Cosmic-ray diffusion in collisionless plasmas including pressure anisotropy

TL;DR

This study investigates how pressure anisotropy in collisionless plasmas influences magnetic turbulence and proton diffusion, revealing significant variations in diffusion coefficients across different plasma regimes.

Contribution

It introduces a hybrid kinetic MHD model to simulate turbulence in collisionless plasmas with pressure anisotropy, highlighting its impact on particle diffusion.

Findings

Pressure anisotropy affects magnetic field statistics.

Diffusion coefficients vary significantly across models.

Kinetic instabilities influence proton diffusion.

Abstract

Using a hybrid kinetic magnetohydrodynamic formalism incorporating the effects of pressure anisotropy, we simulate the evolution of a turbulent collisionless plasma in six different models covering the sub/super-sonic and sub/super-Alfv\'enic regimes. Based on the power spectrum of the simulated magnetic field, we compute the particle diffusion coefficients for protons with kinetic energy in the $50-500$ MeV range, and compare them to those obtained within standard magnetohydrodynamics. Our results show that the differences in the statistical properties of the magnetic field, generated by pressure anisotropy and its associated kinetic instabilities, have an appreciable impact on the diffusion coefficients of energetic protons. Moreover, the values of the diffusion coefficients that we obtain within each of the six models considered vary significantly.