FG: Kinetic Plasma Processes in the Magnetotail during Substorm Dynamics

Full FG Proposal

Title

Kinetic Plasma Processes in the Magnetotail during Substorm Dynamics

Abstract

The magnetotail contains many non-ideal processes that affect the evolution of substorm dynamics and its consequences in the ionosphere and inner magnetosphere. However, the substorm pre- and post- onset conditions have been understudied despite their determination of the manner in which energy is stored in the magnetotail and ultimately influence the onset mechanism, location, and timing. This focus group will study plasma populations, thin current sheets, and particle energization in fast flows from the far- to mid- and inner-edge magnetotail.

Topic Overview

The Earth’s magnetotail is where magnetic field energy is stored during a substorm growth phase and finally released during the expansion phase, either through magnetic reconnection or some other kinetic instability, resulting in charged particle acceleration and plasma heating. Being less accessible in comparison with the inner Earth’s magnetosphere for monitoring with ground-based facilities and having a much more dynamic magnetic field configuration, this region is mostly investigated via in-situ spacecraft measurements and remote sensing. Ambiguity of spatial and temporal structures is an intrinsic property of such measurements, that together with mapping/projection problems in highly non-dipolar and dynamic magnetic fields complicates the investigation of the magnetotail. Thus, the optimal approach for determination of the dominant magnetotail current sheet modes and dynamical characteristics consists in combining global numerical simulations, modern theoretical/analytical approaches, and multi-spacecraft observations organized via classical statistical approaches and modern machine learning and/or data mining (ML/DM) approaches. The magnetotail current sheet is a principally important element of the magnetosphere, where the substorm grows and its onset is finally triggered by external drivers and/or internal plasma instabilities. A several hour long substorm growth phase with clear memory effects preconditions the magnetotail and determines the location and timing of substorm onset. Plasma transport from the distant tail and ionospheric outflow can significantly alter properties of the magnetotail and cross-magnetopause plasma transport and convective plasma heating affects the magnetotail dawn-dusk asymmetry. Similar global processes significantly alter magnetotail dynamics: plasma transport by fast plasma flows may contribute to both adiabatic (controlled by magnetic field dynamics) and nonadiabatic (determined by local plasma kinetics) charged particle acceleration. All of the aforementioned processes are also influenced by ionospheric outflow of heavy ions such as oxygen and field-aligned anisotropic electron and ion populations that are frequently detected in the near-Earth and mid-tail. This group is focused on various cross-scale couplings and processes (e.g., reconnection, buoyancy, dipolarization fronts, thin current sheets, etc.) which can occur prior to substorm onset, at the onset, and throughout the expansion phase. The primary focus is on the magnetotail current sheet embedded within the plasma sheet, its formation by plasma populations of different origins and its dynamics affecting pre-onset stability and post-onset magnetotail reconfiguration associated with energy transfer and, in particular, charged particle acceleration. The following topics are largely motivated by recent results about the principal role which non-ideal plasma processes play in substorm dynamics from pre-onset through expansion phase. Combining modern global magnetosphere models, including dynamical ionospheric feedback and test particles, ML/DM techniques of reconstructing the magnetic field configuration and plasma parameters, theory and multiple spacecraft observational datasets, we plan to address the following outstanding questions:

[1] Magnetotail plasma populations: origins and role in substorm dynamics.

● What is the role played by ionospheric outflow, transport across the flank magnetopause, and convection from the distant magnetotail in populating the near-Earth tail?

● What are the dynamics of different plasma sources before and after substorm onset?

[2] Thin current sheet dynamics (including formation, stability, and destruction).

● Which non-ideal effects due to ions (including heaving ions) and electrons play a role in thin current sheet formation?

● What determines the location and dynamics of substorm onset in the magnetotail?

● How do thin current sheets interact with mesoscale plasma sheet structures?

[3] Role of fast plasma flows in charged particle transport and acceleration.

● What is the dominant mechanism(s) of charged particle acceleration in the magnetotail, prior to injection into the inner magnetosphere?

● Are plasma kinetics (parallel electric fields, wave-particle resonant interactions, nonadiabatic dynamics) important for charged particle acceleration in the magnetotail?

