Midnight variations of spreading of ionospheric sporadic E-layers before earthquakes

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.