Angular distributions of suprathermal electrons observed at geosynchronous orbit

Six months of low‐energy electron plasma data collected using the Los Alamos National Laboratory magnetospheric plasma analyzer (MPA) onboard the geosynchronous satellite 1989–046 have been surveyed. The MPA instrument measures the three‐dimensional energy per unit charge distributions of cold ions and electrons, allowing for the simultaneous determination of the angular distribution and ambient plasma regime. Suprathermal electrons in the energy range 15 to 200 eV were characterized by local time of occurrence, angular distribution, and ambient plasma regime. Results indicate a local time dependence in angular distributions, with trapped distributions, i.e., enhanced fluxes of particles with pitch angles near 90° and 270°, primarily being observed in the morning, coincident trapped and field‐aligned angular distributions (enhanced fluxes of particles with pitch angles near 90° and 270° coincident with enhanced fluxes of particles with pitch angles near 0° and 180°) occurring around noon, a lack of detectable low‐energy electron fluxes near dusk, and a complex combination of angular distributions on the nightside. When both trapped and field‐aligned angular distributions are present, the field‐aligned component generally has lower energy than the trapped component. Dayside field‐aligned angular distributions are interpreted as being photoelectrons from the ionosphere, while trapped angular distributions are from the low‐energy tail of the plasma sheet distribution. A plasma regime dependence in angular distributions was also observed. Plasma sheet angular distributions are generally isotropic or trapped. The plasma trough angular distributions are trapped or coincident trapped and field‐aligned. Plasmaspheric electrons commonly have energies below our 15 eV threshold.