MAGNETIC-FLUX TRANSPORT AND THE SUNS DIPOLE-MOMENT - NEW TWISTS TO THE BABCOCK-LEIGHTON MODEL

We discuss the mechanism for the generation of the Sun's poloidal magnetic field in the general framework of the Babcock-Leighton model. There are two distinct contributions to this process, the first being associated with the axial tilts of bipolar magnetic regions (BMRs) as they erupt at the solar surface, the second arising from the subsequent transport of the erupted flux over the surface. We suggest that the action of Coriolis forces on rising, expanding flux loops can account for both the magnitude and the latitudinal variation of the average BMR tilt angles. After a BMR has erupted, supergranular diffusion and meridional flow acting in combination can cause its axial dipole moment to grow or decay, depending mainly on the BMR's latitude of eruption. For realistic rates of diffusion and flow, the net effect of flux transport over the sunspot cycle is to reduce the Sun's axial dipole below the strength that it would have attained in the absence of any transport. In this sense, the "alpha-effect" in the Babcock-Leighton dynamo is inherent in the axial tilts of the BMRs; the surface transport acts mainly to redistribute the poloidally oriented component of the BMR flux, producing highly concentrated polar fields. We find that ephemeral regions have no effect on the evolution of the Sun's axial dipole moment, nor do they give rise to an effective diffusive transport of flux. Finally, we discuss the relationship between the surface flux and the subsurface toroidal field.