Examples of the influence of strong magnetic storms on the equatorial F-layer

During extremely strong magnetic storms a sudden anomalous decrease in the F-layer critical frequency foF2 is sometimes noticed above Huancayo (12.0 S, 75.3 W) between 10 and 14 h local time. The duration of the phenomenon is from some tens of minutes to some hours. During the study period July 1958-June 1960, seven phenomena were noted. In all cases a magnetic storm had commenced a few hours earlier and exceeded, at least occcasionally, the 1000 gamma level in the AE index. The sudden decrease of foF2 is roughly simultaneous with some of the maxima in the AE index. A possible explanation for the phenomenon is that the electric field related to the Birkeland current system in the auroral zone extends to the equator and temporarily greatly enhances the equatorial anomaly in the F-layer. Recent model calculations show that under certain conditions the electric field originating from the auroral zone current system may have a correct orientation in the equatorial zone to produce the phenomenon observed.