A semikinetic model for early stage plasmasphere refilling: 2, Effects of wave‐particle interactions

Abstract

We treat the early stages of plasmasphere refilling along an initially depleted L=4 magnetic flux tube through a semikinetic model. The companion paper by Wilson et al. (this issue) describes the plasma evolution for a “baseline” refilling situation in which Coulomb collisions play a central role. Here we focus on the effects of wave‐particle interactions in which stochastic diffusion of ions in perpendicular velocity due to equatorially concentrated electromagnetic ion cyclotron waves plays a central role. We examine characteristic individual ion trajectories, as well as the evolution of bulk parameters and ion distribution functions when equal “polar wind” streams are injected at the northern and southern ionospheres. In the ion trajectories, it is found that relatively modest and realistic perpendicular electric field power levels lead to decreased mirror latitudes, substantial acceleration, and equatorial entrainment of these ions. After about 8 hours, significant general accumulation of plasma occurs all along the flux tube, with equatorial densities attaining levels of 15 ions/cm³, and equatorial parallel and perpendicular temperatures of around 2.4 and 60 eV, respectively. Incorporating an electron temperature distribution which is calculated as a fraction of the local ion temperature, and therefore also peaks at the magnetic equator, leads to an ambipolar electric field which acts to reflect incoming ions back toward the ionospheres. In this case, we find results strikingly similar to those of Olsen et al. (1987), wherein trapped 50 eV ion distributions are seen at the magnetic equator, while incoming and reflected, cool, 1–2 eV, field‐aligned streams are seen on both sides of the equator. A substantial equatorial density depletion is also found, in good accord with the results of Olsen (this issue).