= Equal-diffusivity interface dynamo
{title2=$D=V\alpha/(\eta^2k^3)$}
An ideal planar <alpha-Omega dynamo> places poloidal regeneration above an interface and shear-generated toroidal induction below it. With equal constant <diffusivities>, decay away from the interface and ordinary electromagnetic matching, its surface-wave branch grows when $D>32$ for positive $k,\alpha,V$. The model separates the two regeneration regions while allowing diffusive coupling. The threshold refers to this model, not every solar interface dynamo or a full isotropic alpha closure.
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