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.

Tests of Sunspot Number Sequences: 1. Using Ionosonde Data

“…The fact that R BB matches the best-fit ionospheric data better than the other series after the Waldmeier discontinuity reveals a very important implication. This improvement is possible because R BB corrects for a drift in the k-values for the Locarno Wolf numbers (Clette et al, 2015). This drift was found by research aimed at explaining why the relationship between the F 10.7 radio flux and international sunspot number (Johnson, 2011) broke down so dramatically just after the long and low activity minimum between Cycles 23 and 24.…”

“…We note that k-values for different observers can differ by a factor as large as three (Clette et al, 2015), therefore it is absolutely essential to accurately estimate k to derive the accurate sunspot number. To extend the data series to times before those when both N G and N S were recorded systematically, Schatten (1994, 1998) defined the group sunspot number R G to be…”

“…Thus this would imply a 41.5 % drift in the calibration of R in just three solar cycles. Clette et al (2015) used the R-foF2 plots of the Washington data for Cycles 17 and 18, as published by Ostrow and PoKempner (1952), to attribute all of the change between them to the Waldmeier discontinuity in R and, indeed, this will have made some contribution.…”

Tests of Sunspot Number Sequences: 4. Discontinuities Around 1946 in Various Sunspot Number and Sunspot-Group-Number Reconstructions

“…Like R BB , this uses daisy-chaining of calibrations and, also like R BB , gives higher values for the eighteenth and nineteenth centuries (Clette et al, 2015). A less "root-and-branch" approach to correcting R ISNv1 was taken by Lockwood, Owens, and Barnard (2014a), who made simple corrections for errors to generate a "corrected" series [R C ].…”

“…A less "root-and-branch" approach to correcting R ISNv1 was taken by Lockwood, Owens, and Barnard (2014a), who made simple corrections for errors to generate a "corrected" series [R C ]. It should be noted that because R C makes corrections at only two dates in the series, other errors in R ISNv1 , such as the recently revealed error in modern data that is due to the drift in the Locarno standard (Clette et al, 2015), are carried forward and not corrected.…”