Ionospheric phenomena before strong earthquakes

A statistical analysis of several ionospheric parameters before earthquakes with magnitude M >= 5.5 located less than 500 km from an ionospheric vertical sounding station is performed. Ionospheric effects preceding "deep" (depth h > 33 km) and "crust" (h <= 33 km) earthquakes were analysed separately. Data of nighttime measurements of the critical frequencies f oF2 and f oEs, the frequency f bEs and Es-spread at the middle latitude station Dushanbe were used. The frequencies f oF2 and f bEs are proportional to the square root of the ionization density at heights of 300 km and 100 km, respectively. It is shown that two days before the earthquakes the values of f oF2 averaged over the morning hours (00: 00LT-06: 00 LT) and of f bEs averaged over the nighttime hours (18:00LT-06: 00 LT) decrease; the effect is stronger for the "deep" earthquakes. Analysing the coefficient of semi-transparency which characterizes the degree of small-scale turbulence, it was shown that this value increases 1-4 days before "crust" earthquakes, and it does not change before "deep" earthquakes. Studying Es-spread which manifests itself as diffuse Es track on ionograms and characterizes the degree of large-scale turbulence, it was found that the number of Es-spread observations increases 1-3 days before the earthquakes; for "deep" earthquakes the effect is more intensive. Thus it may be concluded that different mechanisms of energy transfer from the region of earthquake preparation to the ionosphere occur for "deep" and "crust" events.