The electron foreshock at high-Mach-number nonrelativistic oblique shocks

TL;DR

This paper investigates electron-beam instabilities in the foreshock regions of high-Mach-number nonrelativistic oblique shocks using Particle-in-Cell simulations, revealing dominant electrostatic and whistler instabilities consistent with linear theory.

Contribution

It provides the first detailed simulation-based analysis of electron-beam instabilities in the foreshock of high-Mach-number shocks, aligning with theoretical predictions.

Findings

Electrostatic electron-acoustic instability dominates outer foreshock.

Gyroresonant oblique-whistler instability driven by denser inner foreshock beams.

Simulation results agree with linear dispersion analysis predictions.

Abstract

In the Universe matter outside of stars and compact objects is mostly composed of collisionless plasma. The interaction of a supersonic plasma flow with an obstacle results in collisionless shocks that are often associated with intense nonthermal radiation and the production of cosmic ray particles. Motivated by simulations of non-relativistic high-Mach-number shocks in supernova remnants, we investigate the instabilities excited by relativistic electron beams in the extended foreshock of oblique shocks. The phase-space distributions in the inner and outer foreshock regions are derived with a Particle-in-Cell simulation of the shock and used as initial conditions for simulations with periodic boundary conditions to study their relaxation towards equilibrium. We find that the observed electron-beam instabilities agree very well with the predictions of a linear dispersion analysis: the electrostatic electron-acoustic instability dominates in the outer region of the foreshock, while the denser electron beams in the inner foreshock drive the gyroresonant oblique-whistler instability.