What do we know of the long-term change of the Earth's ionosphere?

In this paper, long-term change of the Earth's ionosphere is discussed. It can most effectively be studied by means of ionospheric parameters recorded on ionograms obtained by vertical incidence sounding of the ionosphere. These measurements supply the longest ionospheric data series recorded by the greatest number of ionospheric stations. It has been found that in the F2 region long-term changes are due to geomagnetic activity and dynamical processes, from the 1970s effect of greenhouse gases is dominant [Mikhailov, A.V. Ionospheric long-term trends: can the geomagnetic control and the greenhouse hypothesis be reconciled? Ann. Geophys. 24, 2533-2541, 2006]. Spatial variation of hmF2 trends may be related to up- and downwelling at the neighbourhood of the boundary continent-sea. In the E and F1 regions, the sign of the foE, foF1 and h′E corrected trends, foF1 and foE increasing, h′E decreasing hint at greenhouse effect, which results in enhanced electron density and reduced height due to shrinking of the atmosphere as a consequence of cooling. Long-term changes in the lower ionosphere studied by LF phase height and ionospheric absorption measurements can be attributed to the effect of greenhouse gases [Bremer, J., Berger, U. Mesospheric temperature trends derived from ground-based LF phase-height observations at mid-latitudes: comparison with model simulations. J. Atmos. Solar-Terr. Phys. 64, 805-816, 2002].