University of Oulu

Chatzistergos, T., Usoskin, I., Kovaltsov, G., Krivova, N., Solanki, S. (2017) New reconstruction of the sunspot group numbers since 1739 using direct calibration and “backbone” methods. Astronomy and Astrophysics, 602 (), A69. doi:10.1051/0004-6361/201630045

New reconstruction of the sunspot group numbers since 1739 using direct calibration and “backbone” methods

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Author: Chatzistergos, Theodosios1; Usoskin, Ilya G.2,3; Kovaltsov, Gennady A.4;
Organizations: 1 Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-weg 3, 37077 Göttingen, Germany
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
5School of Space Research, Kyung Hee University, Yongin, 446-701 Gyeonggi, Republic of Korea
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 1.1 MB)
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Language: English
Published: EDP Sciences, 2017
Publish Date: 2017-06-21


Context: The group sunspot number (GSN) series constitute the longest instrumental astronomical database providing information on solar activity. This database is a compilation of observations by many individual observers, and their inter-calibration has usually been performed using linear rescaling. There are multiple published series that show different long-term trends for solar activity.

Aims: We aim at producing a GSN series, with a non-linear non-parametric calibration. The only underlying assumptions are that the differences between the various series are due to different acuity thresholds of the observers, and that the threshold of each observer remains constant throughout the observing period.

Methods: We used a daisy chain process with backbone (BB) observers and calibrated all overlapping observers to them. We performed the calibration of each individual observer with a probability distribution function (PDF) matrix constructed considering all daily values for the overlapping period with the BB. The calibration of the BBs was carried out in a similar manner. The final series was constructed by merging different BB series. We modelled the propagation of errors straightforwardly with Monte Carlo simulations. A potential bias due to the selection of BBs was investigated and the effect was shown to lie within the 1σ interval of the produced series. The exact selection of the reference period was shown to have a rather small effect on our calibration as well.

Results: The final series extends back to 1739 and includes data from 314 observers. This series suggests moderate activity during the 18th and 19th century, which is significantly lower than the high level of solar activity predicted by other recent reconstructions applying linear regressions.

Conclusions: The new series provides a robust reconstruction, based on modern and non-parametric methods, of sunspot group numbers since 1739, and it confirms the existence of the modern grand maximum of solar activity in the second half of the 20th century.

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Series: Astronomy and astrophysics
ISSN: 0004-6361
ISSN-E: 1432-0746
ISSN-L: 0004-6361
Volume: 602
Pages: 1 - 18
Article number: A69
DOI: 10.1051/0004-6361/201630045
Type of Publication: A1 Journal article – refereed
Field of Science: 115 Astronomy and space science
Funding: T.C. acknowledges a postgraduate fellowship of the International Max Planck Research School on Physical Processes in the Solar System and Beyond. This work was performed in the framework of the ReSoLVE Center of Excellence (the Academy of Finland, project No. 272157). This work was partly supported by the BK21 plus programme through the National Research Foundation (NRF) funded by the Ministry of Education of Korea.
Academy of Finland Grant Number: 272157
Detailed Information: 272157 (Academy of Finland Funding decision)
Copyright information: Credit: Chatzistergos, T., Usoskin, I., Kovaltsov, G., Krivova, N., Solanki, S. (2017) Astronomy and Astrophysics, 602, A69. reproduced with permission, © ESO 2017