Modelling cosmic-ray transport: magnetised versus unmagnetised motion in astrophysical magnetic turbulence
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| Main Authors: | , , , , , , |
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| Format: | Preprint |
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2025
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| _version_ | 1866911314559893504 |
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| author | Lübke, Jeremiah Reichherzer, Patrick Aerdker, Sophie Effenberger, Frederic Wilbert, Mike Fichtner, Horst Grauer, Rainer |
| author_facet | Lübke, Jeremiah Reichherzer, Patrick Aerdker, Sophie Effenberger, Frederic Wilbert, Mike Fichtner, Horst Grauer, Rainer |
| contents | 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. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2505_18155 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Modelling cosmic-ray transport: magnetised versus unmagnetised motion in astrophysical magnetic turbulence Lübke, Jeremiah Reichherzer, Patrick Aerdker, Sophie Effenberger, Frederic Wilbert, Mike Fichtner, Horst Grauer, Rainer Plasma Physics High Energy Astrophysical Phenomena 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. |
| title | Modelling cosmic-ray transport: magnetised versus unmagnetised motion in astrophysical magnetic turbulence |
| topic | Plasma Physics High Energy Astrophysical Phenomena |
| url | https://arxiv.org/abs/2505.18155 |