Magnetic field amplification and evolution in turbulent collisionless MHD: an application to the ICM

TL;DR

This study investigates magnetic field amplification in the turbulent intracluster medium using a collisionless MHD model with pressure anisotropy relaxation, finding that rapid relaxation yields results similar to collisional MHD, while no relaxation hampers dynamo action.

Contribution

It introduces a collisionless MHD simulation framework with pressure anisotropy relaxation to model magnetic field evolution in the ICM, bridging the gap between collisionless and collisional plasma descriptions.

Findings

Fast anisotropy relaxation reproduces collisional MHD turbulence statistics.

Without relaxation, small-scale fluctuations increase and dynamo amplification fails.

Magnetic field amplification is similar to collisional MHD when relaxation is rapid.

Abstract

The amplification and maintenance of the observed magnetic fields in the ICM are usually attributed to the turbulent dynamo action. This is generally derived employing a collisional MHD model. However, in the ICM the ion mean free path between collisions is of the order of the dynamical scales, thus requiring a collisionless MHD description. Unlike collisional MHD simulations, our study uses an anisotropic plasma pressure with respect to the direction of the local magnetic field, which brings the plasma within a parameter space where collisionless instabilities should take place. Within the adopted model these instabilities are contained at bay through the relaxation term of the pressure anisotropy which simulates the feedback of the mirror and firehose instabilities. This relaxation acts to get the plasma distribution function consistent with the empirical studies of collisionless plasmas. Our 3D numerical simulations of forced transonic turbulence motivated by modeling of the turbulent ICM are performed for different initial values of the magnetic field intensity, and different relaxation rates of the pressure anisotropy. We found that in the high beta plasma regime corresponding to the ICM conditions, a fast anisotropy relaxation rate gives results which are similar to the collisional-MHD model as far as the statistical properties of the turbulence are concerned. Also, the amplification of seed magnetic fields due to the turbulent dynamo action is similar to the collisional-MHD model. Our simulations that do not employ the anisotropy relaxation deviate significantly from the collisional-MHD results, and show more power at the small-scale fluctuations of both density and velocity representing the results of the instabilities. For these simulations the large scale fluctuations in the magnetic field are mostly suppressed and the turbulent dynamo fails in amplifying seed magnetic fields.