Observations of Electromagnetic Perturbations Shortly Before Earthquakes Onboard a Low-Orbiting Spacecraft

We present a statistical study of intensity of VLF (up to 10 kHz) electromagnetic waves in the top side ionosphere above the earthquakes. More than 2.5 years of data recorded by the French micro-satellite DEMETER (altitude of orbit 700 km) and a specially developed two-step data processing method have been used. In the first step, a map of electromagnetic emissions that contains a statistical description of wave intensity at a given point of the satellite orbit under given conditions is constructed. In the second step, the wave intensity close to earthquakes is evaluated. Changes of wave intensity related to the seismic activity are investigated and their statistical significance is analyzed. Earthquakes with magnitude larger than 4.8 that occurred all over the world during the analyzed period (~ 9000 earthquakes) have been included in the study. It is shown that the observed intensity of electromagnetic waves decreases during the night by 4-6 dB shortly before earthquakes (up to 4 hours before the time of the main shock). Spatial scale of the affected area is approximately 300 km. A possible explanation of the observed effects is suggested.