1. Introduction

[2]  Manifestations of perturbations of magnetospheric origin in the Earth's ionosphere have extremely complex character. Magnetospheric storms and substorms cause some drastic disturbances of the ionospheric structure creating the difficulties for information transmission through HF radio channels especially in the high latitudes [Angling et al., 1998; Blagoveshchensky and Borisova, 2000; Blagoveshchensky and Rodger, 1999; Blagoveshchensky et al., 1992; Milan et al., 1998; Wagner et al., 1995]. There are not enough statistical experimental data on the radio paths to predict any disturbed conditions. Also, the ionospheric models and calculations of propagation (ray tracing) are not corrected during disturbed periods.

[3]  The aims of this paper are (1) consideration of features of variations of the operational frequency range on the HF radio path of the oblique ionosphere sounding (OIS) Heiss Island-St. Petersburg during the great magnetic storm on 14-17 May 1997; (2) estimation of the ionospheric parameters (for example, f_oF2 or f_oE_s ) by the OIS data during a magnetic storm period; and (3) analysis of variations of the frequency range during a great disturbance that could enable to reveal the character of signal passing through a channel for radio communication needs.

[4]  The principal cause to formulate the main tasks of the paper in the mentioned form is that the OIS method allows to observe the variations of the ionospheric F2 layer concentration and other parameters during disturbances due to a magnetic storm practically without losses. In contrast, this information cannot be obtained by the vertical ionospheric sounding (VIS) data because it is known from sounding practice that a sharp decrease of the VIS data amount takes place during disturbed conditions [Milan et al., 1997]. The ionospheric parameters can be estimated just indirectly by the use of the frequency range. The highest limit of the range, the MOF (the maximum observed frequency) characterizes during daytime the degree of the ionospheric F2 layer ionization. The larger the MOF is the higher the ionization in the F2 layer is. At night, particularly during disturbed periods, the MOF can be determined by the sporadic E layer ionization [Hunsucker et al., 1996]. On the contrary, the lower limit of the range, the LOF (the lowest observed frequency) characterizes the degree of absorption in the low ionosphere. The higher the LOF is the higher is the concentration in the ionospheric D layer (in the daytime) or the absorption level in the low ionosphere (at night) [Hunsucker, 1992].