[2] For the creation of a dynamical model of the lower ionosphere a detailed study of spatial-time variations in the ionospheric D region caused by disturbances of various nature is needed. Chromospheric solar flares are among the most important natural sources of disturbances [Mitra, 1974]. In the ground-based observations, solar flares are manifested as a result of pulse ionizing impact on the atmosphere of the Earth: splashes of X-ray and ultraviolet emissions, cosmic ray fluxes, subrelativistic protons of the polar cap, and auroral electrons. Their interaction to the atmosphere leads to a series of effects: sudden increase of the electron concentration in the lower ionosphere, changes in the D-region structure, increase in the absorption of MF and HF radio waves, and others [Al'pert, 1972; Belikovich et al., 1975; Garmash et al. 1999; Mitra, 1974]. Occurrence of the flare is detected on the basis of such radio events as solar radio emission bursts, sudden phase anomaly (SPA) at a very low frequency, sudden enhancement in atmospherics (SEA) and sudden absorption of the sudden cosmic noise absorption (SCNA) [Davies, 1990; Hargreaves, 1995; Hunsucker and Hargreaves, 2003].
[3] Studies of these effects provide information on the main physical and chemical processes occurring in the ionosphere under the influence of the ionizing radiation of solar flares. For example, measurements of the electron concentration vertical profiles and fluxes of X-ray radiation from solar flares make it possible to find the effective recombination coefficient [Belikovich and Itkina, 1972; Belikovich et al., 1976; Hunsucker and Hargreaves, 2003; Mitra, 1974]. The analysis of the simultaneous measurements of the effective loss rate of electrons and the electron concentration observed experimentally during flares led to the conclusion on the significant role of cluster ions in the loss processes [Danilov, 1989; Mitra, 1974]. However, there are still very few such measurements during flares, especially at high altitudes. Moreover, every solar flare presents a unique event. Therefore, in order to increase the reliability of the available information on the main parameters and processes in the lower ionosphere an increase in the observations is needed. The goal of this paper is studying of the reaction of the polar lower ionosphere to solar flares of the M class using the data of the partial reflection installation of the Polar Geophysical Institute.
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