A semikinetic model for early stage plasmasphere refilling: 1, Effects of Coulomb collisions

Abstract

Recently we have modified our collisionless, time‐dependent, kinetic plasma model (Wilson et al., 1990) to include the effects of Coulomb self‐collisions in a theoretically rigorous fashion. The algorithm employed (Takizuka and Abe, 1977) faithfully mimics a Fokker‐Planck operator and conserves both energy and momentum. This collisional kinetic model has been applied to the problem of baseline plasmasphere refilling of an initially depleted flux tube, neglecting the effects of wave‐particle interactions. The companion report by Lin et al. (this issue) examines the effects of wave‐particle interactions on refilling. We have performed refilling calculations for various flux tubes (L = 3, 4, 5, 6) and for different ionospheric plasma fluxes and temperatures. In each case considered, the same set of events occurs. Initially, two polar wind outflows develop from each hemisphere and set up counterstreaming beams. With time the vacant phase space region between these beams fills, primarily because of to collision‐induced particle diffusion but also because of lowering ambipolax potential drops resulting from the increasing density in the plasmasphere. In contrast to all previous hydrodynamic approaches, we find no formation of shocks. The plasma first evolves an isotropic nearly Maxwellian velocity distribution in a region that starts near the ionosphere and moves outward toward the equator. We find that for reasonable topside ionospheric temperatures and fluxes the thermal plasma all along an L shell will become nearly isotropic in 6 to 30 hours (longer times for smaller fluxes or larger L shells), consistent with the observations of Horwitz et al. (1984).