Abstract
The recalibration of the sunspot number series, the primary long-term record of the solar cycle, requires the recovery of the entire collection of raw sunspot counts collected by the Zurich Observatory for the production of this index between 1849 and 1980.Here, we report about the major progresses accomplished recently in the construction of this global digital sunspot number database, and we derive global statistics of all the individual observers and professional observatories who provided sunspot data over more than 130 years.First, we can announce the full recovery of long-lost source-data tables covering the last 34 years between 1945 and 1979, and we describe the unique information available in those tables. We then also retrace the evolution of the core observing team in Zurich and of the auxiliary stations. In 1947, we find a major disruption in the composition of both the Zurich team and the international network of auxiliary stations.This sharp transition is unique in the history of the Zurich Observatory and coincides with the main scale-jump found in the original Zurich sunspot number series, the so-called “Waldmeier” jump. This adds key historical evidence explaining why methodological changes introduced progressively in the early 20th century could play a role precisely at that time. We conclude on the remaining steps needed to fully complete this new sunspot data resource.
Highlights
Our knowledge of the long-term evolution of the solar cycle is largely based on the historical observations of sunspots since the newly invented telescope was aimed at the Sun for the first time in 1610
The Zurich sunspot number produced by Wolf and his successors is based on three types of data: i) the raw counts from the Zurich staff: essentially, the director and the assistants in Zurich, and in the course of the 20th century, other assistants stationed in the Arosa and Locarno observatories in southern Switzerland
Waldmeier started collaborating with the Zurich Observatory in 1936, he did not contribute during three years, from 1939 to 1941 because of the onset of World War II
Summary
Our knowledge of the long-term evolution of the solar cycle is largely based on the historical observations of sunspots since the newly invented telescope was aimed at the Sun for the first time in 1610. Page 3 of 19 137 to a huge amount of detailed information and to essential metadata The recovery of this vast collection can feed full statistical analyses by current state-of-the-art methods and lead to an improved index, independent of all assumptions and practices adopted over the years by Wolf and his successors at the Observatory of Zurich. This is what motivated a collective effort to recover and digitize all those original source data.
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