Cosmic Ray Parallel and Perpendicular Transport in Turbulent Magnetic Fields

TL;DR

This study uses numerical simulations to analyze cosmic-ray diffusion in turbulent magnetic fields, revealing anisotropic behavior, superdiffusion at small scales, and the dependence of diffusion coefficients on turbulence properties.

Contribution

First numerical derivation of the M_A^4 dependence of perpendicular diffusion coefficient in turbulent magnetic fields.

Findings

No resonance gap at 90° pitch angle.

Mirror interactions dominate scattering for most pitch angles.

CRs exhibit superdiffusion on scales smaller than the injection scale.

Abstract

A correct description of cosmic-ray (CR) diffusion in turbulent plasma is essential for many astrophysical and heliospheric problems. This paper aims to present physical diffusion behavior of CRs in actual turbulent magnetic fields, model of which has been numerically tested. We perform test particle simulations in compressible magnetohydrodynamic turbulence. We obtain scattering and spatial diffusion coefficients by tracing particle trajectories. We find no resonance gap for pitch-angle scattering at 90$^\circ$. Our result confirms the dominance of mirror interaction with compressible modes for most pitch angles as revealed by the nonlinear theory. For cross-field transport, our results are consistent with normal diffusion predicted earlier for large scales. The diffusion behavior strongly depends on the Alfvenic Mach number and particle's parallel mean free path. We, for the first time, numerically derive the dependence of M_A^4 for perpendicular diffusion coefficient with respect to the mean magnetic field. We conclude that CR diffusion coefficients are anisotropic in sub-Alfvenic turbulence and spatially correlated to the local turbulence properties. On scales smaller than the injection scale, we find that CRs are superdiffusive. We emphasize the importance of our results in a wide range of astrophysical processes, including magnetic reconnection.