Fig. 2
The longitude versus UTC (in a time cadence of 15 min) variations of ionospheric perturbation (∆TEC) for the (a and c) northern (10° to 20° MLAT) and (b and d) southern (− 10° to − 20° MLAT) EIA crest regions during geomagnetic storms on (a and b) 6–11 September 2017 and (c and d) 24–29 August 2018. The gray dashed lines represent the local noon at different longitudes