Day-time variations of foF2 connected to strong earthquakes

The statistical analysis of the characteristic frequency foF2 of the Earth's ionosphere averaged over midday hours - from 11:00 till 17:00 h LT - is carried out. Disturbances of foF2 connected to earthquakes are considered on the background of seasonal, geomagnetic, 11-years and 27-days solar variations. A special normalized parameter F is introduced, which represents the almost seasonal-independent part of foF2. Days with high solar (Wolf number > 100) and geomagnetic (Sigma K-p > 30) activity are excluded from the analysis. Events with magnitude M> 5, distance from the sounding station R < 500 km and depth h < 70 km are taken into account. The superimposed epoches' method is used to determine the temporal dependence of F. It is found that F increases about 3-6 days before the earthquakes and then decreases one day - two days before the shock. The decreased values of F continue to exist two-three days after events with M>5.5. The obtained phenomenon depends on the magnitude of the earthquake. For events with M>5.5, the reliability of the effect is larger than 95%. For data of more than 80 earthquakes in the vicinity of Petropavlovsk-Kamchatsky and more than 200 earthquakes in the vicinity of Tokyo analogous results are obtained.