Limits on the ions temperature anisotropy in turbulent intracluster medium

TL;DR

This paper investigates how turbulence in the intracluster medium influences ion temperature anisotropy, showing that turbulence sustains anisotropies near instability thresholds, affecting magnetic field amplification.

Contribution

It compares turbulence-driven anisotropy growth with plasma instability relaxation rates, highlighting the role of AMHD models in describing ICM turbulence.

Findings

Turbulence maintains anisotropies close to instability thresholds.

AMHD models effectively describe Alfvenic turbulence in the ICM.

Anisotropy levels are limited by plasma instabilities.

Abstract

Turbulence in the weakly collisional intracluster medium of galaxies (ICM) is able to generate strong thermal velocity anisotropies in the ions (with respect to the local magnetic field direction), if the magnetic moment of the particles is conserved in the absence of Coulomb collisions. In this scenario, the anisotropic pressure magnetohydrodynamic (AMHD) turbulence shows a very different statistical behaviour from the standard MHD one and is unable to amplify seed magnetic fields, in disagreement with previous cosmological MHD simulations which are successful to explain the observed magnetic fields in the ICM. On the other hand, temperature anisotropies can also drive plasma instabilities which can relax the anisotropy. This work aims to compare the relaxation rate with the growth rate of the anisotropies driven by the turbulence. We employ quasilinear theory to estimate the ions scattering rate due to the parallel firehose, mirror, and ion-cyclotron instabilities, for a set of plasma parameters resulting from AMHD simulations of the turbulent ICM. We show that the ICM turbulence can sustain only anisotropy levels very close to the instabilities thresholds. We argue that the AMHD model which bounds the anisotropies at the marginal stability levels can describe the Alfvenic turbulence cascade in the ICM.