A 22-YEAR CYCLE IN THE F-LAYER IONIZATION OF THE IONOSPHERE

The double-sunspot-cycle variation in terrestrial magnetic activity ha s been well known for about 30 years. In 1990 we examined and compared the low-solar-activity (LSA) part of two consecutive cycles and predi cted from this database and from published results the existence of a double-sunspot-cycle variation in total electron content (TEC) of the ionosphere too. This is restricted to noontime when the semi-annual co mponent is well developed. Since 1995 we have had enough data for the statistical processing for high-solar-activity (HSA) conditions of two successive solar cycles. The results confirm the LSA findings. The an nual variation of TEC shows a change from an autumn maximum in cycle 2 1 to a spring maximum during the next solar cycle. Similar to the aa i ndices for geomagnetic activity the TEC data show a phase change in th e 1-year component of the Fourier transform of the annual variation. A dditionally we found the same behaviour in the F-layer peak electron d ensity (N-max) over four solar cycles. This indicates that there exist s a double-sunspot-cycle Variation in the F-layer ionization over Euro pe too. It is very likely coupled with the 22-year cycle in geomagneti c activity.