Statistical study of ELF/VLF emissions observed in the vicinity of earthquakes by the DEMETER spacecraft

Results of a statistical survey of low-frequency electromagnetic waves observed by the DEMETER spacecraft (launched in June, 2004) in the vicinity of earthquakes are presented. The satellite has a low orbit (altitude 720 km) with an inclination of 98 degrees and its primary goal is to study ionospheric perturbations connected with seismic and/or man-made activity. The data consist of spectra of one electric and one magnetic field component with a frequency resolution of 19.5 Hz and time resolution of 1 or 4 seconds. In order to look for a correlation between wave intensity and the occurrence of earthquakes, data processing in two steps is used. In the first step, histograms of power spectral densities of ELF/VLF emissions as a function of the position of the spacecraft and natural conditions are constructed. These histograms are then used in the second step of the data processing, which enables us to distinguish exceptional phenomena possibly connected with seismic activity. The shape and dimensions of the affected area as well as the possible ways of signal propagation from the earthquake region up to the ionosphere (along the magnetic field line vs. perpendicular to the Earth's surface) are discussed and a statistical reliability of obtained results is estimated.