  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.
