is not a necessary condition. Tail reconnection, dipolarization fronts, and BBFs may develop in a thick current sheet. San agreed with Misha’s point and stressed that the thickness of a dipolarization front is determined by ion kinetic physics and that from observations, the width of a front is complicated and might have to do with the scale of the responsible X-line or with the conductance in the ionosphere or both.

San then showed results from the ANGIE3D global hybrid model. Slava Merkin asked: what determines the scale size of the X-line? San didn’t know but stressed that it was not resistivity but likely an inherent property of the X-line itself, perhaps due to a non-uniform magnetotail. Shin asked why dipolarization fronts moved dawnward, and San replied that it was just a result of ExB drift. Mostafa asked if there is a correspondence between sizes of X-lines and dipolarization fronts/flows? Can larger X-lines produce smaller flows or vice versa? There was some disagreement and discussion between whether or not dipolarizing flux bundles should get smaller as they move inwards. Joachim brought up that when reconnection starts, an X-line might be extended in the tail due to solar wind driving conditions and the distribution of resistivity in the model, but over time the active X-line narrows to a few RE due to entropy reduction and the system becoming unstable to ballooning. He discussed how the tearing mode and ballooning mode can either compete or act in concert, and ultimately, that the cross tale scale depends on the region of the outflow where the BBFs go to.

Jimmy brought up an analogy to seismology and terrestrial earthquakes. He stressed that an earthquake in one place on the planet can trigger another earthquake 1000s of miles away. He thinks that one active X-line can similarly trigger reconnection elsewhere in the plasma sheet. The formation of the active X-line changes the entire environment in the tail; it is a disruptive event. This is of course all driven by changes in the solar wind too, which further complicates the picture. He pointed to auroral arcs as evidence that there is likely no preferred scale size for X-lines and the dipolarization fronts they spawn.

The Transition Region: How do we Move Forward?

Christine next turned the discussion to question 3. Misha kicked off the discussion on that and stressed that we do have a comprehensive picture of the transition region from a collection of many, many years of observations throughout it. He argues that with data mining, relying on observations from many, many similar cases, we have full coverage of the region. From his empirical model, which employs data mining, he finds that the transition region expands downtail from ~-8 to -18 RE during substorm dipolarization. From here, Andrei asked how Misha defines a substorm, to which Misha replied with the AL index. This sparked a debate on how to define substorms.

Katie changed the subject to stress that plasma pressure in the inner magnetosphere has to be captured correctly to properly model the transition region. This requires that plasma sheet models be coupled to accurate inner magnetosphere models. She again stressed the important role of the ionosphere, and how that can help dictate how far into the inner magnetosphere a dipolarizing flux bundle can travel and the properties of its rebound and oscillations as it comes to rest there.

Larry Lyons brought up that we had not discussed the ground based observations point of view. He asked how we can connect where reconnection is occurring in the models to what we are seeing in the aurora with streamers. He stressed that in the aurora, much of it is east/west aligned, which corresponds to azimuthal drifts in the inner magnetosphere, and streamers are the only features that can correspond to dipolarizing flux bundles and BBFs. San agreed and mentioned that localized reconnection and dipolarization fronts may be the consequences of dayside streamers loading small, localized portions of the tail. Jimmy agreed and stressed that models might be capturing the east/west features but that we just haven’t focused on analyzing them. Jimmy stressed too that we had to be careful, because there is a filter effect with the ionosphere too. Not everything seen in the aurora/ionosphere is reflecting what is happening in the magnetosphere.

      • From this panel discussion, we established a GEM challenge: modelers are challenged to simulate three different cases: storm-time substorm, isolated substorm, and magnetotail reconnection during steady magnetospheric convection. From the simulation results, how well can a given model capture the observed similarities and differences between these different cases? How will models be constrained so that they do not start reconnection prematurely? This challenge will be further developed and fully defined at the mini-GEM meeting at AGU 2018 and will be conducted in partnership with the focus group on mesoscale aurora, polar cap dynamics, and substorms.***


Session 5. Contributed Talks

The Dipolarization FG held a second session immediately following the panel, chaired by Drew Turner, to allow for contributed talks. Also attended by about 70-80 GEM members, the session had ten contributed talks and excellent discussion:

1. Chih-Ping Wang presented on “RCM simulations of entropy reduction caused by plasma bubbles from different MLT locations”. He showed that the earthward transport of the simulated plasma bubble qualitatively explains the two-point THEMIS observation of a BBF event. He showed that the simulated entropy reduction caused by a plasma bubble varies significantly with the bubble’s initial MLT and background convection. A plasma bubble starting at 23 MLT results in an entropy reduction that extends closer to the Earth and azimuthally wider than does a bubble starting at 1 MLT.

2. Ryan Dewey presented on "Dipolarization effects at Mercury and comparisons to Earth". He used MESSENGER observations at Mercury to identify dipolarizations in Mercury's near magnetotail, and discussed the statistical characteristics of these events. He showed that dipolarization fronts are short-lived (~2 s) enhancements of the northward component of the magnetotail field (~30 nT) and are associated with fast sunward flows, energetic particle acceleration, and thermal plasma heating/depletion. He discussed that these signatures are analogous to those at Earth, however, he showed that dipolarizations are most frequently observed in the post-midnight plasma sheet at Mercury, opposite to that at Earth.

3. Joachim Birn presented results from an analysis of the stopping region of DFBs, both from a fluid and a particle point of view, based on test particles combined with an MHD simulation. The stopping region was characterized by pileup of plasma sheet flux tubes ahead of the DFB, leading to an excess of pressure gradient force. Particle distributions were characterized by perpendicular ion and electron anisotropy with a high-energy electron ring, all originating from the inner plasma sheet particles.

