  The 1-D mean-field equation describing the evolution of the subsurface
toroidal field can be used together with the observed surface radial field to
model the subsurface toroidal flux density. We aim to test this model and
determine the relationship between the observationally inferred surface
toroidal field (as a proxy for flux emergence), and the modelled subsurface
toroidal flux density. We use a combination of sunspot area observations, the
surface toroidal field inferred from WSO line-of-sight magnetic field
observations, and compare with the results of a 1-D mean-field evolution
equation for the subsurface toroidal field, driven by the observed radial field
from the National Solar Observatory/Kitt Peak and SOLIS observations. We derive
calibration curves relating the subsurface toroidal flux density to the
observed surface toroidal field strengths and sunspot areas. The calibration
curves are for two regimes, one corresponding to ephemeral region emergence
outside of the butterfly wings, the other to active region emergence in the
butterfly wings. We discuss this in terms of the size and vertical velocity
associated with the two types of flux emergence.
