ZONAL DIFFERENCES IN THERMOSPHERIC AIR DENSITIES AT MID-LATITUDES

This study investigated longitudinal differences in the thermospheric air mass density (ρ) during the solar minimum period from 2007 to 2009 at magnetic mid-latitudes. Both observations by CHAMP satellite and model simulations using the Global Ionosphere-Thermosphere Model were analyzed. It was found that ρ exhibited wave-1 structure in its longitudinal profile (i.e., one peak and one trough), which was almost 180° out of phase between the two hemispheres. The phase of the wave changed with the local time, exhibiting a clear diurnal variation. The value of ρ exhibited few longitudinal variations in the eastern part of the northern hemisphere, where the electron density showed some longitudinal differences. The reason for this was that the variation in the electron density was too slight to affect ρ in the Europe-Asia sector via ion drag. As confirmed by the simulation, the longitudinal difference in ρ was reduced by almost 40%∼50% in terms of the maximum value of the peak-trough difference when ion drag was negligible. Thus, both the model and the observations confirmed the important role of ion drag in the longitudinal structure of neutral density. This study further indicated the obvious longitudinal difference in the solar heating at magnetic mid-latitudes. The peak-to-trough difference in the solar zenith angle could reach 20°∼30°. The non-uniformity of solar irradiation heating could be another cause of the longitudinal structure of the air mass density.