Challenges in Establishing the “True” Sunspot Number Time Series

The sunspot number (SN) is the earliest and longest (1610-present) direct measure of the overall level of solar activity. The SN is 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.  The SN also provides a link between past and modern observations of solar activity.  The classical definition of the SN 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 SN 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.   Recently, several research groups went through an exercise of recalibrating past time series of sunspot numbers and expanding them to include newly discovered records of sunspot drawings. The outcome of this work was released by the World Data Center SILSO (Sunspot Index and Long-term Solar Observations) in July 2015.  This unprecedented revision immediately led to the production of several other independent versions of this time series with varying levels of agreement.   In order to better coordinate the current and future revisions, an informal group was recently formed to analyze each of the new time series in detail in order to understand the reasons for disagreements and to resolve the differences. The end goal is to develop a single unified sunspot number time series with error bars as a community-accepted reference dataset. In this presentation we will provide an update on early results from this working group.