Since July 1st 2015, the original Sunspot number data are replaced by a new entirely revised data series. On this occasion, the data are presented in a new array of files, containing additional values that were not present in the original series. For proper tracking of those changes, the present version is numbered 2.0. All files are now labeled with their version number. The past version, now numbered 1.0, remains accessible for reference in our new Version Archive section. In addition to the Sunspot Number data, we now also distribute the new Group Number series, accessible in a dedicated section. Further information can be found at the bottom of this page.Each file can be downloaded by clicking on the grey button corresponding to the chosen format. All files are in ASCII text and can be opened in any text editor. CSV files allow direct import in various applications, like spreadsheets.
Data Credits: The Sunspot Number data can be freely downloaded. However, we request that proper credit to the WDC-SILSO is explicitely included in any publication using our data (paper article or book, on-line Web content, etc.), e.g.: "Source: WDC-SILSO, Royal Observatory of Belgium, Brussels".
More information about the various diagnostics and corrections applied to the Sunspot Number and Group Number series in version 2.0 can be found in: 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. All presentations given at the occasion of four dedicated Sunspot Number Workshops are also accessible at: http://ssnworkshop.wikia.com/wiki/Home
Three main innovations come with the new sunspot number series:
- New conventional scale: the most prominent change in the Sunspot Number values is the choice of a new reference observer, A.Wolfer (pilot observer from 1876 to 1928) instead of R. Wolf himself. This means dropping the conventional 0.6 Zürich scale factor, thus raising the scale of the entire Sunspot Number time series to the level of modern sunspot counts. This major scale change may thus strongly affect some user applications. Still, it allows to remove a persisting ambiguity caused by the systematically low sunspot numbers compared to all modern sunspot counts made since the late 19th century.
- Error values: the new numbers now come with the value of the standard deviation of the base counts included in the calculations, as well as the number of data included in the calculation. This allows to assess the precision of the value (statistical uncertainty). The absolute accuracy itself is not given as part of the data files, as it is determined only over long periods of time and not for single observations. This aspect is developed in the scientific litterature.
- Version numbers: as future updates of the series can be expected based on new recovered data and progresses in the methods, we initiated a version serial number that will allow to cleanly keep track of the incremental changes. The first number indicates a new version of the series where the sunspot numbers were collectively modified by any kind of correction. The second sub-version number is used to indicate secondary modifications that do not change the numbers themselves (e.g. new error estimates) or that involve only isolated modifications of the values (e.g. a typo found in a single daily value). Each incremental change will be documented in our archive section.