The relationship between bipolar magnetic regions and their sunspots

TL;DR

This study establishes an empirical relationship between magnetic flux in bipolar magnetic regions and sunspot area, improving surface flux transport models and enhancing understanding of solar magnetic activity.

Contribution

It introduces a novel empirical method to relate BMR magnetic flux to sunspot area using disc-integrated magnetic flux data, bypassing the need for a detailed BMR database.

Findings

The derived BMR flux-area relationship improves flux transport model accuracy.

Facular and network magnetic flux saturate at higher sunspot flux levels.

The ratio of facular to sunspot flux indicates a near balance, with faculae slightly dominating.

Abstract

The relationship between bipolar magnetic regions (BMRs) and their sunspots is an important property of the solar magnetic field, but it is not well constrained. One consequence is that it is a challenge for surface flux transport models (SFTMs) based on sunspot observations to determine the details of BMR emergence, which they require as input, from such data. We aimed to establish the relationship between the amount of magnetic flux in newly emerged BMRs and the area of the enclosed sunspots. Earlier attempts to constrain BMR magnetic flux were hindered by the fact that there is no proper database of the magnetic and physical properties of newly emerged BMRs currently available. We made use of the empirical model of the relationship between the disc-integrated facular and network magnetic flux and the total surface coverage by sunspots reported in a recent study. The structure of the model is such that it enabled us to establish, from these disc-integrated quantities, an empirical relationship between the magnetic flux and sunspot area of individual newly emerged BMRs, circumventing the lack of any proper BMR database. Applying the constraint on BMR magnetic flux derived here to an established SFTM retained its ability to replicate various independent datasets and the correlation between the model output polar field at the end of each cycle and the observed strength of the following cycle. The SFTM output indicates that facular and network magnetic flux rises with increasing sunspot magnetic flux at a slowing rate such that it appears to gradually saturate, analogous to earlier studies. The activity dependence of the ratio of facular and network flux to sunspot flux is consistent with the findings of recent studies: although the Sun is faculae-dominated, it is only marginally so as facular and network brightening and sunspot darkening appear to be closely balanced.