Physics of auroral phenomena : proceedings of the 34th Annual seminar, Apatity, 01 - 04 March, 2011 / [ed.: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2011. - 231 с. : ил.

"Physics o fAuroral Phenomena ", Proc. XXXIV Annual Seminar, Apatity, pp. 125 -1282011 © Kola Science Centre, Russian Academy of Science, 2011 Polar Geophysical Institute C LA S S IF ICA T IO N OF IONOSPHERIC D ISTURBANCES A T MIDDLE LAT ITUD ES BASED ON LONG -TERM OBSERVAT ION S O.M. Barkhatova ['2, A.E. Levitin3,1.A. Dodonova2, N.V. Kosolapova2 1. Nizhniy Novgorod State University o fArchitecture and Civil Engineering, Nizhniy Novgorod, Russia 2. Nizhniy Novgorod State Pedagogical University, Nizhniy Novgorod, Russia 3. Pushkov Institute o f Terrestrial Magnetism, Ionosphere and Propagation o fRadio Waves (IZMIRAN), Russian Academy o f Sciences, Troitsk, Russia Abstract. The foundations of ionospheric disturbances classification caused by Solar-geomagnetic activity on critical frequencies of the F2 are developed. To study the variation of ionospheric layer F2 critical frequency the data for the full cycle of Solar activity from 1975 to 1986 at the ionospheric observatory Moscow (55°45' N, 37°37' W) were used. The level of Solar activity was estimated by the values of the index F10.7. Geomagnetic disturbance was determined by indices Kp*10, Dst and AE. According to established classification the index of ionospheric activity is introduced. 1. Introduction Investigation of ionospheric disturbances, depending on the level of heliogeophysical activity is the present-day geophysical problem. One of the effective tools for diagnosing the state of ionosphere are critical frequencies, since they contain information about the electrons and ions concentration in the ionosphere. Among the studies that analyze the critical frequencies behavior in the different Solar and magnetospheric activity conditions should be mark out work on the classification of ionospheric disturbance and the creation of ionospheric weather index [Kutiev and Muchtarov, 2001; Bremer et al., 2006; Gulyaeva et al., 2008] . This problem is most topical because of universal index performance will allow to realize the operational control of ionospheric disturbances and forecast the state of the ionosphere under varying heliogeophysical conditions. This paper analyzes the critical frequency of ionospheric F2 layer over the station in Moscow a year minimum (1975) and maximum (1982) of Solar activity. This study main purpose is creation of a method for ionospheric mid­ latitude disturbances classification and subsequent development o f ionospheric activity index on the basis of the established classes. The advantage of this method is to use "cleaned" data of critical frequencies which include only ionospheric disturbances and it does not contain long-term variations (such as annual, seasonal variation, variation related to the Sun rotation, etc.). Also from consideration the daily variation of the critical frequency is excluded. However the proposed classification takes into account the level of ionospheric disturbances associated with the year cycle o f Solar activity and global geomagnetic disturbances. The provided classes thereupon can be considered universal. 2. Experimental data and their processing The study of ionospheric F2 layer critical frequency variations have been carried out on data for the full cycle of Solar activity from 1975 to 1986 obtained at the ionospheric station Moscow (55° 45' N, 37° 37' W) at 1 hour resolution. Omissions of critical frequencies were filled by cubic interpolation. To determine the level of Solar activity index F10.7 is used. The level of geomagnetic disturbance was defined by indices Kp*10, Dst and AE with 1 hour resolution. In order to identify ionospheric disturbances which are not directly related to seasonal and diurnal variations the input data of the critical frequency was cleared. For that the cleaning method of critical frequencies was developed based on spectral analysis and Fourier transform. The analysis was performed for time intervals up to a month. At the first stage o f cleaning were made critical frequency spectra based on fast Fourier transform. In the second stage was carried out a removing a number of harmonics with periods of 24 hours, 12 hours and more than 24 hours out of the critical frequency signal. For this purpose has been made the decomposition of original signal into its spectral components using the method of harmonic analysis, described in detail in [Yanovski, 1978, P. 389-393]. The final stage of cleaning was performed signal convolution —the sum of the remaining harmonics. As a result we obtain new values of "cleared" critical frequency (CCF) which contains only the variations directly related to ionospheric disturbances. Fig. 1 shows the spectra example of such CCF for January 1972. 125