Pitch-angle scattering in magnetostatic turbulence. I. Test-particle simulations and the validity of analytical results

TL;DR

This study compares Monte-Carlo simulations with analytical theories to assess the validity of pitch-angle diffusion coefficients in magnetostatic turbulence, revealing good agreement in slab turbulence but deviations in isotropic turbulence.

Contribution

It provides a detailed validation of analytical pitch-angle diffusion models using test-particle simulations across different turbulence geometries.

Findings

Excellent agreement in slab turbulence

Deviations observed in isotropic turbulence

Fokker-Planck coefficients diminish at high times

Abstract

Context. Spacecraft observations have motivated the need for a refined description of the phase-space distribution function. Of particular importance is the pitch-angle diffusion coefficient that occurs in the Fokker-Planck transport equation. Aims. Simulations and analytical test-particle theories are compared to verify the diffusion description of particle transport, which does not allow for non-Markovian behavior. Methods. A Monte-Carlo simulation code was used to trace the trajectories of test particles moving in turbulent magnetic fields. From the ensemble average, the pitch-angle Fokker-Planck coefficient is obtained via the mean square displacement. Results. It is shown that, while excellent agreement with analytical theories can be obtained for slab turbulence, considerable deviations are found for isotropic turbulence. In addition, all Fokker-Planck coefficients tend to zero for high time values.