The physical mechanisms for the sunrise enhancement of equatorial ionospheric upward vertical drifts

Satellite and incoherent scatter radar observations have shown frequently a strong enhancement of upward vertical E x B drifts in the equatorial ionospheric F region near sunrise. Previous studies suggested that this enhancement is associated with the local equatorial wind dynamo, the same process used to explain the prereversal enhancement after sunset. However, this hypothesis has never been tested in a first-principles way. In this study, we explore the physical mechanisms responsible for the sunrise enhancement of equatorial upward vertical drifts using the Thermosphere-Ionosphere-Electrodynamics General Circulation Model. The model well reproduces the sunrise enhancement of upward vertical drifts observed at Jicamarca on 10 June 2004. The simulation results show that large eastward zonal electric fields occur around sunrise at all latitudes but with a peak at middle latitudes. Further numerical experiments reveal that the equatorial sunrise enhancement at solar minimum is primarily driven by the E region zonal wind dynamo at middle latitudes rather than by the local dynamo effect in the equatorial region. Specifically, the intensity of the equatorial eastward electric field near sunrise is determined by the magnitudes of westward wind and its longitudinal gradient at middle latitudes and the declination of the dawn termination.