Sound-Wave Instabilities in Dilute Plasmas with Cosmic Rays: Implications for Cosmic-Ray Confinement and the Perseus X-ray Ripples

TL;DR

This paper analyzes how cosmic rays can induce acoustic instabilities in weakly collisional, magnetized plasmas, potentially affecting cosmic-ray scattering and observed X-ray ripples in galaxy clusters.

Contribution

It introduces a linear stability analysis of plasmas with cosmic rays, revealing a new acoustic instability driven by pressure anisotropy and cosmic-ray pressure in high-beta environments.

Findings

Cosmic rays offset viscous damping, leading to wave growth.

Instability occurs when CR pressure exceeds a threshold related to plasma parameters.

Potential impact on cosmic-ray scattering and X-ray fluctuations in galaxy clusters.

Abstract

Weakly collisional, magnetised plasmas characterised by anisotropic viscosity and conduction are ubiquitous in galaxies, halos and the intracluster medium (ICM). Cosmic rays (CRs) play an important role in these environments as well, by providing additional pressure and heating to the thermal plasma. We carry out a linear stability analysis of weakly collisional plasmas with cosmic rays using Braginskii MHD for the thermal gas. We assume that the CRs stream at the Alfv\'en speed, which in a weakly collisional plasma depends on the pressure anisotropy ($\Delta p$) of the thermal plasma. We find that this $\Delta p$-dependence introduces a phase shift between the CR-pressure and gas-density fluctuations. This drives a fast-growing acoustic instability: CRs offset the damping of acoustic waves by anisotropic viscosity and give rise to wave growth when the ratio of CR pressure to gas pressure is $\gtrsim \alpha \beta^{-1/2}$, where $\beta$ is the ratio of thermal to magnetic pressure, and $\alpha$, typically $\lesssim 1$, depends on other dimensionless parameters. In high-$\beta$ environments like the ICM, this condition is satisfied for small CR pressures. We speculate that the instability studied here may contribute to the scattering of high-energy CRs and to the excitation of sound waves in galaxy-halo, group and cluster plasmas, including the long-wavelength X-ray fluctuations in \textit{Chandra} observations of the Perseus cluster. It may also be important in the vicinity of shocks in dilute plasmas (e.g., cluster virial shocks or galactic wind termination shocks), where the CR pressure is locally enhanced.