Hemispheric Asymmetry in the Thermosphere Revealed by GOLD Observations

Due to the limited observations, previous studies have focused mainly on the temporalevolution of thermospheric composition and temperature at a fixed location or on their globalaverage. Their annual and semiannual variations are well documented. However, the spatialvariations of these thermosphere parameters have not been adequately investigated, especiallythe LT/MLT variations of the hemispheric asymmetries. In this work, we analyzed GOLDthermospheric measurements to obtain the monthly average distributions of thethermospheric composition and temperature in both geomagnetic and geographic coordinatesat midlatitudes. We calculated the hemispheric asymmetry indexes and determined theirseasonal, LT, MLT, latitude, and magnetic latitude dependence. Main findings are: (1) Theasymmetry index is larger in the morning sector than that in the afternoon sector. (2) Theasymmetry index is larger at higher latitudes. (3) During both equinoxes, the southernhemisphere has larger and larger temperature.