Electron acceleration at a low-Mach-number perpendicular collisionless shock

TL;DR

This study uses large-scale particle simulations to show that rippling at low-Mach-number perpendicular shocks enhances electron acceleration through electrostatic waves and surfing acceleration, revealing a common acceleration mechanism.

Contribution

It demonstrates that shock rippling significantly influences electron acceleration via electrostatic waves and surfing acceleration at low-Mach-number perpendicular shocks.

Findings

Large-amplitude electric fields are excited at the shock front.

Reflected ions are accelerated upstream at rippled regions.

Electrostatic waves facilitate electron surfing acceleration.

Abstract

A full particle simulation study is carried out on the electron acceleration at a collisionless, relatively low Alfven Mach number (M_A=5), perpendicular shock. Recent self-consistent hybrid shock simulations have demonstrated that the shock front of perpendicular shocks has a dynamic rippled character along the shock surface of low-Mach-number perpendicular shocks. In this paper, the effect of the rippling of perpendicular shocks on the electron acceleration is examined by means of large-scale (ion-scale) two-dimensional full particle simulations. It has been shown that a large-amplitude electric field is excited at the shock front in association with the ion-scale rippling, and that reflected ions are accelerated upstream at a localized region where the shock-normal electric field of the rippled structure is polarized upstream. The current-driven instability caused by the highly-accelerated reflected ions has a high growth rate to large-amplitude electrostatic waves. Energetic electrons are then generated by the large-amplitude electrostatic waves via electron surfing acceleration at the leading edge of the shock transition region. The present result suggests that the electron surfing acceleration is also a common feature at low-Mach-number perpendicular collisionless shocks.