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.BACK