Ionospheric Response to the St. Patrick's Day Space Weather Events in March 2012, 2013, and 2015 at Southern Low and Middle Latitudes

The changes in critical frequency of the F-2 layer (f(o)F(2)) and f(o)F(2) deviation (Delta f(o)F(2)) have been determined for three geomagnetic storms in March of the years 2012, 2013, and 2015 at low-latitude stations, Darwin (geomag. lat. 21.96 degrees S) and Townsville (28.95 degrees S), and midlatitude stations, Brisbane (36.73 degrees S), Canberra (45.65 degrees S), and Hobart (54.17 degrees S). The moderate storm during 15-16 March 2012 (Dst = -87 nT) showed a decrease in f(o)F(2) at midlatitude and no effect at low-latitude stations. For the intense storm of 17-18 March 2013 (Dst = -132 nT) and the super storm of 17-18 March 2015 (Dst = -222 nT), some middle- to low-latitude stations showed a short-duration increase in f(o)F(2), but all stations showed a long-duration decrease in f(o)F(2) during the recovery phases with Delta f(o)F(2)% varying from 26% (Darwin) to 36.6% at Hobart for the March 2013 storm and above 40% for the March 2015 storm at all of the stations. Short-duration (similar to 2-4 hr) increase in f(o)F(2) seems to be associated with the prompt penetrating electric fields. Long-duration (>6 hr) decrease in f(o)F(2) is mainly accounted to the decrease in thermospheric O/N-2 density ratio because of storm-induced high-latitude circulation of gas with depleted O/N-2 density ratio to lower latitudes and partly due to disturbance dynamo electric fields. A comparison of ionosonde given f(o)F(2) for equinoctial storms (March 2013 and 2015) with similar strength Southern Hemisphere winter storms (July 2012 and June 2015) has been made with the IRI-2016 model f(o)F(2) for Darwin, Brisbane, and Canberra stations.