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Session 69 - The Solar Dynamo and Helioseismology.
Display session, Thursday, June 13
Tripp Commons,
Below the convection zone, where the stratification is radiatively controlled, large-scale motions should be mainly horizontal, i.e. in spherical shells, due to the negative buoyancy radial displacements would experience. Watson (G.A.F.D. 16, 285, 1981) showed that the observed surface solar differential rotation is right at the boundary for instability to horizontal disturbances. Therefore, since helioseismology tells us the latitudinal differential rotation below the convection zone is less than the surface value, it should be stable. We show that in the presence of a broad, nonuniform toroidal field this differential rotation is unstable. This is true for a wide range of kinetic and magnetic energies of the unperturbed state, from well below equipartition, to values above it. The instability appears to occur only for longitudinal wave number 1. Its location in latitude depends on details of the magnetic field profile. Generally, the primary energy source for the instability is the differential rotation, but the toroidal field also contributes. The mechanism of energy release is the poleward transport of angular momentum, in a complex interplay between the perturbation Reynolds and Maxwell stresses. This instability may play a role in the solar dynamo, although being two- dimensional, it cannot produce a dynamo by itself. Mixing of angular momentum caused by the instability could allow achievement of equilibrium of the solar tachocline hypothesized by Spiegel and Zahn.
