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Session 74 - The Quiet & Active Sun.
Display session, Friday, January 09
Exhibit Hall,

[74.06] 2-D Solar Dynamo Models in Spherical Geometry

J. A. Markiel (U. Rochester)

We have developed a 2-D finite-difference code for solving the kinematic mean field dynamo equations in spherical geometry. We apply this code to the solar dynamo, considering interface dynamo models of the type initiated by Parker (1993, ApJ 408, 707) but including the full solar rotation profile as determined by helioseismology. The regeneration of the poloidal field by the alpha-effect occurs in the lower part of the convection zone, while the production of toroidal field by the differential rotation takes place below the convection zone. The surface rotation rate persists to the base of the convection zone, where a transition to rigid rotation of the core takes place within a thin layer of thickness \leq 0.1 R_ødot below the convection zone. The diffusivity changes discontinuously across the interface from its large turbulent value in the convection zone to a smaller uniform value in the core. Diffusivity ratios between 0.001 and 0.1 are considered. Both positive and negative alpha-effect are allowed, and the growth of the dynamo is limited by a nonlinear quenching of the alpha-effect based on the ratio of magnetic to kinetic energy density.

If the shear layer is sufficiently thin so that the radial gradient of the rotation dominates the latitudinal gradient, then modes propagating along the interface are produced. The direction of propagation is towards the equator (pole) if the product \alpha \cdot \partial Ømega/\partial r is negative (positive), as expected. However, the radial gradient changes sign at midlatitudes, which has two effects: separate bands of field are produced in the equatorial and polar regions, propagating in different directions, and (2) the latitudinal gradient always dominates in the intermediate region where \partial Ømega/\partial r \sim 0. The latter effect can produce different types of modes which alter or destroy the interface modes excited in the rest of the hemisphere.

This research is supported by NSF grant AST-9528398.

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