On variations of f oF2 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 f oF2 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, f oF2 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 f oF2 analysis, the Wolf number is less than 100 and the index Σ Kp is smaller than 30, and in case of the F-spread study a Wolf number less than 80 and Σ K p smaller than 17 are chosen.