Since July 2015, we decided to distribute the Group Number time series as part of our data sets, although this series was produced completely independently from the Sunspot Number until now. There are three main reasons for this extension:

- this data series was also submitted to a full revision, simultaneously with the Sunspot Number. As no maintenance of successive versions was foreseen previously, this will allow to implement a version tracking scheme similar to the new Sunspot Number.

- various past analyses have shown that a different information is contained in the Sunspot and Group numbers, although these are very close quantities, being both derived from direct sunspot observations. Therefore, it is appropriate to maintain both series in parallel and also to distribute both data sets from a common data portal.

- in comming months, the WDC-SILSO will start routinely producing Group Numbers by the same method as the Sunspot Number and using the same base observations from our worldwide network of observers. This will thus extend the original series that were limited to their production date (1995 for the original V1.0 series and 2014 for the new V2.0 series)

### Version 1.0

This is the original Group Number series. This first version was normalized to the Sunspot Number over the interval 1900-1995, by a factor 12.08.

A full description of the construction of this series can be found in:

- Hoyt, D. V. and Schatten, K. H., 1998a: "Group Sunspot Numbers: A New Solar Activity Reconstruction", Solar Physics, 179, 189-219.

- Hoyt, D. V. and Schatten, K. H., 1998b: "Group Sunspot Numbers: A New Solar Activity Reconstruction", Solar Physics, 181, 491-512.

In preparation: We will include here all the original files, converted to the standard format used for our other Sunspot Number files.

For the moment, the original files can be downloaded from the NOAA/NCEI Web site at:

http://www.ngdc.noaa.gov/stp/space-weather/solar-data/solar-indices/sunspot-numbers/group/

or byt FTP at:

ftp://ftp.ngdc.noaa.gov/STP/space-weather/solar-data/solar-indices/sunspot-numbers/group/

(NB: Some slight differences exist between the numbers contained in those files and the values given in the paper publications)

### Version 2.0

This is the new version accessible by the link at the top of this page.

Version 2.0 is a fully recomputed series, using a new approach for linking the scales of successive observers based on five long-duration reference observers. Some additional observers were also added to the original archive assembled by Hoyt and Schatten. In particular, many new observers were used over the 20th century in replacement of the photographic data from the Royal Greenwich Observatory. The extension from 1995 to 2014 mainly used the group counts contained in the SILSO database.

A full description of this reconstruction is available in the following publications:

- Svalgaard L., Schatten K. H.: "Reconstruction of the Sunspot Group Number: the Backbone Method",Contribution to Topical Issue of Solar Physics on “Recalibration of the Sunspot Number”

http://www.leif.org/research/Reconstruction-of-Group-Number-1610-2015.pdf

- Svalgaard, L., 2013: "Building a Sunspot Group Number Backbone Series", SSN-Workshop 3 (Tucson) http://www.leif.org/research/SSN/Svalgaard11.pdf

- Clette, F., Svalgaard, L., Vaquero, J.M., Cliver, E. W.,“Revisiting the Sunspot Number. A 400-Year Perspective on the Solar Cycle”, Space Science Reviews, Volume 186, Issue 1-4, pp. 35-103.

Currently only yearly mean numbers are available.

As the ratio between the Sunspot and Group numbers is actually variable with time, the new numbers are scaled to raw group counts without any normalization factor.