= Collapse of a planar magnetic separatrix
For a planar <magnetic field> $\mathbf B=(\psi_y,-\psi_x,0)$, ideal induction advects the <Cartesian magnetic flux function>: $D_t\psi=0$. <Incompressible flow> also preserves the area inside each material contour. During relaxation, the arms of a magnetic <separatrix> can approach each other while their field-line connectivity remains fixed. A transition of width $\epsilon$ with a finite change of tangential field has <electric current density> of order $\epsilon^{-1}$ and can approach a <current sheet>. This is a possible limiting mechanism, not a claim that every planar equilibrium is singular.
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