Cosmic ray Transport in Magnetohydrodynamic turbulence

TL;DR

This study investigates cosmic ray transport in MHD turbulence through simulations, revealing that fast modes dominate propagation, with different diffusion behaviors on various scales, especially super diffusion and Richardson diffusion.

Contribution

The paper provides detailed simulation-based analysis of cosmic ray interactions with different MHD turbulence modes, highlighting the dominance of fast modes and scale-dependent diffusion behaviors.

Findings

Fast modes dominate cosmic ray propagation.

Super diffusion occurs on small scales.

Richardson diffusion observed in Alfvén modes.

Abstract

This paper studies cosmic ray (CR) transport in magneto hydrodynamic (MHD) turbulence. CR transport is strongly dependent on the properties of the magnetic turbulence. We perform test particle simulations to study the interactions of CR with both total MHD turbulence and decomposed MHD modes. The spatial diffusion coefficients and the pitch angle scattering diffusion coefficients are calculated from the test particle trajectories in turbulence. Our results confirm that the fast modes dominate the CR propagation, whereas Alfv\'en and slow modes are much less efficient and have shown similar pitch angle scattering rates. We investigate the cross field transport on large and small scales. On large/global scales, normal diffusion is observed and the diffusion coefficient is suppressed by $M_A^\zeta$ compared to the parallel diffusion coefficients, with $\zeta$ closer to 4 in Alfv\'en modes than that in total turbulence as theoretically expected. For the CR transport on scales smaller than the turbulence injection scale, both the local and global magnetic reference frames are adopted. Super diffusion is observed on such small scales in all the cases. Particularly, CR transport in Alfv\'en modes show clear Richardson diffusion in the local reference frame. Our results have broad applications to CRs in various astrophysical environments.