FG: Particle Heating and Thermalization in Collisionless Shocks in the MMS Era

Contents

Focus Group Chairs

1. Lynn Wilson / lynn.b.wilsoniii@gmail.com / NASA Goddard Space Flight Center
2. Ivan Vasko / vaskoiy@berkeley.edu / Space Sciences Laboratory, University of California at Berkeley
3. Li-Jen Chen / lijen@mailaps.org / Astronomy Department, University of Maryland, College Park
4. Katy Goodrich / katherine.goodrich@mail.wvu.edu / Department of Physics and Astronomy, West Virginia University

2019 GEM Summer Workshop

1. Steve Schwartz (Tutorial on quasi-potential fields in shocks)
2. Li-Jen Chen (Quasi-potential fields or not: MMS perspectives)
3. Jonathan Ng (Simulations of microinstabilities leading to electron heating at the bow shock)
4. Shan Wang (Reconnection in the shock transition region)
5. Naoki Bessho (PIC simulations of shocks and reconnection)
6. Ilya Kuzichev (PIC simulation of the whistler heat flux instability in the interplanetary shocks)
7. Anton Artemyev (Nonlinear wave-particle interaction in application to bow shock physics)
8. Terry Liu (Particle acceleration by magnetosheath jet-driven bow waves)

2020 Virtual GEM

Session 1

Attendance: ~30+ people

1. Lynn Wilson: Gave a talk on the partition of energy at interplanetary shocks. The main conclusion is that electrons contribute significantly to the energy budget of low Mach number (Mf < 6) collisionless shock waves.

1. Terry Liu: Gave a talk on magnetic reconnection in foreshock transients showing that reconnection aided/affected the particle energization in these environments.

1. Vadim Roytershteyn: Gave a talk on high resolution PIC simulations showing that PIC simulations consistently underestimate the amplitude of high frequency electrostatic waves while over estimating the amplitude of quasi-static electric fields. These results are critically important to understanding particle dynamics in collisionless shocks.

1. Hadi Madanian: Gave a talk on the modulation of high Mach number shocks by reflected ions. The basic idea is that ion reflection induces nonstationarity rather than maintaining a stationary discontinuity in high Mach number shocks.

1. Ivan Vasko: Gave a talk on electrostatic solitary waves and their origin in collisionless shock waves. The work illustrates that not only are these waves not consistent with previous assumptions (i.e., they are not electron phase space holes), they likely arise indirectly from an ion/ion two stream instability.

1. Mike Liemohn: Gave a quick dog-and-pony show advertisement for his GEM modeling and resource group.

Session 2

Attendance: ~30+ people

1. Drew Turner: Gave a talk on MMS in situ observations of the formation of a collisionless shock. If correct, this would be the first time such a phenomena has been observed.

1. Naoki Bessho: Gave a talk on PIC simulations of magnetic reconnection occuring within a quasi-parallel collisionless shock. The idea is to test whether this phenomena is present in the Earth's quasi-parallel bow shock. So far as they can tell, it does seem like reconnection plays an important role in the energy dissipation in quasi-parallel collisionless shocks.

1. Andrew Dimmock: Gave a quick presentation on the Solar Orbiter working group focusing on shock waves and particle energization.

1. Alexandra Brosius: Gave a talk on the importance of how one implements MVA on the quality of the output results. Her work shows that one needs to take great care to determine the proper time intervals and frequency filters otherwise the results will not only be inaccurate, they can be completely misleading.

1. Colby Haggerty: Gave a talk on hybrid simulation work that shows really high Mach number shocks that generate cosmic rays actually start to modify the traditional Rankine-Hugoniot relations. That is, the density compression ratio can exceed 4 and the power law spectra of cosmic rays undergoing diffusive shock acceleration becomes steeper, not flatter.

1. Katy Goodrich: Gave an impromptu talk on some new Parker Solar Probe observations of the Cytherian bow shock. She is finding that there are more magnetic holes and double-layers within the shock than one finds at Earth, which is currently unexplained.

Session Style: An informal virtual session. People were free to ask questions as they pleased and all talks generated some good discussion.

2021 Virtual mini-GEM

Attendance: ~30+ people Date: January 2021

1. Aaron Tran: Gave a presentation on electron heating in quasi-perpendicular shocks using 1D and 2D PIC simulations specifically looking at the cross-shock electric fields. The work investigated the competition between whistler precursors and quasi-static fields in energizing the electrons across collisionless shocks and future work necessary to resolve this issue.

