http://arxiv.org/abs/1605.06224

The key elements of the Babcock-Leighton dynamo are the generation of poloidal field through the decay of tilted bipolar active regions and the generation of toroidal field through the observed differential rotation. There are two classes of Babcock-Leighton models: flux transport dynamos where an equatorward flow at the bottom of the convection zone (CZ) is responsible for the equatorial propagation of the butterfly wings, and dynamo waves where the radial gradient of differential rotation and the $\alpha$ effect act in conjunction to produce the equatorial propagation. Here we investigate the role of downward magnetic pumping near the surface using a kinematic Babcock-Leighton model. We find that the pumping causes the poloidal field to become predominately radial in the near-surface shear layer. This allows the negative radial shear in the near-surface layer to effectively act on the radial field to produce a toroidal field. Consequently, we observe a clear equatorward migration of the toroidal field at low latitudes even when there is no meridional flow in the deep CZ. Both the dynamo wave and flux transport dynamo are thus able to reproduce some of the observed features of solar cycle including the 11-year periodicity. The main difference between the two types of dynamo is the value of $\alpha$ required to produce dynamo action. In flux transport dynamo the equatorward flow near the bottom of CZ advects toroidal field to cause the equatorward migration in butterfly wings and this advection makes the dynamo easier by transporting strong toroidal field to low latitudes where $\alpha$ effect works. Second conclusion of this paper is that the magnetic pumping suppresses the diffusion of fields through the photospheric surface which helps to achieve the 11-year dynamo cycle at a moderately larger value of magnetic diffusivity $\eta$ than has previously been used.

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B. Karak and R. Cameron
Mon, 23 May 16
5/55

Comments: Submitted in ApJ. Comments are welcome