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

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 l(x), l(y), and l(z). The ratio rho/lambda of the particle Larmor radius rho over the turbulence correlation length lambda is also varied. It is found that for l(x),l(y)> l(z), and for rho/lambda less than or similar to 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 l(x)/l(z), 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)less than or similar to rho/lambda less than or similar to 1 because of increased pitch angle diffusion. New anomalous superdiffusive regimes appear when rho/lambda greater than or similar to 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. (c) 2007 American Institute of Physics.