A Theoretical Study of the Build-up of the Sun's Polar Magnetic Field by Using a 3D Kinematic Dynamo Model

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Monday, January 9, 2017

We develop a three-dimensional kinematic self-sustaining model of the solar dynamo in which the poloidal field generation is from tilted bipolar sunspot pairs placed on the solar surface above regions of strong toroidal field by using the SpotMaker algorithm and then the transport of this poloidal field to the tachocline is primarily caused by turbulent diffusion. We obtain a dipolar solution within a certain range of parameters.

Time evolution of two bipolar magnetic regions image
Time evolution of two bipolar magnetic regions (BMRs) located at a latitude of +/- 5 degrees latitude and a longitude difference of 90 degrees. As time goes on, the BMRS are distorted and dispersed by the differential rotation, meridional circulation, and turbulent diffusion. Within several years, a small fraction of the trailing flux (about 0.2%) makes it to the poles, generating a surface dipole moment. Meanwhile, the leading flux migrates to the equator and slowly diffuses.

We use this model to study the build-up of the polar magnetic field and show that some insights obtained from surface flux transport (SFT) models have to be revised. We present results obtained by putting a single bipolar sunspot pair in a hemisphere and two symmetrical sunspot pairs in two hemispheres.We find that the polar fields produced by them disappear due to the upward advection of poloidal flux at low latitudes, which emerges as oppositely- signed radial flux and which is then advected poleward by the meridional flow. We also study the effect that a large sunspot pair violating Hale’s polarity law would have on the polar field. We find that there would be some effect—especially if the anti-Hale pair appears at high latitudes in the mid-phase of the cycle—though the effect is not very dramatic.

Submitted to Astrophysical Journal