## Abstract

The hourly values of the *F*-layer critical frequency from the ionospheric sounder in Dourbes (50.1°N, 4.6°E) during the time interval from 1957 to 2010, comprising five solar cycles, were analyzed for the effects of the solar activity. The hourly time series were reduced to hourly monthly medians which in turn were used for fitting a single station *foF*_{2} monthly median model. Two functional approaches have been investigated: a statistical approach and a spectral approach. The solar flux *F*_{10.7} is used to model the dependence of *foF*_{2} on the solar activity and is incorporated into both models by a polynomial expression. The statistical model employs polynomial functions to fit the *F*-layer critical frequency while the spectral model is based on spectral decomposition of the measured data and offers a better physical interpretation of the fitting parameters. The daytime and nighttime *foF*_{2} values calculated by both approaches are compared during high and low solar activity. In general, the statistical model has a slightly lower uncertainty at the expense of the larger number of fitting parameters. However, the spectral approach is superior for modeling the periodic effects and performs better when comparing the results for high and low solar activity. Comparison with the International Reference Ionosphere (IRI 2012) shows that both local models are better at describing the local values of the *F*-layer critical frequency.

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