Acceleration of ions of ionospheric origin in the plasma sheet during substorm activity

The ionic mass and charge composition of the suprathermal (10-230 keV/e) population of the plasma sheet has been measured using the Suprathermal Energy Ionic Charge Analyzer (SULEICA) on AMPTE/IRM before and after a substorm onset on April 8, 1985. Simultaneously with the substorm onset and strong earthward plasma flows, a flux increase of the suprathermal ions and a significant hardening of the spectra are observed that are most pronounced for O(+). The enhancement ratios can be best organized in terms of energy per charge. The observations are interpreted as an indication of particle acceleration by electric fields and additional injection of ionospheric plasma into the plasma sheet after substorm onset.