Solar cycle variations in equatorial ionospheric zonal electric fields near sunrise

In this study, we investigate the solar cycle dependence of the sunrise ionospheric zonal electric fields at the equator under geomagnetically quiet conditions. Simulations using the Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM) reveal that the equatorial eastward electric field at sunrise decreases with the increase in solar activity, independent of longitude, season, and lower atmospheric tides. The solar cycle dependence of the sunrise zonal electric field is mainly related to the zonal wind dynamo. Moreover, this solar cycle dependence of sunrise electric fields at the equator is dominated by the corresponding variation in the F-region dynamo because the response of conductivity and neutral winds near sunrise to increasing solar flux is stronger in the F-region than in the E-region, although the sunrise eastward enhancement of electric fields is mainly driven by the E-region zonal wind dynamo. Specifically, the westward gradient of low-latitude F-region neutral winds near the dawn terminator tends to produce westward electric fields in the equatorial region that are more pronounced at solar maximum, whereas the midlatitude E-region dynamo induces an eastward enhancement of sunrise electric fields at the equator that decreases slightly with increasing solar activity. This study also reveals that the reason the eastward enhancement of equatorial zonal electric fields near dawn and dusk terminators show opposite solar cycle dependence is because of their different generation mechanisms.