Cosmic-ray pitch-angle scattering in imbalanced MHD turbulence simulations

TL;DR

This study investigates how cosmic-ray particles scatter in imbalanced MHD turbulence, revealing the influence of turbulence imbalance and electric field structures on scattering anisotropy through simulations and theoretical comparisons.

Contribution

It provides the first detailed simulation-based analysis of cosmic-ray pitch-angle scattering in imbalanced MHD turbulence, highlighting the role of electric field structures over wave coherence.

Findings

Cross helicity alters the shape of the pitch-angle diffusion coefficient.

Scattering anisotropy is mainly due to electric field spatial structure, not wave coherence.

Simulation results align with quasilinear theory predictions for idealized spectra.

Abstract

Pitch-angle scattering rates for cosmic-ray particles in magnetohydrodynamic (MHD) simulations with imbalanced turbulence are calculated for fully evolving electromagnetic turbulence. We compare with theoretical predictions derived from the quasilinear theory of cosmic-ray diffusion for an idealized slab spectrum and demonstrate how cross helicity affects the shape of the pitch-angle diffusion coefficient. Additional simulations in evolving magnetic fields or static field configurations provide evidence that the scattering anisotropy in imbalanced turbulence is not primarily due to coherence with propagating Alfven waves, but an effect of the spatial structure of electric fields in cross-helical MHD turbulence.