Reconstruction of the Sunspot Number Source Database and the 1947 Zurich Discontinuity

TL;DR

This paper details the reconstruction of the historical sunspot number database from Zurich, revealing a major 1947 disruption linked to the Waldmeier jump, and discusses progress in recovering raw data for long-term solar activity analysis.

Contribution

It presents the full recovery of previously lost sunspot data from 1945 to 1979 and analyzes the 1947 disruption in the Zurich sunspot observations.

Findings

Recovered 34 years of lost sunspot data

Identified a major 1947 disruption in observation data

Linked the disruption to the Waldmeier jump in the series

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 $20^{th}$ century could play a role precisely at that time. We conclude on the remaining steps needed to fully complete this new sunspot data resource.