Full--Sphere Simulations of a Circulation--Dominated Solar Dynamo: Exploring the Parity Issue

Abstract

We explore a two-dimensional kinematic solar dynamo model in a full sphere, based on the helioseismically determined solar rotation profile and with an $\alpha$ effect concentrated near the solar surface, which captures the Babcock--Leighton idea that the poloidal field is created from the decay of tilted bipolar active regions. The meridional circulation, assumed to penetrate slightly below the tachocline, plays an important role. Some doubts have recently been raised regarding the ability of such a model to reproduce solar-like dipolar parity. We specifically address the parity issue and show that the dipolar mode is preferred when certain reasonable conditions are satisfied, the most important condition being the requirement that the poloidal field should diffuse efficiently to get coupled across the equator. Our model is shown to reproduce various aspects of observational data, including the phase relation between sunspots and the weak, diffuse field.