Researching of quasiperiodic variations in the amplitude of VLF electromagnetic signals from lightning discharges passing over strong earthquakes

Abstract

The specific features of a method for radiosounding the lower ionosphere over earthquake epicenters using LF electromagnetic signals of thunderstorm sources (atmospherics) have been considered. The effects of shallow-focus earthquakes with magnitudes larger than 4.0 and their precursors manifest themselves in amplitude characteristics of atmospherics. It has been assumed that variations in the signal characteristics are related to disturbances in the lower ionosphere. According to the results of azimuthal scanning, cross sectional dimensions of disturbed regions, as a rule, correspond to the dimensions of the first two Fresnel zones for signals at a frequency of 10 kHz. Azimuthal scanning also indicated that the positions of disturbed regions during and before earthquakes could have a certain dynamics and differ from the projection onto the earthquake epicenter. Quasi-periodic variations of signal amplitude of lightning discharges passing over earthquake epicenters are in details considered. It has been obtained that the period of amplitude modulation makes up 2-3 hours, i.e. in the ionosphere D-layer the same values of the periods of wave phenomena are registered as which are observed in a higher F-region of the ionosphere. The results obtained during the work specify that, at least, in the part of events the seismic disturbances in the ionosphere can be caused by AGW, propagating upwards from the epicentral area.