Nightside Ionospheric Oxygen Outflows During Geomagnetic Storms: Van Allen Probes Statistics

Abstract

Oxygen ions (O+) contribute dominantly to the energy density of the ring current during geomagnetic storms, thereby driving the evolution of storms. The majority of magnetospheric oxygen ions originate from the high latitude ionosphere in the form of ionospheric outflows. In this work, we employ Van Allen Probes data from January 2013 to December 2018 to analyze ionospheric oxygen outflows during storms. A superposed epoch analysis discovered that the statistical oxygen ion fluence enhanced more than twice during storms, with the total outflow rate reaching 4.36 × 10²⁴ s⁻¹ in the main phase and 2.34 × 10²⁴ s⁻¹ in the recovery phase compared to 0.46 × 10²⁴ s⁻¹ in quiet times. An examination of the magnetic latitude-local time distributions of these outflows indicates that this enhancement stems from both an increase in outflow flux at specific locations and an increase in the area of outflow. Statistical analysis shows that the ionospheric footprints of these outflows migrate equatorward with increasing geomagnetic activity. However, the equatorward expansion of the auroral oval is more pronounced, resulting in a growing proportion of outflows originating from within the auroral oval as geomagnetic activity develops. Further investigation of the geomagnetic indices combining local magnetic and electric fields relates this enhancement to: (a) the increased geomagnetic disturbance during storms; (b) enhanced plasma waves in the ultra-low-frequency range and near oxygen ion gyrofrequency, whose power shows a positive correlation with oxygen outflow rates. Besides, the distributions by season show that the total outflow rate more than doubles in local summer compared to other seasons.