Journal Article

Adiabatic and Non‐Adiabatic Electron Heating at Quasi‐Perpendicular Collisionless Shocks

Khotyaintsev, Yu. V., Lalti, A., Graham, D. B.

2024

Overview

The relative contribution of adiabatic and non‐adiabatic processes to electron heating across collisionless shocks remains an open question. We analyze the evolution of suprathermal electrons across 310 quasi‐perpendicular shocks with Alfvénic Mach numbers in the normal‐incidence frame MA−NIF$\\left({M}_{A-NIF}\\right)$ranging from 1.7 to 48, using in situ measurements of Earth's bow shock by the Magnetospheric Multiscale (MMS) spacecraft. We introduce a novel non‐adiabaticity measure derived from the electron distribution function and based on Liouville's theorem. Our results reveal, for the first time, that the electron heating mechanism is governed by the Alfvénic Mach number in the de Hoffman‐Teller frame MA−HT$\\left({M}_{A-HT}\\right)$ , with a transition from predominantly adiabatic to non‐adiabatic heating occurring at MA−HT≳30${M}_{A-HT}\\gtrsim 30$ . Furthermore, by examining the spectral index of the suprathermal electron distribution, we find that for shocks exhibiting dominant non‐adiabatic electron dynamics, the observed electron heating is consistent with the predictions of the stochastic shock drift acceleration (SSDA) mechanism. Plain Language Summary Understanding how electrons get heated across shock waves in space is a challenging scientific question. These shocks can heat electrons through different processes: some involve smooth, gradual changes (adiabatic), while others involve more chaotic interactions (non‐adiabatic). In this study, we looked at data from 310 shock events near Earth using the Magnetospheric Multiscale (MMS) spacecraft, focusing on shocks with a normal vector almost perpendicular to the direction of the magnetic field. We developed a new way to measure how much of the heating is due to non‐adiabatic processes by studying the patterns in how the electrons are distributed in energy. Our findings show that the way electrons are heated is mainly controlled by a dimensionless parameter called the Alfvénic Mach number, which describes how fast the shock is moving compared to a specific speed in the plasma, in a particular frame of reference (the de Hoffman‐Teller frame). We discovered that when this Mach number is above about 30, the heating changes from being mostly adiabatic to mostly non‐adiabatic. Additionally, we found that when non‐adiabatic heating is dominant, it matches well with a known process called stochastic shock drift acceleration (SSDA). Key Points We analyze electron heating across 310 quasi‐perpendicular shocks observed by MMS We use a Liouville mapping technique to show the electron heating mechanism is controlled by the Mach number in the de Hoffmann‐Teller frame We find that electron heating at shocks with dominant nonadiabatic dynamics aligns with the stochastic shock drift acceleration mechanism

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Acceleration / Adiabatic / Adiabatic flow / Adiabatic heating / Adiabatic processes / Dimensionless numbers / Distribution functions / Drift / Earth / Earth magnetosphere / Electron distribution / Electron heating / Electrons / Heating / In situ measurement / Liouville theorem / Mach number / Magnetic field / Magnetic fields / Shock waves / Spacecraft / Stochastic processes