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

Journal article

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.

Authors: Lübke, J; Reichherzer, P; Aerdker, S; Effenberger, F; Wilbert, M

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Cambridge University Press
• Journal: Journal of Plasma Physics
• Volume: 91
• Issue: 5
• Article number: E147
• Publication date: 2025-10-30
• Acceptance date: 2025-09-08
• DOI: 10.1017/s0022377825100883
• EISSN: 1469-7807
• ISSN: 0022-3778
• Language: English
• Keywords: astrophysical plasmas; plasma flows; plasma simulation