Vertical Variations in O/N2 Using SSULI Tomographic Reconstructions during 2015 St. Patrick’s Day Storm

Column integrated ∑O/N2 has been widely utilized to monitor thermosphere composition and ionospheric plasma response to storm-time conditions. However, drivers such as atmospheric waves and tides, solar and geomagnetic heating, and upwelling/downwelling can produce altitude-dependent changes in thermospheric composition (Yu et al. 2020; Rishbeth, 1998). As a result, ∑O/N2 may not provide a good measure of the vertical response to a storm. In this study, we investigate altitude-dependent variations in thermospheric composition during the St. Patrick’s Day Storm of 2015 using limb observations from the Special Sensor Ultraviolet Limb Imager (SSULI) onboard Defense Meteorological Satellite Program (DMSP) satellites. SSULI’s unique observing scenario allows for tomographic reconstructions of ionosphere and neutral thermosphere volume emission rates (VER) along the orbit path. We utilize measurements of OI 135.6 nm and N2 Lyman–Birge–Hopfield (LBH) to produce vertical VER profiles (100-500 km).

Using these reconstructions, we investigate both the quiet and storm-time effects on the altitudinal variations of O and N2 and their impacts on ∑O/N2. Our analysis shows that during the 2015 storm, the vertical structures of both O and N2 VER change; O emission extends into lower altitudes (below 150 km), and N2 emission reaches higher altitudes than observed in quiet-time conditions. The resulting ∑O/N2 ratio does not capture this vertical variability, illustrating the importance of acquiring altitude-resolved observations of thermospheric O and N2 during storm-time conditions. Our results highlight that the current assumptions for computing and interpreting ∑O/N2 may miss important altitude-dependent composition changes that can provide insight into the mechanisms that drive thermospheric response to geomagnetic activity.