Trends in the thermosphere derived from global ionosonde observations Original Research Article

Long-term ionospheric data series have been used to derive trends in the E region (foE and h′E), in the F1 region (foF1), and in the F2 region (foF2, hmF2). To detect these small trends from ionospheric data the solar and geomagnetically induced variations have carefully to be eliminated. Different methodical approaches have been developed and are discussed in this paper. The trends in the E layer (decrease of h′E and increase of foE) and in the F1 layer (increase of foF1) are in general agreement with model predictions assuming an increasing atmospheric greenhouse effect. Especially in the E layer the experimental trends are, however, markedly stronger than the theoretical values. In the F2 layer the scatter of the derived experimental trends for the different stations is rather high and no significant mean global trends could be estimated.