Seasonal Variation of O/N₂ on Different Pressure Levels From GUVI Limb Measurements

The ratio of O number density to N₂ number density (O/N₂) is an important parameter to describe thermospheric composition changes and its effects on the ionosphere. Based on Global Ultraviolet Imager (GUVI) limb measurements, we investigate the seasonal behaviors of O/N₂ volume density ratio on different constant pressure levels during geomagnetically quiet periods. The global O/N₂ shows the prominent annual and semiannual variations with solar activity dependence. An empirical model considering the solar activity and annual/semiannual variations can reasonably reproduce the original O/N₂. The modeled O/N₂ captures the hemispheric asymmetry of the annual variations in both length and magnitude. Global maps of the seasonal harmonic components of the modeled O/N₂ indicate the latitudinal and altitudinal dependence of O/N₂ seasonal variations. The annual component dominates over the semiannual component at mid-latitudes, but it is smaller than the semiannual component at low latitudes. In the Northern Hemisphere, and at low geomagnetic latitudes of the Southern Hemisphere, the annual component peaks around December solstice at all altitudes, whereas at middle geomagnetic latitudes of the Southern Hemisphere, it peaks around June solstice. The semiannual component peaks at the equinoxes in almost all regions over the globe at all altitudes. The annual and semiannual amplitudes both increase with altitude. In addition, O/N₂ annual variations and solar activity dependence are more influenced by the thermal expansion and contraction.