A north-south asymmetry in thermospheric density

Utilizing the opportunity that our previously analyzed drag material covers the whole latitude interval between 89°N and 89°S, we have reanalyzed the 16 000 observed total density data (orbital decay of 20 satellites from 1965 till 1977 in the height interval 180 - 430 km) comparing them with corresponding CIRA-72 and CIRA-86 (MSIS-86) model densities respectively and investigated the residuals as a function of different parameters. A significant asymmetric dependence has been found on both magnetic and geographic latitudes, namely the residuals decrease continuously and evenly from the North Pole to the South Pole. The effect is significant also in several subgroups - e.g. selected only from geomagnetically disturbed periods - or even using data derived from the decay of single satellites. Finally a simple formula is suggested which can be incorporated into a new CIRA model in order to compensate this newly discovered effect.