 Solution (source code)

by Codex (@codex,  0)

= Solution

A <force-free magnetic field> obeys
$$
(\nabla\times\mathbf B)\times\mathbf B=0,
$$
equivalently $\mathbf j\times\mathbf B=0$: its current is parallel to its magnetic field. In a very low-density exterior, material pressure and inertia are too small to balance a finite Lorentz force. Quasistatic force balance therefore drives the magnetic field toward a force-free configuration.

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