1. Lynn Wilson: Gave a short presentation on the discrepancy between observations and simulations of electric fields, based upon his recent work found at: https://doi.org/10.3389/fspas.2020.592634. The work illustrates the large divide between current PIC simulations and observations in regards to electric fields.

1. Savvas Raptis: Gave a presentation on magnetosheath jets close to the bow shock observed by the MMS spacecraft. The work suggests that shock ripples and SLAMS may play a role in the generation of magnetosheath jets.

1. Ivan Vasko: Discussed observations of electrostatic solitary waves (ESWs), measured by MMS, showing that most are actually ion phase space holes not electron holes. This is significant because prior to this, all ESWs observed outside the auroral acceleration region were assumed to be electron holes. They also showed that the holes do not propagate strictly along the magnetic field, again in contrast to previous work. These finds are important for energy dissipation in collisionless shocks.

1. Michael Gedalin: Discussed the issue of whether small-scale electrostatic waves could affect the cold, fast incident ion populations. The talk was based upon his recent publication found at: https://doi.org/10.3847/1538-4357/ab8af0

Open discussion: The talks were followed by a ~20 minute open discussion on outstanding problems in collisionless shock research. This involved topics of measurement limitations from spacecraft and the gap we need to bridge for closure between electric field measurements and PIC simulation results.

Session Style: An informal virtual session. People were free to ask questions as they pleased and all talks generated some good discussion.

2021 Virtual GEM Summer Workshop

Session 1 (July 27)

1. Ilya Kuzichev: Whistler Wave Stability Around Interplanetary Shocks
2. Ahmad Lalti: Source of Whistler Precursor Waves At Quasi-Perpendicular Super-Critical Shocks
3. Brent Page: Whistler Waves in the Earth’s Bow Shock
4. Anton Artemyev: Whistler Waves Acceleration of Electrons in Shocks
5. Ivan Vasko: Electrostatic Waves in the Earth’s Bow Shock
6. Artem Bohdan: Electron heating at high Mach number planetary and astrophysical shocks; PIC simulations
7. Vadim Roytershteyn: Electric fields in shocks [?]; PIC simulations

Session 2 (July 28)

1. Andrew Dimmock: An MMS bow shock database using machine learning: EU H2020 SHARP project
2. Steven Schwartz: Evaluating the de Hoffmann-Teller cross-shock potential at real collisionless shocks
3. Alexandra Brosius Foreshock
4. Terry Liu: Statistical study of foreshock ion conditions for hot flow anomalies and foreshock bubble
5. Nehpreet Walia: Study of Slow-mode Shocks in Magnetic Reconnection Based on Hybrid Simulations and Satellite Observations
6. Naoki Bessho: Electron acceleration by magnetic reconnection in the Earth’s bow shock

$Katy Goodrich: A new mission on shocks

2021 Virtual mini-GEM

Attendance: ~20 people Date: mid December 2021

1. Andreas Johlander: Quasi-parallel Shock Reformation Seen by MMS and Ion-kinetic Simulations
2. Ahmad Lalti: Measurement of high frequency of electrostatic waves in the shock ramp of quasi-perpendicular shocks.
3. Sergey Kamaletdinov: Theoretical analysis of electron scattering by electrostatic fluctuations in the Earth’s bow shock
4. Rachel Wang: Electrostatic fluctuations in the Earth's bow shock: bow shock ion holes vs. magnetotail ion holes
5. Robert Allen: HelioDISC

2022 GEM Summer Workshop

Attendance: TBD Date: 10:30 am to 12 pm on June 24, 2022 Location: Honolullu, Hawaii

1. Lynn Wilson: Intro/Opening Statements
2. Terry Liu: Foreshock ion motion across discontinuities: formation of foreshock transients
3. Ahmad Lalti: Measurement of Debye scale electrostatic waves with MMS EDP instrument: problems and possible mitigation
4. Rachel Wang: An in-depth discussion of solitary waves in the Earth's bow shock
5. Ivan Vasko: Ion-acoustic waves in the Earth’s bow shock
6. Sergey Kamaletdinov: Quasi-linear and nonlinear interactions of solar wind electrons and ion acoustic waves in the Earth’s bow shock
7. Paul Cassak: Entropic approach to study energy conversion in shocks
8. M. Hasan Barbhuiya: HORNET - A New Measure of Kinetic-Scale Energy Conversion

Attendance: TBD Date: 1:30 pm to 3 pm on June 24, 2022 Location: Honolullu, Hawaii

