QUIET TIME DENSITIES OF HOT IONS AT GEOSYNCHRONOUS ORBIT

Densities of hot (100 eV-40 keV) ions vary with local time at geosynch ronous orbit. Typically, the density is maximum near midnight, falls o ff toward both terminators, and is minimum near noon. This local time dependence is routinely observed in the Los Alamos geosynchronous plas ma data. We use a month of these data (June 1996) and a simple model o f particle drifts within the magnetosphere to explore its origins. We first show that the nightside density profile is shaped according to t he drift path topology: trapped orbits are depleted, and only particle s drifting on open trajectories contribute to this population, particu larly Then, we highlight spectral absorption in dayside fluxes, which occurs when the particles pass through the geocorona. A model of charg e exchange accounts for the shape and energy range of the absorption f or observations taken at times of low K-p. signatures observed at low K-p for varying pitch angles. The density absorption computed versus l ocal time matches the observations. Hence these basic principles of pa rticle motion in the terrestrial magnetosphere readily explain the loc al time dependence of the hot ion density at geosynchronous orbit.