Variations of the ion composition during an intense magnetic storm and their consequences

Abstract

In March 1991, a great magnetic storm occurred, with [Dst] reaching peak values of more than 300 nT. We present energetic ion measurements from the Magnetospheric Ion Composition Spectrometer (MICS) on board the Combined Release and Radiation Effects Satellite (CRRES), which monitored the dramatic compositional variations in the inner magnetosphere during the storm. While the inner magnetosphere was dominated by protons before the storm, the MICS observations show that the abundance of ionospheric-origin O' ions progressively increased during the main phase of the storm. Eventually the O+ ions dominated, contributing more than 70% of the total particle energy density near storm maximum, which is to be compared to a quiet-time contribution of less than 10%. We suggest that the final deep drop in Dst at storm maximum, is due to a new population that is effectively fed into the inner magnetosphere from the polar ionosphere. The main agent of this ionospheric “intervention” is O+, which becomes the dominant ion species at the maximum epoch of great storms, at least around solar maximum.