1. Katy Goodrich: Terrestrial Shock Observations: Past, Present, and Future
2. Ilya Kuzichev: Quasi-parallel whistler waves around interplanetary shock waves
3. Shan Wang: Electron heating associated with reconnection and magnetic holes in foreshock waves: PIC analysis
4. Xiaofei Shi: Whistler waves in the foreshock transients
5. Anton Artemyev: On importance of nonlinear resonant effects for electron interaction with whistler-mode waves around the bow shock
6. Hadi Madanian: Foreshock bubbles upstream of Mars: Implications of a relatively small bow shock
7. Naoki Bessho: Electron acceleration and heating due to magnetic reconnection in a quasi-parallel shock
8. Craig Pollock: Dynamics of Earth’s bow shock under near-radial interplanetary magnetic field conditions

2022 GEM-Shine joint session

Attendance: TBD Date: 10:30 am to 12 pm on June 25, 2022 Location: Honolullu, Hawaii

1. Lynn Wilson: Intro
2. Takanobu Amano: Theory of nonthermal electron acceleration and its implication for in-situ observations
3. Colby Haggerty: solar abundances of helium/ entropy at shocks
4. Damiano Capriolli: revised theory of DSA
5. Ivan Vasko: Review of electrostatic waves in the Earth’s bow shock

2023 GEM Summer Workshop

Attendance: ~35 people in room, ~12 online Date: 10:30 am to 12 pm on June 15, 2023 Location: San Diego, California

1. Ahmad Lalti: Statistical analysis of electron heating at quasi-perpendicular shocks
2. Sergei Kamaletdinov: Electron scattering by ion-acoustic waves in the Earth's bow shock
3. Muhammad Shamir: Statistical study of double layers in the Earth’s bow shock
4. Aaron Tran: Electron Heating in 2D PIC Shocks: Parallel Potential Structure
5. Sasha Lukin: On two approaches for inclusion of wave-particle resonant effects into global test particle simulations
6. Xiaofei Shi: Electron resonant interactions with whistler-mode waves around the Earth's bow shock

Attendance: ~20 people in room, ~10 online Date: 1:30 pm to 3 pm on June 15, 2023 Location: San Diego, California

1. Andrew Dimmock: Ion reflection observed at an IP shock by Solar Orbiter
2. Oleksiy Agapitov: The entropy dynamics across the plasma shocks
3. Terry Liu: Modeling discontinuity-driven foreshock transients
4. Kun Zhang: The early-phase growth of ULF waves in the ion foreshock observed in a hybrid-Vlasov simulation
5. Zubair Shaikh: Electrostatic Solitary Waves : Magnetosheath vs. Bow Shock
6. Hector Salinas: Checking for electric current closure across the magnetosheath, from the bow shock to the magnetopause

2023 mini-GEM meeting

Attendance: ~20 people in room Date: 3:30 pm to 5 pm Pacific Time, December 10, 2023 Location: The Holiday Inn San Francisco – Golden Gateway, San Francisco

1. Neha Srivastava: Global MHD Simulation of Solar Wind Tangential Discontinuities Interacting with Earth's Bow Shock using OpenGGCM
2. Vicki Toy-Edens: Clustering MMS: labeling 4 plasma regimes in 8 years of dayside mission data
3. Wenli Mo: Comparison of Observed and Modeled Dayside Bow Shock and Magnetopause Locations in 8 Years of MMS Data.
4. David Tonoian: Electron resonant interaction with whistler-mode waves around the Earth’s bow shock: the mapping technique
5. Xiaofei Shi: Multi-component mechanism of relativistic electron acceleration in the foreshock region
6. Zubair Shaikh: Electrostatic fluctuations in the Earth's magnetosheath
7. Sergey Kamaletdinov: Nonlinear electron scattering by electrostatic waves in collisionless shocks

Term: Five years (2019-2024)

Introduction to the focus group

Topic

Particle heating and thermalization in collisionless shocks are important unsolved problems, because of the challenges to resolve the kinetic scales with past in-situ data and simulations. Further progress in modeling collisionless shocks and validating MHD, hybrid, and PIC simulations requires resolving particle heating and thermalization processes from the ion to electron kinetic scales. Understanding these processes to the level with predictive capabilities will advance simulations of collisionless shock waves and wave-particle interactions, whether occurring near-Earth or other regions of space. The broad goals of the FG are to address (1) the structure of the quasi-static electric fields in collisionless shocks and their role in particle heating, (2) waves/structures in collisionless shocks and their generation mechanisms, (3) contributions of quasi-static and high-frequency electric fields to particle heating and thermalization and (4) enabling advances of MHD, hybrid, and PIC simulations to model the Earth’s bow shock and magnetosheath plasma. We expect to have strong collaborations with other focus groups that study the phenomena sensitive to the solar wind input.

