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Astron. Astrophys. 322, 311-319 (1997)

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6. Remarks and conclusions

The research of double peaks in solar activity maxima, starting from Gnevyshev's results, has been extended on a global scale (the photosphere, chromosphere and corona) to the on-going solar activity cycle. Results of our preliminary work on the subject suggest that:

1. structured maxima (with two or more peaks) generally occur in each activity cycle and are to be considered the result of dynamical effects superimposed on the quasi-periodic 11-year trend. The standard deviation of monthly averages with respect to the yearly averages is a good tool to stress the activity peaks in time series of data; in particular, the first intense peak is detected in the ascending phase of the cycle and the second one in the early years of the descending phase (Gnevyshev 1977).

2. The duration of each peak ([FORMULA] year) suggests that it can not be seen as the result of impulsive activity effects due to localised centers but a phenomenon which arises from a large-scale restructuration of the solar magnetic field and associated effects. We notice that Bieber & Rust (1995), evaluating the magnetic flux escape from the Sun, found that its characteristic time-scale is typically [FORMULA] 6 months (with annual averages included between 0.3 and 1.1 year).

3. The double-peak appearance concerns each solar layer, even if an analysis on a finer time scale is necessary to evaluate the synchronic or delayed occurrence of each peak, when moving from the lower to the upper atmosphere.

4. The bimodal behaviour of solar activity occurs separately in each solar hemisphere, i.e. double peaks can not be considered a result of anisotropic solar activity.

5. The double peak appearance is strongly related to the growing event importance (i.e. clusters of high energy and long-lived events) while low energy phenomena tend to follow an 11-year cycle.

6. An evident link between the occurrence of major solar events and the strength of the heliomagnetic field energy seems to exist.

The Gnevyshev gap has great importance, because a reduction of large-scale and intense dynamical phenomena are expected in such a period. Its role in solar activity forecasting could be very important and future studies must concentrate on clarifying its origin and time-length occurrence.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998