Could Cosmic Rays Affect Instabilities in the Transition Layer of Nonrelativistic Collisionless Shocks?

TL;DR

This study uses particle-in-cell simulations to investigate whether cosmic rays influence the development of instabilities in nonrelativistic collisionless shocks, finding no clear evidence of such effects within the explored parameters.

Contribution

It is the first to systematically analyze the impact of cosmic rays on shock instabilities using PIC simulations with varied parameters.

Findings

No unambiguous cosmic-ray effects detected on instabilities.

Simulation noise can be affected by energetic particle distribution.

Increasing computational particles reduces artificial noise effects.

Abstract

There is an observational correlation between astrophysical shocks and non-thermal particle distributions extending to high energies. As a first step toward investigating the possible feedback of these particles on the shock at the microscopic level, we perform particle-in-cell (PIC) simulations of a simplified environment consisting of uniform, interpenetrating plasmas, both with and without an additional population of cosmic rays. We vary the relative density of the counterstreaming plasmas, the strength of a homogeneous parallel magnetic field, and the energy density in cosmic rays. We compare the early development of the unstable spectrum for selected configurations without cosmic rays to the growth rates predicted from linear theory, for assurance that the system is well represented by the PIC technique. Within the parameter space explored, we do not detect an unambiguous signature of any cosmic-ray-induced effects on the microscopic instabilities that govern the formation of a shock. We demonstrate that an overly coarse distribution of energetic particles can artificially alter the statistical noise that produces the perturbative seeds of instabilities, and that such effects can be mitigated by increasing the density of computational particles.