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

TL;DR

This study evaluates the active day fraction (ADF) method for calibrating sunspot group numbers, revealing its dependence on solar activity levels and limitations during grand minima.

Contribution

It provides a quantitative analysis of how the ADF calibration varies with solar activity, highlighting its biases and applicability limits.

Findings

ADF accurately estimates sunspot groups during moderate activity

Underestimates during strong activity by 0.5-1.5 groups

Overestimates during weak activity by up to 2.5 groups

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 ($\leq$10\,\%) for strong and very strong activity, and is strongly overestimated by up to 2.5 groups ($\leq$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.