Re-calibration of the Sunspot Number: Status Report

TL;DR

This paper reports on the progress of recalibrating the historical sunspot number series, aiming to improve the accuracy and consistency of solar activity records by refining databases and reconstruction methods.

Contribution

It provides an update on the ongoing recalibration efforts of sunspot numbers, emphasizing database improvements and methodological developments for better historical solar activity reconstructions.

Findings

Significant progress in database expansion and refinement.

Development of multiple reconstruction methods underway.

Upcoming releases of updated sunspot number series in 2024.

Abstract

We report progress on the ongoing recalibration of the Wolf sunspot number (SN) and Group sunspot number (GN) following the release of version 2.0 of SN 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 SN and GN re-construction 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 SN and (primarily) GN 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 SN and GN will shortly become available for years through 2022 and we anticipate the release of next versions of these two time series in 2024.