Changes in the critical frequency foF2 and their interpretation

The results of the recent publication by Qiann and Mursula who have found an increase with time in the ratio of the modelled and observed values of the thermospheric density at satellite heights are considered. It is assumed that that increase is related to the existence of the negative trend in the density that is not described by the model properly. To confirm this concept, the change with time in the modelled and observed values of the F2-layer critical frequency foF2, foF2(mod)/foF2(obs) based on the observations at the Northern and Southern hemisphere stations is considered. It is shown that the same increase is observed for this ratio as for the ratio of densities. It is found that the rate of this increase in foF2 correlate well with the foF2 trends (in MHz/year) in winter months when the negative trends are small. In the winter months when the foF2 trends are small, there is almost no foF2(mod)/foF2(obs) increase. All that allows us to assume that the results of Qiann and Mursula show that the model incompletely describes the negative trends in the density.