Sunspot Numbers and Areas from the Madrid Astronomical Observatory (1876-1986)

TL;DR

This paper digitizes and analyzes a century-long dataset of sunspot numbers and areas from Madrid, revealing high data reliability and a notable ratio change between 1946 and 1972.

Contribution

It provides a comprehensive digitized dataset of Madrid sunspot observations and identifies a significant ratio change period, enhancing historical solar activity records.

Findings

High correlation with international sunspot indices

Reliable data over a century-long period

Detected a ratio change in sunspot observations from 1946 to 1972

Abstract

The solar program of the Astronomical Observatory of Madrid started in 1876. For ten solar cycles, observations were made in this institution to determine sunspot numbers and areas. The program was completed in 1986. The resulting data have been published in various Spanish scientific publications. The metadata allowed four periods of this program (with different observers and instruments) to be identified. In the present work, the published data were retrieved and digitized. Their subsequent analysis showed that most of these data can be considered reliable given their very high correlation with international reference indices (International Sunspot Number, Group Sunspot Number, and Sunspot Area). An abrupt change emerged in the spots/groups ratio in 1946 which lasted until 1972.