Lefèvre, Laure ; Mathieu, Sophie ; Clette, Frederic

Talk presented at IUGG 2019, Montreal on 2019-07-15

Abstract: Following the recalibration of the Sunspot Number in 2015, we present the digitization of original sunspot data opening the possibility of a reconstruction of the sunspot series from scratch. So far, the largest part of the sunspot number series was based on the sunspot counts of a single reference station (Zurich). Further progress will require a broader statistical analysis of the raw sunspot data from past sunspot observers. Therefore, since 2018, we have progressively digitized all data gathered at the Zurich Observatory. We report on the digitization of all data from auxiliary stations published in the Mittheilungen of the Zurich Observatory, from 1856 to 1945. We recently started a search for the original data in the Zurich archives, which led to the recovery of long-lost data tables containing all data between 1945 and 1980. The information included in those new tables bridges the gap between the Mittheilungen and the current database of the WDC-SILSO (http://www.sidc.be/silso) that starts in 1980, a key missing link connecting the historical series with the ongoing modern sunspot index. Another potential link connecting the historical series to the modern sunspot index is methodological: the determination of a complete statistical model of sunspot counts (including error bars). In addition to giving us tools to monitor the sunspot series, error bars could also be used to improve the accurate reconstruction of the long-term evolution of solar activity. In this context, we present key results obtained by the VALUSUN project (sidc.be/valusun) concerning the statistical modeling of the Sunspot Numbers.

Note: Session: Old Data for New Knowledge: Preservation and Utilization of Historical Data in the Geosciences
Funding: BR/165/A3/VAL-U-SUN