Drift-shell splitting of energetic ions injected at pseudo-substorm onsets

Abstract

One feature of a magnetospheric substorm is the injection of energetic particles into closed drift orbits. Injections are routinely observed by geosynchronous satellites and have been used to identify the occurrence of substorms and the local time of particle energization. In this study we examine pitch angle distributions of ion injections in the 50- to 300-keV energy range observed by the Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer (AMPTE/CCE) satellite, hereinafter CCE. In a dipole field, all pitch angles follow the same drift shell, but the day-night asymmetry of the magnetospheric magnetic field introduces a pitch angle dependence in particle drift orbits, so that particles with different pitch angles disperse radially as they drift. The effect is known as drift-shell splitting. For satellite observations near noon at a fixed geocentric distance, the guiding center orbits of ions detected at small pitch angles intersect the midnight meridian at larger geocentric distances than do ions with near-90° pitch angles. The ion pitch angle distributions detected on the dayside therefore provide information about the radial distance of the nightside acceleration region. We apply this principle to study ion injection events observed on September 17-18, 1984, in association with pseudo-substorm onsets. CCE was at 13 hours local time near its apogee (8.8 RE) and observed a series of ion flux enhancements. Energy dispersion of the timing of the flux increases assures that they are due to injections on the nightside. The flux increases were observed only at pitch angles from 0° to 60°. We calculate drift orbits of protons using the Tsyganenko 89c magnetic field model and find that the drift orbits for 60° pitch angle protons observed at the satellite pass through midnight at 9 RE, well outside of geostationary orbit, indicating that the ion injections occurred tailward of 9 RE. Energetic ion data from geostationary satellites for the same time interval show no evidence of injections at 6.6 RE, consistent with the calculated inner boundary of the injections. Model calculations are presented demonstrating that dispersive injections observed near noon outside geosynchronous orbit provide the greatest sensitivity to drift-shell splitting effects and are therefore most suitable for remote sensing the radial boundaries of substorm injections. Show more