Non-Markovian Cosmic-Ray Pitch-Angle Transport from Mirror Interactions

TL;DR

This paper develops a Langevin model for cosmic-ray pitch-angle transport in MHD turbulence, revealing non-Markovian effects from magnetic mirroring and the transition between anomalous and normal diffusion regimes.

Contribution

It introduces a guiding-center Langevin model explicitly incorporating mirror drift and gyroresonant diffusion, accurately capturing complex particle transport behaviors in 3D turbulence.

Findings

Magnetic mirroring causes anomalous diffusion at large pitch angles.

Particles exhibit a transition from anomalous to normal diffusion as scattering increases.

Compressible turbulence dominates the pitch angle transport mechanisms.

Abstract

Cosmic-ray pitch-angle transport in magnetohydrodynamic (MHD) turbulence is governed by the interplay between magnetic mirroring and gyroresonant scattering. We develop a guiding-center (GC) Langevin model with explicit mirror drift and gyroresonant diffusion to describe the pitch angle evolution. This model accurately captures our test-particle simulation results in three-dimensional MHD turbulence, driven both solenoidally and compressively. We find that magnetic mirroring can drive anomalous pitch-angle diffusion at large pitch angles (including $90^\circ$) with non-Markovian memory effects, which arises from trapping of particles in magnetic wells. Gyroresonant scattering controls the escape rate from these wells. Across $M_{\rm A}$, large-pitch-angle particles are jointly regulated by mirror trapping and gyroresonant escape, exhibiting a transition from anomalous to normal diffusive pitch-angle transport as scattering strengthens, whereas small-pitch-angle particles remain gyroresonance-dominated and diffusive throughout. The pitch angle transport is found to be dominated by the compressible perturbations with marginal influence from Alfv\'en modes. In compressible turbulence with realistic damping accounted for, transit time damping (TTD) treatment fully recovers mirror interactions.