University of Oulu

Willamo, T., Usoskin, I.G. & Kovaltsov, G.A. Sol Phys (2018) 293: 69.

A test of the active-day fraction method of sunspot group number calibration : dependence on the level of solar activity

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Author: Willamo, T.1; Usoskin, I.G.2,3; Kovaltsov, G.A.4,2
Organizations: 1Department of Physics, University of Helsinki, 00014 Helsinki, Finland
2Space Climate Research Unit, University of Oulu, Oulu, Finland
3Sodankylä Geophysical Observatory, University of Oulu, Oulu, Finland
4Ioffe Physical-Technical Institute, St. Petersburg, Russia
Format: article
Version: accepted version
Access: open
Online Access: PDF Full Text (PDF, 0.1 MB)
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Language: English
Published: Springer Nature, 2018
Publish Date: 2019-04-12


The method of active-day fraction (ADF) was proposed recently to calibrate different solar observers to standard observational conditions. The result of the calibration may depend on the overall level of solar activity during the observational period. This dependency is studied quantitatively using data of the Royal Greenwich Observatory by formally calibrating synthetic pseudo-observers to the full reference dataset. It is shown that the sunspot group number is precisely estimated by the ADF method for periods of moderate activity, may be slightly underestimated by 0.5 – 1.5 groups (≤10%) for strong and very strong activity, and is strongly overestimated by up to 2.5 groups (≤30%) for weak-to-moderate activity. The ADF method becomes inapplicable for the periods of grand minima of activity. In general, the ADF method tends to overestimate the overall level of activity and to reduce the long-term trends.

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Series: Solar physics
ISSN: 0038-0938
ISSN-E: 1573-093X
ISSN-L: 0038-0938
Volume: 293
Issue: 4
Article number: 69
DOI: 10.1007/s11207-018-1292-7
Type of Publication: A1 Journal article – refereed
Field of Science: 115 Astronomy and space science
Funding: This work was carried out in the framework of the ReSoLVE Centre of Excellence (Academy of Finland, project no. 272157).
Academy of Finland Grant Number: 272157
Detailed Information: 272157 (Academy of Finland Funding decision)
Copyright information: © Springer Science+Business Media B.V., part of Springer Nature 2018. This is a post-peer-review, pre-copyedit version of an article published in Solar Physics. The final authenticated version is available online at: