On diurnal dependence and spatial scales of seismo-ionospheric effects in the E-layer

In the present work, disturbances of the f_bE_s-frequency of the sporadic E-layer of the ionosphere are investigated in connection with earthquakes. The f_bE_s-frequency is proportional to the square root of the maximum ionisation density of the sporadic E-layer. In this work, it is shown that two days before a seismic shock with magnitude M>5.5, and during the shock, an increase of the f_bE_s-frequency is obtained around midnight at distances from the epicentre R<expM+100 km in the case that the focus of the shock was situated at depths smaller than 60 km. Data obtained by the three ionospheric sounding stations "Kokubunji", "Akita" and "Yamagawa" are analysed, which were recorded during a total time of 42 years. The superimposed epoches method is applied for a few tens of earthquakes.