Severe ionospheric disturbances caused by the sudden response of evening subequatorial ionospheres to geomagnetic storms

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Abstract
By monitoring C band beacon signals from geostationary satellites in Japan, we have observed anomalously strong ionospheric scintillations several times during three years from 1978 to 1980. These severe scintillations occur associated with geomagnetic storms and accompany sudden and intense ionospheric perturbations in the low‐latitude region. Through the analysis of these phenomena we have identified a new type of ionospheric disturbances characterized by intensifications of equatorial anomalies and successive severe ionospheric scintillations that extend to the C band range. The events occur only during a limited local time interval after the sunset, when storm time decreases of midlatitude geomagnetic fields in the same meridian take place during the same time interval. From the viewpoint of ionospheric storms, these disturbances precede the occurrence of midlatitude negative phases and storm time depressions of equatorial anomalies to indicate that the cause of the events is different from disturbed thermospheric circulations. The timing and magnitude of substorms at high‐latitudes not always correlate with the events. We have concluded that the phenomena are closely related with penetrations toward low‐latitudes of electric fields owing to the partial closure of asymmetrical ring currents.