Revisiting the Sunspot Number

Abstract

Abstract:Our knowledge of the long-term evolution of solar activity and of its primary modulation, the 11-year cycle, largely depends on a single direct observational record: the visual sunspot counts that retrace the last 4 centuries, since the invention of the astronomical telescope. Currently, this activity index is available in two main forms: the International Sunspot Number initiated by R. Wolf in 1849 and the Group Number constructed more recently by Hoyt and Schatten (1998a,b). Unfortunately, those two series do not match by various aspects, inducing confusions and contradictions when used in crucial contemporary studies of the solar dynamo or of the solar forcing on the Earth climate. Recently, new efforts have been undertaken to diagnose and correct flaws and biases affecting both sunspot series, in the framework of a series of dedicated Sunspot Number Workshops. Here, we present a global overview of our current understanding of the sunspot number calibration.After retracing the construction of those two composite series, we present the new concepts and methods used to self-consistently re-calibrate the original sunspot series. While the early part of the sunspot record before 1800 is still characterized by large uncertainties due to poorly observed periods, the more recent sunspot numbers are mainly affected by three main inhomogeneities: in 1880-1915 for the Group Number and in 1947 and 1980-2014 for the Sunspot Number.After establishing those new corrections, we then consider the implications on our knowledge of solar activity over the last 400 years. The newly corrected series clearly indicates a progressive decline of solar activity before the onset of the Maunder Minimum, while the slowly rising trend of the activity after the Maunder Minimum is strongly reduced, suggesting that by the mid 18 th century, solar activity had already returned to levels equivalent to those observed in recent solar cycles in the 20 th century. We finally conclude with future prospects opened by this epochal revision of the Sunspot Number, the first one since Wolf himself, and its reconciliation with the Group Number, a long-awaited modernization that will feed solar cycle research into the 21 st century.

Assessment of different sunspot number series using the cosmogenic isotope 44 Ti in meteorites

“…Another disadvantage of this series is that when Wolfer took it over from Wolf, he continued its compilation following different sunspot selection criteria, and instead of revisiting the entire reconstructed series by Wolf, he only introduced a scaling factor based on a comparison of his own and Wolf's observations over the period of the overlap (Clette et al 2014). Furthermore, Waldmeier also changed the method of constructing the SN series introducing a weighting factor for each individual sunspot based on their size.…”

“…Furthermore, Waldmeier also changed the method of constructing the SN series introducing a weighting factor for each individual sunspot based on their size. However, no full revision of the entire WSN series can be done, with only a scaling correction introduced (Clette et al 2014). Thus, the way this series was constructed makes it hardly verifiable and not always consistent.…”

“…At present, there are a handful of independent SN reconstructions, all differing in their assessments of the level of solar activity in the past (e.g. Hoyt & Schatten 1998;Clette et al 2007;Clette et al 2014;Lockwood, Owens & Barnard 2014b;Svalgaard & Schatten 2016;Usoskin et al 2016a;Friedli 2017). These SN series are based on a significant number of fragmentary observations by individual observers, each subject to different E-mail: ilya.usoskin@oulu.fi (IGU); m.j.owens@reading.ac.uk (MJO) processing techniques.…”

Level and length of cyclic solar activity during the Maunder minimum as deduced from the active-day statistics

“…Many of these records corresponded to solar meridian observations (Vaquero 2007;Clette et al 2014) and should be used with caution for the reconstruction of solar activity, as shown by , who analysed sunspot records taken during systematic solar meridian observations performed at the Royal Observatory of the Spanish Navy from 1833 to 1840. Moreover, as Carrasco et al (2015) suggest, based on an analysis of sunspot records by Hevelius in the 17th century, the GSN index may be underestimated during the MM owing to a large number of "zero" sunspot records taken from solar meridian observations.…”

“…Nevertheless, Hoyt & Schatten (1998a) adopted solar observations recorded in this source as no-spot reports, which is not correct. It has been discussed that, while the definition of sunspot numbers and even sunspot groups is not very reliable in the earlier part of the GSN Article published by EDP Sciences A71, page 1 of 6 series (Clette et al 2014;Zolotova & Ponyavin 2015), solar activity during the MM can be reliably represented by the fraction of active days (Kovaltsov et al 2004;Vaquero et al 2012Usoskin 2013). Despite the overall level of activity, the parameters of the solar cyclic variability during MM are also important to know.…”