Disturbances in the F2 layer of the ionosphere preceding the onset of earthquakes with $ M\geq 6.0$ in the Kamchatka region

The paper presents an analysis of data from hourly measurements of the critical frequency of the ionospheric F2 layer (foF2), carried out over the time interval 01.07.1968-30.07.1997 at the vertical radio sounding station of the ionosphere PK553 (Paratunka, Kamchatka region) in order to identify seismoionospheric anomalies preceding earthquakes with magnitudes $M \geq 6.0$. During the analysis, 45 earthquakes of corresponding magnitudes were considered, in the preparation zone of which, determined by the Dobrovolsky formula, the PK553 ionospheric station was located. The vertical radio sounding station of the ionosphere MG560 (Stekolny, Magadan region) was chosen as a control station. The procedure for detecting seismic-ionospheric disturbances consisted of a joint analysis of the foF2 critical frequency data measured at the reference and control ionospheric stations for vertical radio sounding of the ionosphere. An ionospheric disturbance recorded at a reference station was considered as a possible ionospheric precursor of earthquakes if the deviation of the reference station's foF2 values from the boundaries of the background range of values was observed under conditions of low geomagnetic activity, while the foF2 values of the control station were within its range of background values. Anomalies in foF2 values of the reference station preceded 20% of the considered seismic events. The quantitative characteristics of ionospheric disturbances were made: the maximum relative deviations of the anomalous foF2 values from the median values and from the boundaries of the background range, the lead time of the ionospheric disturbance at the moment of the earthquake. Empirical dependencies were obtained that connect the epicentral distances of earthquakes and the lead time of ionospheric disturbances at the moment of the onset of earthquakes with their magnitude. The identified disturbances of the critical frequency foF2 may be associated with the processes of preparation of the corresponding earthquakes, i.e. may be possible ionospheric precursors of earthquakes.