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. Plain Language Summary: Annual and semiannual variations are the most prominent phenomenon standing out on the seasonal scale in the thermosphere‐ionosphere system. Despite being recognized for decades, the characteristics of the AO and SAO are not yet well delineated, and relevant physics is not fully understood, due largely to limited observations and sampling constraints of low‐Earth‐orbit (LEO) satellites. For instance, distinguishing the local time versus longitude variations in the thermosphere is not feasible with LEO satellites alone. Leveraging the synoptic observations enabled by the Global‐scale Observations of Limb and Disk (GOLD) mission, we identified the AO and SAO in the column density ratios of atomic oxygen to molecular nitrogen at low and middle latitudes. More importantly, since GOLD provides full coverage of daytime local times on a daily basis, we quantified the local time dependence of seasonal variations in composition. Prevalent local time modulations could be driven by atmospheric tides. Key Points: Pronounced annual and semiannual oscillations (AOs and SAOs) are seen in Global‐scale Observations of Limb and Disk ∑O/N2AO is predominant at mid‐latitudes, and SAO is evident in both low‐latitudes and southern mid‐latitudesLocal time modulations of seasonal variations are prominent at low‐ and mid‐latitudes [ABSTRACT FROM AUTHOR]