Poststorm Thermospheric NO Overcooling?

Abstract

A mechanism of the neutral gas density decrease at middle latitudes during the recovery storm phase is discussed. It is shown that well-known F2-layer storm-mechanism is sufficient to explain the observed neutral gas density changes. Severe geomagnetic storms in equinoctial, summer, and winter seasons along with CHAMP and Swarm neutral gas density observations are analyzed. Our recently proposed method to retrieve thermospheric parameters from ionospheric observations is used for the analysis. Storm-induced atomic oxygen variation is shown to be the controlling process. In accordance with the F2-layer storm mechanism during summer and equinoctial seasons the disturbed thermospheric circulation brings at middle latitudes the air with low atomic oxygen concentration and satellites observe a decrease in neutral gas density during the recovery storm phase. Low atomic oxygen concentration corresponds to negative F2-layer storm effect. In winter the disturbed neutral composition is restricted to high latitudes and no decrease of neutral gas density takes place at middle latitudes even during severe storms. Unchanged neutral composition corresponds to positive NmF2 deviations or median conditions in the mid-latitude daytime F2-layer. Therefore well-known F2-layer storm morphology and the post-storm decrease of neutral gas density reflect the same storm-induced variations of atomic oxygen abundance in the upper atmosphere.