Conical distributions of low‐energy ion fluxes at synchronous orbit

Abstract

Low‐energy ion fluxes observed at synchronous orbit often display conical pitch angle distributions, that is, peak fluxes between 0° and 90° to the magnetic field direction. They are observed at all local times and are an especially common occurrence near dusk. The conic peaks normally occur in the 15°–35° pitch angle range. Many of the low‐energy ion conics in the late afternoon‐dusk sector appear to be associated with low‐energy plasma from the vicinity of the dusk plasmasphere bulge region. Several hypotheses for the production of these conics are advanced, and it is judged likely that cyclotron acceleration of ions at geocentric distances of 3.1 to 5.1 RE is operative in many cases. It is suggested that cyclotron acceleration in this altitude range may be an effective mechanism in trapping ionospheric ions within the magnetosphere.