Statistical analysis of storm-time $O^+$ outflows as observed from the Van Allen Probes

The outflow of low-energy O$^+$ ions into inner magnetosphere is a key contributor to the warm plasma cloak, and O+ torus, which can also contribute to storm time ring current particularly after local acceleration. Although O$^+$ outflows are well observed, their source and transport in the inner magnetosphere remain unclear. Using HOPE mass spectrometer data from the Van Allen Probes, we systematically examine O$^+$ outflows during geomagnetic storms. We developed an automated event identification method to detect outflows, estimate onset times, and record occurrence details. With this event set, we performed a statistical analysis of occurrence characteristics and storm phase dependence, separately for O$^+$ outflows parallel and anti-parallel to the magnetic field. Our findings show: (1) outflows are mostly detected off the magnetic equator; (2) with the understanding that the parallel outflows are from the Southern Hemisphere, while anti-parallel from the Northern Hemisphere, we observed more events in the hemisphere within which the source of O+ outflow lies; (3) the peak of the occurrence rate in MLT is derived at mid-night with skewing towards dawn; (4) during the main phase of the storms, outflows events are concentrated at post-midnight and dawn. Decrease in the occurrence rate is observed in the recovery phase. Hemispheric and dawn-dusk asymmetries are evident in the event distribution patterns; (5) a preference for outflows from the summer hemisphere. These results offer new insights into nightside, low-energy O$^+$ outflows. Our large event dataset enables us to explore the sources of energization and their transport better.