Reconstructing the Sunspot Number : challenges and impact

We will present the international effort that led to the first-ever revision of the Sunspot and Group Numbers (let us call them "the Sunspot Series"), a well-known series that had never been put into question since its creation by Rudolf Wolf in 1849. We will review the process and its challenges (ISSI review paper, https://www.issibern.ch/teams/sunspotnoser/). 

At this point in time, the number of Group Number series available is still based on different techniques that enable the stitching of datasets of various quality over wildly different periods, and although they present an undeniable improvement over the original Group Number from Hayt & Schatten (1998), the resulting series remain to be extensively tested in order to be clearly validated by the scientific community.

Since this first revision in 2015 (Solar Physics Topical Issue, 2016), the Sunspot Series have become living datasets that require constant monitoring since more source data are being recovered regularly (Arlt & Vaquero, 2020). With the team from ISSI, we are currently driving a large effort to gather raw data from all around the world. At the Royal Observatory of Belgium, within the WDC-SILSO (https://wwwbis.sidc.be/silso/) where the original Mittheilungen have been digitized (2017-2019) we also have 2 PhD students working on stitching historical and modern sunspot numbers and evaluating the quality of the reconstructed series through advanced statistical techniques.

After this review of previous efforts, we will focus more specifically on the reconstruction of the International Sunspot Number from raw sunspot data (Bhattacharya et al., 2021, 2022, FARSUN belgian project) end present the potential impact of these revisions on end users (e.g. F10.7, Clette, 2021).