Authors: M. Rempel, M. Dikpati, K. MacGregor
Affiliation: National Center for Atmospheric Research, High Altitude Observatory
Recently flux-transport dynamos have been successful in reproducing various
observed features of the large-scale solar magnetic field. However, these studies
addressed the transport of magnetic fields by the meridional circulation in
a purely kinematic regime. The toroidal field strength at the base of the solar
convection zone inferred from studies of rising magnetic flux tubes is around
100 KG and thus orders of magnitude larger than the equipartition field strength
estimated from a meridional flow velocity of a few m/s. Therefore it is crucial
for flux-transport dynamos to address the feedback of the jxB on the meridional
flow. In this paper we present two approaches: 1) A kinematic approach in which
we parametrize this feedback in terms of a non-linear quenching of the meridional
flow in regions of strong field. 2) A MHD approach in which we solve the full
set of hydrodynamic equations together with the dynamo equations. Since we focus
in the present paper only on the large-scale flow field, the anisotropic turbulent
transport of angular momentum responsible for the differential rotation is parametrized.
From both studies we conclude that Babcock-Leighton flux-transport dynamos work
even with strong feedback of the jxB force, primarily because of two reasons:
1) The transport of the weak poloidal magnetic field, which is the sources of
strong toroidal field, is not affected strongly. 2) The meridional flow results
from a small difference between large forces, so that the transport capability
is much larger than a simple estimate based on equipartition arguments.
This work is partially supported by NASA grants W-10107 and W-10175. The National
Center for Atmospheric Research is sponsored by the National Science Foundation.