Detection of Possible Short-Term Ionospheric Precursors of Strong Earthquakes Based on Changes in Diurnal Es Characteristics

Abstract

The changes in two characteristics of the sporadic Е layer are studied for a pair of stations: the probability of the occurrence PEs and the limiting frequency of the ordinary wave of the sporadic E layer of the ionosphere foEs during a 10-day period of the preparation of 19 crustal earthquakes in the Pacific region with М = 6.5–7.4. The stations are located hundreds of kilometers from each other, but they fall within the zone of the preparation of a particular earthquake (the sizes of the earthquake preparation zone are estimated with formulas known in the scientific literature that relate the size of the radius of the earthquake preparation zone and the earthquake magnitude). The measurement data obtained at the ground stations of ionospheric vertical sounding are analyzed. The deviations of diurnal values of PEs (δPEs) from the median over the studied time interval and the integral diurnal values of the total irregular fluctuations in foEs (ΔfEsΣ) are used to identify possible ionospheric earthquake precursors. The coincidence of the time of appearance of the deviation maxima for both parameters before the earthquakes at each of the stations on the same day (from 1 to 4 days before the earthquake day) is recorded in the diurnal changes in the indicated values during the preparation periods of all of the considered earthquakes. The criteria for the identification of a short-term ionospheric earthquake precursor is discussed. Comparison of the analysis results for manual and automatic ionogram processing showed the prospects for the use the proposed parameters obtained according to the data of the distant ionosondes to identify the short-term ionospheric precursors of an earthquake with М = 6.5–7.0.