[2] Ionospheric storms are one of manifestations of space weather disturbances. The disturbances are caused by nonstationary processes on the Sun: solar flares and solar storms accompanied by ejection of coronal mass (CME) and magnetic clouds, enhancement of the dynamical pressure of the solar wind, variations in the parameters of the interplanetary magnetic field (IMF), etc. These disturbances covering the interplanetary space and the Sun-Earth system influence the processes in various regions of the near-Earth environment. On the Earth, the storms cause malfunctions in operation of powerful energetic systems, navigation and remote radio communication systems, and influence the weather [German and Goldberg, 1981], and possibly the human health and the state of the entire biosphere [Vladimirsky et al., 1995]. A large number of publications has been dedicated to studies of ionospheric disturbances accompanying geomagnetic storms (see, e.g., reviews by Danilov and Morozova [1985], Prölss [1995], and Buonsanto [1999]). The accumulated material demonstrates a variety and complicated interaction of the processes forming storms. The latter fact makes each storm a unique event and due to this it is difficult to forecast the ionospheric disturbances. The peculiarity of ionospheric storms is their global character. They cover the entire ionosphere from high latitudes to the equator. However, the manifestation of their development depends on many geophysical factors. The complexity of physical processes and relative contribution of different physical mechanisms at different geographical regions lead to a large variety of the observed phenomena at different locations. The analysis of each storm provides valuable information for further studies in the solar-terrestrial physics and also for forecasting of the ionospheric response in the particular region to disturbances on the Sun.
[3] The goal of this paper is the comparative analysis of the peculiarities in the ionospheric behavior during three geomagnetic storms different by their development character and intensity. The observations have been carried out by the Kharkov incoherent scatter (IS) radar.
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