Flux‐Transport Dynamos with α‐Effect from Global Instability of Tachocline Differential Rotation: A Solution for Magnetic Parity Selection in the Sun

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Abstract
We propose an αΩ flux-transport dynamo for the Sun that is driven by a tachocline α-effect. This α-effect comes from the global hydrodynamic instability of latitudinal differential rotation in the tachocline, as calculated using a shallow-water model. Growing, unstable shallow-water modes propagating longitudinally in the tachocline create vortices that correlate with radial motion in the layer to produce a longitude-averaged net kinetic helicity and, hence, an α-effect. We show that such a dynamo is equally successful as a Babcock-Leighton-type flux-transport dynamo in reproducing many large-scale solar cycle features. The success of both dynamo types depends on the inclusion of meridional circulation of a sign and magnitude similar to that seen on the Sun. Both α-effects (the Babcock-Leighton-type and tachocline α-effect) are likely to exist in the Sun, but it is hard to estimate their relative magnitudes. By extending the simulation to a full spherical shell, we show that the flux-transport dynamo driven by the tachocline α-effect selects a toroidal field that is antisymmetric about the equator, while the Babcock-Leighton flux-transport dynamo selects a symmetric toroidal field. Since our present Sun selects antisymmetric fields, we argue that the tachocline α-effect must be more important than the Babcock-Leighton α-effect.