LAUSR

Abstract In this paper, the critical frequency of F2 layer of the ionosphere (foF 2) and the total electron content (TEC) recorded at mid- and low-latitude observation sites near 120°E 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 foF 2 and TEC did not show obvious perturbation during the main phase. Severe depletion of foF 2 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°N, 119.3°E). The minimum percentage deviations of decrease in foF 2 reached −65% at Sanya (18.1°N, 109.3°E) and Mohe (53.5°N, 122.3°E). This was an infrequent negative effect that foF 2 and TEC sustained throughout the day with extremely low level on March 18. The O/N 2 rate showed a distinct reduction on March 18 in the China zone, which may be mainly responsible for the severe depletion of foF 2 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.