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Will the original Sunspot Number data serie (V1.0) be continued in the future?

Submitted by lemaitre on

With the introduction of the new series, we stop the extension of the original series as the new series gives a better number, cleaned from past defects. It turns out that the main defect in the series was the use of weighted counts by the main reference station in Zürich and now Locarno. As such counts were not used for the whole series before 1947, it produced a strong inhomogeneity. As we will now use unweighted counts following the original Wolf definition, it would be impossible to reconcile the newly computed monthly numbers with the original series, which is about 17% too high until May 2015. There would be an artificial step between May and June 2015.

Therefore, the original series is kept accessible to users only for reference when making comparisons with past published studies that used the original sunspot number. So, just like the original Group Number series was never extended beyond 1995, the archived version 1.0 of the Sunspot Number will end in May 2015.

Transition to the new Sunspot Number successfully completed

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.

New Data pages with archive of past versions

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.

Advancing towards the July 1st transition: no change for our observers

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.

Major change of data set on July 1st, 2015

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.

2014 : maximum year for solar cycle 24

Now that we completed the definitive sunspot numbers for 2014, we can conclude that the maximum of solar cycle 24 was reached in April 2014, with a maximum of the 13-month smoothed sunspot number at 81.8. Since then, solar activity has steadily declined (monthly mean sunspot number now around 40), but remained above 70 over many months, probably indicating that the annual mean for 2014 will also mark a yearly maximum at 78.9.

The SILSO upgrade continues: new prediction files and plots

Over past weeks, various upgrades took place in the backstage. We further improved the primary scripts for the sunspot number calculation. We also improved the internal data flow. The biggest transition was also the full migration from our old server, which served for the past 10 years, to the new one after a full year of cross-validation (double calculation in parallel). Although those deep changes are largely invisible to the outside world, they now put a fully new engine into our sunspot number production!

News 002: From SIDC to SILSO, why a new name?

Over the past month, we already got very positive feedback about our new Web site. Like for all new Web sites, we had to fix a few initial defects but now, the web site seems to be on its tracks. Sorry for any inconvenience!
On the other hand, giving a new name to our World Data Center (WDC) seems to have caused some confusion. Will SIDC disappear? What will it change in the SIDC and sunspot services? So, let us clarify this point.

Which time tag should be attached to each daily International Sunspot Number?

Submitted by lemaitre on

The daily sunspot number is based on an average of all Wolf numbers reported during the 24h time bin of a UT day, i.e. between 0h00 and 24:00 UT. 
Therefore, the most logical time tag for each daily sunspot number is 12h00 UT.

Moreover, this time is also reflecting the current distribution of stations in the worldwide SILSO network. Indeed, by an historical heritage, there is a higher concentration of stations in Europe. Most of these, including the Locarno pilot station, thus actually observe around the middle of the UT day. The resulting index, which involves a time averaging over a whole day, is thus weighted in favour of the actual sunspot status near 12h00 UT.

What is the special role of the pilot station ? (Specola Solare Ticinese, Locarno)

Submitted by lemaitre on

Until 1980, traditionally, the sunspot number was essentially the Wolf number provided by the Zürich observatory, with some cross-validation relative to a network of supporting stations. 
Since 1981, in Brussels, we decided to derive the daily sunspot number from an average of all observations from a large worldwide network. This prevents anomalous index values due to a problem at a single station. However, as the network is evolving over time, with the inclusion of new stations and the departure of other ones, there is a risk of introducing a long term drift of the index. Therefore, prior to the averaging of daily values, the scale of each contributing station is adjusted on a monthly basis on a single reference station. The raw Wolf numbers are thus multiplied by a monthly average k personal coefficient. Therefore, the long-term scaling of the sunspot index is still attached to the average level of a single station, namely the Specola Solare Ticinese in Locarno (Switzerland). 
The Specola was choosen as reference station in 1981, at the occasion of the transfer of the World Data Center from Zürich to Brussels. Indeed, the Specola observatory was established back in 1955 as an alternate observatory to the Zürich observatory, providing the reference Wolf number when Zürich could not observe. It was thus the best reference to ensure a seamless transition between the original Zürich number and the modernized Brussels sunspot number. Now 31 years later, a look back shows that indeed no significant jump can be detected in the sunspot number at the time of the 1981 transition.

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