The origin of the near-Earth plasma population during a substorm on November 24, 1996

We investigate the origins and the transport of ions observed in the near-Earth plasma sheet during the growth and expansion phases of a magnetospheric substorm that occurred on November 24, 1996. Ions observed at Geotail were traced backward in time in time-dependent magnetic and electric fields to determine their origins and the acceleration mechanisms responsible for their energization. Results from this investigation indicate that during the growth phase of the substorm, most of the ions reaching Geotail had origins in the low-latitude boundary layer and had already entered the magnetosphere when the growth phase began. Late in the growth phase and in the expansion phase a higher proportion of the ions reaching Geotail had their origin in the plasma mantle. Indeed, during the expansion phase, more than 90% of the ions seen by Geotail were from the mantle. The ions were accelerated enroute to the spacecraft; however, most of the ions' energy gain was achieved by nonadiabatic acceleration while crossing the equatorial current sheet just prior to their detection by Geotail. In general, the plasma mantle from both southern and northern hemispheres supplied nonadiabatic ions to Geotail, whereas the LLBL supplied mostly adiabatic ions to the distributions measured by the spacecraft. Distribution functions computed at the ion sources indicate that ionospheric ions reaching Geotail during the expansion phase were significantly heated. Plasma mantle source distributions indicated the presence of a high-latitude reconnection region that allowed ion entry into the magnetosphere when the interplanetary magnetic field was northward. These ions reached Geotail during the expansion phase. Ions from the traditional plasma mantle had access to the spacecraft throughout the substorm.