Investigation of the ΣO/N2 depletion with latitudinally tilted equatorward boundary observed by GOLD during the geomagnetic storm on April 20, 2020

A long-lived (>10 hr) O/N-2 column density ratio (Sigma O/N-2) depletion at middle latitudes was observed by the Global-scale Observations of the Limb and Disk (GOLD) during the 20 April 2020 geomagnetic storm. The observed Sigma O/N-2 depletion tilts latitudinally at equatorward boundary with the lowest latitude of similar to 20 degrees N at similar to 75 degrees W, and changes orientation near 75 degrees W, with a north-westward tilt to the west of similar to 75 degrees W and a north-eastward tilt to the east of similar to 75 degrees W. The National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model shows the similar Sigma O/N-2 depletion patterns and magnitudes, but the longitudes with the most equatorward were similar to 100 degrees W and moved westward from similar to 100 degrees W to similar to 140 degrees W during the storm recovery period. Horizontal winds play major role in forming this middle-latitude Sigma O/N-2 depletion structure. The strongest equatorward winds in the longitude sector near the magnetic pole resulted in the most equatorward expansion of all longitudes, generating the depletion with latitudinally tilted equatorward boundary. In addition, the high-latitude westward zonal wind and the middle-latitude eastward zonal wind maintained the long existence of this depletion in the GOLD Field of View. The further analysis on neutral winds shows that storm-time meridional winds at middle latitudes in the mid-low thermosphere are mainly dominated by pressure gradient force, while zonal winds are determined primarily by horizontal momentum advection.