A severe negative response of the ionosphere to the intense geomagnetic storm on March 17, 2015 observed at mid- and low-latitude stations in the China zone

In this paper, the critical frequency of F2 layer of the ionosphere (foF2) and the total electron content (TEC) recorded at mid- and low-latitude observation sites near 120\textdegreeE in the China zone were used to investigate the response to a severe geomagnetic storm on March 17, 2015 (the minimum Dst -223 nT at 23 UT). The results showed that the strong geomagnetic storm caused a massive effect on the ionosphere. The characteristics of foF2 and TEC did not show obvious perturbation during the main phase. Severe depletion of foF2 and TEC was observed at all stations during the storm recovery period. The maximum absolute discrepancy in TEC compared with the past 27-day average value was 78 TECU, and the minimum percentage deviations reached -71\% at Fuzhou (26.1\textdegreeN,\ 119.3\textdegreeE). The minimum percentage deviations of decrease in foF2 reached -65\% at Sanya (18.1\textdegreeN,\ 109.3\textdegreeE) and Mohe (53.5\textdegreeN,\ 122.3\textdegreeE). This was an infrequent negative effect that foF2 and TEC sustained throughout the day with extremely low level on March 18. The O/N2 rate showed a distinct reduction on March 18 in the China zone, which may be mainly responsible for the severe depletion of foF2 and TEC. The spread-F seemed to be developed at first but was then suppressed to some extent during the main phase. During the recovery phase, the spread-F was suppressed at Sanya, while it developed at Wuhan and Mohe. The disturbance electric fields and thermospheric circulation may contribute to this phenomenon.