Attenuation of electromagnetic waves at the frequency ~1.7 kHz in the upper ionosphere observed by the DEMETER satellite in the vicinity of earthquakes

David Píša David Píša,František Němec František Němec,Ondřej Santolík Ondřej Santolík,Michel Parrot Michel Parrot

doi:10.4401/ag-5276

Abstract

The DEMETER satellite was the first satellite specifically dedicated to the recording of electromagnetic phenomena connected with seismic activity. Almost 6.5 years of measurements provide good opportunities to analyze a unique dataset with global Earth coverage. We present the results of a statistical study of the intensity of very low frequency electromagnetic waves recorded in the upper ionosphere. Robust two-step data processing has been used. The expected unperturbed distribution of the power spectral densities of electromagnetic emissions was calculated first. Then, the power spectral densities measured in the vicinities of earthquakes are compared with the unperturbed distribution and are examined for the presence of uncommon effects related to seismic activity. The statistical significance of the observed effects is evaluated. We confirm the previously reported results of a very small, but statistically significant, decrease in wave intensities a few hours before times of main shocks using this much larger dataset. The wave intensity decrease at a frequency of about 1.7 kHz is observed only during the night and only for shallow earthquakes. This can potentially be explained by increases in the cut-off frequency of the Earth ionosphere waveguide caused by imminent earthquakes.

Highlights

Over the past two decades, considerable progress has been made in the field of seismo-electromagnetic effects
In the first step of the data processing, it is necessary to describe the distribution of the intensity of electromagnetic waves observed by DEMETER using all of the available data
It can be seen that the main observed feature is a decrease in the normalized probabilistic intensity at the frequency of about 1.7 kHz shortly before (04 h) the time of the main shocks

Summary

Introduction

Over the past two decades, considerable progress has been made in the field of seismo-electromagnetic effects. Various types of possible precursors have been reported (e.g. changes in temperature or conductivity), electromagnetic perturbations are probably one of the most promising candidates. These have been observed both using ground-based measurements and satellites, and they are believed to occur across a wide range of frequencies, from direct current to very high frequency [Parrot and Mogilevsky 1989, Larkina et al 1989, Tate and. There is still controversy concerning the actual existence of these seismo-electromagnetic effects, and there are quite a few studies that dispute their existence [Henderson et al 1993, Rodger et al 1996, Clilverd et al 1999]

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