Advances in Radio Science www.adv-radio-sci.net 1684-9965 1684-9973 2 Kleinheubacher Berichte 2003 2004 10.5194/ars-2-253-2004 http://www.adv-radio-sci.net/2/253/2004/ http://www.adv-radio-sci.net/2/253/2004/ars-2-253-2004.html http://www.adv-radio-sci.net/2/253/2004/ars-2-253-2004.pdf 253 258 2005-05-27 Investigations of long-term trends in the ionosphere with world-wide ionosonde observations* J. Bremer Leibniz-Institut für Atmosphärenphysik, Schloss-Str.6, D-18225 Kühlungsborn, Germany Dedicated to Prof. Dr. K. Rawer on the occasion of his 90th birthday. Basing on model calculations by Roble and Dickinson (1989) for an increasing content of atmospheric greenhouse gases in the Earth’s atmosphere Rishbeth (1990) predicted a lowering of the ionospheric F2- and E-regions. Later Rishbeth and Roble (1992) also predicted characteristic longterm changes of the maximum electron density values of the ionospheric E-, F1-, and F2-layers. Long-term observations at more than 100 ionosonde stations have been analyzed to test these model predictions. In the E- and F1-layers the derived experimental results agree reasonably with the model trends (lowering of <i>h</I>'E and increase of ƒ<i>o</i>E and ƒ<i>o</i>F1, in the E-layer the experimental values are however markedly stronger than the model data). In the ionospheric F2-region the variability of the trends derived at the different individual stations for <i>hm</i>F2 as well as ƒ<i>o</i>F2 values is too large to estimate reasonable global mean trends. The reason of the large differences between the individual trends is not quite clear. Strong dynamical effects may play an important role in the F2-region. But also inhomogeneous data series due to technical changes as well as changes in the evaluation algorithms used during the long observation periods may influence the trend analyses.