Stochastic simulation of charged particle transport on the massively parallel processor

A Monte Carlo formulation which is closely related not only to the finite-difference formulation but also to the underlying physics of transport phenomena is developed to avoid the problems of instabilities, inaccuracies, and artifacts in calculations of cosmic-ray transport. Computations of charged particle transport along a large-scale guiding magnetic field whose spatial variations are characterized by the focusing length are described. The magnetic fields are visualized as static, and there is no interaction among particles in an extremely tenuous distribution of charged particles. This situation differs from those considered by plasma physics, but it is closely analogous to those treated by classical transport theory. This formulation includes all important effects except those of the convective motion of the background medium, which significantly affect the slow variations of cosmic-ray modulation but play a minor role during the rapid evolution of solar particle events. It includes the two essential aspects of charged particle transport. These are a strong inhibition of transport perpendicular to the guiding field and a strong anisotropy of the pitch-angle scattering by random fields. Results obtained on the massively parallel processor with the aid of Monte Carlo methods are equivalent in every detail to those based upon careful use of more traditional methods, but they are less subject to error and are closer to physics