Anomalous, non-Gaussian transport of charged particles in anisotropic magnetic turbulence

Abstract

The transport of energetic particles in a mean magnetic field and in the presence of anisotropic magnetic turbulence is studied numerically, for parameter values relevant to astrophysical plasmas. A numerical realization of magnetic turbulence is set up, in which the degree of anisotropy is varied by changing the correlation lengths lx, ly, and lz. The ratio ρ∕λ of the particle Larmor radius ρ over the turbulence correlation length λ is also varied. It is found that for lx,ly⪢lz, and for ρ∕λ≲10−2 transport can be non-Gaussian, with superdiffusion along the average magnetic field and subdiffusion perpendicular to it. In addition, the spatial distribution of particles is clearly non-Gaussian. Such regimes are characterized by a Levy statistics, with diverging second-order moments. Decreasing the ratio lx∕lz, nearly Gaussian (normal) diffusion is obtained, showing that the transport regime depends on the turbulence anisotropy. Changing the particle Larmor radius, normal diffusion is found for 10−2≲ρ∕λ≲1 because of increased pitch angle diffusion. New anomalous superdiffusive regimes appear when ρ∕λ≳1 showing that the interaction between particles and turbulence decreases in these cases. A new regime, called generalized double diffusion, is proposed for the cases when particles are able to trace back field lines. A summary of the physical conditions which lead to non-Gaussian transport is given.