Re-examination of the Daily Number of Sunspot Groups for the Royal Observatory, Greenwich (1874 – 1885)

D M Willis,M N Wild,J S Warburton

doi:10.1007/s11207-016-0856-7

Abstract

The daily number of sunspot groups on the solar disk, as recorded by the programme of sunspot observations performed under the aegis of the Royal Observatory, Greenwich, UK, and subsequently the Royal Greenwich Observatory (RGO), is re-examined for the interval 1874 – 1885. The motivation for this re-examination is the key role that the RGO number of sunspot groups plays in the calculation of Group Sunspot Numbers (Hoyt and Schatten in Solar Phys. 179, 189, 1998a; Solar Phys. 181, 491, 1998b). A new dataset has been derived for the RGO daily number of sunspot groups in the interval 1874 – 1885. This new dataset attempts to achieve complete consistency between the sunspot data presented in the three main sections of the RGO publications and also incorporates all known errata and additions. It is argued that days for which no RGO solar photograph was acquired originally should be regarded, without exception, as being days without meaningful sunspot data. The daily number of sunspot groups that Hoyt and Schatten assign to days without RGO photographs is frequently just a lower limit. Moreover, in the absence of a solar photograph, the daily number of sunspot groups is inevitably uncertain because of the known frequent occurrence of sunspot groups that exist for just a single day. The elimination of days without photographs changes the list of inter-comparison days on which both the primary RGO observer and a specified secondary comparison observer saw at least one sunspot group. The resulting changes in the personal correction factors of secondary observers then change the personal correction factors of overlapping tertiary observers, etc. In this way, numerical changes in the personal correction factors of secondary observers propagate away from the interval 1874 – 1885, thereby potentially changing the arithmetical calculation of Group Sunspot Numbers over an appreciably wider time interval.

Highlights

An accurate measure of the varying levels of solar activity is a prerequisite in a wide range of scientific studies, including investigations of the solar dynamo (Charbonneau, 2010), space weather (Pulkkinen, 2007), space climate (Barnard et al, 2011), solar–terrestrial physics (Lockwood, 2013), global terrestrial climate change (Gray et al, 2010; Lockwood, 2012), and regional terrestrial climate change (Lockwood, 2012)
The daily number of sunspot groups on the solar disk (GRGO = G332), as determined by the programme of sunspot observations conducted under the aegis of the Royal Observatory, Greenwich, and subsequently the Royal Greenwich Observatory (RGO), has been reexamined for the interval 1874 – 1885
The motivation for such a re-examination is the key role that the RGO number of sunspot groups plays in the calculation of Group Sunspot Number [RG] in the procedure adopted by Hoyt and Schatten (Hoyt, Schatten, and Nesmes-Ribes, 1994; Hoyt and Schatten 1998a, 1998b)

Summary

Introduction

An accurate measure of the varying levels of solar activity is a prerequisite in a wide range of scientific studies, including investigations of the solar dynamo (Charbonneau, 2010), space weather (Pulkkinen, 2007), space climate (Barnard et al, 2011), solar–terrestrial physics (Lockwood, 2013), global terrestrial climate change (Gray et al, 2010; Lockwood, 2012), and regional terrestrial climate change (Lockwood, 2012). Until almost the start of the twenty-first century, the International Sunspot Number, referred to in the past as the Wolf or Zürich Sunspot Number, was the primary time series used to record the varying levels of solar activity (Clette et al, 2007). The International Sunspot Number, which is based on telescopic sunspot observations made by a large number of solar observers over the past four centuries, was for many years the longest quantitative record of solar activity (Waldmeier, 1961; McKinnon, 1987; Clette et al, 2007). The Hoyt and Schatten reconstruction of past solar activity was designed to be more self-consistent internally (i.e. less dependent upon seeing the tiniest spots) and less noisy than the International Sunspot Number. Much attention has been focused recently on attempts to reconcile the discrepancies between the International and Group Sunspot Numbers (Cliver, Clette, and Svalgaard, 2013; Clette et al, 2014; Cliver et al, 2015; Cliver and Ling, 2016)

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