Abstract
Recently Clette et al. (Space Sci. Rev. 186, 35, 2014) completed the first revision of the international sunspot number SSN(V2) since its creation by Wolf in 1849 SSN(V1) starting in 1700 and ending in May 2015. The yearly values of SSN(V2) are larger than those of SSN(V1) but the secular trend in their timelines both exhibit a gradual descent after Cycle 21 minimum resulting in greatly reduced activity for Cycle 24. It has two peaks; one in 2012 due to activity in the north hemisphere (NH) and the other in 2014 due to excess activity in the south hemisphere (SH). The N–S excess of hemispheric SSNs is examined for 1950 – 2014, in relation to the time variations of the solar polar field for 1976 – 2015, covering five complete solar cycles (19 – 23) and parts of the bordering two (18, 24). We find that SH tends to become progressively more active in the declining phase of the cycles reaching an extreme value that gave rise to a second higher peak in October 2014 in the smoothed SSNs accompanied by a strong solar polar field in SH. There may be a Gleissberg cyclicity in the asymmetric solar dynamo operation. The continuing descent of the secular trend in SSNs implies that we may be near a Dalton-level grand minimum. The low activity spell may last well past 2060, accompanied by a stable but reduced level of the space weather/climate. Fourier spectrum of the time domain of SSNs shows no evidence of the 208 year/cycle (ypc) (DeVries/Suess cycle) seen in the cosmogenic radionuclide (\({}^{10}\mathrm{Be}\)) concentration in the polar ice cores and \({}^{14}\mathrm{C}\) record in trees indicating that 208 ypc peak may be of non-solar origin. It may arise from the climate process(es) that change(s) the way radionuclides are deposited on polar ice. It should be noted that we only have \({\sim}\,400~\mbox{years}\) of SSN data, so it is possible that DeVries/Suess cycle is really driven by the Sun but for now we do not have any evidence of that; there is no known physical process linking 208 ypc to solar dynamo operation.
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Acknowledgements
We thank Frédéric Clette for sharing the hemispheric sunspot number data, Laure Lefèvre for comments on the Greenwich photographic results starting in 1874, and the reviewer for two references and helpful comments.
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Ahluwalia, H.S., Ygbuhay, R.C. Salient Features of the New Sunspot Number Time Series. Sol Phys 291, 3807–3815 (2016). https://doi.org/10.1007/s11207-016-0996-9
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DOI: https://doi.org/10.1007/s11207-016-0996-9