Ionospheric responses to two large geomagnetic storms over Japanese and Indian longitude sectors

The physical processes including the prompt penetration electric field, disturbance dynamo originated electric field, disturbed thermospheric winds and composition changes can play a significant role in restructuring the equatorial, low, mid and high-latitude ionosphere during storm-time. However, it has not yet been revealed that the contribution of individual physical processes, their interactions and impacts on that restructuring (Maruyama et al., 2005) is primarily due to the lack of continuous observational facilities. In this present research, the electric field (measured indirectly) and thermospheric wind (derived from an empirical disturbance wind model) components are effectively utilized as alternate database to ascertain the individual role of physical processes by studying the ionospheric response over Japanese and Indian longitude sectors during two geomagnetic storms occurring on 31 March, 2001 and 20 November, 2003 using ground (ionospheric parameters scaled from ionosondes and global ionospheric maps of total electron content measured by the ground-based GPS receivers) and satellite-borne (in-situ electron density data measured by the Planar Longmuir Probe onboard CHAMP satellite) measurements. It has been found that the equatorial ionization anomaly is expanded and intensified during the main phase of these two storms, which is believed to be caused by the prompt penetration electric field according to the current theory. In addition, the storm associated thermospheric wind is propagating equatorward (with an average velocity of ~230. m/s) during the recovery phase of these two storms that is responsible for a height rise in the virtual height of the F-layer (h'F) starting from mid to low and equatorial latitudes with a consistent time delay. The empirical model derived winds corroborate the equatorward propagation, suggesting that this wind model data could be used as an alternate database particularly during the space weather events in order to discuss the global dynamical state of the ionosphere. Further, an important observation is that the ionospheric irregularities are found in the electron densities in the form of depletions nearly at anomaly crest region (~23°N) as measured by the CHAMP satellite over the Japanese longitude sector during the main phase of the 20 November, 2003 storm during the pre mid night period that correspond to the time of rapid decrease (~-30-35. nT/h) in Sym-H index due to prompt penetration of eastward electric fields into the low latitudes.