Estimation of the Ionospheric Effects of Geomagnetic Disturbances Based on Data from Wakkanai Station

Ionosonde measurements of the critical frequency of the ionospheric F2 layer (foF2) at Wakkanai station (Japan) are analyzed with the method of superposed epochs. The station is situated in a region of high seismicity. Winter periods with low sunspot and geomagnetic activity and high seismic activity are considered. The goals of this work were the quantitative characterization and comparison of ionospheric disturbances induced by the development of geomagnetic storms/geomagnetic disturbances. Geomagnetic disturbances refer to manifestations of geomagnetic activity with an intensity that is higher than "quiet" geomagnetic background but not higher than the threshold characteristic of weak geomagnetic storms. It is ascertained that weak geomagnetic storms and geomagnetic disturbances, from the time of the main trough in the Dst index after the onset of a geomagnetic storm/geomagnetic disturbance to the end of the third day, result in only positive foF2 deviations. The deviations exceed 6% of the background; the maximal relative deviations from the background (delta foF2(max)) are similar to 17% and similar to 14%, respectively. For geomagnetic disturbances, the most significant response of the ionosphere is observed similar to 7 h after the main trough in the Dst index. The resulting assessments can be used to filter out false ionospheric earthquake precursors with analysis of the ionospheric data from Wakkanai station.