Survey of ring current composition during magnetic storms

During geomagnetic storms, the ring current is substantially modified and intensified. We use data from the MICS instrument on CRRES to investigate composition changes in magnetospheric ions in the energy range 20-400 keV/e. These are related to the Dst signature of the storm. Long term survey plots of MICS data show that during storms there is an initial increase of ionospheric material at around L=3. The peak in the ratio of ionospheric material rises to higher L-shells during the storm recovery. By contrast, solar wind material remains predominantly at higher L-shells, and a belt of ring current alpha particles forms around L=4. The L-shell of the peak fraction of ionospheric material is a strong function of Dst. Substorm composition reflects the background composition at that L-shell. These results are emphasised by a superposed epoch analysis of substorm injection composition. It shows that the composition of storm time injections is similar to a set of oxygen rich injections. We conclude that, during the solar maximum interval which CRRES observed, substorms are not directly responsible for the ring current oxygen population.