An Etymological Dictionary of Astronomy and Astrophysics

In → magnetohydrodynamics, in the limit of zero → resistivity, the → magnetic field  B satisfies the → induction equation  ∂B/∂t = ∇ x (v x B), then for a → plasma confined by a perfectly conducting boundary, the → magnetic helicity is conserved. If the normal field is fixed on the boundary, the minimum-energy state is the linear → force-free magnetic field that conserves the total magnetic helicity.

See also: Named after the Dutch astrophysicist Lodewijk Woltjer (1930-2019), who discovered the phenomenon in 1958 while studying the → Crab Nebula; → theorem.