Inner magnetospheric heavy ion composition during high-speed stream-driven storms

Ion composition data, taken by the Combined Release and Radiation Effects Satellite Magnetospheric Ion Composition Spectrometer instrument, are investigated across eight high-speed solar wind-stream-driven storms (HSSs) during 1991. The HSSs are identified using solar wind data from OMNI alongside geomagnetic indices, and the behavior of ions in the energy range 31.2-426.0keV is investigated. A case study of the single HSS event that occurred on 30 July 1991 is performed, and superposed epoch analyses of five events are conducted. The data show evidence of a local minimum (dropout) in the flux and partial number density of ionic species H+, He+, He++, and O+ close to the onset of magnetospheric convection. The flux and number density rapidly fall and then recover over a period of hours. The initial rapid recovery in number density is observed to consist primarily of lower-energy ions. As the number density reaches its maximum, the ions show evidence of energization. Heavy ion-to-proton ratios are observed to decrease substantially during these HSS events.