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

Contents

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)

Focus Group Chairs

1. Lynn Wilson, NASA Goddard Space Flight Center (lynn.b.wilsoniii@gmail.com)
2. Li-Jen Chen, Astronomy Department, University of Maryland, College Park (lijen@mailaps.org)
3. Katherine Goodrich, Space Sciences Laboratory, University of California at Berkeley (katygoodrich@berkeley.edu)
4. Ivan Vasko, Space Sciences Laboratory, University of California at Berkeley (ivan.vasko@ssl.berkeley.edu)

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.