Westward Electric Fields in the Afternoon Equatorial Ionosphere During Geomagnetically Quiet Times

The daytime average zonal electric field is well known to be eastward in the equatorial ionosphere. Geomagnetic observations have revealed that the equatorial electrojet or zonal electric field occasionally becomes westward in the afternoon sector during quiet times. We present the first analysis of the dependence of the afternoon downward plasma drifts on the season, longitude, solar activity, and lunar phase using the ROCSAT-1 satellite observations during 1999-2004. The results show that there are similar to 20% of downward plasma drifts at 13-17 LT. The occurrence rate of the downward plasma drift shows a peak around January and a secondary peak in July. The occurrence rate presents a slight decrease trend with the solar activity. Especially, the occurrence rate can attain similar to 40-50% around 60 degrees E in June solstice and 145-245 degrees E in December solstice, which is nearly comparable with the occurrence rate of the upward plasma drifts in these longitudes. The downward plasma drift occurs most frequently around the full moon in the afternoon sector. The geomagnetic declination induces the hemispheric differences in the dynamo current in the conjugate regions and controls the occurrence of the westward electric fields. A wavenumber-4 longitudinal structure also appears to be present in the occurrence rate in June-September, and the wavenumber-4 peak coincides with the diurnal eastward tidal wind, which drives the generation of the westward electric fields.