Ionospheric oxygen outflows (IOOs) are frequently observed in Earth's inner magnetosphere. As a potential fast mass source of the inner magnetosphere, they have been extensively studied for decades. In this study, we identified 271 IOO events from 6-year Van Allen Probes (VAPs) data and conducted a statistical study to reveal when, where, and how they occur. All the events are observed in the nightside magnetosphere, with the occurrence rate peaking at the L-shells corresponding to the auroral oval. The spatial scale of IOOs in the equatorial plane is estimated from the two-spacecraft configuration of the VAP mission. The results show that, in statistics, IOOs are 3.9-hr wide in magnetic local time and 1.0-Earth radius wide in L-shell. Further investigation shows that IOOs would shift to lower L-shells and occupy a larger L-shell extent as the AE index increases. Analysis of the environment conditions reveals that IOOs are statistically associated with AE-index enhancements, ultra-low-frequency (ULF) waves, and magnetic field dipolarization, but not with any systematic variations in the SYM-H index and the solar wind parameters. From this observation, we suggest that geomagnetic substorms and substorm-associated processes (e.g., ULF waves) are potential triggers of inner-magnetosphere IOOs.
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