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

Posted: May 17, 2017 by tallbloke in Analysis, Dataset, Solar physics, solar system dynamics

Ilya Usoskin has kindly sent me the data for the new group sunspot number series he and his colleagues have published. I’ve done a rough and ready plot below. Excel file here in case you have problems wit the links below.


Group sunspot number average value. Missing values given as zero

T. Willamo1, I. G. Usoskin2,3 and G. A. Kovaltsov4

1 Department of Physics, University of Helsinki, 00014 Helsinki, Finland
2 Space Climate Research Unit, University of Oulu, 90014 Oulu, Finland
e-mail: Ilya.Usoskin@oulu.fi
3 Sodankylä Geophysical Observatory, University of Oulu, 90014 Oulu, Finland
4 Ioffe Physical-Technical Institute, 194021 St. Petersburg, Russia

Received: 4 October 2016
Accepted: 6 March 2017


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.

Key words: Sun: activity / sunspots

Monthly values of the reconstructed sunspot are available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr ( or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/601/A109

  1. oldbrew says:

    The G-values declined rapidly after the 1988-1992 burst of high numbers.


    Mostly below 2 in 1995-96 (end of cycle) compared to mainly double figures from late 88 to early 92.

  2. USteiner says:

    What is the reason for the end in 1996, 20 years and almost 2 cycles ago?

  3. tallbloke says:

    Usteiner, here’s part of an email I got from Frédéric Clette, Director of the World Data Center SILSO earlier today:

    The Group number was created in 1998 by Hoyt & Schatten using archived data going up to 1995. They never considered extending this series as they were only interested in the reconstruction of past centuries, in particular the historical period before 1800, when telescopes were of poor quality and the counts of individual spots were less reliable. Even the more recent versions of the series keep this philosophy. (NB: you can find all the group numbers series in our dedicated page: http://www.sidc.be/silso/groupnumberv3)

    However, given the renewed interest for this series, we thought that it would be worth starting the routine production of group numbers extending this series, in order to provide a longer overlap with all other modern solar indices, which only appeared in recent decades. So, this is in preparation in our World Data Center and it will be implemented probably in (early?) 2018. Note however that this group number would be part of the production of the existing international sunspot number that our WDC-SILSO is routinely producing using the counts from our worldwide network of observers (85 stations). It is just a simpler “byproduct” quantity that only uses the group counts, while the sunspot number uses both the group and spot counts (the latter give a measure of the actual size of the groups).

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