Preliminary Results of Methodology for Forecasting the Minimum Virtual Height of Ionospheric F-Region for Sofia City

Using the virtual height for the calculation of the minimum and maximum usable frequency of a given radio path is well known. The purpose of the present investigation is to illustrate a methodology for determining the values of the minimum virtual height (h’F) of the ionospheric F-region. The dependence of the entire ionosphere on the level of solar activity is one of the main factors in the empirical modelling of the ionosphere. Correlation analysis was used to assess the relationship between the quantities. The results show a significant correlation, confirming the well-known relationship between solar activity and its impact on the ionosphere. In order to analyze the most appropriate dependence between the solar activity represented by F107 and h’F, a regression analysis was performed by month. The results show that the optimal dependence is close to linear. As a result, an empirical median model for forecasting h’F for the territory of Sofia city, depending on solar activity and season, is proposed. In its concluding part, this study also presents examples of the model's performance in selected months. The resulting errors are ME=0.000 km and RMS=8.180 km, respectively, which is accurate enough for practical purposes.