= Linear force-free magnetic field
{title2=$\nabla\times\mathbf B=\lambda\mathbf B,\quad\lambda\text{ constant}$}
A <force-free magnetic field> is linear when the proportionality between its <curl> and itself is a spatial constant. The <curl of the curl identity> and zero <divergence> give the vector <Helmholtz equation> $\nabla^2\mathbf B+\lambda^2\mathbf B=0$. Here $\lambda$ has inverse-length units; specifying a constant current-to-field coefficient with another normalization changes its numerical definition.
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