On ionospheric precursors of earthquakes in scales of 2-3 h

The probability of 2-3 h variation in electron concentration at different regions of ionosphere 1-3 days before earthquakes at distances R < 500 km from vertical sounding stations was analyzed. Using 12 months of data sampled each 15 min at one ionospheric station it was revealed that before earthquakes with M > 4.5 the number of variations with time scales of 2-3 h in the lower F-layer of the ionosphere 000 (real altitude of about 250 km; virtual altitude of 300 km) increases for 10 earthquakes, decreases for 4 and it does not changes for 2, on 16 cases we analyzed. Using the hourly data of 10 ionospheric stations collected for 20-30 years it was revealed that the number of disturbances in the foF2-frequency (real altitudes of about 300-350 km corresponding to the maximal electron density of F2 layer) with time scales of 2-3 h does not change before earthquakes with M less than or equal to 5.5 and slightly decreases (on 9%) before more strong earthquakes (M > 5.5). Possible models able to justify the phenomenology we observed are presented. (C) 2004 Elsevier Ltd. All rights reserved.