4.4 Grand maxima of solar activity

4.4.1 The modern episode of active sun

We have been presently living in a period of very high sun activity with a level of activity that is unprecedentedly high for the last few centuries covered by direct solar observation. The sunspot number was growing rapidly between 1900 and 1940, with more than a doubling average group sunspot number, and has remained at that high level until recently (see Figure 1View Image). Note that growth comes entirely from raising the cycle maximum amplitude, while sunspot activity always returns to a very low level around solar cycle minima. While the average group sunspot number for the period 1750 – 1900 was 35 ± 9 (39 ± 6, if the Dalton minimum in 1797 – 1828 is not counted), it stands high at the level of 75 ± 3 since 1950. Therefore the modern active sun episode, which started in the 1940s, can be regarded as the modern grand maximum of solar activity, as opposed to a grand minimum (Wilson, 1988bJump To The Next Citation Point).

Is such high solar activity typical or is it something extraordinary? While it is broadly agreed that the present active sun episode is a special phenomenon, the question of how (a)typical such upward bumps are from “normal” activity is a topic of hot debate.

4.4.2 Grand maxima on a multi-millennial timescale

The question of how often grand maxima occur and how strong they are, cannot be studied using the 400-year-long series of direct observations. An increase in solar activity around 1200 AD, also related to the Medieval temperature optimum, is sometimes qualitatively regarded as a grand maximum (Wilson, 1988bde Meyer, 1998), but its magnitude is lower than the modern maximum (Usoskin et al., 2003cJump To The Next Citation Point). Accordingly, it was not included in a list of grand maxima by Eddy (1977a,b).

Table 2: Approximate dates (in –BC/AD) of grand maxima in the SN-L series. (after Usoskin et al., 2007Jump To The Next Citation Point).

No. center duration
1† 1960 80
2 –445 40
3 –1790 20
4 –2070 40
5 –2240 20
6 –2520 20
7 –3145 30
8 –6125 20
9 –6530 20
10 –6740 100
11 –6865 50
12 –7215 30
13 –7660 80
14 –7780 20
15 –7850 20
16 –8030 50
17 –8350 70
18 –8915 190
19 –9375 130

† Center and duration of the modern maximum are preliminary since it is still ongoing.

A quantitative analysis is only possible using proxy data, especially cosmogenic isotope records. Using a physics-based analysis of solar-activity series reconstructed from 10Be data from polar (Greenland and Antarctica) archives, Usoskin et al. (2003c2004) stated that the modern maximum is unique in the last millennium. Then, using a similar analysis of the 14C calibrated series, Solanki et al. (2004Jump To The Next Citation Point) found that the modern activity burst is not unique, but a very rare event, with the previous burst occurring about 8 millennia ago. An update (Usoskin et al., 2006a) of this result, using a more precise paleo-magnetic reconstruction by Korte and Constable (2005) since 5000 BC, suggests that an increase of solar activity comparable with the modern episode might have taken place around 2000 BC, i.e., around 4 millennia ago. The result by Solanki et al. (2004Jump To The Next Citation Point) has been disputed by Muscheler et al. (2005Jump To The Next Citation Point) who claimed that equally high (or even higher) solar-activity bursts occurred several times during the last millennium, circa 1200 AD, 1600 AD and at the end of the 19th century. We note that the latter claimed peak (ca. 1860) is not confirmed by direct solar or geomagnetic data. However, as argued by Solanki et al. (2005), the level of solar activity reconstructed by Muscheler et al. (2005) was overestimated because of an erroneous normalization to the data of ground-based ionization chambers (see also McCracken and Beer, 2007). This indicates that the definition of grand maxima is less robust than grand minima and is sensitive to other parameters such as geomagnetic field data or overall normalization.

Keeping possible uncertainties in mind, let us consider a list of the largest grand maxima (the 50 year smoothed sunspot number stably exceeding 50), identified for the last 11,400 years using 14C data, as shown in Table 2 (after Usoskin et al., 2007Jump To The Next Citation Point). A total of 19 grand maxima have been identified with a total duration of around 1030 years, suggesting that the sun spends around 10% of its time in an active state. A statistical analysis of grand-maxima–occurrence time suggests that they do not follow long-term cyclic variations, but like grand minima, are defined by stochastic/chaotic processes. The distribution of the waiting time between consecutive grand maxima is not as clear as that for grand minima, but also hints at a deviation from exponential law. The duration of grand maxima has a smooth distribution, which nearly exponentially decreases towards longer intervals. Most of the reconstructed grand maxima (about 75%) were not longer than 50 years, and only four grand minima (including the modern one) have been longer than 70 years. This suggest that the probability of the modern active-sun episode continuing is low5 (cf. Solanki et al., 2004Abreu et al., 2008).UpdateJump To The Next Update Information

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