2. Used Methods of Observations, Additional Data

[5]  The mentioned radio path is a high-latitude (transauroral) one because its transmitting station, Heiss Island, is located in the polar cap area but its receiving station, St. Petersburg, is located in the subauroral area. The path is directed southward and has the length D = 2500 km. The reflection point of the path is situated at the geomagnetic latitude 67^o for quiet geomagnetic conditions. An OIS session is being carried out hourly. Two signal parameters, the MOF and the LOF, were being discovered at the receiving station. One is inclined to think that during magnetic storm occurred on 14-17 May 1997 the belt of irregularities is southward the auroral oval where precipitation is quite possible and ionization changes in the ionosphere exert some influence on the path.

[6]  For solution of the main goal of the paper, it is necessary to compare the geophysical data (riometer absorption by Finnish chain Sodankyla, Ouluyarvi and Yuvaskyula, AE index and B_z component of IMF) with the MOF and the LOF data on the path as well as with data of vertical ionospheric sounding at St. Petersburg and Sodankyla. Comparison of geophysical data to signal parameters, first, give the possibility to understand the physical phenomena, Second, such consideration enables to estimate the degree of distortions in the ionosphere structure during disturbances for solving Space Weather problems, and, thirdly, it contributes to diagnose and forecast such events.