Solar Cycle and Seasonal Variation of Thermospheric Density and Composition

Simulations with the NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM) have been conducted from solar maximum to solar minimum using measurements made by the TIMED Solar Extreme-ultraviolet Experiment (SEE) as input. These simulations have demonstrated good correspondence with satellite drag measurements of thermospheric density. However, in order to obtain this agreement, the semi- annual and annual terms of seasonal variation in the thermosphere must be accounted for. Large-scale thermospheric dynamics have been proposed as a mechanism for the semiannual variation, but cannot explain the annual variation observed as global differences between December and June densities shown in satellite drag measurements. In the MSIS model, seasonal changes are represented by empirical variations of thermospheric composition and temperature. For these TIEGCM simulations, seasonal variation of the eddy diffusivity parameter in the lower thermosphere is imposed. This alters density and temperature throughout the thermosphere by influencing the atomic/molecular balance. In this presentation, we explore the question of whether this description of seasonal variation in density is consistent with observations of thermospheric composition. Measurements from the TIMED Global Ultraviolet Imager (GUVI) of the atomic oxygen to molecular nitrogen ratio (O/N2) are analyzed as a function of solar cycle and season, and compared to calculations using the TIEGCM with TIMED/SEE data. Through these comparisons, we examine the effects of solar forcing, eddy diffusion, and thermospheric large-scale dynamics on thermospheric O/N2 and their roles in producing global seasonal variation in the thermosphere.