Kalugodu, Chandrashekhar ; Lefèvre, Laure ; Mampaey, Benjamin ; Valliappan, Senthamizh Pavai ; Hayakawa, Hisashi ; Ritter, Christian

Poster presented at AGU Fall Meeting 2025, New Orleans, USA on 2025-12-16

Abstract: The Sunspot Number ($S_{N}$) is one of the longest continuous records of solar activity. $S_{N}$ V2.0 significantly improved the historical series through targeted recalibrations, but recent studies have shown that some scale changes—such as the 1849 transition—can only be fully understood by returning to the original raw observations. This motivates a complete reconstruction ($S_{N}$ V3.0) in which uncertainties and calibration steps arise directly from the quality of each dataset rather than from global statistical corrections. To enable this reconstruction, the historical data foundation has expanded dramatically in recent years. WDC–SILSO digitized the Mittheilungen (1610–1944) between 2017 and 2019, converting Wolf’s original Zürich journals into machine-readable datasets with modern metadata and provenance. As part of FARSuN, the Zürich observation tables for 1945–1979 were digitized from 2023 to 2025, completing the recovery of the full Zürich legacy. In parallel, major community efforts (e.g., Hayakawa, Arlt, Vaquero, Carrasco) have located, transcribed, and published numerous additional early telescopic sunspot records since 2010. Together, these datasets greatly expand the number of overlapping observations and allow independent cross-checks across observers and epochs. This richer observational base strengthens scale-transfer analyses, improves calibration robustness, and provides the foundation for an uncertainty-quantified reconstruction of the long-term sunspot series. These advances make a defensible $S_{N}$ V3.0 possible and ensure that the Sunspot Number remains a reliable reference for solar activity studies.

Keyword(s): Sunspot Number ; SN V3.0 ; Solar activity reconstruction ; FARSuN ; Historical sunspot observations ; Mittheilungen ; Zürich tables ; Gruithuisen manuscripts ; Adams sunspot drawings ; Solar cycle calibration ; Observer bias ; Scale transfer ; Uncertainty quantification ; Space climate ; Solar-terrestrial relations