4. Brian Swiger presented a talk entitled, “Do different substorm strengths accelerate keV electrons the same?” He showed that from X=-6 to -25 RE, for all electron energies between ~5-52 keV, the average flux increase was greater for larger AE events.

5. Andrei Runov presented THEMIS and LANL observations in the near-Earth plasma sheet and at GEO, respectively, during events of prolonged, extreme solar wind/IMF driving. Events with IMF Bz <-10 nT during longer than 5 hours were selected. THEMIS measurements indicate that the magnetotail responded by a set of thinning-dipolarization events with a duration of 1 hour, which resemble the sawtooth events. The dipolarizations were accompanied by ion and electron injections in energy ranges ~50 to 500 and ~20 to 200 keV, respectively. Dispersionless and dispersed injections in these energy ranges were also detected by LANL spacecraft at GEO.

6. Sasha Ukhorskiy presented on ion acceleration and transport from the tail to the inner magnetosphere, the effects of trapping, adiabaticity, and the role of charge. (See Ukhorskiy et al., 2017.) Recent analysis showed that the buildup of hot ion population in the inner magnetosphere largely occurs in the form of localized discrete injections associated with sharp dipolarizations of magnetic field, similar to dipolarization fronts in the magnetotail. Because of significant differences between the ambient magnetic field and the dipolarization front properties in the magnetotail and the inner magnetosphere, the physical mechanisms of ion acceleration at dipolarization fronts in these two regions may also be different. He discussed an acceleration mechanism enabled by stable trapping of ions at the azimuthally localized dipolarization fronts, and showed that trapping can provide a robust mechanism of ion energization in the inner magnetosphere even in the absence of large electric fields.

7. Anton Artemyev discussed regimes of ion energization during injections: adiabatic vs. nonadiabatic acceleration. The canonical approach for the guiding center theory was proposed, and using this approach the particle equations of motion were rewritten in the coordinate frame with vanishing inductive electric field (a non-inertial coordinate system). Using these equations of motion, Anton discussed three regimes of plasma acceleration: the hot plasma in a large background Bz field, the cold plasma in a small background Bz field, and the intermediary plasma/background Bz field. He referenced Zhou et al. [2018] to discuss mass dependence on energization, with more massive particles (e.g., O+) able to gain the most energy. He showed that ions of different charges at ~5-6 keV will gain a similar amount of energy, but that ions with greater positive charge (e.g., O+6 vs. O+) at ~20 keV can gain more energy.

8. Xiangning Chu discussed broadband waves on plasmapause induced by deep penetration of dipolarization front. He showed that most plasmapause observations with broadband waves are centered around pre-midnight, similar to the distribution of flows/dipolarization fronts. He also found parallel electron fluxes around the same time. He found that AE was larger when the waves were observed at the plasmapause than when no waves were observed at the plasmapause.

9. Shin Ohtani presented on “Spatial structure and development of dipolarization in the near-Earth region”. By statistically comparing the relative timing of dipolarizations at two satellites, he found that the dipolarization region expands earthward as well as away from midnight at r <= 6.6 Re. The expansion velocity was estimated at several tens of km/s, noticeably slower than outside geosynchronous orbit. He suggested that this earthward expansion of the dipolarization region can be attributed to a two-wedge current system with a R2-sense wedge moving earthward and a R1-sense wedge staying outside of geosynchronous orbit.

10. Tetsuo Motoba reported on "A near-Earth dipolarization event observed by MMS (r ~13 Re)". In the course of the dipolarization, MMS observed multiple dipolarization fronts (DFs, < 1min), energetic particle injections (> 70 keV), and oscillating flows. The injected energetic ions were field-aligned accelerated with pitch angle asymmetry, while no apparent pitch angle asymmetry was found for the energetic electrons. The MMS-GOES and MMS-ground comparisons revealed good correlation between the dipolarizations at MMS and GOES and between the oscillating flows and low-latitude Pi2 pulsations, respectively.



GEM 2017

The “Magnetotail Dipolarizations and their Impact on the Inner Magnetosphere” Focus Group kicked off its inaugural year with two joint sessions (combined with the Midtail and Reconnection Focus Groups, with ~ 70 attendees), two panel-led “controversy” sessions (each with ~35 attendees), and one contributed session (~45 attendees). The over-arching theme of this year’s discussion was defining dipolarization, including how different scale-sizes relate and impact the magnetosphere. The panel on the “controversy sessions” consisted of R. McPherron, J. Birn, A. Runov, S. Ohtani, M. Sitnov, X. Li, R. Wolf. Through dialogue with each other and the audience, they addressed the following questions: 1. How do you define dipolarization? 2. Is there a difference between small- and large-scale dipolarization? a. If there is a difference, how do the two types compare/contrast? b. If there is a difference, do the two types impact the inner magnetosphere differently? (Or similarly?) Specifically, on injections/particles? 3. How are current models doing at modeling dipolarizations (small and/or large scale)? Should they be modeled differently? 4. What key observations are required to constrain/test current models?

Definitions and Paradigms

Bob McPherron began by reminding us that the original definitions (in a 1972 Planetary and Space Science paper, and his 1979 paper) was “a return to dipolar orientation”. Using GEO spacecraft, they saw each onset causes an increase in magnetic field, or “dipolarization”—data that looks very similar to what THEMIS now presents around 10 RE. Baumjohann et al. [1999] later discussed the tailward moving dipolarization front that reaches the near-Earth neutral line distance downtail about 45 minutes after onset. This definition of the “dipolarization front” differs from the “front” discussed in Nakamura et al. [2002], Sitnov et al. [20??], and Runov et al. [2009; 2011], which is the earthward-propagating boundary between the ambient plasma sheet and the hot, tenuous plasma following reconnection.

