GOLD Synoptic Observations of Thermospheric Annual and Semiannual Variations in Composition During Solar Minimum Years

This study investigates thermospheric seasonal variations composed of annual and semiannual oscillations (AO and SAO) in the column density ratio of atomic oxygen to molecular nitrogen observed by Global-scale Observations of Limb and Disk. On the basis of multiple analysis approaches, it is revealed that the AO is the predominant component at middle latitudes, while the SAO tends to be increasingly important toward the lower latitudes. The SAO is also evident in the southern mid-latitudes. Quantitative analyses demonstrate that AO (SAO) peaks around winter solstices (equinoxes). Additionally, the seasonal variation at low-latitudes exhibits greater amplitudes in the early morning and around noon, whereas it gradually weakens from early morning toward late afternoon at mid-latitudes. A hypothesis is that seasonal variations are likely modulated by atmospheric tides, including those generated in the thermosphere and those propagating upward from the lower atmosphere.

Automated summary

This study examines the seasonal variations in the thermosphere, specifically the annual and semiannual oscillations, using observations of the column density ratio of atomic oxygen to molecular nitrogen. The results show that the annual oscillation dominates in middle latitudes, while the semiannual oscillation becomes more important at lower latitudes and is also evident in the southern mid-latitudes. The analysis also reveals that the annual oscillation peaks around winter solstices, while the semiannual oscillation peaks around equinoxes. The study also suggests that atmospheric tides, both in the thermosphere and propagating upward from the lower atmosphere, likely modulate these seasonal variations.