On variations of foF2 and F-spread before strong earthquakes in Japan

Other literature type English OPEN
Liperovskaya, E. V. ; Parrot, M. ; Bogdanov, V. V. ; Meister, C.-V. ; Rodkin, M. V. ; Liperovsky, V. A. (2006)

The statistical analysis of the variations of the dayly-mean frequency of the maximum ionospheric electron density <i>fo</i>F2 is performed in connection with the occurrence of (more than 60) earthquakes with magnitudes <i>M</i>&gt;6.0, depths <i>h</i>&lt;80 km and distances from the vertical sounding station <i>R</i>&lt;1000 km. For the study, data of the Tokyo sounding station are used, which were registered every hour in the years 1957&ndash;1990. It is shown that, on the average, <i>fo</i>F2 decreases before the earthquakes. One day before the shock the decrease amounts to about 5%. The statistical reliability of this phenomenon is obtained to be better than 0.95. <P> Further, the variations of the occurrence probability of the turbulization of the F-layer (F spread) are investigated for (more than 260) earthquakes with <i>M</i>&gt;5.5, <i>h</i>&lt;80 km, <i>R</i>&lt;1000 km. For the analysis, data of the Japanese station Akita from 1969&ndash;1990 are used, which were obtained every hour. It is found that before the earthquakes the occurrence probability of F spread decreases. In the week before the event, the decrease has values of more than 10%. The statistical reliability of this phenomenon is also larger than 0.95. Examining the seismo-ionospheric effects, here periods of time with weak heliogeomagnetic disturbances are considered. For the <i>fo</i>F2 analysis, the Wolf number is less than 100 and the index &Sigma;<i>K<sub>p</sub></i> is smaller than 30, and in case of the F-spread study a Wolf number less than 80 and &Sigma;<i>K<sub>p</sub></i> smaller than 17 are chosen.
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