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M. Rempel, M. Dikpati, K. MacGregor (HAO/NCAR, Boulder, CO)
Recently, flux-transport dynamos have been successful in reproducing various observed features of the large scale solar magnetic fields. 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. We present a "dynamic" dynamo model, in which we couple a mean-field Reynolds-stress approach for the differential rotation and meridional circulation with the axisymmetric dynamo equations. This provides a self-consistent model that allows to study the back-reaction of the mean-field Lorentz force of the dynamo generated field on differential rotation and meridional circulation. This model gives an estimate of the magnetic field strength up to which a transport of magnetic field by the weak meridional flow and amplification by the shear in the differential rotation is possible. Additional to this the model also provides solar cycle variations in differential rotation and meridional circulation, which can be compared to helioseismic data. We also show that the feedback of the Lorentz-force on the meridional flow can be included into a kinematic dynamo model in terms of a "quenching" of the stream function, which deflects the flow from regions of strong toroidal magnetic field. From both studies we conclude that 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 argument.
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.
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Bulletin of the American Astronomical Society, 36 #2
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