What is the effect of substorms on the ring current ion population during a geomagnetic storm?

Abstract

We consider the properties of substorms during periods of high and low Dst, and their effect on Dst. Data from the MICS instrument on CRRES are used to examine composition changes of ring current ions in the energy range 72-400 keV/e. These we relate to the Dst index, generally taken to characterise the storm. A superposed epoch analysis using high time resolution Dst data shows that on a timescale of 30 minutes, Dst is largely unchanged by substorm onset. It also shows that there is some increase in the number density of ionospheric oxygen relative to solar wind origin He++. There is no systematic difference in the energisation between different species. The changes in energy density are primarily the result of changes in number density. We show that while the underlying ring current composition is a strong function of Dst, the energisation and increase in species number density at substorm onset is independent of Dst. Examination of long term survey plots of MICS data shows that during storms the bulk ring current composition changes. We see the emergence of a concentration of ionospheric material at low L-shells, and its rise to higher L-shells during the recovery. By contrast, solar wind material remains mainly at higher L-shells. The altitude of the peak in ionospheric material is a strong function of Dst. These major changes are always at lower L-shells than substorm particle injections. Overall, we conclude that there is no qualitative difference between stormtime substorms and those from less disturbed periods. During solar maximum when these measurements were made, substorms are not directly responsible for the injection of oxygen ions into the ring current. Although substorms may be important in initiating ionospheric upflow, substorm injected particles themselves play no part in modifying ring current composition during storms.