This topical issue is broken into five sections:
Historical Context and Data (8 articles)
New Time Series (5 articles)
Critiques (9 articles)
Correlates (9 articles)
Modern Sunspot Observations (5 articles)
In Section i), Stenflo (2016) gives a fascinating personal account of the circumstances attending the transfer of the responsibility for constructing the official sunspot number from Zürich to Brussels in 1980. The next seven articles deal with historical sunspot observations. Arlt and Vaquero are the acknowledged leaders in this field. Arlt and collaborators have digitized and analyzed the sunspot records of Staudach (Arlt 2008), Schwabe (Arlt 2011; Arlt et al.
2013), and Spörer (Diercke, Arlt, and Denker 2015). In this section, Vaquero and colleagues have contributions on the sunspot observations of Flamsteed during the Maunder Minimum (Carrasco and Vaquero 2016), as well as those from the Ebro Observatory from 1910 – 2014 (Curto et al.
2016), and by Aguilar for 1914 – 1920 (Lefèvre et al.
2016). Willis, Wild, and Warburton (2016) and Willis et al. (2016) examine the records for the early years (1874 – 1885) of the Royal Greenwich Observatory sunspot patrol in detail, as does Friedli (2016) for Rudolf Wolf’s (1849 – 1876) sunspot observations. Svalgaard (2017) argues that modern group-splitting will increase Staudach’s 1749 – 1799 sunspot group counts by 25 % on average.
Section ii) contains the new time series of Clette and Lefèvre (2016) and Svalgaard and Schatten (2016) as well as the independently derived series of Usoskin et al. (2016) and Friedli (2016, 2017). While the Clette and Lefèvre (2016) and Svalgaard and Schatten series hew more closely to the original WSN than do the Hoyt and Schatten (1998a, 1998b) GSN, the series of Usoskin et al. (2016) and Friedli (2017), as well as that of Lockwood, Owens, and Barnard (2014) and Lockwood et al. (2016b, 2016d), are more closely aligned with the GSN. Also in this section, Dudok de Wit, Lefèvre, and Clette (2016) provide a first determination of uncertainties in the WSN series, with new insights about the temporal variations of these uncertainties.
Section iii) contains critiques and corrections of the various new and old sunspot-number time series, mostly focusing on specific inhomogeneities and time intervals. Clette et al. (2016) report on the effect of the irregular drift in the sunspot counts of the Locarno reference station that began in 1982 shortly after the transition from Zürich to Brussels. Cliver and Ling (2016) examine the original Hoyt and Schatten (1998a, 1998b) time series and conclude that the lower values of the GSN relative to the WSN before \(\approx 1885\) are primarily due to an inhomogeneity from 1874 – 1915 in the Royal Greenwich Observatory (RGO) record of sunspot groups that Hoyt and Schatten used as their reference observer (cf. Willis et al.
2016). During the sunspot-number workshops, Svalgaard for the first time brought the sunspot weighting that was apparently instituted by Waldmeier in 1947 to the general attention of the solar community. Svalgaard, Cagnotti, and Cortesi (2017) assess the effect of that weighting on the post-1946 WSN series. In a series of articles in this topical issue (Lockwood, Owens, and Barnard 2016; Lockwood et al.
2016a, 2016b, 2016c) and elsewhere (Lockwood et al.
2016d), Lockwood and colleagues i) use comparisons with ionospheric, geomagnetic, and auroral data to question the amplitude of the 1947 scale jump in the WSN time series, and ii) challenge the validity of the standard-observer normalization procedure used by Svalgaard and Schatten (2016) and also by Hoyt and Schatten (1998a, 1998b), which relies on linear-regression comparisons of secondary observers with a standard observer based on the complete interval of observer overlap (see also Usoskin, Kovaltsov, and Chatzistergos 2016). Zolotova and Ponyavin (2016)Footnote 1 question the extreme low values of the GSN during the Maunder Minimum (cf. Usoskin et al.
2015). In a comparison of several of the new and old sunspot-number time series, Cliver (2016) points out incongruities in the normalization scheme of the Usoskin et al. (2016) GSN series and a lack of fidelity between that SN series and a recent construction of the long-term solar-wind magnetic-field strength (Owens et al.
Section iv) contains correlations between sunspot-number time series and other solar-activity measures. These include sunspot area (Li et al.
2016; Carrasco et al.
2016a; Muraközy, Baranyi, and Ludmány 2016), total solar irradiance (Kopp et al.
2016), Ca ii K index (Bertello et al.
2016), extreme ultra-violet flux (Svalgaard 2016), X-ray flare and CME rates (Winter, Pernak, and Balasubramaniam 2016), solar-modulation potential derived from cosmogenic nuclides (Muscheler et al.
2016), and solar-cycle timing characteristics (Carrasco et al.
Section v) contains articles on modern sunspot-number observations. The first article in this section, by Vaquero et al. (2016), represents one of the key outputs of the sunspot-number workshops: the revised Hoyt and Schatten (1998a, 1998b) group sunspot number data base (1610 – present). This section also contains valuable reports from the long-standing observatories at Locarno (since 1957; Cortesi et al.
2016), Debrecen (since 1958; Baranyi, Gyori, and Ludmány 2016), and Kanzelhöhe (since 1943; Pötzi et al.
2016), as well as a description of the ≈ ten-year record (2003 – 2012) of sunspot numbers obtained by the ISOON telescope (Balasubramaniam and Henry 2016).
A list of all of the articles in this topical issue, organized by section, is given in the Table of Contents section of the References.