On spread-E_s effects in the ionosphere before earthquakes

Abstract. The spread-E_s phenomenon which appears as diffusivity of the traces of sporadic E-layers on the ionogrammes of vertical sounding stations and which reflects the turbulization of the sporadic layers -E_s is studied in connection with earthquake preparation processes. Spread-E_s data obtained at night every 15 min by the midlatitudinal vertical sounding station of the ionosphere in Dushanbe (φ=38.5° N, λ=68.8° E) are analysed. Groups of earthquakes with different magnitudes and different distances between the epicenter and the sounding station are considered. A statistical analysis of the obtained results is performed. It is shown that during the three nights before an earthquake, spread-E_s phenomena are observed more often than during the forth, fifth and sixth nights before the event. This effect is found to depend on both the magnitude of the earthquake and the distance between the epicenter and the sounding station.