 Linear force-free magnetic field

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.