Large‐scale O+ depletions observed by ICON in the post‐midnight topside ionosphere: Data/model comparison

The Ion Velocity Meter instrument on the Ionospheric Connection Explorer (ICON) satellite measured large depletions in the O+ density (but not H+) in the post-midnight topside ionosphere on December 23, 2019. This phenomenon has been observed previously but has yet to be explained. The authors provide for the first time a physical explanation for this observation based on a simulation study using the first-principles, ionosphere/plasmasphere code SAMI3 (Sami3 is also a model of the ionosphere) with neutral atmosphere inputs (neutral wind, density, and temperature) from the whole atmosphere model Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM-X). The physical processes responsible for these observations are (1) the nighttime O+/H+ transition altitude is below the ICON orbit and (2) a prolonged downward E x B drift in the post-midnight/pre-sunrise time sector. Interestingly, the authors also find that equatorial plasma bubbles develop in the African and Pacific sectors.