Long-period variations of wind parameters in the mesopause region and the solar cycle dependence

A solar dependence of wind parameters below 100 km was found by Sprenger and Schminder on the basis of long-term continuous ionospheric drift measurements (D1) in the l.f. range. For winter they obtained for the prevailing wind a positive correlation with solar activity and for the amplitude of the semi-diurnal tidal wind a negative correlation. Later on this result was confirmed by radar meteor wind measurements (D2) at Obninsk and further D1 measurements at KÜhlungsborn and Collm.However, after the years 1973–1974 a change in the behaviour of the zonal prevailing wind was observed. At this time we found a significant negative correlation with solar activity with an indication of a new change after 1983. This was obtained from D1 results in Collm and D2 results in Kühlungsborn not only for winter, but also for summer and even for annual averages. We conclude that this long-term behaviour points rather to a climatic variation with an internal atmospheric cause than to a direct solar control. The negative correlation with solar activity of the semi-diurnal tidal wind in winter remained unchanged (up to 1984) and also proved to be the same in summer and for annual averages. Recent satellite data of the solar u.v. radiation and the upper stratospheric ozone have shown that the possible variation of the thermal tidal excitation during the solar cycle amounts to only a few per cent. This is, therefore, insufficient to account for the 40–70% variation of the tidal amplitudes. Some other possibilities of explaining this result are discussed.