Evolution of the Sunspot Number and Solar Wind \(B\) Time Series

Abstract

The past two decades have witnessed significant changes in our knowledge of long-term solar and solar wind activity. The sunspot number time series (1700-present) developed by Rudolf Wolf during the second half of the 19th century was revised and extended by the group sunspot number series (1610–1995) of Hoyt and Schatten during the 1990s. The group sunspot number is significantly lower than the Wolf series before ∼1885. An effort from 2011–2015 to understand and remove differences between these two series via a series of workshops had the unintended consequence of prompting several alternative constructions of the sunspot number. Thus it has been necessary to expand and extend the sunspot number reconciliation process. On the solar wind side, after a decade of controversy, an ISSI International Team used geomagnetic and sunspot data to obtain a high-confidence time series of the solar wind magnetic field strength (\(B\)) from 1750-present that can be compared with two independent long-term (> ∼600 year) series of annual \(B\)-values based on cosmogenic nuclides. In this paper, we trace the twists and turns leading to our current understanding of long-term solar and solar wind activity.

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Notes

  1. 1.

    The Wolf number is also referred to as the Zürich number because of the long period during which it was made in Switzerland, and, more recently, as the international sunspot number following the transfer of the curatorship to the Royal Observatory of Belgium (ROB) in 1981 (Clette et al. 2007; Stenflo 2016).

  2. 2.

    This practice may have been implemented on a trial basis before Waldmeier (Friedli 2016; Svalgaard et al. 2017) but the effect is not apparent until after 1946. The weighting was stopped with the publication of the new WSN (Clette et al. 2015). Thus the formula in Eq. (1) is correct again.

  3. 3.

    Lockwood et al. (1999) also employed the “Ulysses result” (Balogh et al. 1995; Smith and Balogh 1995; Lockwood et al. 2004; Lockwood and Owens 2009) that \(Br\) (normalized to 1 AU) is independent of latitude when averaged over a rotation cycle or more, thus permitting the determination of OSF from a point measurement of \(Br\).

  4. 4.

    For IDV, as well as IHV, restriction to night time hours removes contamination by the EUV-driven regular ionospheric variation.

  5. 5.

    Spörer (1412.0, 1420.0, 1421.0, 1436.0, 1442.1, 1448.4, 1450.3, 1451.1, 1560.1, 1461.9, 1531.6); Maunder (1681.0, 1692.7, 1693.6, 1694.6, 1696.0, 1697.0, 1697.8, 1698.7, 1704.6, 1705.9); Dalton (1809, 1810).

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Acknowledgements

E.W.C. thanks Daniel Baker, André Balogh, Tamás Gombosi, Hannu Koskinen, Rudolf von Steiger, and Astrid Veronig for organizing the ISSI Workshop on The Scientific Foundation of Space Weather and the invitation to speak on long-term solar variability. He is grateful to ISSI for supporting both this Workshop and, previously, the Svalgaard, Lockwood, and Beer International Team on Long-term reconstruction of Solar and Solar Wind Parameters. He thanks Leif Svalgaard for the translation of the Wolf (1856) article. K.H. thanks Jürg Beer, Ken McCracken, Bernd Heber, and Raimund Muscheler for their support and many insightful discussions over the past years.

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Cliver, E.W., Herbst, K. Evolution of the Sunspot Number and Solar Wind \(B\) Time Series. Space Sci Rev 214, 56 (2018). https://doi.org/10.1007/s11214-018-0487-4

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Keywords

  • Sunspot number
  • solar wind magnetic field strength