Andrei Runov expressed some regret at the word-choice, given that the terminology is now a bit confusing (not to mention the fact that a Google search will alter the search term to “depolarization”). To reduce this confusion, he suggested to change our way of thinking regarding the phenomenon. Instead of discussing magnetic field, total magnetic field elevation angle, etc., we should discuss the phenomenon in terms of currents. He pointed out that there are clearly two, distinct current systems. One, the substorm current wedge, is responsible for the global dipolarization. The other, a local current system generated in a high beta regime, supports the “dipolarization front”. This locally generated diamagnetic current flows on the boundary between rarefied, hot plasma coming from reconnection and compressed, colder plasma ahead of the front.

Runov also explained the difference between the “dipolarization front” and the “dipolarizing flux bundle”. The former is the sharp boundary (about one thermal ion gyroradius thin) separating two plasma populations, whereas the latter follows the front, lasting ~40-50 seconds, and is the region where the electric field enhances. Joachim Birn also included the caveat that these events have to be sufficiently fast, agreeing that they last on the order of minutes. Tying in the Baumjohann et al. tailward-propagating front with the transient earthward-propagating front, he expressed that the earthward-propagating dipolarization event piles up in the near-Earth, transition region. He agreed that the region of enhanced Bz behind the front is the dipolarizing flux bundle (DFB), but views the flow channel behind the DFB (where the magnetic field is not enhanced) as separate. Birn explained that there is a “snowplow effect” before the front, observed as in increase in pressure, but behind the front is reduced entropy. He pointed out that most people now see the transient, small-scale dipolarization and the global dipolarization as two different stages of the same thing.

Misha Sitnov shared his observation that we usually pay attention to the final result of the process that occurs within ~9 RE, what he referred to as “substorm scale dipolarizations” lasting ~20-60 minutes. However, he noted, similar structures are seen by MMS at 25 RE. THEMIS has even observed the tailward-retreating front expanding all the way to lunar distances. Sitnov expressed his opinion that the conversation surrounding “dipolarization” is semantics; meaning, it is simply some way that the field becomes more dipolar. The method could be a front, a DFB, a substorm, or something completely different. Because the inner magnetosphere has such a large background magnetic field, he pointed out that the phenomenon is more pronounced in the particles. Xinlin Li shared a similar view, pointing out that one can model the dispersionless injection associated with dipolarization in order to infer information about the dipolarization. Models allow for making the dipolarized region narrow or wide in order to fit the dispersion observed in injections. Shin Ohtani expressed that in the past, dipolarization was a very simple concept that simply explained that the magnetic field went from a more stretched state to a more dipolar state. He explained that using the auroral definition [e.g., Akasofu 1972; Friedrick 2001], tail stretching and ensuing dipolarization was observed as the poleward boundary moving equatorward, then expanding poleward. The magnetic field at the equator increases sharply close to Earth, then gradually farther out.

Ohtani also pointed out the conundrum of the term “dipolarization” in the near-Earth region where the intense ring current contributes to a field that is “more dipolar” than a dipole. In essence, it is strange to call something “dipolarization” when the field becomes stronger than a dipole. Continuing the topic of conundrums, and perhaps similar to points made about semantics, Ohtani expressed that it is difficult to demarcate between scale sizes: there is no clear line between “large” and “small”. On the extreme “large scale”, we have sawtooth events, which are larger than the substorm dipolarization for example. His preference, therefore, is to use “substorm” as part of the definition when discussing dipolarization. The original definition was a substorm-related reconfiguration of the near-Earth magnetic field, and thus a change in tail current which appears in the ionosphere and which forms the substorm current wedge. Dick Wolf, on the other hand, agreed with Birn’s analysis and distinguished between two stages in the dipolarization process. He pointed out that if the ionosphere is perfectly conducting (such that the field-line feet are fixed), a localized, depleted flux tube will come to rest in a shortened form. It will have a different shape from the background, a downward parallel current on the eastside and upward on the westside. The equatorial motion involves just an induction electric field which doesn’t map to the ionosphere. However, if the collapse is narrow across Y and conductance is finite, then parallel current leads to westward potential electric field in the ionosphere and in the equatorial plane. The ionospheric foot points move equatorward, and the equatorial crossing point moves earthward. The depleted flux bubbles take the same shape as the background. The time-integrated potential electric field is typically at least as big as the time-integrated inductive electric field. The currents map to the sides of the narrow channel in the ionosphere, and an intense potential electric field exists in the channel. This process would not work for a wide injection, as the currents would map to widely separated spots in the ionosphere. This would not result in an intense potential electric field, and therefore no strong flows.

Entropy and Bubbles

Matina Gkioulidou brought up the question of entropy and how it plays a role in bubble formation and propagation. Wolf explained that although you cannot measure entropy directly, it can still be the agent behind the physics. Ohtani shared that he was against any definition based on physics (e.g., referring to the small-scale dipolarizations as entropy “bubbles”). In such cases that the physics behind the phenomenon is later discovered to be different, the field would be stuck with an incorrectly named process. Instead, he advocated to defining phenomena based on morphology (what it looks like in the data), after which the physics can be discussed.

