Variations of Thermospheric Compositions Using the TIMED/GUVI Dayglow Limb Measurements

Understanding and predicting thermospheric variability is essential for the understanding and prediction of the ionosphere, as the thermosphere and ionosphere are one inseparable coupled system. In addition, thermospheric density variability is vital for the orbit prediction of space objects and spacecraft lifetimes. The Global UltraViolet Imager (GUVI) onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) provides a limb dataset of thermosphere composition (O, N₂ and O₂) and temperature vertical profiles from 2002-2007. This adds to the understanding of the thermosphere composition, complementing the conventional disc measurement of column integrated O/N₂ ratio. We carried out seasonal analysis of the GUVI limb data, in particular, the annual and semiannual variations of O, N₂ and O/N₂ as a function of height or pressure. This yields new insights into the AO, SAO, QBO-like and solar cycle variations of thermosphere compositions. For instance, global O density is larger in winter than that in summer throughout the thermosphere. O/N₂ is larger in summer than winter below 200 km, but O/N₂ in the upper thermosphere is larger in summer than that in winter. We also examined the latitude dependence of O, N₂ and O/N₂ and their temporal variations. O in the lower thermosphere shows strong AO at mid latitudes and SAO at lower latitudes. Also O in the tropics shows QBO-like oscillation. O in the upper thermosphere is dominant by the solar cycle effect, with strong AO near solar maximum and strong SAO near solar minimum.