The major focus of the Focus Group is particle heating and thermalization processes in collisionless shocks at kinetic scales, through the Earth’s bow shock and interplanetary shock observations and modern numerical simulations. Understanding and modeling these fundamental processes are essential to modeling geospace as the bow shock controls the solar wind input into the magnetosphere. The proposed FG will primarily build on the synergy of the unprecedented observation and modeling capabilities that have only come into effect in the past few years.

Figure 1 The two upper panels present the Earth’s bow shock crossing by MMS presented by Chen et al., PRL, 2018 that is selected as a challenge event for the FG during the first year. Panels (a)-(f) in the left figure show the standard behavior of various quantities across the shock - density and magnetic field increase, ion bulk velocity decrease, electron parallel and perpendicular temperature growth resolved with the high temporal resolution by the FPI instrument, electron and ion spectra with unprecedented time resolution. Panel (g) presents the quasi-static electric field along the normal to the shock and indicates that this electric field may consist of high-amplitude short-scale electric field spikes associated with noticeable particle heating as seen in the spectra. The right figure shows the evolution of 30 ms time resolution electron distribution function across the ramp of the bow shock and associated high-frequency waves. The role of quasi-static vs. high-frequency large-amplitude electric field fluctuations in particle heating and thermalization is the fundamental question to be addressed in the frame of the FG. The bottom left panel from Wilson et al., JGR, 2014 (THEMIS observations) shows that the high-frequency wave activity in collisionless shocks may consists of different waves (ion-acoustic waves, Bernstein modes, solitary waves and whistler waves). The mechanisms of their generation and role in particle heating and thermalization remain unresolved and represent another objective of the FG. The bottom right figure shows the results of modern PIC simulations (Matsumoto et al., ApJ, 2012) of a collisionless shock. The important point is that the current simulations do not reproduce the realistic amplitudes and profiles of both the quasi-static electric field and high-frequency waves/structures observed in the Earth’s bow shock.

Goals & Deliverables

The broad goal of the Focus Group is to:

• establish the distribution and properties of the quasi-static electric field in collisionless shocks, resolve particle heating by the quasi-static field, quantitatively compare terms in Ohm’s law;
• analyze microscopic fluctuations (small-scale & high-frequency electric and magnetic fields) across subcritical and supercritical shock waves: census of waves/structures contributing to the microscopic fluctuations; mechanisms of fluctuation generation;
• establish contributions of quasi-static and high frequency fields in particle heating and thermalization in collisionless shocks; and
• establish the limits of applicability of existing MHD, hybrid, and PIC simulations of collisionless shocks to improve simulations of the terrestrial bow shock to accurately reproduce the plasma properties of the magnetosheath.

The deliverables include:

• quantify the contributions of macro- and microscopic fields in particle heating and thermalization at the terrestrial bow shock;
• quantify constraints and limits of applicability of existing simulations of collisionless shocks; and
• quantified constraints will motivate improvements for MHD, hybrid, and PIC simulations with the goal of reproducing the microscopic processes in the sheath downstream of collisionless shocks.

Expected activities

The following activities and session topics for the duration of the proposed focus group are planned

• Year-1: We will focus on the structure and properties of the macroscopic (quasi-static) electromagnetic fields in the terrestrial bow shock. The major objective is to quantify the relevance of quasi-static fields in particle heating, quantify the spatial scales and amplitude of the quasi-static field, and determine their influence on particle heating and thermalization. Particular attention will be focused on modelling the challenge event (see Figure 1), where small-scale features in the quasi-static field and associated particle heating have been clearly resolved. The participation of modellers will make it possible to understand what features of the quasi-static shock structure are reproducible in modern simulations and what processes should be incorporated to reproduce the observed heating and thermalization.

• Year-2: The focus will be on the properties and occurrence rates of waves/structures observed in the Earth’s bow shock. Theoretical and numerical instability analysis of different waves/structures to identify free energy sources/generation mechanisms will be performed. Compare observations and simulations to determine the physical processes missing in the simulations.

• Year-3 and Year-4: The focus will be on comparing the importance of quasi-static vs. high frequency fields on particle heating and thermalization in the Earth’s bow shock. The inclusion of interplanetary shocks (generally subcritical) observed by Parker Solar Probe will expand the range of shock parameters to help establish the factors controlling the heating and thermalization. Comparison between in situ observations and simulations will enable the identification of the key features absent in the simulations

• Year-5: A statistically significant number of shock crossings will provide a solid basis for testing numerical simulation results (improved over the FG activity period) to reproduce the sheath plasma properties downstream of collisionless shocks.