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 31.2–426.0 keV 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. Key Points Ion species H+, He+, He++ and O+ show evidence of a dropout during HSS events. Recovery of ions is rapid. Densities become greater following convection onset. Recovery begins with low energy ions which then spread to higher energies.