Long-term trends in different ionospheric layers

The paper deals with investigations of long-term changes in different parts of the Earth's ionosphere to find answers for two questions: (1) Are there trends in the ionospheric plasma which may become important for practical applications? and (2) Can long-term changes in the ionosphere give hints to anthropogenic influences as e. g. an increasing atmospheric greenhouse effect? In the F2 layer, trends have been derived from worldwide ionosonde observations. The estimated global mean values of the foF2 and hmF2 trends are relatively small and can therefore be neglected for practical purposes. At individual stations, however, the ionospheric trends may be markedly stronger. The reason of the strong differences between the individual observations is quite unknown at the current time. In the lower ionospheric layers (Fl, E, and D), the derived trends of different ionospheric parameters are more consistent and can at least partly be explained by an increasing greenhouse effect.