EC response during severe geomagnetic storms near the crest of equatorial ionization anomaly

The responses of the ionosphere near equatorial anomaly crest as observed at Bhopal (geographic 23.2\textdegreeN, 77.4\textdegreeE, and magnetic latitude 14.2\textdegreeN), India for geomagnetic storms during 15 May 2005 (SSC: 0239 hrs UT, SymH: -305 nT) and 24 August 2005 (SSC: 0615 hrs UT, SymH: -179 nT) has been studied using total electron content (TEC). TEC data is recorded by GPS Ionospheric Scintillation and TEC Monitor (GISTM) GSV4004A at Bhopal. The temporal variations of vertical total electron content (VTEC) during these storms were analysed. Fifteen minute average of VTEC is considered for analysis. More than +68\% and +70\% deviation in VTEC was observed for 15 May 2005 and 24 August 2005 storms, respectively, relative to average of five quiet days of the respective months. Significant deviations in VTEC during the main phase of the storms were found to be associated with the prompt penetration of electric fields which originate due to the under-shielding and over-shielding conditions. Peaks like structure in EIA region were observed for both the storms. Peaks like structure well before the fountain effect in EIA region were observed for 15 May 2005. Latitudinal enhancement of anomaly peak in EIA region was also observed for both the storms. Suppressed equatorial electrojet strength and consequently VTEC, observed on 25 August 2005 during recovery phase of the storm, are mainly due to the thermospheric composition changes and also possibly due to the delayed disturbance dynamo electric fields in this region. The range error is also discussed in the present paper.