Longitudinal variations of equatorial ionospheric electric fields near sunrise

In this work, we carry out a comprehensive modeling study, using the Thermosphere-Ionosphere-Electrodynamics General Circulation Model, to explore the physical processes by which the longitude-dependent geomagnetic field drives the longitudinal variations of the sunrise enhancement of the zonal electric fields at the dip equator near the June solstice. Numerical experiments and diagnostic analyses of the electrodynamics equation show that the longitudinal differences of the equatorial zonal electric fields near sunrise are primarily associated with the longitudinal variations in the zonal wind dynamo, with those from the meridional wind dynamo contributing secondarily. Furthermore, the longitudinal differences of the wind dynamo near sunrise are mainly related to the longitudinal variations of UxB and conductance, which are caused primarily by the direct influence of the longitudinal structures of magnetic field declination and strength. Meanwhile, the longitudinal variations of neutral winds, which also result in moderate UxB longitudinal variations, play a secondary role in the longitudinal variations of the neutral wind dynamo, while plasma density, which has minor longitudinal differences near sunrise, contributes slightly by modifying the conductance. Overall, the sunrise enhancement in June is more significant at the longitudes where the magnetic field strength and distortion are larger or the magnetic field declination is smaller in the Northern Hemisphere.