Abstract
In the present study, ionospheric phenomena caused by earthquakes of magnitudes M >4.0 were investigated. Night-time observations of the spreading of sporadic E-layers (Es-spread) performed every 15 min by the Dushanbe and Petropavlovsk-Kamchatsky (middle Asia) vertical sounding stations were studied. The mean relative occurrence frequency of Esspread at different values of the blanketing frequency fbEs was considered, and the dependence of Es-spread on the season as well as on the year through an 11-yr solar activity cycle were studied. The fbEs characterizes the maximum plasma density of the Es-layer. The analysis shows that 1- 3 days before seismic shocks in the Earth crust at depths of h <80 km, the occurrence frequency of the Es-spread increases a few hours before midnight. This effect is characteristic of a strengthening of the turbulization of the E-layer plasma. On the basis that the radius of the earthquake preparation region (RD) is estimated by the Dobrovolsky formula RD ≈ exp(M) km, it was found that Es-spread is observed more often when the distance between the epicenter and the radar station is not greater than RD + 150 km. In cases of earthquakes at greater distances and depths, no midnight effect was found. The authors act on the assumption that the Es-spread might be caused by acoustic waves with periods of 20 s to 5 min. When such acoustic disturbances propagate from the Earth surface they will have maximum amplitudes if they move nearly vertically to greater altitudes.
Highlights
In a series of studies, the phenomenon of spreading of sporadic E-layers (Es-spread) has been studied in connection with earthquake preparation processes [Liperovsky et al 2000, Liperovsky et al 2008, Meister et al 2011]
The main aim of the present study was to investigate the intensification of Es-spread in dependence on the daytime during periods of earthquake preparation using data from the vertical sounding station of Dushanbe, and to compare this with results obtained based on observations by the vertical sounding station of Petropavlovsk-Kamchatsky
The analysis of Es-spread data observed by the Dushanbe vertical sounding station shows that 1-3 days before earthquakes in the Earth crust at depths h
Summary
In a series of studies, the phenomenon of spreading of sporadic E-layers (Es-spread) has been studied in connection with earthquake preparation processes [Liperovsky et al 2000, Liperovsky et al 2008, Meister et al 2011]. The Es-spread effect apparently reflects the processes of large-scale (a few hundreds of meters) instabilities in the sporadic layers. This effect is a consequence of the turbulization of sporadic E-layers [Bowman 1985, Whitehead 1989]. The effect is above all observed in summer, especially in the years of minimum activity of the solar cycle
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