“…Although theoretical models (e.g., Crowley et al, 1996;Daniell et al, 1995) could be considered as powerful tools for physical analyses providing real input in the understanding of the mechanisms that govern the ionospheric formation under various geophysical conditions, they hardly offer real contribution in operational applications (Mikhailov et al, 2007). On the other hand, the empirical approach based on the correlation between the ionospheric disturbances and the level of the geomagnetic activity as it is described by various geomagnetic activity indices (Fuller-Rowell et al, 2000b;Fuller-Rowell et al, 2002;Muhtarov, 2001 a, b, 2003;Muhtarov and Kutiev, 1999;Muhtarov et al, 2002;Tsagouri and Belehaki, 2006), is widely used in practice.…”