5.5 Long-term variations from radioisotope studies

The solar cycle modulation of cosmic rays (Section 3.8) leaves its imprint in the concentration of the radioisotopes 14C in tree rings and 10Be in ice cores (Section 3.9). The connection between solar activity and radioisotope concentrations is complicated by the transport and storage of these radioisotopes. Nonetheless, estimates of solar activity levels over time-scales much longer than the 400-year sunspot record can be obtained (see Usoskin, 2008, for a review).

These reconstructions of solar activity reveal Grand Minima like the Maunder Minimum as well as Grand Maxima similar to the last half of the 20th century. The reconstructions suggest that the Sun spends about 1/6th of its current life in a Grand Minimum phase and about 1/10th in a Grand Maximum.

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