A model of the 'steady' and 'fluctuating' parts of the sun's internal magnetic field

Abstract

We model the structure of the 'steady' (slowest varying) part of the sun's internal poloidal magnetic field assuming it to be (for given reasons and in the relevant domain) a current-free field whose field lines 'isorotate' according to the sun's internal rotation given by helioseismology. This part of the field can be described as that of a central dipole and a central hexapole both parallel to the rotation axis and embedded in a uniform external field. The field structure contains a critical surface (running along the base of the convection zone in the low latitudes), where a discontinuity of about 7 nHz per unit flux in the gradient of rotation may be winding a poloidal field into a toroidal field about 2 MG in about 10 exp 5 - 10 exp 9 yr. Small deviations from isorotation indicate presence of MHD perturbations whose latitude structure and time scales may be similar to those dominant in the solar cycle.