Creation and evolution of magnetic helicity

Projecting a non-Abelian SU(2) vacuum gauge held-a pure gauge constructed from the group element U-onto a fixed (electromagnetic) direction in isospace gives rise to a nontrivial magnetic field, with nonvanishing magnetic helicity, which coincides with the winding number of LI. Although the helicity is not conserved under Maxwell (vacuum) evolution, it retains one-half its initial value at infinite time.