Signatures of the universal time effect in the thermosphere and ionosphere

Thermospheric neutral and ionospheric electron densities measured in situ by the CHAMP satellite at nearly constant local time show significant longitudinal/universal time variations under geomagnetically quiet conditions. An NCAR-Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) run under the same geophysical conditions as the CHAMP measurements reproduced these observed longitudinal variations. Further analyses of the TIEGCM results demonstrate that the displacement of the geomagnetic equator from the geographic equator is the cause of this longitudinal variability. A discussion of the physical processes involved, including neutral wind circulation and electrodynamics, indicate that the dynamo is critical in producing these variations.