Abstract:
Based on 10 years' of vertical total electron content (VTEC) data in solar cycle 24 from Ahmedabad (23.0°N, 72.6°E, dip angle 35.2°), a station under the crest region of equatorial ionization anomaly (EIA), it is shown that both the integrated residual and total post-sunset enhancements in VTEC are conspicuous during Equinox and December solstice when solar flux level exceeds 110 solar flux unit (sfu) with the exception of the year 2012-2013. The post-sunset enhancements are absent in June solstice at this local time even if the solar flux level exceeds 110 sfu. The integrated residual and total VTEC enhancements during post-sunset hours are found to be linearly correlated with the solar flux level in Equinox and December solstice. It is noted that a parabolic fit works better for the integrated total VTEC enhancement during December solstice suggesting a possible saturation of VTEC enhancements at high solar flux levels. Based on these observations and Thermosphere Ionosphere Electrodynamics-General circulation model (TIE-GCM) outputs, it is argued that the pre-reversal enhancement (PRE) in the equatorial F region zonal electric field works in tandem with latitudinal gradient in the F region plasma density to determine the degree of VTEC enhancement over the EIA crest region during post-sunset hours. These results highlight the solar flux dependence of the post-sunset enhancement of VTEC over the crest region and show that sudden stratospheric warming events in 2012-2013 suppressed these enhancements in December solstice even if solar flux levels exceeded 110 sfu.