On the diffusivity of cosmic ray transport

A common assumption in particle transport theory is the assumption of diffusive propagation, corresponding to a linear increase of the mean square displacement of particle orbits with time. In recent years, however, several authors have discovered nondiffusive transport in observations and in test particle simulations. Some analytical theories also predict subdiffusion or superdiffusion of charged particles. In this article, a recently developed test particle code is employed to explore diffusive and nondiffusive transport regimes. For slab, two‐component, and isotropic turbulence the simulations are performed for different particle energies. It is demonstrated that, in most cases, cross‐field transport is indeed subdiffusive. The level of subdiffusivity is sensitively controlled by the turbulence geometry.