Hybrid Simulations of Supersonic Shear Flows: II) Cosmic Ray Viscosity

TL;DR

This study uses 2D hybrid simulations to explore how cosmic rays influence shear layer turbulence and energy transfer, revealing that CRs can enhance momentum exchange and affect energy partitioning.

Contribution

It introduces the role of cosmic ray viscosity in shear flows and examines its effects across subsonic to supersonic regimes.

Findings

CRs promote momentum transfer between shear layers.

Increasing CR energy density enhances shear dissipation.

CRs influence energy partitioning and particle acceleration.

Abstract

In this second paper in a series dedicated to characterizing shear layers via 2D hybrid (kinetic ions -- fluid electrons) simulations, we study the dynamical role of nonthermal particles (cosmic rays, CRs), either spontaneously generated or pre-existing. We initialize Kolmogorov-type sinusoidal velocity shear flows unstable to the Kelvin--Helmholtz instability, which evolve nonlinearly into turbulence. Particles with large gyroradii act as long-range messengers that promote momentum exchange between layers, hence introducing a form of CR viscosity. Even when not energetically dominant, increasing the CR energy density generally enhances momentum transfer, provided that their gyroradii are smaller than the shear lengthscale. We consider flows ranging from subsonic to supersonic and assess the rate of shear dissipation, the partition of the initial kinetic energy among heating, thermal ion acceleration, CR reacceleration, and magnetic-field amplification, and the maximum energy attained by accelerated particles.