The longitudinal variation of the daily mean thermospheric mass density

This study uses the GRACE (Gravity Recovery And Climate Experiment) and CHAMP(CHAllenging Minisatellite Payload) accelerometer measurements from 2003 to 2008. These measurements gave thermospheric mass densities at ~480km (GRACE) and ~380km (CHAMP), respectively. We found that there are strong longitude variations in the daily mean thermospheric mass density. These variations are global and have the similar characteristics at the two heights under geomagnetically quiet conditions (Ap<10). The largest relative longitudinal changes of the daily mean thermospheric mass density occur at high latitudes from October to February in the Northern Hemisphere and from March to September in the Southern Hemisphere. The positive density peaks locate always near the magnetic poles. The high density regions extend toward lower latitudes and even into the opposite hemisphere. This extension appears to be tilted westward, but mostly is confined to the longitudes where the magnetic poles are located. Thus, the relative longitudinal changes of the daily mean thermospheric mass density have strong seasonal variations and show an annual oscillation at high and middle latitudes but a semiannual oscillation around the equator. Our results suggest that heating of the magnetospheric origin in the auroral region is most likely the cause of these observed longitudinal structures. Our results also show that the relative longitude variation of the daily mean thermospheric mass density is hemispherically asymmetric and more pronounced in the Southern Hemisphere.