Asymmetric behaviour and geomagnetic dependencies of zonal winds in the middle latitude upper thermosphere

Abstract

In this study we use the CHAMP wind observations from 2002 to 2004 to investigate features of mid-latitude thermospheric winds at low and high geomagnetic activity levels. The winds are observed between the geomagnetic latitudes 40–50 north or south within an altitude range of 370–450 Km and averaged longitudinally. The low geomagnetic activity level wind generally leads eastward in both hemispheres for all the seasons. Maximum westward speeds going above 100 m/s are observed at all levels of geomagnetic activity. Most investigations on F-region parameters assume the equinoxes are fundamentally the same. A first time separation of Equinox season using CHAMP winds reveals equinoctial asymmetric behavior of the wind speed at the two levels considered. Interhemispheric asymmetry in neutral wind behaviour is clearly evident in the solstice seasons with local winds considered in each of the hemispheres. This is consistent with asymmetry in the amount of solar illumination received in each of the hemispheres and differences in the magnetic field strength at magnetically conjugate regions