Dissipation in intercluster plasma

TL;DR

This paper examines dissipation mechanisms in galaxy cluster plasmas, highlighting electron-dominated viscous damping at small scales and the role of magnetic field instabilities in plasma heating.

Contribution

It challenges the applicability of Braginsky theory to ICM and introduces the significance of electron viscosity and pressure instabilities in plasma dissipation.

Findings

Electron viscosity dominates damping at scales ≤ 1 kpc.

Magnetic field variations induce pressure instabilities affecting dissipation.

Dissipation rate scales inversely with plasma beta parameter.

Abstract

We discuss dissipative processes in strongly gyrotropic, nearly collisionless plasma in clusters of galaxies (ICM). First, we point out that Braginsky theory, which assumes that collisions are more frequent that the system's dynamical time scale, is inapplicable to fast, sub-viscous ICM motion. Most importantly, the electron contribution to collisional magneto-viscosity dominates over that of ions for short-scale Alfvenic motions. Thus, if a turbulent cascade develops in the ICM and propagates down to scales $\leq 1$ kpc, it is damped collisionally not on ions, but on electrons. Second, in high beta plasma of ICM, small variations of the magnetic field strength, of relative value $\sim 1/\beta$, lead to development of anisotropic pressure instabilities (firehose, mirror and cyclotron). Unstable wave modes may provide additional resonant scattering of particles, effectively keeping the plasma in a state of marginal stability. We show that in this case the dissipation rate of a laminar, subsonic, incompressible flows scales as inverse of plasma beta parameter. We discuss application to the problem of ICM heating.