4.1 The overall activity level

Here we consider the 14C-based decade reconstruction of sunspot numbers (shown in Figure 17View Image). It is identical to that shown in Figure 14View Image, but includes also a Gleissberg 1-2-2-2-1 filter in order to suppress noise and short-term fluctuations. This series forms the basis for the forthcoming analysis, while differences related to the use of other reconstructions are discussed.

First, we analyze the distribution of the occurrence frequency of sunspot-number values. The histogram for sunspot-number distribution (for the series shown in Figure 17View Image) is shown in Figure 18View Image. The over-decades filtered sunspot numbers range between 0 and 95. The bulk of the distribution can be roughly approximated by a normal Gaussian distribution with a mean of 31 and standard deviation of σ = 30. However, there are apparent excesses both at very low (SN < 10) and high (SN > 70) sunspot numbers. These excesses are very unlikely to be a result of random fluctuations or noise in the data and, as argued in Section 4.3, correspond to special states of the solar dynamo, namely, the grand minima and grand maxima. It is important that the entire distribution is more-or-less consistent with the directly-observed sunspot series after 1610, suggesting that the latter can serve as a representative sample for sunspot-activity statistics, including a grand minimum (the Maunder minimum) and the modern maximum.

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Figure 17: Sunspot activity (over decades, smoothed with a 12221 filter) throughout the Holocene, reconstructed from 14C by Usoskin et al. (2007Jump To The Next Citation Point) using geomagnetic data by Yang et al. (2000). Blue and red areas denote grand minima and maxima, respectively.
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Figure 18: Histogram of sunspot-number distribution for the series shown in Figure 17View Image. Hatched areas correspond to directly-observed sunspots after 1610. The curve represents the best fit normal distribution.

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