The inapplicability of spatial diffusion models for solar cosmic rays

Because of the rapid temporal evolution of the particle density, the interplanetary propagation of solar cosmic rays cannot be accurately described by a spatial diffusion equation. With the use of a simple numerical integration scheme, we show that the pitch-angle scattering equation gives intensity-time profiles and anisotropies very different from the spatial diffusion results. In many cases, the reasons for the failure of the diffusion limit, and an explanation for why some temporal profiles of solar particle events look diffusive, are given.