Ionospheric variations over Irkutsk before strong earthquakes in Tuva

Measurements made at two closely-spaced vertical-incidence sounders (Irkutsk, Tory) and in the low-inclined path Usolye-Tory have yielded data on changes of ionospheric parameters occurring several days before the earthquakes of magnitude M>6 in Tuva on December 27, 2011 and February 26, 2012. The measurements were carried out with different time resolutions that allowed us to examine changes both in diurnal variations of main ionospheric parameters and in an inhomogeneous structure at heights 100-200 km. We have found out that 1-2 days before earthquakes there are considerable deviations both in the sporadic E-layer and in the F2-layer critical frequency. Also noted is an increase in the degree of inhomogeneity in the lower ionosphere.