Diffusion of cosmic rays in MHD turbulence with magnetic mirrors

TL;DR

This paper investigates how magnetic mirroring effects in MHD turbulence influence cosmic ray diffusion, revealing a new type of mirror diffusion that depends on turbulence properties and affects cosmic ray transport in astrophysical environments.

Contribution

It introduces the concept of mirror diffusion caused by magnetic mirroring in MHD turbulence and analyzes its impact on cosmic ray diffusion coefficients and energy scalings.

Findings

Mirror diffusion is generally slower than gyroresonant scattering diffusion.

The critical pitch angle determines the diffusion behavior of cosmic rays.

Diffusion coefficients exhibit non-universal energy scalings depending on turbulence properties.

Abstract

As the fundamental physical process with many astrophysical implications, the diffusion of cosmic rays (CRs) is determined by their interaction with magnetohydrodynamic (MHD) turbulence. We consider the magnetic mirroring effect arising from MHD turbulence on the diffusion of CRs. Due to the intrinsic superdiffusion of turbulent magnetic fields, CRs with large pitch angles that undergo mirror reflection, i.e., bouncing CRs, are not trapped between magnetic mirrors, but move diffusively along the turbulent magnetic field, leading to a new type of parallel diffusion, i.e., mirror diffusion. This mirror diffusion is in general slower than the diffusion of non-bouncing CRs with small pitch angles that undergo gyroresonant scattering. The critical pitch angle at the balance between magnetic mirroring and pitch-angle scattering is important for determining the diffusion coefficients of both bouncing and non-bouncing CRs and their scalings with the CR energy. We find non-universal energy scalings of diffusion coefficients, depending on the properties of MHD turbulence.