Diagnosing and calibrating the multi-century Sunspot Number Series

Visual sunspot observations form the longest scientific record of solar activity, spanning over four centuries. Long term solar studies  are crucial to predict the future evolution of the solar cycles and at improving our understanding of the solar influence on Earth climate change.

As an important step for the ongoing sunspot number recalibration throughout the entire solar physics community, (Lefevre et al., 2018), SN Version 2 was released in July, 2015 (Clette et al., 2016), which helped in shedding new light to long-term solar variations and instabilities of the 11-year solar cycle. However, uncertainties remain and errors in past historical data need to be further revised using the data at our disposal  for a robust long-term series.

In 1843, Professor Rudolf Wolf  who coined the term “Sunspot Number”, founded a journal called the "Mittheilungen der Naturforschenden Gesellschaft in Berne" where he published yearbooks with all of his findings, including sunspot observations as far back as Galileo (Wolf, 1861). The journal was maintained from 1848 (Wolf, 1848) until his death (Wolf and Wolfer, 1894). The Sunspot records collected by him from his European colleagues and his auxiliary observers are all tabulated in this journal. The Royal Observatory of Belgium (https://www.astro.oma.be/en/), particularly the WDC-SILSO, conducted a mission between 2017 and 2019 to digitize all the data contained in the published Mittheilungen.

In this study we exploit this database along with other available recounts of various observers to identify scale discrepancies or inhomogeneities that happened along the Sunspot number series over time. We also introduce statistical techniques to implement confidence bands or errors on daily Sunspot Numbers, an information that existing versions lack. The long-term aim is a complete reconstruction of the Sunspot Number series from the available raw data instead of a recalibration.