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 (Δ 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°S) and Townsville (28.95°S), and midlatitude stations, Brisbane (36.73°S), Canberra (45.65°S), and Hobart (54.17°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 Δ 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 (~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.