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

•Observations of the fbEsfbEs-frequency of the vertical sounding stations “Kokubunji”, “Akita”, and “Yamagawa” are analysed.•It is found that the ionisation density of sporadic E-layers increases after sunset on the two days before earthquakes and on the day of the seismic shock.•This result is valid for earthquakes with magnitudes M>5M>5, focal depths smaller than 60 km, and distance between the epicentre and the vertical sounding station smaller than R

In the present work, disturbances of the fbEsfbEs-frequency of the sporadic E-layer of the ionosphere are investigated in connection with earthquakes. The fbEsfbEs-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>5.5M>5.5, and during the shock, an increase of the fbEsfbEs-frequency is obtained around midnight at distances from the epicentre R