## 4 Discussion
The contribution of the overshielding electric field would be stronger for the 30 September-3 October and 7-10 October storms which had a stronger (5 to 10 nT) northward component of IMF \(B_{Z}\) during their recovery phases as compared to the other four storms that had a week northward component of IMF \(B_{Z}\) which was mostly southward with weak intensity. The northward turning of the IMF \(B_{Z}\) marks dominance of the overshielding field which penetrates in a few seconds from high latitudes down to low/equatorial latitudes and can occur both for short and long periods [12; 23]. The strength (Table 2) and durations (e.g., Figures 1-6) of ionospheric storms clearly show the latitude dependent effect of these mechanisms (undershielding PPEFs, DDEFs, overshielding field, and storm-induced circulation) with maximum effect at the mid-latitude station (Canberra) which needs further investigation. During intense geomagnetic storms, the high-latitude Joule heating (JH) could persist from a few hours to several days which raises the temperature of the upper thermosphere and generates large scale atmospheric gravity waves (AGWs) with a phase speed of about 600 m s\({}^{-1}\)[26]. AGWs launched from the auroral regions due to JH propagate toward equatorial latitudes. At the ionospheric height, AGWs generate traveling ionospheric disturbances (TIDs) [5] via the neutral-plasma interaction and are identified by wave-like fluctuations in the ionospheric parameters [27]. TIDs can also contribute to the changes in \(f\)0F2 during the main phase of the storm indicated by the high value of the \(AE\) index during these storms (Table 1). The JH also drives equatorward storm-time thermospheric neutral winds with decreased O/N\({}_{2}\) density ratio due to upwelling of this gas at high latitude moving toward the low latitudes [28; 29] and downwelling of gas with reduced O/N\({}_{2}\) ratio at low latitudes. In our study high values of the \(AE\) index associated with these storms (Table 1) show the reasonably high \(JH\). The \(JH\) and \(AE\) indices are linearly related as \(JH=0.19\ AE\)[30] and \(JH=0.33\ AE\)[31] where \(JH\) is in gigawatts and \(AE\) in nanotesla. This \(JH\) would result in a negative ionospheric storm (decrease in \(f\)oF2) during the recovery phase of storm at mid and low-latitude stations due to composition changes that occur due to an equatorward geomagnetic storm-induced circulation of gas with a depleted O/N\({}_{2}\) ratio. At the ionospheric height, [PERSON] et al. [5] also studied the physical mechanisms of the ionospheric storms during the recovery phase of geomagnetic storms using CHAMP Ne, GPS-TEC data, and the Sheffield University plasmasphere ionosphere model, and reported that storm-time equatorward neutral wind could be the major cause of ionospheric effects. The AGWs are launched from the auroral regions due to JH and propagate toward equatorial latitudes producing TIDs. Variation in the ionospheric electron density is because of the enhanced JH caused by the auroral electrojet current, where the energy is transported to the equatorial region with a delay of 24 h [10; 32]. [PERSON] and [PERSON] [7] found a strong long-duration decrease in the \(f\)oF2 during the recovery phase of the St. Patrick's Day storms in March 2012, 2013, and 2015 in the southern hemisphere. They based on the thermospheric O/N\({}_{2}\) density data measured by the global ultraviolet imager (GUVI) onboard the thermosphere, ionosphere, mesosphere energetics, and dynamics (TIMED) satellite accounted decrease in \(f\)oF2 to the decrease in thermospheric O/N\({}_{2}\) density ratio at lower latitudes and partly due to DDEFs. [PERSON] et al. [32] studied TIDs by measuring TEC derived from the global navigational satellite system and radio occultation during the same storms that we have studied here for the geographic latitudinal coverage of 10-40\({}^{\circ}\) S within a longitude sector of 10-40\({}^{\circ}\) E. A common result portrayed to all storms was the presence of large scales TIDs during the storm main phases.