In 1981, the production of the international Sunspot Number moved from the Zürich Observatory to the Royal Observatory of Belgium, with a new pilot station, the Specola Solare Ticinese Observatory in Locarno. This marked a very important transition in the history of the Sunspot Number. Those recent decades are particularly important as they provide the link between recent modern solar indices and fluxes and the entire Sunspot Number series extending back to the 18 th century. However, large variations have been recently identified in the scale of the Sunspot Number between 1981 and the present. Here, we refine the determination of those recent inhomogeneities by reconstructing a new average Sunspot Number series S N from a subset of long-duration stations between 1981 and 2015. We also extend this reconstruction by gathering long-time series from 35 stations over 1945-2015, thus straddling the critical 1981 transition. In both reconstructions, we also derive a parallel Group Number series G N built by the same method from exactly the same data set. Our results confirm the variable trends associated with drifts of the Locarno pilot station, which start only in 1983. We also verify the scale of the resulting 1981-2015 correction factor relative to the preceding period , which leads to a fully uniform S N series over the entire 1945-2015 interval. By comparing the new S N and G N series, we find that a constant quadratic relation exists between those two indices. This proxy relation leads to a fully constant and cycle-independent S N /G N ratio over cycles 19 to 23, with the exception of cycle 24 where it drops to a lower value. Several comparisons with other indices show a very good agreement between our reconstructed G N and the new "backbone" Group Number (Clette et al., 2014) but reveal inhomogeneities in the original Group Number as well as the F 10.7 radio flux and the American sunspot number R a over the period 1945-2015.

Abstract:Waldmeier in 1947 introduced a weighting (on a scale from 1 to 5) of the sunspot count made at Zurich and its auxiliary station Locarno, whereby larger spots were counted more than once. This counting method inflates the relative sunspot number over that which corresponds to the scale set by Wolfer and Brunner. Svalgaard re-counted some 60,000 sunspots on drawings from the reference station Locarno and determined that the number of sunspots reported were 'over counted' by 44% on average, leading to an inflation (measured by a weight factor) in excess of 1.2 for high solar activity. In a double-blind parallel counting by the Locarno observer Cagnotti, we determined that Svalgaard's count closely matches that of Cagnotti's, allowing us to determine the daily weight factor since 2003 (and sporadically before). We find that a simple empirical equation fits the observed weight factors well, and use that fit to estimate the weight factor for each month back to the introduction of weighting in 1947 and thus to be able to correct for the overcount and to reduce sunspot counting without weighting to the Wolfer method in use from 1893 onwards.Keywords: Sunspot weighting; Waldmeier sunspot weight factor; .Correcting the Sunspot Number; Locarno sunspot drawings. -----------------------------------------------------------------------------------------------------------"Hoc opus, hic labor" "When we wish to correct with advantage, and to show another that he errs, we must notice from what side he views the matter, for on that side it is usually true, and admit that truth to him, but reveal to him the side on which it is false. He is satisfied with that, for he sees that he was not mistaken, and that he only failed to see all sides. Now, no one is offended at not seeing everything; but one does not like to be mistaken, and that perhaps arises from the fact that man naturally cannot see everything, and that naturally he cannot err in the side he looks at, since the perceptions of our senses are always true."Blaise Pascal, Pensées (1670).2

Specola Solare Ticinese is an observatory dedicated to Sunspot Number counting, which was constructed in 1957 in Locarno, Southern Switzerland, as an external observing station of the Zurich Federal Observatory. When in 1981 the responsibility of the determination of the International Sunspot Number was assumed by the Royal Observatory of Belgium, Specola Solare Ticinese was given the role of pilot station, with the aim of preserving the continuity in the counting method. We report the observing procedure and counting rules applied in Locarno.

The recalibration of the sunspot number series, the primary long-term record of the solar cycle, requires the recovery of the entire collection of raw sunspot counts collected by the Zurich Observatory for the production of this index between 1849 and 1980.Here, we report about the major progresses accomplished recently in the construction of this global digital sunspot number database, and we derive global statistics of all the individual observers and professional observatories who provided sunspot data over more than 130 years.First, we can announce the full recovery of long-lost source-data tables covering the last 34 years between 1945 and 1979, and we describe the unique information available in those tables. We then also retrace the evolution of the core observing team in Zurich and of the auxiliary stations. In 1947, we find a major disruption in the composition of both the Zurich team and the international network of auxiliary stations.This sharp transition is unique in the history of the Zurich Observatory and coincides with the main scale-jump found in the original Zurich sunspot number series, the so-called “Waldmeier” jump. This adds key historical evidence explaining why methodological changes introduced progressively in the early 20th century could play a role precisely at that time. We conclude on the remaining steps needed to fully complete this new sunspot data resource.

Sunspot observations and counting are carried out at the Specola Solare Ticinese in Locarno since 1957 when it was built as an external observing station of the Zurich observatory. When in 1980 the data center responsibility was transferred from ETH Zurich to the Royal Observatory of Belgium in Brussels, the observations in Locarno continued and Specola Solare Ticinese got the role of pilot station. The data collected at Specola cover now the last 6 solar cycles.The aim of this presentation is to discuss and give an overview about the Specola data collection, the applied counting method and the future archiving projects. The latter includes the publication of all data and drawings in digital form in collaboration with the ETH Zurich University Archives, where a parallel digitization project is ongoing for the document of the former Swiss Federal Observatory in Zurich collected since the time of Rudolph Wolf.