Seasonal Variation Analysis of Thermospheric Composition in TIMED/GUVI Limb Measurements

Knowledge of thermospheric variability is essential to the understanding and forecasting of ionospheric behavior and space weather. As well, thermospheric density variability is a vital ingredient for prediction of space objects orbital changes and the lifetime of spacecraft. The Global UltraViolet Imager (GUVI) onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite provides the first global dataset of thermosphere composition (O, N₂ and O₂ densities) and temperature vertical profiles from 2002-2007. We carried out a seasonal analysis of the GUVI limb O density, N₂ density and volume number density ratio O/N₂ as a function of pressure. The GUVI limb retrieval from dayglow emissions was done at tangent points with solar zenith angle (SZA) lower than 80°, thus leading to very limited local time coverage. We first examine the latitudinal dependence of GUVI O, N₂ and O/N₂ and their temporal variations at different pressure levels. O in the lower thermosphere shows a strong AO at mid latitudes and a clear SAO at lower latitudes, so does O/N₂. The diurnal and global mean MSIS composition shows the expected Annual Oscillation (AO) or global O/N₂ larger in January than in July and Semiannual Oscillation (SAO) or O/N₂ greater during equinoxes than solstices in the thermosphere. On the other hand, both the GUVI composition data and NRLMSIS-00 empirical model sampled at the GUVI geolocations and times show skewed seasonal variations. Global averaged O/N₂ in the upper thermosphere from the GUVI limb measurement is larger in July than in January due to incomplete local time and latitude sampling and strong local time and latitudinal variation of composition.