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This work presents an empirical model for forecasting the hourly values of the ionospheric E-region critical frequency (foE) over Bulgaria in dependence of both: the index of the solar activity described by F10.7 and the cosine of solar zenith angle. The dependence of foE on solar activity and solar zenith angle is studied by correlation and regression analysis. The obtained relationships are used as a base for the synthesis of the model. The model is produced by using the hourly values of foE obtained in the iono...

The dependence of the relative deviation of the critical frequencies of the ionospheric F2- layer (foF2) and the total electron content (TEC) on geomagnetic disturbances was investigated based on data from an ionosonde station at Sofia (National Institute of Geophysics, Geodesy and Geography - Bulgarian Academy of Sciences) for the period of 1995-2014. The local time dependence is presented by a Fourier decomposition including four harmonics of the diurnal course. The average daily dependencies for each calendar m...