As a consequence of alterations of atmospheric chemical composition due to anthropogenic emissions, Earth's ionosphere and thermosphere are expected to change. A number of authors tried to detect signs of global change in their ionospheric data, but many findings remain controversial. We briefly review long-term trends observed in the critical frequencies of the ionospheric E and F2 layers as well as in the height of the F2-peak, i.e. the layer of maximum electron density. Using 48 years of F2-layer critical frequency data from Sodankyla, Finland, we demonstrate how the sign and amplitude of the detected trends depend upon the choice of model fitted to the data and suggest a method to choose the best possible model
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