Parity of solar global magnetic field determined by turbulent diffusivity

H. Hotta1* and T. Yokoyama1
1Department of Earth and Planetary Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

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We investigate the criterion for the solar dipole-field in a kinematic flux-transport dynamo model. The sun has a dipole-like global magnetic field. This field is thought to be generated by the dynamo action of the solar internal plasma. The flux-transport dynamo succeeds to reproduce some features of solar cycle, e.g. poleward the migration of the general magnetic field and the butterfly diagram. The parity, however, of the global magnetic field significantly depends on parameters in the flux-transport dynamo. It is known that the coupling of the magnetic field between hemispheres due to turbulent diffusivity is an important factor for the solar parity issue, but the detailed criterion for the generation of the dipole field has not been investigated. Our conclusions are as follows. (1) The stronger diffusivity near the surface is more likely to cause the magnetic field to be a dipole. (2) The thinner layer of the strong diffusivity near the surface is also more apt to generate a dipolar magnetic field. (3) The faster meridional flow is more prone to cause the magnetic field to be a quadrupole, i.e., symmetric about the equator. The result (1) is consistent with our previous work (Hotta & Yokoyama 2010a), which is on the effect of the surface diffusivity for the observed weak polar field.

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Keywords : Sun: dynamo – Sun: activity