Understanding Streaming Instabilities in the Limit of High Cosmic Ray Current Density

TL;DR

This study uses kinetic simulations to explore how high cosmic ray currents affect streaming instabilities, revealing less magnetic amplification than expected and highlighting the role of electron-scale modes.

Contribution

It investigates the high-current regime of the Bell instability, showing deviations from traditional assumptions and the impact of anisotropic heating.

Findings

Less magnetic field amplification at high CR currents.

Electron-scale modes cause moderate electron heating.

High-current regime alters instability evolution.

Abstract

A critical component of particle acceleration in astrophysical shocks is the non-resonant (Bell) instability, where the streaming of cosmic rays (CRs) leads to the amplification of magnetic fields necessary to scatter particles. In this work we use kinetic particle-in-cells simulations to investigate the high-CR current regime, where the typical assumptions underlying the Bell instability break down. Despite being more strongly driven, significantly less magnetic field amplification is observed compared to low-current cases, an effect due to the anisotropic heating that occurs in this regime. We also find that electron-scale modes, despite being fastest growing, mostly lead to moderate electron heating and do not affect the late evolution or saturation of the instability.