Runov explained that he did try to address it with observations. Using Wolf’s formula for the entropy function, he found it significantly dropped behind the front. The physics is there; however, because he could not obtain concrete numbers, the study did not progress past reviewers. The conversation opened up to the idea, though, that there may be away to estimate it using multi-spacecraft data combined with models. The idea is to translate to the language of local forces (i.e., Li et al., 2011). From their work, the DFB was clearly propelled earthward by curvature force, stopping when the gradient of total pressure became comparable to the magnetic tension force. Vassilis Angelopoulos pointed out that the entropy description allows us to estimate a final state given the initial state, but it doesn’t describe the forces (as the force balance does).

Misha Sitnov slightly disagreed, saying that most processes are driven by interchange such that reconnection ends up as the final point, after interchange instability. Mike Wiltberger disagreed, stating that his model shows reconnection occurring first.

The Relationship between Scale Sizes

We then shifted the topic of conversation to the relationship between scale sizes: how do they fit together? Do multiple DFBs make the large-scale, global dipolarization? Shin Ohtani started us off by pointing to Tanskanen et al. [2002] and Akasofu [2013]. From these works, he believes that 10,000 BBFs are required to compile the energy of the substorm—meaning, the large-scale morphology cannot be simply the compilation of multiple BBFs. Angelopoulos questioned whether or not Akasofu included the thermal energy in his calculation, a question that was followed by Joachim Birn who explained that the major energy source is in the thermal speed, not flow speed. In that case, the accumulation of multiple BBFs/DFBs should suffice. McPherron agreed that his favorite idea is a cumulative effect. Sitnov, on the other hand, stated that dipolarization fronts have no relation to substorms, and that substorms have no relation to storms.

Runov emphasized his earlier point that we are dealing with two sub-systems, or two distinct plasma regimes. What happens in the magnetotail regarding dipolarization fronts may or may not be created by reconnection, it doesn’t matter so much as the fact that it is a high beta regime. Time scales are different than in the low beta regime. In the high beta regime, transient structures are supported. These are connected to a local current system, which propagate earthwards, create field-aligned currents, and connect to a low beta region. The major deposit of energy goes to heating of the local, ambient plasma. The VxB channel accelerates particles around it, which builds localized pressure inside, providing field-aligned current and connecting the low beta regime to the high beta regime.

Runov further explained that when these processes power enough to create a sustained system—and the ionosphere is responsive—then global dipolarization happens. If it isn’t powerful enough, or the ionosphere is not responsive, then global dipolarization does not happen. Pointing to Mercury as an example of a planet with transient dipolarizations but no substorms nor sustained dipolarization, he suggested that the low beta regime is not involved because there is no ionosphere to maintain the current system. Meanwhile, at Earth, each DFB twists flux tubes and creates field-aligned current that the ionosphere then maintains even after the DFB is gone. This allows the time response to be much longer than the actual DFB’s lifetime.

Ryan Dewey explained that currents close through Mercury’s conducting core, not an ionosphere. Runov asked him if he knew why dipolarizations occur in Mercury’s post-midnight sector, because at Earth they start in the pre-midnight sector. Dewey explained that the running hypothesis is that the higher concentration of sodium on Mercury’s duskside could affect local reconnection rates, modifying the asymmetry.

McPherron explained that he used to think the substorm current wedge originated at the X-line. However, no current wedges form on the dayside—even though there is dayside reconnection—which is evidence that flow bursts are an essential feature. He pointed out that in his MHD simulation, he only saw two flow bursts coming in. This opened discussion on the fact that during a substorm, there are 2-3, sometimes up to 6, flow bursts, and that only 2-3 are enough for flux to build up. Runov, in response, underscored that the effect could be cumulative, BUT it has to be more than that. The process must include the currents, which will sustain the build-up.

Because most flows do not make it in to geosynchronous orbit, Ohtani was still uncomfortable with the idea. Citing Pulkkinen [1992] and Kaufmann [1987], he pointed out that most current enhancement occurs within 10 RE. Therefore, the current must somehow intensify just outside GEO…how? McPherron suggested that as the magnetic field strength goes up, the flow velocity decreases to below instrument measurement levels.

Unconvinced, Ohtani pointed to his 2006 paper that demonstrated the magnetic field at geosynchronous orbit can continue to be stretched even though Geotail observed the flow at large distances. He concluded that the magnetic field measured at GEO by GOES is determined by a more global current system. For example, in a psuedobreakup, Geotail observed the dipolarization front and fast earthward flow. Meanwhile, at GOES, the magnetic field became more stretched. Then, after substorm onset, there was dipolarization. He therefore sees localized and large-scale dipolarizations as completely different events that may have no physical connection.

This concluded Session 1.

Session 2

We began session 2 by recapping session 1, and answering an audience question about bursty bulk flows (BBFs). Angelopoulos explained that they are fast flows lasting over ten minutes with a series of distinct dipolarizations and dawn-dusk Ey. He also explained that the most efficient flux transport occurs as the DFB (75% within the BBFs). Runov further detailed the BBFs by saying that many observations have shown the cross-scale structure of the DFBs are only one to a few RE wide. In terms of plasma physics, that’s a few tens ion inertial lengths. Along with them, Christine Gabrielse answered a question that yes, electrons could be transported all the way from the reconnection region, although ions have different drift motions. Drew Turner expressed that all of these terms are related to the small-scale. Using ground magnetograms, McPherron obtained substorm parameters, such as that it can expand East and West. The difficulty is that you must know the onset time in order to do the inversion. Plus, you’re looking at changes on the ground, which is an indirect observation. If the onset is isolated, it is easier to accomplish.

Entropy Part II

The discussion about entropy and how models deal with it resurfaced at this point. Sitnov shared his opinion that in a global simulation with a boundary, entropy is stable—it is decreasing with R—which makes it different than reality. Birn explained that when you compress the tail and assume some closed field boundary, you get flux tubes.

