Kinetic Simulations of Cosmic-Ray-Modified Shocks I: Hydrodynamics

TL;DR

This paper uses hybrid simulations to study how cosmic rays influence shock hydrodynamics, revealing upstream and downstream modifications, magnetic fluctuation behaviors, and increased shock compression ratios in collisionless plasma shocks.

Contribution

It provides the first detailed assessment of non-linear magnetic fluctuations and their effects on shock structure and cosmic-ray acceleration in collisionless shocks.

Findings

Presence of upstream precursor and downstream postcursor in shocks.

Magnetic fluctuations travel away from the shock at Alfvén speed.

Shock compression ratio exceeds the standard value of 4 in strong shocks.

Abstract

Collisionless plasma shocks are efficient sources of non-thermal particle acceleration in space and astrophysical systems. We use hybrid (kinetic ions -- fluid electrons) simulations to examine the non-linear feedback of the self-generated energetic particles (cosmic rays, CRs) on the shock hydrodynamics. When CR acceleration is efficient, we find evidence of both an upstream precursor, where the inflowing plasma is compressed and heated, and a downstream postcursor, where the energy flux in CRs and amplified magnetic fields play a dynamical role. For the first time, we assess how non-linear magnetic fluctuations in the postcursor preferentially travel away from the shock at roughly the local Alfv\'en speed with respect to the downstream plasma. The drift of both magnetic and CR energy with respect to the thermal plasma substantially increases the shock compression ratio with respect to the standard prediction, in particular exceeding 4 for strong shocks. Such modifications also have implications for the spectrum of the particles accelerated via diffusive shock acceleration, a significant result detailed in a companion paper, Caprioli, Haggerty & Blasi 2020, arXiv:2009.00007 [astro-ph.HE].