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

In March 1991, a great magnetic storm occurred, with \Dst\ reaching peak va lues of more than 300 nT. We present energetic ion measurements from the Ma gnetospheric Ion Composition Spectrometer (MICS) on board the Combined Rele ase and Radiation Effects Satellite (CRRES), which monitored the dramatic c ompositional 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 progres sively 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 o f less than 10%. We suggest that the final deep drop in Dst at storm maximu m, is due to a new population that is effectively fed into the inner magnet osphere from the polar ionosphere. The main agent of this ionospheric "inte rvention" is OC, which becomes the dominant ion species at the maximum epoc h of great storms, at least around solar maximum. (C) 1998 Elsevier Science Ltd. All rights reserved.