Revisiting the Sunspot Number

By F. Clette, L. Svalgaard, J. Vaquero, E. Cliver. 2014

397 citations recorded by the source corpus.

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 (Sol. Phys. 179:189–219, 1998a, 181:491–512, 1998b). 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 18th century, solar activity had already returned to levels equivalent to those observed in recent solar cycles in the 20th 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 21st century.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/1f65b27a970a7016e267a754e68456177f80b7db

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Citations in the corpus

The Maunder minimum (1645–1715) was indeed a grand minimum: A reassessment of multiple datasets (2015, background): “However, the exact level of solar activity in the 17th century remains somewhat uncertain (e.g. Vaquero & Vázquez 2009; Vaquero et al. 2011; Clette et al. 2014), leaving room for discussion and speculation.”
IRI the International Standard for the Ionosphere (2003, methodology): “A recent review of the way R is determined from observations of spots and groups of spots on the Sun resulted in a recalibration of the R computation (Clette et al., 2014).”
A Revised Collection of Sunspot Group Numbers (2016, methodology): “This database includes all observations collected on a monthly basis from the worldwide network coordinated by the WDC–SILSO, for a current total of more than 530,000 individual daily observations.”
Progress in Solar Cycle Predictions: Sunspot Cycles 24–25 in Perspective (2020, methodology): “…revised sunspot time series but have left the numbers unchanged for predictions published in the year 2016 or thereafter; i.e., we have assumed that predictions published in the year 2016 and thereafter have been calibrated with the new sunspot time series released in 2015 (Clette et al., 2015).”
A New Calibrated Sunspot Group Series Since 1749: Statistics of Active Day Fractions (2015, methodology): “…is a composite series based on observations of the Sun by a large number of individual observers with instruments of different quality and different techniques, it is always a problem to produce a homogeneous series, which requires an inter-calibration of the observers (Clette et al. , 2014).”
Solar Forcing for CMIP6 (v3.1) (2016, methodology): “For the CMIP6 composite, we decided to rely on version 1.0 of the international sunspot number (from http://www.sidc.be/silso), even though a newer version 2.0 recently came out (Clette et al., 2014).”
International Reference Ionosphere 2016: From ionospheric climate to real‐time weather predictions (2017, methodology): “A recent review of the way R is determined from observations of spots on the Sun resulted in a recalibration of the R computation [Clette et al., 2014].”
Reconstruction of the Sunspot Group Number: The Backbone Method (2015, methodology): “The present paper constitutes publication in the open literature of the details of that method, and should be viewed as a step on the road to the long-needed and epoch-making recalibration of the venerable sunspot number ‘distance scale’ (Cliver et al., 2013, Clette et al., 2014).”
Solar activity during the Holocene: the Hallstatt cycle and its consequence for grand minima and maxima (2016, background): “6 the comparison of the sunspot number series for the period 1600 – 1900 AD reconstructed here from 14C with other series based on sunspot counts and drawings (Hoyt & Schatten 1998; Lockwood et al. 2014; Clette et al. 2014; Svalgaard & Schatten 2015).”