= Gravity-equivalent magnetic surface density
{title2=$\Sigma_M=-B_{z0}/\sqrt{\pi G\mu_0}$}
For an isolated razor-thin disk, rescale its exterior <magnetic scalar potential> to satisfy $\sqrt{4\pi G/\mu_0}\mathbf B=-\nabla\Phi_M$. Comparing the normal magnetic boundary derivative with $\partial_z\Phi=2\pi G\Sigma$ identifies the displayed equivalent signed <surface density>. Both exterior potentials satisfy <Laplace's equation>. The equivalent <surface density> is a representation of the magnetic boundary data, rather than physical negative mass.
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