An Assessment of Sunspot Number Data Composites over 1845-2014

Abstract

New sunspot data composites, some of which are radically different in the character of their long-term variation, are evaluated over the interval 1845-2014. The method commonly used to calibrate historic sunspot data, relative to modern-day data, is "daisy-chaining", whereby calibration is passed from one data subset to the neighbouring one, usually using regressions of the data subsets for the intervals of their overlap. Recent studies have illustrated serious pitfalls in these regressions and the resulting errors can be compounded by their repeated use as the data sequence is extended back in time. Hence the recent composite data series by Usoskin et al. (2016), $R_{UEA}$, is a very important advance because it avoids regressions, daisy-chaining and other common, but invalid, assumptions: this is achieved by comparing the statistics of "active day" fractions to those for a single reference dataset. We study six sunspot data series including $R_{UEA}$ and the new "backbone" data series $R_{BB}$, recently generated by Svalgaard and Schatten (2016) by employing both regression and daisy-chaining. We show that all six can be used with a continuity model to reproduce the main features of the open solar flux variation for 1845-2014, as reconstructed from geomagnetic activity data. However, some differences can be identified that are consistent with tests using a basket of other proxies for solar magnetic fields. Using data from a variety of sunspot observers, we illustrate problems with the method employed in $R_{BB}$ which cause it to increasingly overestimate sunspot numbers going back in time and we recommend using $R_{UEA}$ because it employs more robust procedures that avoid such problems.

arXiv commentIn press, Astrophys. J., 2016

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Cited by in corpus (14)

• Magnitudes and Timescales of Total Solar Irradiance Variability
• New reconstruction of the sunspot group number since 1739 using the direct calibration and "backbone" methods
• Re-calibration of the Sunspot Number: Status Report
• Uncertainties in the Sunspot Numbers: Estimation and Implications
• Updated sunspot group number reconstruction for 1749-1996 using the active day fraction method
• Space climate and space weather over the past 400 years: 2. Proxy indicators of geomagnetic storm and substorm
• Scrutinizing the relationship between plage areas and sunspot areas and numbers
• Dependence of the Sunspot-group Size on the Level of Solar Activity and its Influence on the Calibration of Solar Observers
• Sunspot Observations Made by Hallaschka during the Dalton Minimum
• Sunspot observations by Charles Malapert during the period 1618-1626: a key dataset to understand solar activity before the Maunder minimum
• Revisiting the Amplitude of Solar Cycle 9: The Case of Sunspot Observations by W.C. Bond
• Revisiting the prediction of solar activity based on the relationship between the solar maximum amplitude and max-max cycle length
• Solar Irradiance Reconstruction over the Telescopic Era Using a Revised Photospheric Magnetic Field Model
• Iterative construction of the optimal sunspot number series