Cosmic ray transport in MHD turbulence

TL;DR

This paper develops an analytical framework for cosmic ray transport in MHD turbulence, incorporating nonlinear effects and turbulence characteristics, to improve understanding of cosmic ray propagation in astrophysical environments.

Contribution

It introduces an analytical approach using scaling laws for MHD modes and nonlinear effects to describe cosmic ray transport, complementing numerical simulations.

Findings

Transport properties depend on plasma beta and Coulomb mean free path.

Perpendicular transport is influenced by turbulence scale and Alfvénic Mach number.

Subdiffusion occurs only with slab waves, below a critical scale.

Abstract

Numerical simulations shed light onto earlier not trackable problem of magnetohydrodynamic (MHD) turbulence. They allowed to test the predictions of different models and choose the correct ones. Inevitably, this progress calls for revisions in the picture of cosmic ray (CR) transport. It also shed light on the problems with the present day numerical modeling of CR. In this paper we focus on the analytical way of describing CR propagation and scattering, which should be used in synergy with the numerical studies. In particular, we use recently established scaling laws for MHD modes to obtain the transport properties for CRs. We include nonlinear effects arising from large scale trapping, to remove the 90 degree divergence. We determine how the efficiency of the scattering and CR mean free path depend on the characteristics of ionized media, e.g. plasma $\beta$, Coulomb collisional mean free path. Implications for particle transport in interstellar medium and solar corona are discussed. We also examine the perpendicular transport of CRs. Perpendicular transport depends on the comparison of parallel mean free path and the injection scale of the turbulence, as well as the Alfv\'enic Mach number. Normal turbulence does not allow subdiffusion unless there are slab waves. The critical scale below which subdiffusion applies is provided. These results can be used to compare with the numerical calculations, provided that these calculations use the structure of magnetic field which is consistent with the numerical studies of MHD turbulence.