Early-time velocity autocorrelation for charged particles diffusion and drift in static magnetic turbulence

TL;DR

This study uses simulations to analyze how charged particles' velocity correlations decay over time in static magnetic turbulence, revealing insights into their diffusion and drift behaviors relevant to space physics.

Contribution

It provides new simulation-based insights into early-time velocity autocorrelation and diffusion coefficients for charged particles in static magnetic turbulence.

Findings

Perpendicular diffusion coefficient depends on turbulence energy and particle energy.

Decorrelation times differ for parallel and perpendicular motions.

Results applicable to cosmic ray and interplanetary particle transport.

Abstract

Using test-particle simulations, we investigate the temporal dependence of the two-point velocity correlation function for charged particles scattering in a time-independent spatially fluctuating magnetic field derived from a three-dimensional isotropic turbulence power spectrum. Such a correlation function allowed us to compute the spatial coefficients of diffusion both parallel and perpendicular to the average magnetic field. Our simulations confirm the dependence of the perpendicular diffusion coefficient on turbulence energy density and particle energy predicted previously by a model for early-time charged particle transport. Using the computed diffusion coefficients, we exploit the particle velocity autocorrelation to investigate the time-scale over which the particles "decorrelate" from the solution to the unperturbed equation of motion. Decorrelation time-scales are evaluated for parallel and perpendicular motions, including the drift of the particles from the local magnetic field line. The regimes of strong and weak magnetic turbulence are compared for various values of the ratio of the particle gyroradius to the correlation length of the magnetic turbulence. Our simulation parameters can be applied to energetic particles in the interplanetary space, cosmic rays at the supernova shocks, and cosmic-rays transport in the intergalactic medium.