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Using test-particle simulations, we investigate the temporal dependence ofthe two-point velocity correlation function for charged particles scattering ina time-independent spatially fluctuating magnetic field derived from athree-dimensional isotropic turbulence power spectrum. Such a correlationfunction allowed us to compute the spatial coefficients of diffusion bothparallel and perpendicular to the average magnetic field. Our simulationsconfirm the dependence of the perpendicular diffusion coefficient on turbulenceenergy density and particle energy predicted previously by a model forearly-time charged particle transport. Using the computed diffusioncoefficients, we exploit the particle velocity autocorrelation to investigatethe time-scale over which the particles "decorrelate" from the solution to theunperturbed equation of motion. Decorrelation time-scales are evaluated forparallel and perpendicular motions, including the drift of the particles fromthe local magnetic field line. The regimes of strong and weak magneticturbulence are compared for various values of the ratio of the particlegyroradius to the correlation length of the magnetic turbulence. Our simulationparameters can be applied to energetic particles in the interplanetary space,cosmic rays at the supernova shocks, and cosmic-rays transport in theintergalactic medium.

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