Calibration of Sunspot Numbers

TL;DR

This paper revisits Waldmeier's 1971 analysis, demonstrating that the relationship between F10.7 solar radio flux and sunspot number has significantly changed over time, indicating potential calibration drift or changes in sunspot visibility.

Contribution

It provides an updated analysis of the F10.7 and sunspot number relationship using recent data, highlighting the need for re-calibration or understanding secular changes.

Findings

The relationship between F10.7 flux and sunspot number has changed significantly.

Possible causes include calibration drift or secular changes in sunspot visibility.

Recommends re-evaluating sunspot number calibration over time.

Abstract

Waldmeier [1971] found a very tight relationship between the F10.7 solar radio flux and the sunspot number and suggested using the flux for an objective calibration of the sunspot number. He suggested that if this relationship changed later on, the sunspot number should be re-calibrated, assuming that the calibration must have drifted with time. I repeat his analysis using data up to the present and it is, indeed, clear that the relationship has changed significantly. This could be due to a drift of the calibration or to a secular change in the visibility of sunspots, or both.