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Abstract

We report progress on the ongoing recalibration of the Wolf sunspot number ($S_{\mathrm{N}}$) and group-sunspot number ($G_{\mathrm{N}}$) following the release of version 2.0 of $S_{\mathrm{N}}$in 2015. This report constitutes both an update of the efforts reported in the 2016 Topical Issue of Solar Physics and a summary of work by the International Space Science Institute (ISSI) International Team formed in 2017 to develop optimal $S_{\mathrm{N}}$and $G_{\mathrm{N}}$reconstruction methods while continuing to expand the historical sunspot-number database. Significant progress has been made on the database side while more work is needed to bring the various proposed $S_{\mathrm{N}}$and (primarily) $G_{\mathrm{N}}$reconstruction methods closer to maturity, after which the new reconstructions (or combinations thereof) can be compared with (a) “benchmark” expectations for any normalization scheme (e.g., a general increase in observer normalization factors going back in time), and (b) independent proxy data series such as F10.7 and the daily range of variations of Earth’s undisturbed magnetic field. New versions of the underlying databases for $S_{\mathrm{N}}$and $G_{\mathrm{N}}$will shortly become available for years through 2022 and we anticipate the release of the next versions of these two time series in 2024.