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

Abstract. The statistical analysis of the variations of the dayly-mean frequency of the maximum ionospheric electron density foF2 is performed in connection with the occurrence of (more than 60) earthquakes with magnitudes M>6.0, depths h<80 km and distances from the vertical sounding station R<1000 km. For the study, data of the Tokyo sounding station are used, which were registered every hour in the years 1957–1990. It is shown that, on the average, foF2 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.

Further, the variations of the occurrence probability of the turbulization of the F-layer (F spread) are investigated for (more than 260) earthquakes with M>5.5, h<80 km, R<1000 km. For the analysis, data of the Japanese station Akita from 1969–1990 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 foF2 analysis, the Wolf number is less than 100 and the index ΣK_p is smaller than 30, and in case of the F-spread study a Wolf number less than 80 and ΣK_p smaller than 17 are chosen.