Assessment of the Failure of Active Days Fraction Method of Sunspot Group Number Reconstructions

TL;DR

This paper critically evaluates the Active Days Fraction method for sunspot group number reconstructions, showing it fails to produce consistent results for equivalent observers and questioning its reliability for long-term solar activity studies.

Contribution

The study demonstrates the failure of the ADF method in accurately reconstructing sunspot numbers for equivalent observers, challenging its validity for long-term solar activity analysis.

Findings

ADF method does not produce nearly identical series for equivalent observers.

The method is unreliable during the critical period at the end of the 19th century.

The results cast doubt on the use of ADF for long-term solar activity reconstruction.

Abstract

We identify several pairs of 'equivalent' observers defined as observers with equal or nearly equal 'observational threshold' areas of sunspots on the solar disk as determined by the 'Active Days Fraction' method [e.g. Willamo et al., 2017]. For such pairs of observers, the ADF-method would be expected to map the actually observed sunspot group numbers for the individual observers to two reconstructed series that are very nearly equal and (it is claimed) represent 'real' solar activity without arbitrary choices and deleterious, error-accumulating 'daisy-chaining'. We show that this goal has not been achieved (for the critical period at the end of the 19th century and the beginning of the 20th), rendering the ADF-methodology suspect and not reliable nor useful for studying the long-term variation of solar activity.