Modelling cosmic-ray transport: magnetised versus unmagnetised motion in astrophysical magnetic turbulence

TL;DR

This study distinguishes two cosmic-ray transport modes in turbulent magnetic fields—magnetised and unmagnetised—using simulations, and develops stochastic models for each to better understand their combined effects.

Contribution

It introduces a novel framework that characterizes and models the two distinct cosmic-ray transport modes in turbulent magnetic environments.

Findings

Identification of magnetised and unmagnetised transport modes.

Development of stochastic models for each mode.

A combined model accurately reproduces observed particle displacements.

Abstract

Cosmic-ray transport in turbulent astrophysical environments remains a multifaceted problem and, despite decades of study, the impact of complex magnetic field geometry -- evident in simulations and observations -- has only recently received more focussed attention. To understand how ensemble-averaged transport behaviour emerges from the intricate interactions between cosmic rays and structured magnetic turbulence, we run test-particle experiments in snapshots of a strongly turbulent magnetohydrodynamics simulation. We characterise particle-turbulence interactions via the gyro radii of particles and their experienced field-line curvatures, which reveals two distinct transport modes: magnetised motion, where particles are tightly bound to strong coherent flux tubes and undergo large-scale mirroring; and unmagnetised motion, characterised by chaotic scattering through weak and highly tangled regions of the magnetic field. We formulate an effective stochastic process for each mode: compound subdiffusion with long mean free paths for magnetised motion, and a Langevin process with short mean free paths for unmagnetised motion. A combined stochastic walker that alternates between these two modes accurately reproduces the mean squared displacements observed in the test-particle data. Our results emphasise the critical role of coherent magnetic structures in comprehensively understanding cosmic-ray transport and lay a foundation for developing a theory of geometry-mediated transport.