Reconstruction of the Sunspot Group Number: The Backbone Method

Abstract

We discuss recent papers very critical of our Group Sunspot Number Series (Svalgaard & Schatten [2016]). Unfortunately, we cannot support any of the concerns they raise. We first show that almost always there is simple proportionality between the group counts by different observers and that taking the small, occasional, non-linearities into account makes very little difference. Among other examples: we verify that the RGO group count was drifting the first twenty years of observations. We then show that our group count matches the diurnal variation of the geomagnetic field with high fidelity, and that the heliospheric magnetic field derived from geomagnetic data is consistent with our group number series. We evaluate the 'correction matrix' approach ] and show that it fails to reproduce the observational data. We clarify the notion of daisychaining and point out that our group number series has no daisy-chaining for the period 1794-1996 and therefore no accumulation of errors over that span. We compare with the cosmic ray record for the last 400+ years and find good agreement. We note that the Active Day Fraction method (of Usoskin et al.) has the fundamental problem that at sunspot maximum, every day is an 'active day' so ADF is nearly always unity and thus does not carry information about the statistics of high solar activity. This 'information shadow' occurs for even moderate group numbers and thus need to be extrapolated to higher activity. The ADF method also fails for 'equivalent observers' who should register the same group counts, but do not. We conclude that the criticism of Svalgaard & Schatten [2016] is invalid and detrimental to progress in the important field of long-term variation of solar activity.

Tests of Sunspot Number Sequences: 1. Using Ionosonde Data

“…The corrected sunspot numbers series [R C ] is given in the supplementary data to the article by , and the backbone sunspot group data [R BB ] were digitised from the article by Svalgaard and Schatten (2016) that accompanied the call for articles for this special issue. We employ the version of the RGO sunspot-group data made available by the Space Physics website of the Marshall Space Flight Center (MSFC), which has been compiled, maintained and corrected by D. Hathaway.…”

“…All long-term sunspot-number data sequences are therefore an observational composite: this is true of the much-used original Wolf/Zurich/International sunspot-number data sequence (version 1 of the International Sunspot Number, here termed R) as published by Solar Influences Data Analysis Center (SIDC, the solar physics research department of the Royal Observatory of Belgium) and hence of all sunspot series based on R with corrections for known or putative discontinuities, for example, the corrected sequence [R C ] suggested by . This is equally the case for the new (second) version of the Wolf/Zürich/International composite recently published by SIDC, the sunspot-group number [R G ] Schatten 1994, 1998), and the "backbone" group number data series [R BB ] proposed by Svalgaard and Schatten (2016).…”

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

“…Roughly, the SN reconstructions can be divided into 'high' and 'low'. The high reconstructions (Svalgaard & Schatten 2016) suggest that the level of sunspot activity, as compared to that in the late 20th century, was high during the 19th and 18th century, and moderate but not low during the Maunder minimum in the second half of the 17th century. The low reconstructions (e.g.…”

“…sidc.be/silso/groupnumberv3, responsible person F. Clette). The new GSN series, SvSc16, was constructed by Svalgaard & Schatten (2016), based on the HS98 data base using a modified k-factor ('backbone') calibration. Another GSN series R G , by L14, corrected for some proposed errors, was constructed by L14b.…”

“…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.…”