Long-term trends in the upper atmosphere and ionosphere: Models and observations

Theoretical models predict a 10 K cooling in the mesosphere and 50 K cooling in the thermosphere in response to doubling of CO2 and CH4 from present day mixing ratios. In the mesosphere this cooling is expected to bring-in considerable changes in the individual ion concentration, but no significant change in the total ion density. In the thermosphere, atmospheric density would decrease and the heights of the ionospheric E- and F2-layers will drop by about 2 and 20 km, respectively. There would be little change in the critical frequency of these layers but electron density will decrease in the topside and increase in the bottomside ionosphere due to this lowering. Early results from some individual ionosonde stations showed the predicted decrease in the height of the F2-peak, but statistical analysis of ionospheric data from stations spread all over the globe did not indicate any significant trend in this parameter, as well as in the height of the E-layer and density of the F2-layer. The E and F1 layers peak densities, however, showed negative trends. Satellite drag data have provided convincing evidence of decrease in atmospheric density in the thermosphere during the last few decades. Better statistical methods are needed to filter out long-term solar activity and magnetic activity influences for detecting long-term trends in the F2 layer. Measurements of low frequency reflection heights from 1959 to 2003 at the mid-latitude station Kuhlungsborn show a long-term decreasing trend, an observation in agreement with the expected cooling in the mesosphere