# High $\beta$ Effects on Cosmic Ray Streaming in Galaxy Clusters

## TL;DR

This paper investigates how high plasma beta values in galaxy clusters enhance MHD wave damping, leading to faster cosmic ray streaming and reduced radio halo brightness, revising previous models of CR transport.

## Contribution

It introduces a revised treatment of MHD wave damping in high beta plasmas, showing increased CR streaming speeds and lower radio halo surface brightness predictions.

## Key findings

- High beta enhances MHD wave damping rates by a factor of beta^{1/2}.
- Faster CR streaming results in decreased radio halo brightness.
- Re-examination of previous simulations highlights the importance of high beta effects.

## Abstract

Diffuse, extended radio emission in galaxy clusters, commonly referred to as radio halos, indicate the presence of high energy cosmic ray (CR) electrons and cluster-wide magnetic fields. We can predict from theory the expected surface brightness of a radio halo, given magnetic field and CR density profiles. Previous studies have shown that the nature of CR transport can radically effect the expected radio halo emission from clusters (Wiener et al. 2013). Reasonable levels of magnetohydrodynamic (MHD) wave damping can lead to significant CR streaming speeds. But a careful treatment of MHD waves in a high $\beta$ plasma, as expected in cluster environments, reveals damping rates may be enhanced by a factor of $\beta^{1/2}$. This leads to faster CR streaming and lower surface brightnesses than without this effect. In this work we re-examine the simplified, 1D Coma cluster simulations (with radial magnetic fields) of Wiener et al. (2013) and discuss observable consequences of this high $\beta$ damping. Future work is required to study this effect in more realistic simulations.