Articles | Volume 9, issue 1
19 Jan 2009
 | 19 Jan 2009

Day-time variations of foF2 connected to strong earthquakes

E. V. Liperovskaya, V. V. Bogdanov, P.-F. Biagi, C.-V. Meister, V. A. Liperovsky, and M. V. Rodkin

Abstract. The statistical analysis of the characteristic frequency foF2 of the Earth's ionosphere averaged over mid-day hours – from 11:00 till 17:00 h LT – is carried out. Disturbances of foF2 connected to earthquakes are considered on the background of seasonal, geomagnetic, 11-years and 27-days solar variations. A special normalized parameter F is introduced, which represents the almost seasonal-independent part of foF2. Days with high solar (Wolf number >100) and geomagnetic (∑Kp>30) activity are excluded from the analysis. Events with magnitude M>5, distance from the sounding station R<500 km and depth h<70 km are taken into account. The superimposed epoches' method is used to determine the temporal dependence of F. It is found that F increases about 3–6 days before the earthquakes and then decreases one day – two days before the shock. The decreased values of F continue to exist two-three days after events with M>5.5. The obtained phenomenon depends on the magnitude of the earthquake. For events with M>5.5, the reliability of the effect is larger than 95%. For data of more than 80 earthquakes in the vicinity of Petropavlovsk-Kamchatsky and more than 200 earthquakes in the vicinity of Tokyo analogous results are obtained.