[2] The ionospheric response to a geomagnetic disturbance is a complex set of events caused by both the upper atmosphere and ionosphere parameters and characteristics of the magnetosphere and solar wind. This response is a subject of many-year studies, the results being presented in numerous reviews [Buonsanto, 1999; Danilov and Lastovicka, 2001; Fuller-Rowell et al., 1996]. The theoretical and experimental studies of the ionosphere during magnetic storms made it possible to find the physical processes determining the electron concentration distribution in the ionosphere at various latitudes and to present the most general picture of an ionospheric storm manifestation. Changes in the neutral composition and system of neutral wind circulation are the most important factors determining ionospheric variations during a geomagnetic storm [Danilov and Belik, 1991; Prolss and Ocko, 2000; Reddy and Mayr, 1988; Rishbeth, 1998]. The experimental data and results of modeling [Forster et al., 1995; Mikhailov and Foster, 1997] show that during geomagnetic storms the ionosphere is enriched by molecular ions. The variations in the ratio of atomic oxygen concentration to molecular nitrogen concentration [O]/[N2] lead to the changes in the phase of an ionospheric storm. The seasonal variations in the ionospheric effects of storms and their dependence on local time are described with the help of the so-called "AC/DC" effect in the maximum of the F2 layer [Rodger et al., 1989; Wrenn et al., 1987]. Using the "disturbance index" determined as the logarithm of the ratio of the electron concentration in the layer maximum in disturbed conditions to the same value in quiet conditions, the authors showed that the seasonal variation or "DC splash" of the disturbance index is caused by the summer-to-winter asymmetry of the thermospheric wind. The "AC" variation during a magnetic storm is caused by the local time changes in the wind. Because of variations in winds and neutral composition, at middle latitudes the negative and positive effects of storms are observed more often in summer and winter, respectively [Field and Rishbeth, 1997; Rodger et al., 1989].
[3] Fuller-Rowell et al. [1994] noted that the ionospheric response to a geomagnetic disturbance in a particular place depends on both, local and universal time. A typical storm consists of an initial positive phase later changed to a negative phase. The duration and intensity of these two phases depend on latitude and season. Disagreement between the geographic and magnetic coordinates complicates the picture of ionospheric disturbances and leads to a longitudinal dependence of the ionospheric effects of geomagnetic storms [Afraimovich et al., 2002; Blagoveshchensky et al., 2003; Pirog et al., 2003; Zherebtsov et al., 2003]. In eastern Asia, the strongest deviation of geographic coordinates from geomagnetic coordinates is observed. The formation of the high-latitude large-scale structure of the ionosphere in this sector occurs on the background of relatively low electron concentration. The latter fact determines an increased interest to this region. Kurkin et al. [2004] studied the variations in the critical frequencies and index of ionospheric disturbances at the stations of Siberia and Far East during the main phases of geomagnetic storms in various seasons and periods of maxima and minima of solar activity. They found that during the main phase of a storm, negative disturbances of various intensities were observed in summer and fall independently of latitude and local time. In winter at high latitudes, intense positive and negative disturbances were observed at night and in the daytime, respectively. The amplitude of ionospheric disturbances was higher at lower solar activity.
[4] In this paper we present the results of a morphological analysis and numerical modeling of the ionospheric state during storms observed in different seasons at various latitudes of the eastern Asia region.
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