## 4 Discussion  
The ionospheric response using the critical frequency of F\({}_{2}\)-region (_f_0F2) to six intense geomagnetic storms at a low latitude (Darwin) and a low-mid latitude (Townsville), and a mid-latitude (Canberra) showed the positive storm effect (positive ionospheric storm) during the main phase of the storms and both positive and negative storm effects (negative ionospheric storms) during the recovery phase of the five storms. The storm of 23-24 April did not produce any ionospheric effect. The negative ionospheric storms were stronger and of much longer duration compared to positive ionospheric storms. Generally, a geomagnetic storm is categorized by its SSC, minimum _Dst_ (storm intensity), main, and recovery phases. The change in the quiet ionospheric electric field plays a major role in the occurrence of ionospheric storms at the low and mid-latitudes. On quiet conditions (normal days), the equatorial ionospheric electric field is eastward during the daytime and westward at nighttime [18], so the quiet-time ionospheric \(\mathbf{E}\times\mathbf{B}\) drifts are upward in the daytime and downward at the nighttime. The normal pattern of \(\mathbf{E}\times\mathbf{B}\) drifts can be seriously affected or even reversed by the storm-time prompt penetration of electric fields, PPEFs [13], the disturbance dynamo electric fields, DDEFs [20,21] of high-latitude origin, and overshielding electric field [12,22,23,24]. The storm-time substorms can also induce both eastward/westward PPEFs under the steady southward/northward Z-component of the interplanetary magnetic field (IMF \(B_{z}\)) conditions [25]. The PPEFs are the potential sources of the positive ionospheric storms (increase in \(f\)0F2) during the main phases of the five storms (excluding the April storm), as shown in Table 1. This is supported by the fact that the main phase of these storms occurred in the local daytime of the stations when the quiet time ionospheric electric fields are eastward hence the \(\mathbf{E}\times\mathbf{B}\) drifts are upward. The main phases of these storms were associated with strong (\(<\)\(-\)5 nT) southward Z-components of IMF \(B_{Z}\) giving rise to eastward PPEFs which would have enhanced the daytime vertical \(\mathbf{E}\times\mathbf{B}\) drifts and strengthened the equatorial plasma fountain [5,18]. The storm of 8-9 March produced a positive ionospheric storm only at mid-latitude station (Canberra) with no effect at Darwin and Townsville stations, indicating that PPEFs did not propagate to the lower latitudes of these two stations. The storms of 7-8 October and 13-14 November produced positive ionospheric storms at the mid-latitude station (Canberra) and the low-mid-latitude station (Townsville) with no effect at Darwin indicating that PPEFs did not propagate to the low latitude of Darwin station. In general, the positive ionospheric effects were stronger at mid-latitude station as compared to low- and low-mid-latitude stations which support the latitudinal dependence of the geomagnetic storm effect. [PERSON] et al. [5] have also reported that during the main phase of geomagnetic storms the daytime eastward PPEFs could be the main driving factors of a positive storm at low and mid-latitudes. [PERSON] et al. [10] reported that the DDEFs caused by the enhanced energy deposited at the high latitudes can perturb the low-latitude ionosphere for one to two days after the main phase of the geomagnetic storm. DDEFs are always opposite to quiet time ionospheric electric fields for both day and night, that is, DDEFs are westward (eastward)