University of Oulu

Willamo, T.; Usoskin, I. G.; Kovaltsov, G. A. Updated sunspot group number reconstruction for 1749–1996 using the active day fraction method. A&A 601, A109 (2017) DOI: 10.1051/0004-6361/201629839

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

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Author: Willamo, T.1; Usoskin, I. G.2,3; Kovaltsov, G. A.4
Organizations: 1Department of Physics, University of Helsinki, 00014 Helsinki, Finland
2Space Climate Research Unit, University of Oulu, 90014 Oulu, Finland
3Sodankylä Geophysical Observatory, University of Oulu, 90014 Oulu, Finland
4Ioffe Physical-Technical Institute, 194021 St. Petersburg, Russia
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 1.2 MB)
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Language: English
Published: EDP Sciences, 2017
Publish Date: 2017-05-18


Aims: Sunspot number series are composed from observations of hundreds of different observers that require careful normalization to standard conditions. Here we present a new normalized series of the number of sunspot groups for the period 1749–1996.

Methods: The reconstruction is based on the active day fraction (ADF) method, which is slightly updated with respect to previous works, and a revised database of sunspot group observations.

Results: Stability of some key solar observers has been evaluated against the composite series. The Royal Greenwich Observatory dataset appears relatively stable since the 1890s but is approximately 10% too low before that. A declining trend of 10–15% in the quality of Wolfer’s observations is found between the 1880s and 1920s, suggesting that using him as the reference observer may lead to additional uncertainties. Wolf (small telescope) appears relatively 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 and 19th centuries.

Conclusions: The new series of the sunspot group numbers with monthly and annual resolution is provided forming a basis for new studies of the solar variability and solar dynamo for the last 250 yr.

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Series: Astronomy and astrophysics
ISSN: 0004-6361
ISSN-E: 1432-0746
ISSN-L: 0004-6361
Volume: 601
Article number: A109
DOI: 10.1051/0004-6361/201629839
Type of Publication: A1 Journal article – refereed
Field of Science: 115 Astronomy and space science
Funding: I.G.U. and G.A.K. acknowledge support by the Academy of Finland to the ReSoLVE Center of Excellence (project No. 272157).
Academy of Finland Grant Number: 272157
Detailed Information: 272157 (Academy of Finland Funding decision)
Copyright information: © ESO 2017. Published in this repository with the kind permission of the publisher.