In our statistics, we choose the cases with rapid southward turnings of the IMF, as shown in Fig. 1. In these cases, IMF turnings from northward to southward occur within 0.5–1 h. We may assume that the shielding effect is negligible within this time interval. We characterized observations of apparent penetration of IEF to low-latitude ionosphere in terms of (A) efficiency (ratio of horizontal east–west electric field component measured in the F-region ionosphere at the magnetic equator to IEF y-component) and (B) duration