SILSO

On July 19, 2021, we added a new time series to our data products: extended hemispheric sunspot numbers starting in 1874. Those reconstructed numbers are based on the sunspot areas listed in the Greenwich photographic catalogue, and are calibrated on the SILSO reference hemispheric series, which starts only in 1992. This newly published series results from a collaboration between SILSO, the University of Graz (Austria) and the Skolkovo Institute of Science and Technology (Moscow, Russia).

In January 2020, the 13-month smoothed sunspot number  rose for the first time since the maximum of cycle 24 (April 2014). Most probably, this indicates that the minimum between cycles 24 and 25 was passed in December 2019.

Starting from a collaboration with the NCEI (NOAA, Boulder USA), we implemented new 12-month ahead predictions based on the McNish and Lincoln method. This rather simple method is based on a single mean cycle profile and is thus of "climatology" type. It was used as a standard for many years at NOAA, and we now add it to our other more advanced Standard Curves and Combined methods, allowing direct comparisons. Likewise, we now also provide a Kalman-filter optimized version of these new ML predictions.

Regular solar observers have noticed that since mid-2016, the Sun has occasionally been devoid of sunspots. As the current solar cycle 24 will gradually give way to the new solar cycle 25, several consecutive days and even weeks without sunspots will become the norm. In order to have an idea on the number of spotless days, and how these numbers compare to past solar cycles, SILSO has created a “Spotless Days page”.

Today marked a triple transition for us: - Uploading the new Sunspot Number archive files containing the daily, monthly and yearly re-calibrated sunspot numbers and the new Group Number series - In our Web site, switching to the new "Data" pages giving access to the new files, to updated graphics and also to the past version of the Sunspot Number - Adapting and running the entire monthly procedure to produce the provisional Sunspot Numbers for June 2015 and the associated 12-months forecast and EISN. Thus a lot of work in a single day for our small team.

After completing the new data files themselves, we are now finalizing the design of the new sections in our data pages. Indeed, although the new Sunspot Number will bring large improvements to the most prominent defects of the original series, it still contains more subtle or local inhomogeneities that will require more work for years to come. Therefore, in order to maintain a full history of present and future changes, we will now attach a new version number to each modification of the Sunspot Number time series.

The preparation of the July 1st transition is progressing at high pace. Hundreds of lines of codes are adapted for the new sunspot number series and for the new array of data files. The corrections can be subtle changes in hard-coded formulae for our two 12-month forecast methods, as well as large-scale modifications in our data pipeline controlling the flux of new data to route them to the proper archive file and database. 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.

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. This is actually the first deep revision of the sunspot number since its creation By R. Wolf in 1849.