4.6 Summary

In this section, solar activity on a longer scale is discussed, based on recent reconstructions.

According to these reconstructions, the sun has spent about 70% of its time during the Holocene, which is ongoing, in a normal state characterized by medium solar activity. About 15 – 20% of the time the sun has experienced a grand minimum, while 10 – 15% of the time has been taken up by periods of very high activity.

One of the main features of long-term solar activity is its irregular behavior, which cannot be described by a combination of quasi-periodic processes as it includes an essentially random component.

Grand minima, whose typical representative is the Maunder minimum of the late 17th century, are typical solar phenomena. A total of 27 grand minima have been identified in reconstructions of the Holocene period. Their occurrence suggests that they appear not periodically, but rather as the result of a chaotic process within clusters separated by 2000 – 2500 years. Grand minima tend to be of two distinct types: short (Maunder-like) and longer (Spörer-like). The appearance of grand minima can be reproduced by modern stochastic-driven dynamo models to some extent, but some problems still remain to be resolved.

The modern level of solar activity (after the 1940s) is very high, corresponding to a grand maximum, which are typical but rare and irregularly-spaced events in solar behavior. The duration of grand maxima resembles a random Possion-like process, in contrast to grand minima.

These observational features of the long-term behavior of solar activity have important implications, especially for the development of theoretical solar-dynamo models and for solar-terrestrial studies.

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