5. Conclusion and discussion
The properties of the solar activity were studied by means of the wavelet technique. Two typical time scales can be obtained in the wavelet analysis, the 11-year cycle and a 100-y cycle.
Furthermore, our analysis shows that there are three events over the last four centuries which show deviations from the normal activity state. One of them results in the well-known Maunder minimum which is the largest minimum observed so far on the Sun, while the other two show local deviations to a lesser degree. These smaller deviations can be compared with the solar activity observed at the end of the deep Maunder minimum (after 1690). Additional evidence of the recovery of solar activity at the end of Maunder minimum is visible in other properties of solar activity, namely the asymmetry of the butterfly diagram and the extension of its wings, the solar rotation rate (Ribes and Nesme-Ribes, 1993; Sokoloff and Nesme-Ribes, 1994). The abrupt transition from active cycles to a grand minimum and the recovering of solar activity have been investigated numerically in the framework of dynamo theory by Brandenburg et al. (1989, 1991).
Other events that show up in the wavelet analysis are the Dalton minimum and an event near 1900. Two of these minima have been mentioned by Ochadlick et al. (1993). However, they are much less visible in their wavelet analysis as they were using yearly sunspot numbers.
One important finding is the existence of two parts within the Maunder minimum: a deep minimum with no cyclic sunspot production, and the end of the Maunder minimum with a resumption of the Schwabe cycle. The transition between the stages is abrupt.
We studied the length-strength correlations of the 11-y cycle, and showed that the decrease in solar activity coincides with the negative derivative of the cycle length (T): the weaker the amplitude the longer the period.
The absence of sunspot cycle during the deep Maunder minimum raises the question of whether or not the dynamo mechanism was operating during this period. An 11-year periodicity was detected in the C14 data throughout the Maunder minimum (Stuiver and Braziunas, 1993) as well as by wavelet analysis of historical solar diameter data (Nesme-Ribes et al., 1995), thereby suggesting that the periodic dynamo was still at work. On the other hand, an oscillatory toroidal magnetic field embedded deep in the Sun can produce sunspots provided the field strength is large enough. So one possible explanation is that the toroidal magnetic field was too weak to create sunspots.
© European Southern Observatory (ESO) 1997
Online publication: March 26, 1998