Evidence For The Significant Differences In Response Times Of Equatorial Ionization Anomaly Crest Corresponding To Plasma Fountains During Daytime And Post-Sunset Hours

Based on 10 years' (2010-2019) of vertical total electron content (VTEC) data from Ahmedabad (23.0 degrees N, 72.6 degrees E, dip angle 35.2 degrees) and campaign based OI 630.0 nm airglow intensity measurements from Mt. Abu (24.6 degrees N, 72.7 degrees E, dip angle 38.0 degrees), it is shown that plasma density over the equatorial ionization anomaly (EIA) crest region increases in varying degrees during post-sunset hours (2000-2100 LT) in magnetically quiet periods. The post-sunset peak in VTEC precedes the corresponding peak in airglow intensity. By comparing post-sunset VTEC enhancements with ionosonde observations from Tirunelveli (8.7 degrees N, 77.7 degrees E, dip angle 1.7 degrees), it is shown that pre-reversal enhancement (PRE) of the zonal electric field causes these enhancements over the EIA crest region. These observations are supported by TEC measurements by GAGAN (GPS Aided Geo Augmented navigation), the Indian Satellite-based Augmentation System (SBAS). Comparison of average VTEC variations with global empirical model drifts reveals that the post-sunset enhancements in VTEC occurs similar to 1.7 h after the PRE and are significant only during December solstice and equinoctial months in high solar activity years similar to seasonal variations in PRE amplitudes. This time delay (response time of EIA crest) is almost half compared to the average response time (3-4 h) associated with the daytime fountain. Based on the latitudinal gradient in SBAS-TEC, it is proposed that the PRE drives plasma from 5 degrees N to 10 degrees N magnetic latitudes to the EIA crest region leading to shorter response time. These results show the important role of the PRE in conditioning the EIA crest region.