The Revised Brussels–Locarno Sunspot Number (1981 – 2015)

Abstract

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, Switzerland. This marked a profound 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 the entire sunspot-number series extending back to the eighteenth century. However, large variations have recently been identified in the scale of the sunspot number during this recent time period. Here, we refine the determination of those recent inhomogeneities by reconstructing a new average sunspot-number series [$S_{\mathrm{N}}$] 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 [$G_{\mathrm{N}}$] 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. They lead to a fully uniform $S_{\mathrm{N}}$-series over the entire 1945 – 2015 interval. By comparing the new $S_{\mathrm{N}}$- and $G_{\mathrm{N}}$-series, we find that a constant quadratic relation exists between those two indices over Cycles 19 to 23. Comparisons with a few other solar indices additionally validate this and reveal some possible undetected problems in those series. Using this new reference $S_{\mathrm{N}}$, we find that observing stations are surprisingly grouped among distinct subsets that share similar personal $k$-scaling coefficients. These various results also open the way to implementing a more advanced method for producing the sunspot number in the future.