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For Sun-like stars, the generation of toroidal magnetic field from poloidalmagnetic field is an essential piece of the dynamo mechanism powering theirmagnetism. Previous authors have estimated the net toroidal flux generated ineach hemisphere of the Sun by exploiting its conservative nature. This onlyrequires observations of the surface magnetic field and differential rotation.We explore this approach using a 3D magnetohydrodynamic dynamo simulation of acool star, for which the magnetic field generation is known throughout theentire star. Changes to the net toroidal flux in each hemisphere were evaluatedusing a closed line integral bounding the cross-sectional area of eachhemisphere, following the application of Stokes-theorem to the inductionequation; the individual line segments corresponded to the stellar surface,base, equator, and rotation axis. The influence of the large-scale flows, thefluctuating flows, and magnetic diffusion to each of the line segments wasevaluated, along with their depth-dependence. In the simulation, changes to thenet toroidal flux via the surface line segment typically dominate the totalline integral surrounding each hemisphere, with smaller contributions from theequator and rotation axis. The bulk of the toroidal flux is generated deepinside the convection zone, with the surface observables capturing this due tothe conservative nature of the net flux. Surface magnetism and rotation cantherefore be used to estimate the net toroidal flux generated in eachhemisphere, allowing us to constrain the reservoir of magnetic flux for thenext magnetic cycle. However, this methodology cannot identify the physicalorigin, nor the location, of the toroidal flux generation. In addition, not alldynamo mechanisms depend on the net toroidal field produced in each hemisphere,meaning this method may not be able to characterise every magnetic cycle.

How well does surface magnetism represent deep Sun-like star dynamo action?