The mesosphere and thermosphere are predicted to cool by 10 K and 50 K, respectively, for doubled concentrations of mesospheric carbon dioxide and methane. Several papers have been published thereafter trying to reveal long- term trends in a number of parameters which could be due to an ionospheric greenhouse effect. Besides temperature measurements by sounding rockets and lidars, the altitude variation of the ionospheric F2-layer peak as observed by ionosondes seems to be one of the most promising indirect approaches.
The F2-peak height (hmF2) is not normally scaled from ionograms, but it can be estimated by means of empirical formulae from the maximum usable frequency factor and the critical frequencies of the E and F2 layer. The resulting hmF2 time series is strongly correlated with solar activity; it also exhibits annual and semi-annual variations. A possible linear trend can be extracted by means of fitting a multi-parameter model, which includes a term for the linear trend as well as terms for other variations.
Here I will discuss how the trend estimate is influenced by the choice of the empirical formula for the hmF2 estimate. Furthermore, different multi-parameter models will be fitted to time series of F2-peak heights and the influence of the choice of the model on the trend magnitude will be studied.