Don Mitchell shared that the phenomenology is similar at Saturn, which doesn’t depend on reconnection. The field is stretched by a different mechanism. In that case, whatever preconditions the system is less important than how the system reacts to the configuration. Sitnov’s thought is that for small-scale fronts and related processes, we need to understand the source region or mechanism (such as reconnection), which can only be simulated with kinetic codes. He stresses that with MMS, it is prime time to understand what causes this ideal process that releases the stresses in the magnetotail. Although he is uncertain that we have a model that can accomplish this, he feels we have enough data to empirically put the substorm sequence together. We are now asking ourselves: What happens to dipolarization fronts when they penetrate the inner magnetosphere? No equilibrium model exists in the inner magnetosphere, so we rely on Wolf’s ring current model and the quasi-static approximation. This works very well in the inner magnetosphere, and MHD works well in the tail; however, we are lacking a robust description for the transition region. One suggested solution is to utilize hybrid models, which can take energy-dependent drifts into account.

It was then pointed out that San Lu is using a hybrid code that is coupled to the transition region using PIC code, which is used to model ~7-10 RE. (However, it doesn’t go farther in than that.) The resolution is as high as the computers can take.

Looking Forward

Sitnov then suggested we compare one global, one hybrid, and one PIC model for the same event to see if the resolution is enough. Anton Artemyev shared that hybrid models produce the current sheet better than kinetic models, because kinetic models are stationary. The problem is the boundary conditions: if the initial state is not correct, the model cannot produce the system’s evolution. He also reminded the audience that there is no such thing as a good or bad model…but good or bad questions to ask the model. Ohtani shared that the question we should ask is whether we can have a substorm without the ionosphere? He reminded us that before the THEMIS era, substorms were called “the two-minute problem” because the resolution of observations allowed for a two minute window of uncertainty. This window of uncertainty is what gave rise to the in-out vs. out-in interpretation of the onset phenomenology. Because of the ionosphere’s importance in this, he asked whether we include the ionospheric effect well enough? What aspect(s) are we missing? How do we associate what we see in the auroral images with phenomena in the tail? Wolf answered the first question by explaining that we cannot neglect the ionosphere: it’s an active participant in the process. He also agreed with earlier points that no one model can get everything correct. McPherron agreed, pointing out from his model that conductance is essential for substorms: if conductivity is increased, bubbles can make it farther earthward.

Eric Donovan shared that as far as modeling goes, one thing that is troubling is that the simulation movies are so different than what he observes in the ionosphere. His perspective is that the movies (with all the fast flows, bubbles, and DFBs) are what happen AFTER the expansion phase occurs in the ionosphere. Learning how to reconcile what the simulations show with the 2D picture from the ionosphere is something that we should take very seriously. For instance, the simulations look very chaotic. However, in the late growth phase, things are very ordered…auroral arcs are very clear. He therefore does not see how BBFs, flux bundles, DFBs, etc. can be causal for the onset in the inner magnetosphere.

Donovan suggested that we make maps of the magnetospheric models in the ionosphere. What would the diffuse aurora look like in the ionosphere? What would the proton aurora be doing? Then compare with the data. He also suggested that a mission with 50 spacecraft in the nightside transition region between 6-12 RE, similarly distributed, would provide the better fidelity required to explore what is really happening.

Runov shared that what he would like to see is an increased fleet of low-orbiting spacecraft equipped with high energy particle detectors and better magnetometers. These would remove the need to remote sense the magnetic configuration. This could be very powerful, but we would require auroral observations to assist with the models in order to complete a comprehensive picture.

To address the question of, “Is the auroral observation an ionospheric source or a magnetospheric source,” Drew Turner suggested conjugate imagers in the Northern and Southern hemispheres. Donovan followed up by stressing that our field has really undervalued imaging. We are willing to spend millions on satellite missions, but balk at spending money on imagers.

In the spirit of forward-looking ideas, Ohtani shared that it would be great to have an EM imager: low energy, stereo imaging. The ENA image could look at the change of topology with the flux enhancement.

Contributed Talks

We had ten contributed talks that discussed dipolarization and its effect on the inner magnetosphere.

