RELATIONSHIP BETWEEN THE CRITICAL FREQUENCIES OF THE IONOSPHERE OVER BULGARIA AND GEOMAGNETIC ACTIVITY

The present study represents the functional dependence of the critical frequency of the ionospheric F2-layer on the geomagnetic activity obtained by regression analysis. The data for both ionospheric parameters foF2 and MUF300 are derived from the ionosonde station Sofia (National Institute of Geophysics, Geodesy and Geography, Bulgarian Academy of Sciences) for the period of 1995-2014. The geomagnetic activity is represented by the planetary Kp-index provided by the NOAA. Some examples determining the use of the filtered values of Kp and the relative deviation of the critical frequencies of the ionosphere are shown in this study. The obtained results reveal that during the winter months the response is weaker than that in the summer and equinoxes. The dependencies turn out to be substantially nonlinear, requiring a third-degree polynomial approximation. It is noted that the range of the geomagnetic activity changes is different during each month of the year which in some cases does not allow reliable mean response values to be obtained particularly for strong geomagnetic disturbances. The differences obtained between the ionospheric responses in day and night conditions indicate the need an empirical model for predicting the state of the ionosphere to be synthesized separately for day and night conditions. All these results can be used for establishing an empirical model intended to predict the state of the ionosphere over Bulgaria.