SILSO: Warning of major changes in the Sunspot Number reference series

American Association of Variable Star Observers (AAVSO)
Mon, 06/22/2015 - 16:33

From the desk of, Dr. Laure Lefevre (SILSO) this morning.

Dear SILSO user,


Over the past 4 years a community effort has been carried out to revise entirely the historical Sunspot Number series. A good overview of the analyses and identified corrections is provided in the recent review paper:

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.


Now that the new data series has been finalized, we are about to replace the original version of our sunspot data by an entirely new data set on July 1st. On this occasion, we decided to simultaneously introduce changes in several conventions in the data themselves and also in the distributed data files.


There are so many diverse changes that we cannot guarantee that everything will work perfectly on the first try. Our team is too small to make full prior simulations. Therefore, multiple careful consistency checks will be done on July 1st itself, which will slow down the processing. So, please anticipate some delays compared to an ordinary month.


The most prominent change in the Sunspot Number will be 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. Be prepared!


Regarding data files, various files will be replaced by new ones, with new more homogeneous names and new internal column formats. The included information will sometimes change: combining data (e.g. hemispheric numbers together with total numbers), separating data (monthly smoothed numbers in a separate file) or adding new values that were not provided previously (standard errors).


All those changes will be explained in the information accompanying our data, on the web site of the World Data Center SILSO. While the primary files will all be replaced in early July, some other changes will still occur in the next two or three months. During this transitory phase, we thus invite you to visit the SILSO Web site to keep track of the changes, as we are preparing this major transition now scheduled for July 1st, 2015.


An important remark for our faithful observers: the current transition in the sunspot number processing does not change anything to the way you enter your data. So, just proceed as usual on July 1st. Your past k personal coefficients will simply be recomputed relative to the new re-calibrated sunspot number. We are working on this right now. By the way, the new processing software will open the way towards a better determination of the evolution of each station and so, a better feedback to our observers will become possible in the future.



In the coming weeks, please visit our SILSO Web site:


In the coming weeks, please visit our SILSO Web site:


For specific technical questions, in particular, if you need to adapt automated data import software used for operational purposes, please contact us by e-mail at



Dr. Laure Lefevre

Royal Observatory of Belgium