  1. Sheng Tian presented on “Poynting flux at the PSBL in conjunction with the ground aurora: dipolarization at L~6”. He created a new mapping perspective using a vertical and a horizontal box, where the vertical box maps to the PSBL while the horizontal box maps to the equator. He showed that the dipolarization front correlates to poynting flux in the PSBL which mapped to the ionosphere where aurora was observed. Enhancement of ion outflow occurred right after the increase in poynting flux.
  2. Grant Stephens presented on “Magnetotail thinning and dipolarization during substorms: Empirical picture”, using an empirical model (TS07D). He replaced uniform equatorial current sheet thickness with multiple current sheets of differing thicknesses to reproduce current sheet thinning in the growth phase. He also utilized a new field aligned current description to reproduce the Harang reversal, which proved to be critical to reproducing the substorm dipolarization. The model is not a statistical average, but a statistical average for specific events.
  3. Katie Garcia-Sage presented on “Global MHD Simulations in the context of Magnetotail Stability Theory”. She showed that a ridge or “hump” in Bz could form downtail, which could be interchange unstable. The ridge corresponds to fast flows at the flanks, and remains stable for a long time before going unstable. She showed that distant reconnection causes flows which break around -20 RE. On average, she sees a nice, smooth entropy profile downtail, but gets a high Cd ridge sitting at the velocity convergence. This builds up in what she calls the “flow braking region”, which is at -25 RE (not at -12 RE where we typically think of flow braking).
  4. Don Mitchell presented on “Ion injections inside geosynchronous orbit: charge- (not mass) dependent (quasi-) adiabatic acceleration”. He found that all ion species were being energized by the same process (adiabatically). A 180 keV O ion behaves like a 180 keV H ion. The energy gain of the O6+ particles is six times that of a singly charged ion.
  5. Kareem Sorathia presented “Ion Transport and Acceleration at Dipolarization Fronts: High-Resolution MHD – Test-Particle Simulations”. Using Mike Wiltberger’s LFM simulation, he followed particles in a convection surge (an increase in earthward flow/azimuthal EY). The inverse magnetic field gradients associated with a localized dipolarization front form magnetic islands that can trap ions in their guiding center trajectories. This trapping enables ions to propagate earthward. When he traced many particles, a core group remained at 90 degrees, even though many were pitch angle scattered. These would be able to continue traveling earthward with the front. Looking at the phase space density evolution, he saw a transition to a kappa distribution.
  6. Tetsuo Motoba presented on “Response of energetic particles to dipolarization with GEO”. He discussed whether large/impulsive dipolarization electric fields are necessary for particle injections. In observations, these fields are azimuthally localized, and range from a few mV/m to tens of mV/m.
  7. Andrei Runov discussed “Ion distributions within dipolarizing flux bundles (DFBs) in the near-Earth plasma sheet and the tail-dipole transition region”. Using THEMIS event studies, PIC simulations, Test Particle Modeling and, he discussed how ion injections associated with DFBs may provide a free energy source for the EMIC and MS wave excitation in the inner magnetosphere because DFBs may bring 90 degree anisotropic distributions into the inner magnetosphere.
  8. Yiqun Yu discussed “Effects of bursty bulk flows on large-scale current systems”. She coupled MHD with ionosphere and ring current using BATSRUS, RCM, and RIM to plot field aligned current patterns. As BBFs break around -10 RE, vortices emerge in pairs on the edge of the breaking region (type 1) and in the inner magnetosphere (type 2), connecting to the substorm current wedge. BBFs continually impinge on the dipolar region and brake, disturbing the pressure distribution and field aligned currents. A new ring current is created as a result of multiple localized BBFs.
  9. Xiangning Chu discussed “Magnetotail flux accumulation leading to auroral expansion and substorm current wedge: A case study”. Because pressure gradient and flux tube volume are hard to obtain from in-situ observations, the SCW cannot be obtained from spacecraft. He explained that the substorm current wedge is generated by accumulated flux from the dipolarized magnetic field lines, which causes poleward expansion. Flow braking and diversion can bend field lines and generate field aligned currents.
  10. Eric Donovan presented his view, in response to the earlier discussion, that it is an instability—not flux pile-up—which causes auroral brightening.

Joint Sessions with “Magnetic Reconnection in the Magnetosphere” and “Tail Environment and Dynamics at Lunar Distances” FGs

The “Magnetic Reconnection in the Magnetosphere” focus group joined with the “Tail Environment and Dynamics at Lunar Distances” and “Magnetotail Dipolarization and Its Effects on the Inner Magnetosphere” FGs on Monday afternoon at GEM this year (06/19/2017). These two joint sessions encouraged cross-focus group interaction, and open ended discussion on the topics including the onset of tail reconnection, the role of cross-tail instabilities, the difference between the tailward and earthward reconnection jets/flux bundles, the interaction of dipolarization fronts with ambient plasmas. There were approximately 70 attendees in these two joint sessions.

Vassilis Angelopoulos kicked off the first session with a tutorial talk. Vassilis provided a broad view of the observation and modeling of the nightside phenomena and substorms. Topics include the ionospheric signature, substorm current wedge (SCW), near-Earth-neutral line, current disruption versus reconnection models, external-driven versus spontaneous onset, dipolarization fronts, bursty-bulk flows (BBFs). In particular, Vassilis challenged global modelers for a quantitative assessment of the rate and intensity of BBFs, which brought up discussion on the time-scale difference of BBFs and SCW. At the end of his talk, Vassilis suggested the idea of employing neural networks, to conjoint statistics of occurrence rates and characteristics from multi-mission datasets.

Misha Sitnov described the internally driven (aka spontaneous) onset of magnetotail reconnection, which is only possible - in the case of electrons magnetized initially by the normal magnetic field - when that field has a region with a tailward gradient. 3D PIC simulations of the corresponding ion tearing instability show that its distinctive features are: 1) spontaneously generated earthward plasma flows that precede the topology change, 2) new Hall pattern, opposite to the classical quadrupole pattern near the X-line; 3) new dissipation region (j*E’>0) at the dipolarization front that may form before the X-line electron dissipation region.”

Heli Hietala presented ARTEMIS two-spacecraft observations of reconnection in the presence of density asymmetry in the lunar distance magnetotail. The observations also indicate the reconnection flow channel had a finite width, of the order of 5 Earth radii.

Andrei Runov discussed kinetic properties of earthward-contracting dipolarizing flux bundles (DFBs) observed by THEMIS in the near-Earth tail and tailward progressing rapid flux transport (RFTs) enhancements observed by THEMIS in the near-tail and by ARTEMIS at lunar orbit, respectively. The DFBs and RFTs are considered as earthward and tailward ejecta from near-Earth reconnection. It was shown that whereas DFBs interacts with near-tail plasma populations and particles within DFBs gain energy from the increasing magnetic field, the RFT particles do not interact with ambient field and plasma and keep the energy gained during reconnection. The plasma state within RFTs is close to isothermal.

