In this review the present knowledge of long-term solar activity on a multi-millennial timescale, as reconstructed using the indirect proxy method, is discussed.
Although the concept of solar activity is intuitively understandable as a deviation from the “quiet” sun concept, there is no clear definition for it, and different indices have been proposed to quantify different aspects of variable solar activity. One of the most common and practical indices is sunspot number, which forms the longest available series of direct scientific observations. While all other indices have a high correlation with sunspot numbers, dominated by the 11-year cycle, the relationship between them at other timescales (short and long-term trends) may vary to a great extent.
On longer timescales, quantitative information of past solar activity can only be obtained using the method based upon indirect proxy, i.e., quantitative parameters, which can be measured nowadays but represent the signatures, stored in natural archives, of the different effects of solar magnetic activity in the past. Such traceable signatures can be related to nuclear or chemical effects caused by cosmic rays in the Earth’s atmosphere, lunar rocks or meteorites. The most common proxy of solar activity is formed by data from the cosmogenic radionuclides, 10Be and 14C, produced by cosmic rays in the Earth’s atmosphere and stored in independently-dated stratified natural archives, such as tree rings or ice cores. Using a recently-developed physics-based model it is now possible to reconstruct the temporal behavior of solar activity in the past, over many millennia. The most robust results can be obtained for the Holocene epoch, which started more than 11,000 years ago, whose stable climate minimizes possible uncertainties in the reconstruction. An indirect verification of long-term solar-activity reconstructions supports their veracity and confirms that variations of cosmogenic nuclides on the long-term scale (centuries to millennia) during the Holocene make a solid basis for studies of solar variability in the past. However, such reconstructions may still contain systematic uncertainties related to unknown changes in the geomagnetic field or climate of the past, especially in the early part of the Holocene.
Measurements of nitrates in polar ice allow the reconstruction of strong solar energetic particle (SEP) events in the past, over the five past centuries. Together with independent measurements of the concentration of different cosmogenic isotopes in lunar and meteoritic rocks, it leads to estimates of the SEP flux on different timescales. Directly space-borne-measured SEP flux for recent decades is broadly consistent with estimates on longer timescales – up to millions of years, and the occurrence of extra-strong events is unlikely.
In general, the following main features are observed in the long-term evolution of solar magnetic activity.
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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