Seasonal Variation of Thermospheric Composition Observed by NASA GOLD

We examine characteristics of the seasonal variation of thermospheric composition using column number density ratio ∑O/N ( 2 ) observed by the NASA Global Observations of Limb and Disk (GOLD) mission from low‐mid to mid‐high latitudes. We also use ∑O/N ( 2 ) derived from the Global Ultraviolet Imager (GUVI) limb measurements onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite and estimated by the NRLMSISE‐00 empirical model to aid our investigation. We found that the ∑O/N ( 2 ) seasonal variation is hemispherically asymmetric: in the southern hemisphere, it exhibits the well‐known annual and semiannual pattern, with highs near the equinoxes, and primary and secondary lows near the solstices. In the northern hemisphere, it is dominated by an annual variation, with a minor semiannual component with the highs shifting toward the wintertime. We also found that the durations of the December and June solstice seasons in terms of ∑O/N ( 2 ) are highly variable with longitude. Our hypothesis is that ion‐neutral collisional heating in the equatorial ionization anomaly region, ion drag, and auroral Joule heating play substantial roles in this longitudinal dependency. Finally, the rate of change in ∑O/N ( 2 ) from one solstice season to the other is dependent on latitude, with more dramatic changes at higher latitudes.