VARIABILITY OF TOTAL ELECTRON CONTENT (TEC) DURING FOUR MAJOR GEOMAGNETIC STORMS OF 2012-2014: A MULTISITE ANALYSIS AT FIXED LONGITUDES

This study investigates the ionospheric response to four major geomagnetic storms that occurred on 14 July 2012, 17 March 2013, 2 October 2013, and 27 February 2014, focusing on variations in the vertical total electron content (VTEC). VTEC data were obtained from 13 GNSS stations distributed across low, mid, and high-latitude regions along longitudes between 20E and 40E. For each event, the mean VTEC of the five geomagnetically quietest days of the corresponding month was used as a reference to characterize storm-time deviations.The results reveal diverse and complex ionospheric responses. The 14 July 2012 storm is characterized by a nighttime VTEC depletion at the equatorial ionization anomaly (EIA) trough, a transient intensification of the EIA, pronounced oscillations, interhemispheric asymmetries, and a subsequent suppression of the EIA, while high latitudes exhibit persistent depletion. The 17 March 2013 storm shows a pre-storm enhancement of VTEC, followed by oscillatory behavior, a transition from positive to negative storm effects, interhemispheric asymmetries, and EIA suppression. Similarly, the 2 October 2013 storm exhibits an early VTEC enhancement, marked oscillations, high-latitude depletion, and a transition from positive to negative effects at mid-latitudes, with relatively weak responses at low latitudes.