A Test of the Active Day Fraction Method of Sunspot Group Number Calibration: Dependence on the Level of Solar Activity

Abstract

The method of active day fraction (ADF) was proposed recently to calibrate different solar observers to the standard observational conditions. The result of the calibration may depend on the overall level of solar activity during the observational period. This dependency is studied quantitatively using data of the Royal Greenwich Observatory, by formally calibrating synthetic pseudo-observers to the full reference dataset. It is shown that the sunspot group number is precisely estimated by the ADF method for periods of moderate activity, may be slightly underestimated by 0.5\,--\,1.5 groups ($\le$10\,\%) for strong and very strong activity, and is strongly overestimated by up to 2.5 groups ($\le$30\,\%) for weak--moderate activity. The ADF method becomes unapplicable for the periods of grand minima of activity. In general, the ADF method tends to overestimate the overall level of activity and to reduce the long-term trends.

arXiv commentSolar Physics, accepted

References in corpus (6)

• Revisiting the Sunspot Number
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• Reconstruction of the Sunspot Group Number: the Backbone Method
• A New Calibrated Sunspot Group Series Since 1749: Statistics of Active Day Fractions
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Cited by in corpus (3)

• Solar cycle prediction
• Re-calibration of the Sunspot Number: Status Report
• Robustness of Solar-Cycle Empirical Rules Across Different Series Including an Updated ADF Sunspot Group Series