Composition signatures in ion injections and its dependence on geomagnetic conditions

Ion composition in substorm injections are investigated in detail by using data obtained from the magnetospheric ion composition spectrometer (MICS) on board the Combined Release and Radiation Effects Satellite (CRRES). A total of 398 injection events are identified from 4 February to 10 October, 1991. It is found that the flux enhancements of O + ions in the “with‐oxygen” injections (O + ions occupy more than 20% of the total energy density) are attributed to the flux intensifications of high‐energy (E/q ≥ 100 keV/e) ions. In contrast, the flux of H + and He ++ ions in the “without‐oxygen” injections (O + ions are less than 20% total energy density) are enhanced by the intensification of lower energy H + and He ++ (around 50 keV/e) ions. Statistical results also demonstrate that the abundance of O + ions in an injection strongly depends on the geomagnetic activity. Without‐oxygen events take place most probably in relatively weak geomagnetic activity conditions ( Dst index is around −50 nT and Kp is around 4). The averaged fractional energy density of O + ions increases roughly linearly with Kp index. All injections are found to be rich of oxygen ions during the strong magnetic storms ( Dst < −100 nT). The present study indicates that the ionospheric oxygen ions are energized more readily during the storm conditions. When the near‐Earth reconnection occurs and expands from the central plasma sheet to the plasma sheet boundary layer or even to the lobe region, much more ionosphere origin O + ions could participate and be energized in the magnetospheric dynamic process. This leads to a with‐oxygen injection in the inner magnetosphere. As to the without‐oxygen injection, it seems that only preexisting oxygen ions in the near‐Earth plasma sheet are accelerated and become a part of the injected ions, thus a rather low abundance of O + ions can be observed.