Study of the ionospheric responses over African and Asian longitudes to the intense geomagnetic storm of August 2018

The third strongest geomagnetic storm of solar cycle 24 occurred from 25 to 31 August 2018 (minimum SYM-H index = –206 nT on 26 August) and was associated with a weak coronal mass ejection (CME) in the low solar activity period. The storm itself has shown some very unusual characteristics in interplanetary space, which unraveled further surprises in its ionospheric and thermospheric responses. This study provides a detailed analysis of the effects of this storm over the vast African-Asian longitude region over both hemispheres. The longitudinal differences in the equatorward boundary of the auroral oval and the ionospheric convection patterns are presented using the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) onboard the Defense Meteorological Satellite Program (DMSP) satellite and SuperDARN observations, respectively. The global ionospheric map (GIM)-based hourly vertical TEC (total electron content) variations show large enhancements and depletions over different regions at different phases of storms. It is found that during the morning hours on 26 August, a large TEC enhancement in the Asian low latitude region is observed, but such an effect is not observed over the African region, establishing a clear longitudinal difference. The space-time structure of the enhanced ion-density observations from Swarm satellites is used to confirm the VTEC differences and their sustenance. Furthermore, a latitudinal variation in the [O/N2] ratio during 25-27 August is analyzed for a UT-dependent response and the north–south asymmetry in the O density during the main and recovery phases of the storm. A competing effect of the prompt penetration electric field and disturbance dynamo is found to dominantly modulate the longitudinal patterns, which created positive/negative ionospheric storms over the vast African-Asian region.