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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...