Kinetic simulations of nonrelativistic perpendicular shocks of young supernova remnants. II. Influence of shock-surfing acceleration on downstream electron spectra

TL;DR

This study uses kinetic simulations to examine how shock-surfing acceleration influences the formation of nonthermal electron populations in young supernova remnants' shocks, highlighting the processes' roles and limitations.

Contribution

It demonstrates the impact of shock-surfing acceleration on electron energization and explores how this effect varies with ion-to-electron mass ratio in nonrelativistic shocks.

Findings

Shock-surfing acceleration contributes to nonthermal electron populations.

Its significance decreases with increasing ion-to-electron mass ratio.

Combined acceleration processes shape electron spectra downstream.

Abstract

We explore electron pre-acceleration at high Mach-number nonrelativistic perpendicular shocks at, e.g., young supernova remnants, which are a prerequisite of further acceleration to very high energies via diffusive shock acceleration. Using fully kinetic particle-in-cell simulations of shocks and electron dynamics in them, we investigate the influence of shock-surfing acceleration at the shock foot on the nonthermal population of electrons downstream of the shock. The shock-surfing acceleration is followed by further energization at the shock ramp where the Weibel instability spawns a type of second-order Fermi acceleration. The combination of these two processes leads to the formation of a nonthermal electron population, but the importance of shock-surfing acceleration becomes smaller for larger ion-to-electron mass ratio in the simulation. We discuss the resulting electron spectra and the relevance of our results to the physics of systems with real ion-to-electron mass ratio and fully three-dimensional behavior.