Effects of electrostatic ion cyclotron wave instability on plasma flow during early stage plasmaspheric refilling

Abstract

Mesoscale models for the plasmasphere refilling reveal that the expansion of the ionospheric plasma into empty flux tubes during the early stage refilling produces highly supersonic counterstreaming ion beams. Such beams are found to meet the conditions for the instability of the electrostatic ion cyclotron (eic) waves in the equatorial plasmasphere. Since the spatial scale of the eic wave instability is determined by the ion Larmor radius and the corresponding timescale is determined by the ion cyclotron period, it is unresolvable by the crude spatial and temporal resolutions afforded by the mesoscale models. Thus the eic waves and the associated microprocesses are left out by the existing mesoscale refilling models based on the fluid and semikinetic approaches. I propose here an algorithm for incorporating the effects of eic waves on mesoscale plasma flow; the algorithm is based on the critical conditions for the eic instability. When the relative drift of the counterstreamig ion beams exceeds a critical value, depending on the beam temperatures, the excess momentum and energy are dissipated into thermal motion of the ions, resulting in ion heating in the degree of freedom perpendicular to the ambient magnetic field at the expense of the parallel ion drift. This leads to the creation of two distinct ion populations consisting of field‐aligned counterstreaming ions and perpendicularly heated warm anisotropic ions trapped in the flux tube. The latter ions are created during the first few hours of the refilling process primarily in the equatorial region, but eventually they spread out all over the flux tube. Highly anisotropic ions are seen near the equator by virtue of being locally created and trapped there. Estimates of the Coulomb collision time show that the warm anisotropic population (WAP) has a lifetime ranging from about 10 hours in the topside ionosphere to several days in the equatorial region. Thus once initially created during early stage refilling, WAP ions become a common feature of the outer plasmasphere. The coexisting multiple ion populations, consisting of anisotropic warm ions and the field‐aligned counterstreaming flows, have been observed from DE 1 on flux tubes undergoing refilling.