Timeliness

The rapid growth in inner magnetosphere-ionosphere-thermosphere coupling capability and increasingly high-resolution global magnetosphere simulations make this focus group timely. Further, recent efforts in incorporating empirical reconstructions into first principles models resulting in so-called gray box models offer a new ability to study the magnetotail. Further, a large fleet of spacecraft monitoring the middle and near-Earth magnetotail (measurements of THEMIS, MMS, ARTEMIS, and high-latitude ERG, POES, ELFIN measurements) provide quite a unique opportunity to verify and revise our theoretical concepts of magnetotail dynamics, charged particle acceleration and transport in this key magnetosphere region. In particular, the new MMS string-of-pearls campaign will probe the magnetotail at multiple scales simultaneously shedding light on how kinetic scales couple to the global scale of the magnetosphere. This focus group aims to collect specialists of spacecraft data analysis (including modern ML/DM techniques), theoreticians, and specialists in numerical simulations (both global and local). The focus group will take advantage of the unique current moment with multiple available datasets and well-developed simulation tools in order to address long-standing questions about magnetotail plasma populations, conditioning of the magnetosphere by plasma flows, and magnetotail current sheet characteristics and dynamics. Relevance to Existing GEM Focus Groups

This proposed FG is strongly related to the following existing GEM FGs:

1. Magnetic Reconnection in the Age of the Heliophysics System Observatory (2018-2024): Reconnection is a crucial mechanism for substorm onset. The proposed FG will investigate the nature of the current sheet in relation to reconnection, both near Earth neutral line and distant neutral line reconnection.

2. Mesoscale drivers of the nightside transition region: ionospheric and magnetotail evaluations (MESO) (2022-2026): The proposed focus group will investigate the origin of these mesoscale phenomena beyond the transition region of the magnetotail and the interaction of mesoscale flows with thin current sheets.

3. Magnetospheric Sources of Particle Precipitation and Their Role on Electrodynamic Coupling of Magnetosphere-Ionosphere-Thermosphere Systems (2022-2026): Dipolarization events that originate beyond the night side transition region can transport and energize particles before they precipitate. R1/R2 FACs, field-aligned potential drops and ionospheric conductance conditions, often associated with this dipolarizing flux, modify cross-tail current patterns in the current sheet during substorms.

4. Comparative Planetary Magnetospheric Processes (2023-2027): While the proposed FG focuses on Earth’s magnetotail, the similarities and difference in fundamental dynamics in comparison to other magnetotails play an important role in our understanding of Earth’s magnetotail dynamics.

5. Self-Consistent Inner Magnetospheric Modeling (2020-2025): Magnetotail substorm processes such as current sheet thinning inject energetic particle distributions which feed into the inner magnetosphere, which plays its own role in the causal chain of events that constitutes a substorm.

Goals and Deliverables

Year 1: We will begin by inviting experts from the community to introduce the main topics of the FG. We will plan to hold joint sessions with the Reconnection FG. Year 2: A GEM challenge will be proposed for this year. We will hold joint sessions with the MESO, SCIMM, and MIT FGs. Year 3: An update and continuation of the GEM challenge will be organized. We will continue to hold joint sessions with the MESO, SCIMM and MIT FGs as well as the Comparative Magnetospheres FG. Year 4: The GEM challenge will be completed and summarized. We will also devote a session to determining what questions remain unanswered. We will present this summary in the form of a review paper at the end of the FG.

Chairs

● Harry Arnold, Johns Hopkins Applied Physics Laboratory, harry.arnold@jhuapl.edu (Expertise - Theory and simulations of magnetic reconnection; PIC simulations; global MHD simulations; grey box modeling)

● Jason Derr, United States Military Academy at West Point, (Expertise - Global magnetospheric dynamics; ballooning-interchange instabilities; particle acceleration via parallel electric fields)

● Akhtar Ardakani, University of New Hampshire (Expertise - Multi-spacecraft data analysis; impact of oxygen on magnetic reconnection, mesoscales, and global dynamics)

● Anton Artemyev, University of California, Los Angeles (Expertise – Spacecraft data analysis; particle acceleration; kinetic instabilities in current sheet)

Research Area

Primary: Magnetotail and Plasma Sheet (MPS), Secondary: Global System Modeling (GSM)

Proposed Length

4 years (2024-2028). The first year will be to achieve a rough consensus on the state of the proposed topics. The remaining years will be devoted to progressing on the GEM challenge.

Expected Activities

● We will borrow methods from other existing and past focus groups to encourage more interactive “GEM-Style” sessions. In particular, the notion of “Controversy Sessions” from the recent Dipolarization FG proved to be quite successful. Thus, we will conduct these sessions at least once per meeting where we introduce two opposing viewpoints led by experts in the field.

● We will issue at least one relevant GEM Challenge and devote one session per year to discussing the model results and data analysis with the goal of reaching community consensus on the dominant physics at play in our stated topics.

● We aim to prioritize early career talks during the sessions. In addition, priority will be given to early career members with regard to post-talk questions.

● Finally we will organize joint sessions with the relevant existing FGs as stated above.