Flux Transport Solar Dynamos with Near-Surface Radial Shear

Abstract

Corbard & Thompson analyzed quantitatively the strong radial differential rotation that exists in a thin layer near the solar surface. We investigate the role of this radial shear in driving a flux transport dynamo operating with such a rotation profile. We show that despite being strong, near-surface radial shear effectively contributes only ~1 kG (~30% of the total) to the toroidal fields produced there unless an abnormally high, surface α-effect is included. While 3 kG spot formation from ~1-2 kG toroidal fields by convective collapse cannot be ruled out, the evolutionary pattern of these model fields indicates that the polarities of spots formed from the near-surface toroidal field would violate the observed polarity relationship with polar fields. This supports previous results that large-scale solar dynamos generate intense toroidal fields in the tachocline, from which buoyant magnetic loops rise to the photosphere to produce spots. Polar fields generated in flux transport models are commonly much higher than observed. We show here that by adding enhanced diffusion in the supergranulation layer (originally proposed by Leighton), near-surface toroidal fields undergo large diffusive decay preventing spot formation from them, as well as reducing polar fields closer to the observed values. However, the weaker polar fields lead to the regeneration of a toroidal field of less than ~10 kG at the convection zone base, too weak to produce spots that emerge in low latitudes, unless an additional poloidal field is produced at the tachocline. This is achieved by a tachocline α-effect, previously shown to be necessary for coupling the north and south hemispheres to ensure toroidal and poloidal fields that are antisymmetric about the equator.