Concentration of toroidal magnetic field in the solar tachocline by   eta-quenching
  We show that if the turbulent magnetic diffusivity used in solar dynamos is assumed to be 'quenched' by increasing toroidal fields, much larger amplitude and more concentrated toroidal fields can be induced by differential rotation from an assumed poloidal field than if there is no quenching. This amplification and concentration mechanism is weakened and bounded by j x B feedbacks on the differential rotation. Nevertheless, it is strong enough to contribute to the creation of ~100 kG toroidal fields near the base of the convection zone, perhaps in conjunction with the 'exploding flux tube' process. Such high fields are necessary for sunspots to occur in low solar latitudes. 
Peter A. Gilman, Matthias Rempel (HAO/NCAR, Boulder CO, USA)