On spread-F in the ionosphere before earthquakes

Occurrence probability of the ionospheric spread-F in connection with earthquakes is analyzed. The F-layer is not close to the Earth (∼400 km), but in situ data could be obtained either by ionospheric sounders or by satellites. Data from the two Japanese ionospheric stations Kokubunji and Akita have been analyzed to find out long-term (a few weeks) variations of spread-F before and after earthquakes. Earthquakes with magnitudes M>5M>5 were taken into account. Only time intervals where geomagnetic variations are weak have been analyzed. It is shown that the probability of spread-F observations starts to decrease approximately 40 days before earthquakes, presents a minimum about 10 days before and then takes 1 month to recover the background level (therefore this increase lasts about 3 weeks after earthquakes). This effect exists if the distance between epicenters and the sounding station is less than 500 km.