Seasonal Variations of O/N2 Volume Density Ratio Retrieved from GUVI Dayside Limb Measurement

O/N₂ (ratio of O number density to N₂ number density) is a crucial parameter for ionospheric/thermospheric indicating, assessment and forecasting precisely. Here, based on Global Ultraviolet Imager (GUVI) limb measurement, we utilize the O/N₂ volume density ratio at constant pressure levels to investigate the seasonal behaviors of thermospheric neutral compositions during geomagnetic quiet day. The O/N₂ shows the obvious annual/semiannual changes with minima around solstices, and maxima during equinoxes. An empirical model (denoted as R model) considering solar activity and seasonal variations reproduces the original O/N₂ reasonably, and the modeling O/N₂ display similar season variations at all heights and solar flux levels. There also exist the hemisphere differences; the minima appear around two solstices in the Southern Hemisphere, differing from the single summer solstice minimum in the Northern Hemisphere. The Global maps of seasonal harmonic components from Fourier expansion of the modeling O/N₂ indicate the latitudinal and altitudinal dependency of seasonal variations. The annual component predominates over the semiannual component at middle latitudes, while it gives way to semiannual component at low latitudes. In the Northern Hemisphere, the annual variations peak around December solstice; in the Southern Hemisphere they peak around December solstice at the low geomagnetic latitudes, whereas peak around June solstice at middle geomagnetic latitudes. The semiannual component has maxima in equinoxes in global regions. On the whole, the stable changes of harmonic components with height reveal the thermospheric stability characteristics. Our study represents the modeling of O/N₂ and their global distribution of seasonal variation, emphasizing on their altitudinal dependency.