The Latest on the Reconstruction of the Sunspot Number

The Zurich (or Wolf) sunspot number (Z) is the earliest and longest (1700-present) curated measure of the level of solar activity. The Z has been used extensively to study past solar activity, as a proxy for changes in solar irradiance, and as a representation of solar input for Earth climate modeling. Thus, Z provides a link between past and modern observations of solar activity. The classical definition of Z is based on a linear combination of the total number of “spots” on the Sun, the number of groups of spots, and a coefficient designed to characterize the differences in measurements from individual observers. This apparent simplicity can be misleading. The detectability of spots depends on many factors including the size and quality of the telescope used, the observer’s visual acuity, evolution in the understanding of sunspots (e.g., definition of spot groups), changes in methodology (e.g., introducing different weights when counting pores and large sunspots with penumbra), and local observing conditions. A group of scientists recommended renormalizing the Wolf series as a first step towards an improved data set. The new series was released in July 2015 as Sunspot Number Version 2 by the World Data Center for Sunspot Index and Long-term Solar Observations (SILSO). This unprecedented revision immediately led to efforts to further improve the sunspot number. In order to better coordinate the current and future revisions, an informal group was formed to analyze in detail each proposed variation to the time series in order to understand the where improvements are necessary and the effect on the sunspot number. The end goal is a single unified sunspot number time series with error bars that is accepted as a community-accepted reference dataset. In this presentation we will provide an update on the latest results from this working group. In brief, more sunspot drawings covering the 1600’s and 1700’s have been discovered and statistical methods for combining disparate sets of data have been compared. Although no new update has been produced, treating the sunspot record has a living data set is proving a productive way to examine this durable index of solar activity.