Electron-Ion Temperature Equilibration in Collisionless Shocks: the Supernova Remnant-Solar Wind Connection

TL;DR

This paper reviews the current understanding of electron heating in collisionless astrophysical shocks, highlighting how electron-ion temperature ratios vary with shock speed and exploring wave-based mechanisms for electron heating.

Contribution

It synthesizes observational data from supernova remnants, solar wind, and galaxy clusters to analyze electron-ion temperature equilibration and discusses potential wave-driven heating mechanisms.

Findings

Electron-ion temperature ratio decreases with shock speed.

Wave damping may heat electrons in collisionless shocks.

Similar mechanisms may operate in solar wind and SNR shocks.

Abstract

Collisionless shocks are loosely defined as shocks where the transition between pre-and post-shock states happens on a length scale much shorter than the collisional mean free path. In the absence of collision to enforce thermal equilibrium post-shock, electrons and ions need not have the same temperatures. While the acceleration of electrons for injection into shock acceleration processes to produce cosmic rays has received considerable attention, the related problem of the shock heating of quasi-thermal electrons has been relatively neglected. In this paper we review that state of our knowledge of electron heating in astrophysical shocks, mainly associated with supernova remnants (SNRs), shocks in the solar wind associated with the terrestrial and Saturnian bowshocks, and galaxy cluster shocks. The solar wind and SNR samples indicate that the ratio of electron temperature to ion temperature declines with increasing shock speed or Alfvenic Mach number. We discuss the extent to which such behavior can be understood via cosmic ray-generated waves in a shock precursor, which then subsequently damp by heating electrons. Finally, we speculate that a similar mechanism may be at work for both solar wind and SNR shocks.