Joachim Birn presented a comparison of ion distributions earthward and tailward of the reconnection site, obtained by a combined MHD/test particle approach. While ions on the earthward side might experience multiple, Fermi or betatron-like, acceleration, leading to multiple beams and ring-like distributions, ions on the tailward side experience only single direct acceleration, adding a beam to an unperturbed population.

The GEM-style forum successfully stimulated active discussions between the presenters and audience, including Bob McPherron, Mostafa El Alaoui, Eric Donovan, Matina Gkioulidou, San Lu, Xiangning Chu, Chih-Ping Wang, Drew Turner, Christine Gabrielse et al.


Resulting Papers

If you wrote a paper that was in part thanks to or discussed in this Focus Group, please let us know and we will add it to the list.

  1. Cramer, W. D., J. Raeder, F. R. Toffoletto, M. Gilson, and B. Hu (Apr 2017), Plasma sheet injections into the inner magnetosphere: Two-way coupled OpenGGCM-RCM model results, Journal of Geophysical Research: Space Physics, 122, 5077–5091, doi:10.1002/2017JA024104.
  2. Gabrielse, C., V. Angelopoulos, C. Harris, A. Artemyev, L. Kepko, and A. Runov (Apr 2017), Extensive electron transport and energization via multiple, localized dipolarizing flux bundles, J. Geophys. Res. Space Physics, 122, 5059–5076, doi:10.1002/2017JA023981.
  3. Keesee, A. M., Katus, R. M., & Scime, E. E. (Aug 2017). The Effect of Storm Driver and Intensity on Magnetospheric Ion Temperatures. Journal of Geophysical Research: Space Physics, 122(9), 9414–9426. https://doi.org/10.1002/2017JA023973
  4. Dewey, R. M., Slavin, J. A., Raines, J. M., Baker, D. N., & Lawrence, D. J. (Nov 2017). Energetic electron acceleration and injection during dipolarization events in Mercury’s magnetotail. Journal of Geophysical Research: Space Physics, 122, 12,170-12,188, doi:10.1002/2017JA024617.
  5. Turner, D. L., Fennell, J. F., Blake, J. B., Claudepierre, S. G., Clemmons, J. H.,Jaynes, A. N.,...Reeves, G. D. (Nov 2017). Multipoint observations of energetic particle injections and substorm activity during a conjunction between Magnetospheric Multiscale (MMS) and Van Allen Probes. Journal of Geophysical Research: Space Physics, 122, 11, 481–11,504. https://doi.org/10.1002/2017JA024554
  6. Nakamura, R., Varsani, A., Genestreti, K. J., Le Contel, O., Nakamura, T., Baumjohann, W., et al. (Feb 2018). Multiscale currents observed by MMS in the flow braking region. Journal of Geophysical Research: Space Physics, 123, 1260–1278. https://doi.org/10.1002/2017JA024686
  7. Lui, A. T. Y. (Feb 2018), Frozen-in condition for ions and electrons: Implication on magnetic flux transport by dipolarizing flux bundles, Geosci. Lett., 5:5, doi.org/10.1186/s40562-018-0104-0.
  8. Pritchett, P. L., & Lu, S. (Mar 2018). Externally driven onset of localized magnetic reconnection and disruption in a magnetotail configuration. Journal of Geophysical Research: Space Physics, 123, 2787–2800. https://doi.org/10.1002/2017JA025094
  9. Dewey, R. M., Raines, J. M., Sun, W., Slavin, J. A., & Poh, G. (Mar 2018). MESSENGER observations of fast plasma flows in Mercury’s magnetotail. Geophysical Research Letters, 45, 10,110–10,118, doi:10.1029/2018GL079056.
  10. Lui, A. T. Y. (Mar 2018). Review on the Characteristics of the Current Sheet in the Earth's Magnetotail. In A. Keiling, O. Marghitu, & M. Wheat-land (Eds.),Electric Currents in Geospace and Beyond(Vol. 235, pp. 155–175). Washington, DC. https://doi.org/10.1002/9781119324522.ch10
  11. Ohtani, S., Motoba, T., Gkioulidou, M., Takahashi, K., & Singer, H. J. (June 2018). Spatial development of the dipolarization region in the inner magnetosphere. Journal of Geophysical Research: Space Physics, 123, 5452– 5463. https://doi.org/10.1029/2018JA025443
  12. Stephens, G. K., Sitnov, M. I., Korth, H., Tsyganenko, N. A., Ohtani, S., Gkioulidou, M., & Ukhorskiy, A. Y. (Jan 2019). Global empirical picture of magnetospheric substorms inferred from multimission magnetometer data. Journal of Geophysical Research: Space Physics, 124. https://doi.org/10.1029/2018JA025843
  13. Eshetu, W. W., Lyon, J. G.,Hudson, M. K., & Wiltberger, M. J. (Feb 2019). Simulations of electron energization and injection by BBFs using high-resolution LFM MHD fields. Journal of Geophysical Research: Space Physics,124, 1222–1238. https://doi.org/10.1029/2018JA025789
  14. Liu, Y.-H., Li, T. C., Hesse, M., Sun, W.-J., Liu, J., Burch, J., et al. (Apr 2019). Three-dimensional magnetic reconnection with a spatially confined X-line extent: Implications for dipolarizing flux bundles and the dawn-dusk asymmetry. Journal of Geophysical Research: Space Physics, 124, 2819–2830. https://doi.org/10.1029/2019JA026539
