= Magnetically supercritical disk
A <magnetically supercritical disk> has a mass-to-magnetic-flux ratio above the critical value for magnetic support in a specified geometry. With surface <mass> density $\Sigma$ and perpendicular <magnetic field> $B_z$, a common idealized sheet convention in Gaussian units is $\lambda=2\pi\sqrt G\,\Sigma/|B_z|$, with supercriticality defined by $\lambda>1$.
The ratio compares self-gravity with magnetic support, not with every possible restoring mechanism. Rotation and <gas> <pressure> can still stabilize some disturbances, and thermal evolution controls nonlinear fragmentation. The numerical critical coefficient depends on the geometry and boundary assumptions, so a mass-to-flux criterion should not be transferred unchanged between a sheet, a sphere and a realistic disk.
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