Strong earthquakes and their influence on the polar lower ionosphere

The experimental data obtained by a partial reflections radar and a vertical ionospheric sounder located at the Kola Peninsula (Tumanny, Russia) and the Northern Scandinavia (Sodankyla, Finland) have been analyzed with the aim to detect the ionosphere response to earthquakes. A violent earthquake with the magnitude of 7.7 on the Richter scale occurred on 17.07.2006 at 08:19:25 UT on the western shore of Indonesia under the very quiet geomagnetic field ΣКр=5). Another earthquake with the magnitude of 9.3 occurred on 26.12.2004 at 00:58:53 UT on Sumatra. On that day the geomagnetic field was disturbed. Before the earthquake on July 17 the amplitude of the ordinary component of a partially-reflected signal at the height of 101 km sharply decreased, whereas the period of signal fluctuations increased. Periods of several hours (3 to 6 hours) are derived from the spectra of the daily amplitudes of the ordinary component of the partially-reflected signal on July 17. The amplitude of frequency fluctuations fmin, which occurred several hours before the beginning of the earthquake increased from 1.0 MHz to 2.2 MHz. The period of 3 to 4 hours for these fluctuations has been derived. Such a period is also found in fluctuations of two other ionospheric parameters, the critical frequency of the sporadic layer (foEs), and the Es layer altitude (h-Es). On the disturbed day, 26.12.04, the amplitude fluctuations with periods of 2 to 5 hours of the signal ordinary component were revealed. From this finding we conclude that the amplitude fluctuations of the ordinary component of the partially-reflected signal and of the other parameters of the lower ionosphere having periods of several hours are produced by the internal gravity waves which are related to the earthquakes.