A critical Mach number for electron injection in collisionless shocks

TL;DR

This paper identifies a critical Mach number threshold in collisionless shocks that enables electron injection into acceleration processes, with implications for understanding supernova remnant shocks.

Contribution

It introduces a new criterion for electron injection based on Mach number and upstream parameters, advancing the understanding of particle acceleration in shocks.

Findings

Electron injection occurs when the Mach number exceeds a critical value.

Whistler waves are excited by electron beams at high Mach numbers.

The critical Mach number depends on upstream plasma conditions.

Abstract

Electron acceleration in collisionless shocks with arbitrary magnetic field orientations is discussed. It is shown that the injection of thermal electrons into diffusive shock acceleration process is achieved by an electron beam with a loss-cone in velocity space that is reflected back upstream from the shock through shock drift acceleration mechanism. The electron beam is able to excite whistler waves which can scatter the energetic electrons themselves when the Alfven Mach number of the shock is sufficiently high. A critical Mach number for the electron injection is obtained as a function of upstream parameters. The application to supernova remnant shocks is discussed.