Wavelet Analysis of the Quasi-27d Oscillations of Solar Index F10.7

By using the daily averages of F10.7 in 1956—2003 and adopting the method of Morelet wavelet transform, the characteristics of the quasi-27d oscillations and their relation with the 11-year cycle of solar activity are analyzed. The results demonstrate that the amplitude and period of the quasi-27d oscillations of F10.7 exhibit obvious phenomena of short-tern variations. The degrees of variations in various years differ greatly. In some years, the undulations are quite violent. In periods of several days to several tens of days, the change of amplitude may attain ten more times, and the period may change abruptly by several days or even ten more days. In some years, the changes of amplitude are rather large, yet the undulations are small and the period is comparatively steady. In the annual averages of the variation of quasi-27d oscillaions there appear evident changes from year to year, and this is conspicuouly related to solar activity. Generally speaking, the higher is F10.7, the larger is the amplitude of quasi-27d oscillations. Nevertheless, in the peak of the 19th cycle the values of F10.7 is higher than those of all the other cycles, yet the amplitude of quasi-27d oscillations is lower than those of other cycles. The period of quasi-27d oscillations also exhibits evident changes from year to year. Except certain individual years (e.g., 1987), the annual averages vary in the range from 24 to 31 days, and this has no evident relation with the period of solar activity. The mean period in 48 years is 27.3d. On the whole, the period shows a tendency of gradual shortening. In 48 years, it has decreased by about 1.5d. The causes of quasi-27d oscillations are very complicated, and this awaits further deepgoing investigation.