Novel aspects of cosmic ray diffusion in synthetic magnetic turbulence

TL;DR

This study uses advanced numerical simulations to analyze cosmic ray diffusion in synthetic magnetic turbulence, revealing that the ratio of perpendicular to parallel diffusion coefficients varies with energy, challenging previous universality assumptions.

Contribution

It provides the first comprehensive numerical evidence that the universality of diffusion coefficients in magnetic turbulence does not hold, with implications for cosmic ray transport models.

Findings

Universality of diffusion coefficients is broken.

The ratio D_perp/D_parallel depends on energy.

Results differ across turbulence models.

Abstract

The diffusive motion of charged particles in synthetic magnetic turbulence with different properties is investigated by using numerical simulations with unprecedented dynamical range, which allow us to ensure that both the inertial range and the long wavelength part of the turbulent spectrum are properly described. This is of particular importance in evaluating previous suggestions that parallel and perpendicular diffusion coefficients differ in their energy dependence, an assertion at odds with the many claims of universality of the $D_{\perp}$ and $D_{\parallel}$ as functions of particle energy. Cases with and without an ordered magnetic field are discussed. Results of the numerical simulations are compared with available theoretical models, for slab, slab/2D and isotropic turbulence. We find widespread evidence that universality is broken, and that the ratio $D_{\perp}/D_{\parallel}$ is not independent of energy. The implications of this finding for the physics of cosmic ray transport are discussed in depth.