Turbulence in collisionless plasmas: statistical analysis from numerical simulations with pressure anisotropy

TL;DR

This study uses 3D CGL-MHD simulations to analyze turbulence in collisionless plasmas, revealing how pressure anisotropy and kinetic instabilities like firehose and mirror influence turbulence properties, with implications for astrophysical environments like the ICM.

Contribution

First statistical analysis of turbulence in collisionless plasmas using CGL-MHD simulations, highlighting the impact of pressure anisotropy and kinetic instabilities on turbulence characteristics.

Findings

Instabilities can significantly alter turbulence statistical properties.

Turbulence in collisionless plasmas may differ greatly from traditional MHD predictions.

Results suggest implications for cosmic ray acceleration and energy transfer.

Abstract

In the past years we have experienced an increasing interest in understanding of the physical properties of collisionless plasmas, mostly because of the large number of astrophysical environments, e.g. the intracluster medium (ICM), containing magnetic fields which are strong enough to be coupled with the ionized gas and characterized by densities sufficiently low to prevent the pressure isotropization with respect to the magnetic line direction. Under these conditions a new class of kinetic instabilities arises, such as firehose and mirror ones, which were extensively studied in the literature. Their role in the turbulence evolution and cascade process in the presence of pressure anisotropy, however, is still unclear. In this work we present the first statistical analysis of turbulence in collisionless plasmas using three dimensional double isothermal magnetohydrodynamical with the Chew-Goldberger-Low closure (CGL-MHD) numerical simulations. We study models with different initial conditions to account for the firehose and mirror instabilities and to obtain different turbulent regimes. We study the probability distribution functions, spectra, structure functions and anisotropy of density and velocity fluctuations. The results indicate that in some cases the instabilities significantly modifies the statistical properties of turbulence and even though preliminary and restricted to very specific conditions, show that the physical properties of turbulence in collisionless plasmas, as those found in the ICM, may be very different from what has been largely believed. Implications can range from interchange of energies to cosmic rays acceleration.