  15. Sitnov, M., Birn, J., Ferdousi, B., Gordeev, E., Khotyaintsev, Y., Merkin, V., Motoba, M., Otto, A., Panov, E., Pritchett, P., Pucci, F., Raeder, J., Runov, A., Sergeev, V., Velli, M. & Zhou, X. (June 2019). Explosive Magnetotail Activity. Space Science Reviews, 215(4), 31. https://doi.org/10.1007/s11214-019-0599-5.
  16. Birn, J., Liu, J., Runov, A., Kepko, L.,& Angelopoulos, V. (July 2019). On the contribution of dipolarizing flux bundles to the substorm current wedge and to flux and energy transport. Journal of Geophysical Research: Space Physics,124,5408–5420. https://doi.org/10.1029/2019JA026658.
  17. Gabrielse, C., Spanswick, E., Artemyev, A., Nishimura, Y., Runov, A., Lyons, L., et al. (July 2019). Utilizing the Heliophysics/Geospace System Observatory to understand particle injections: Their scale sizes and propagation directions. Journal of Geophysical Research: Space Physics, 124, 5584–5609. https://doi.org/10.1029/2018JA025588
  18. Sitnov, M. I., Stephens, G. K., Tsyganenko, N. A., Miyashita, Y., Merkin, V. G., Motoba, T., Ohtani, S. & Genestreti, K. (Oct 2019). Signatures of nonideal plasma evolution during substorms obtained by mining multimission magnetometer data. Journal of Geophysical Research: Space Physics, 124. https://doi.org/10.1029/2019JA027037
  19. Ohtani, S. (Oct 2019). Substorm energy transport from the magnetotail to the nightside ionosphere. Journal of Geophysical Research: Space Physics, 124, 8669–8684. https://doi.org/10.1029/2019JA026964
  20. Merkin, V. G., Panov, E. V., Sorathia, K., & Ukhorskiy, A. Y. (Oct 2019). Contribution of bursty bulk flows to the global dipolarization of the magnetotail during an isolated substorm. Journal of Geophysical Research: Space Physics, 124, 8647-8668. https://doi.org/10.1029/2019JA026872
  21. Fu, H., Grigorenko, E.E., Gabrielse, C. et al. Magnetotail dipolarization fronts and particle acceleration: A review. Sci. China Earth Sci. (Dec 2019), doi:10.1007/s11430-019-9551-y
  22. McPherron, R. L., El-Alaoui, M., Walker, R. J., Nishimura, Y., & Weygand, J. M. (Jan 2020). The relation of N-S auroral streamers to auroral expansion. Journal of GeophysicalResearch: Space Physics, 125, e2019JA027063. https://doi.org/10.1029/2019JA027063.
  23. Nishimura, Y., L. R. Lyons, C. Gabrielse, J. M. Weygand, E. F. Donovan & V. Angelopoulos (July 2020), Relative contributions of large-scale and wedgelet currents in the substorm current wedge. Earth Planets Space 72, 106. https://doi.org/10.1186/s40623-020-01234-x.
  24. Ohtani, S., J. Gjerloev (August 2020), Is the Substorm Current Wedge an Ensemble of Wedgelets?: Revisit to Midlatitude Positive Bays, Journal of Geophysical Research: Space Physics, https://doi.org/10.1029/2020JA027902
  25. McPherron, R.L., M. El-Alaoui, R.J. Walker, and R. Richard (August 2020), Characteristics of Reconnection Sites and Fast Flow Channels in an MHD Simulation, J. Geophys. Res. - Space Physics, https://doi.org/10.1029/2019JA027701.
  26. Ghaffari, R., Cully, C. M., & Gabrielse, C. (2021). Statistical study of whistler-mode waves and expected pitch angle diffusion rates during dispersionless electron injections. Geophysical Research Letters, 48, e2021GL094085. https://doi.org/10.1029/2021GL094085
  27. Runov, A., Angelopoulos, V., Henderson, M. G., Gabrielse, C., & Artemyev, A. (2021). Magnetotail dipolarizations and ion flux variations during the main phase of magnetic storms. Journal of Geophysical Research: Space Physics, 126, e2020JA028470., https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020JA028470
  28. Nishimura, Y., Artemyev, A. V., Lyons, L. R., Gabrielse, C., Donovan, E. F., & Angelopoulos, V. (2022). Space-ground observations of dynamics of substorm onset beads. Journal of Geophysical Research: Space Physics, 127, e2021JA030004. https://doi.org/10.1029/2021JA030004.
  29. Ohtani, S., Motoba, T., Gjerloev, J. W., Frey, H. U., Mann, I. R., Chi, P. J., & Korth, H. (2022). New Insights into the Substorm Initiation Sequence from the Spatio-temporal Development of Auroral Electrojets. Journal of Geophysical Research: Space Physics, 127, e2021JA030114. https://doi.org/10.1029/2021JA030114.
  30. Gabrielse C, Gkioulidou M, Merkin S, Malaspina D, Turner DL, Chen MW, Ohtani S-i, Nishimura Y, Liu J, Birn J, Deng Y, Runov A, McPherron RL, Keesee A, Yin Lui AT, Sheng C, Hudson M, Gallardo-Lacourt B, Angelopoulos V, Lyons L, Wang C-P, Spanswick EL, Donovan E, Kaeppler SR, Sorathia K, Kepko L and Zou S, (2023) Mesoscale phenomena and their contribution to the global response: a focus on the magnetotail transition region and magnetosphere-ionosphere coupling. Front. Astron. Space Sci. 10:1151339, https://doi.org/10.3389/fspas.2023.1151339
Retrieved from "https://gem.epss.ucla.edu/mediawiki/index.php?title=FG:_Magnetotail_Dipolarization_and_Its_Effects_on_the_Inner_Magnetosphere&oldid=6609"