Flux-transport dynamos with alpha-effect from global instability of tachocline differential rotation: A solution for magnetic parity selection in theSun

We propose an alpha Omega flux-transport dynamo for the Sun that is driven by a tachocline alpha -effect. This alpha -effect comes from the global hyd rodynamic instability of latitudinal differential rotation in the tachoclin e, as calculated using a shallow-water model. Growing, unstable shallow-wat er modes propagating longitudinally in the tachocline create vortices that correlate with radial motion in the layer to produce a longitude-averaged n et kinetic helicity and, hence, an alpha -effect. We show that such a dynam o is equally successful as a Babcock-Leighton-type flux-transport dynamo in reproducing many large-scale solar cycle features. The success of both dyn amo types depends on the inclusion of meridional circulation of a sign and magnitude similar to that seen on the Sun. Both alpha -effects (the Babcock -Leighton-type and tachocline alpha -effect) are likely to exist in the Sun , but it is hard to estimate their relative magnitudes. By extending the si mulation to a full spherical shell, we show that the flux-transport dynamo driven by the tachocline alpha -effect selects a toroidal field that is ant isymmetric about the equator, while the Babcock-Leighton flux-transport dyn amo selects a symmetric toroidal field. Since our present Sun selects antis ymmetric fields, we argue that the tachocline alpha -effect must be more im portant than the Babcock-Leighton alpha -effect.