Quiet time densities of hot ions at geosynchronous orbit

Abstract

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