| 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 ΣKp is smaller than 30, and
in case of the F-spread study a Wolf
number less than 80 and ΣKp smaller than 17 are chosen. |