Does Building a Relative Sunspot Number Make Sense? A Qualified 'Yes'

TL;DR

This paper examines the validity of the classical Relative Sunspot Number (SSN) in light of recent changes in sunspot activity and proposes a harmonized series that better reflects current solar magnetic activity.

Contribution

It introduces a new approach to constructing a sunspot number series that accounts for variations in spots per group, aligning better with F10.7 flux measurements.

Findings

The ratio of spots per group has decreased to about 5 from the nominal 10.

The proposed sunspot number series aligns more closely with F10.7 flux.

The new series captures both long-term and short-term solar activity variations.

Abstract

Recent research has demonstrated that the number of sunspots per group ('active region') has been decreasing over the last two or three solar cycles and that the classical Relative Sunspot Number (SSN) no longer is a good representation of solar magnetic activity such as revealed by e.g. the F10.7 cm microwave flux. The SSN is derived under the assumption that the number of spots per group is constant (in fact, nominally equal to 10). When this is no longer the case (the ratio is approaching 5, only half of its nominal value) the question arises how to construct a sunspot number series that takes that into account. We propose to harmonize the SSN with the sunspot Group Count that has been shown to follow F10.7 very well, but also to include the day-to-day variations of the spot count in order to preserve both long-term and short-term variability.