## 3 Results
### Ionospheric Response to the Storm of 7-9 October 2012  
This geomagnetic storm occurred on 8 October with SSC at about 1700 UT on 7 October and its main phase occurred in multiple steps. As shown in Figure 4a, the _Dst_ index decreased to a minimum value of \(-\)99 nT at 1200 UT on 8 October and then recovered and then again decreased and recovered in three consecutive steps and had a minimum value of \(-\)109 nT at 0800 UT on 9 October. Variations in the _fo_F2, _Af_oF2%, and _h'F_ for three stations under this storm are shown in Figures 4-6, respectively. As shown in Figure 4b,d (Darwin station), the _fo_F2 and _h'F_ variations showed no significant change during the first and second steps of the storm development. There occurred a decrease in _fo_F2 during 0200-2000 UT on 9 October with a maximum decrease in _fo_F2 of 7.3-2.8 MHz, at 1600 UT with _Af_oF2% of \(-\)61.6%. During this period, there was a considerable increase in _h'F_ (maximum 237-331 km 1700 UT) on 9 October. Another decrease in _fo_F2 occurred from 1100-2100 UT on 10 October with a maximum _Af_oF2% of about \(-\)47%, which was accompanied by a sharp increase in _h'F_. Figure 5b,d presents the _fo_F2 and _h'F_ variations at Townsville station which during the main phase development of the storm showed aslight increase in _fo_F2 during 0900-1400 UT on 8 October. There was no significant change in \(h^{\prime}F\) during this first step development of the main phase of the storm. Thereafter, on 9 October, _fo_F2 showed a strong and long duration decrease between 0000-2000 UT, with a maximum decrease from 6.3 to 2.2 MHz (\(\Delta\)_fo_F2% of \(-\)65.1%) at 1600 UT. The simultaneous strong increase in \(h^{\prime}F\) (maximum 232-390 km at 1600 UT) was found with a maximum of about 68% from the median value. Another event of a decrease in _fo_F2 occurred between 0400-2300 UT on 10 October, which was accompanied by a small increase in \(h^{\prime}F\) (maximum, 216-272 km, at 1500 UT). As shown in Figure 6b-d, Canberra station during the first step development of the main phase showed a small increase in _fo_F2 (maximum 7.1-9.1 MHz at 1000 UT) on 8 October, which had a \(\Delta\)_fo_F2% of +28% relative to the median value. The _fo_F2 approached medium value at 1300 UT on 8 October, following which a long duration large decrease in _fo_F2 from 1400 UT on 8 October to 0700 UT on 9 October was observed (Figure 6b) during the second step development of the storm and beyond the recovery phase of the storm with a maximum \(\Delta\)_fo_F2% of \(-\)59.6% at 1800 UT on 8 October. The _fo_F2 remained below the median value for most of 9-10 October (Figure 6c) showing a long duration of the negative ionospheric storm at Canberra during the storm recovery phase. Increases occurred in \(h^{\prime}F\) during the storm development and recovery phase of the storm during changes in the _fo_F2, as can be seen from Figure 6d. Table 1 summarizes the _Dst_ and _AE_ indices and southward IMF \(B_{Z}\) and its duration during the main phase of the storms and _fo_F2 response during the main phases of the six intense geomagnetic storms of 2012 at three different stations in the Southern hemisphere (Australian Region). IMF \(B_{Z}\) varied approximately between \(-\)15 to \(-\)20 nT for 10 to 20 h duration with an unusually high duration of about 31 h for the 14-15 July storm. The geomagnetic storms of 14-15 July and 30 September-3 October produced positive ionospheric storms at all three stations. The geomagnetic storms of 7-9 October and 13-14 November produced the positive ionospheric storm at Townville and Canberra stations with no effect at Darwin station. The geomagnetic storms of 8-9 March produced the positive ionospheric storm at Canberra with no effect at Townville and Darwin stations. The positive ionospheric storms were not very strong with \(\Delta\)_fo_F2% varying between 25 to 40%. The storm of 23-24 April did not produce any effect at any of the three stations.