Cosmic ray transport in large-amplitude turbulence with small-scale field reversals

Philipp Kempski, Drummond B. Fielding, Eliot Quataert, Alisa K. Galishnikova, Matthew W. Kunz, Alexander A. Philippov, Bart Ripperda

Research output: Contribution to journalArticlepeer-review

6 Scopus citations

Abstract

The nature of cosmic ray (CR) transport in the Milky Way remains elusive. The predictions of current microphysical CR transport models in magnetohydrodynamic (MHD) turbulence are drastically different from what is observed. These models usually focus on MHD turbulence with a strong guide field and ignore the impact of turbulent intermittency on particle propagation. This motivates our studying the alternative regime of large-amplitude turbulence with δB/B0 ≫ 1, in which intermittent small-scale magnetic field reversals are ubiquitous. We study particle transport in such turbulence by integrating trajectories in stationary snapshots. To quantify spatial diffusion, we use a set-up with continuous particle injection and escape, which we term the turbulent leaky box. We find that particle transport is very different from the strong guide-field case. Low-energy particles are better confined than high-energy particles, despite less efficient pitch-angle isotropization at small energies. In the limit of weak guide field, energy-dependent confinement is driven by the energy-dependent (in)ability to follow reversing magnetic field lines exactly and by the scattering in regions of 'resonant curvature', where the field line bends on a scale that is of the order of the local particle gyro-radius. We derive a heuristic model of particle transport in magnetic folds that approximately reproduces the energy dependence of transport found numerically. We speculate that CR propagation in the Galaxy is regulated by the intermittent field reversals highlighted here and discuss the implications of our findings for CR transport in the Milky Way.

Original languageEnglish (US)
Pages (from-to)4985-4998
Number of pages14
JournalMonthly Notices of the Royal Astronomical Society
Volume525
Issue number4
DOIs
StatePublished - Nov 1 2023

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Keywords

  • ISM: structure
  • cosmic rays
  • galaxies: evolution
  • plasmas

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