Updated sunspot group number reconstruction for 1749-1996 using the active day fraction method

Abstract

Sunspot number series are composed from observations of hundreds of different observers that requires careful normalization of the observers to the standard conditions. Here we present a new normalized series of the number of sunspot groups for the period 1749-1996. The reconstruction is based on the active day fraction (ADF) method, which is slightly updated with respect to the previous works, and a revised database of sunspot group observations. Stability of some key solar observers has been evaluated against the composite series. The Royal Greenwich Observatory dataset appears fairly stable since the 1890s but is about 10% too low before that. A declining trend of 10-15% in the quality of Wolfer's observation is found between the 1880s and 1920s, suggesting that using him as the reference observer may lead to additional uncertainties. Wolf (small telescope) appears fairly stable between the 1860s and 1890s, without any obvious trend. The new reconstruction reflects the centennial variability of solar activity as evaluated using the singular spectrum analysis method. It depicts a highly significant feature of the Modern grand maximum of solar activity in the second half of the 20th century, being a factor 1.33-1.77 higher than during the 18-19th centuries. The new series of the sunspot group numbers with monthly and annual resolution, available also in the electronic format, is provided forming a basis for new studies of the solar variability and solar dynamo for the last 250 years.

Published in Astron. Astrophys

References in corpus (3)

• Level and length of cyclic solar activity during the Maunder minimum as deduced from the active day statistics
• Sunspot Numbers and Areas from the Madrid Astronomical Observatory (1876-1986)
• Dependence of the Sunspot-group Size on the Level of Solar Activity and its Influence on the Calibration of Solar Observers

Cited by in corpus (6)

• Thaddäus Derfflinger's sunspot observations during 1802-1824: A primary reference to understand the Dalton Minimum
• Robustness of Solar-Cycle Empirical Rules Across Different Series Including an Updated ADF Sunspot Group Series
• Solar-Cycle Characteristics in Kodaikanal Sunspot Area: North--South Asymmetry, Phase Distribution and Gnevyshev Gap
• Daniel Mögling's sunspot observations in 1626 - 1629: A manuscript reference for the solar activity before the Maunder Minimum
• Sunspot observations by Charles Malapert during the period 1618-1626: a key dataset to understand solar activity before the Maunder minimum
• Revisiting the Amplitude of Solar Cycle 9: The Case of Sunspot Observations by W.C. Bond