Charged Particle Transport in a Non-Uniform Magnetic Field with a Statistically Homogeneous Random Component: Implications for Cosmic-Ray Modulation Models.

In the description of cosmic ray modulation in the heliosphere it is known that the diffusion process is of utmost importance. However, diffusion theories that provide the analytical results that are used in many of the new generation of ab initio modulation models are all derived for the case of a uniform background magnetic field. We investigate the effects of a non-uniform background magnetic field on the propagation of charged particles by means of direct numerical simulation. In particular, the background magnetic field contains a gradient perpendicular to the mean field direction, while the random part of the magnetic field is described by the so-called composite model of turbulence, which consists of a slab- and 2D component respectively. This model for the turbulence is a fully 3D description of the turbulence, with a 20:80 distribution of energy between the slab- and 2D fluctuations respectively. We find that the presence of a gradient in the magnetic field has a significant influence on particle transport. In particular we find that the same turbulence field can yield diffusive transport in a uniform field, while yielding apparent non-diffusive transport (e.g. superdiffusion) when field gradients are introduced. Supported by NASA grant NNG04GF81G and the National Research Foundation under grant GUN2063412.