Visualization of the challenges and limitations of the long-term sunspot number record

TL;DR

This paper reviews recent efforts to analyze historical sunspot data to understand long-term solar activity variations, highlighting challenges and limitations in current observational records and reconstructions.

Contribution

It provides a visual guide to assess observational coverage and disagreement among sunspot series, emphasizing the limitations of long-term solar activity reconstructions.

Findings

Historical sunspot data show conflicting long-term trends.

Observational coverage varies significantly across periods.

Current reconstructions have notable uncertainties and disagreements.

Abstract

The solar cycle periodically reshapes the magnetic structure and radiative output of the Sun and determines its impact on the heliosphere roughly every 11 years. Besides this main periodicity, it shows century-long variations (including periods of abnormally low solar activity called grand minima). The Maunder Minimum (1645-1715) has generated significant interest as the archetype of a grand minimum in magnetic activity for the Sun and other stars, suggesting a potential link between the Sun and changes in terrestrial climate. Recent reanalyses of sunspot observations have yielded a conflicted view on the evolution of solar activity during the past 400 years (a steady increase versus a constant level). This has ignited a concerted community-wide effort to understand the depth of the Maunder Minimum and the subsequent secular evolution of solar activity. The goal of this Perspective is to review recent work that uses historical data to estimate long-term solar variability, and to provide context to users of these estimates that may not be aware of their limitations. We propose a clear visual guide than can be used to easily assess observational coverage for different periods, as well as the level of disagreement between currently proposed sunspot group number series.