Magnetic helicity reversal of a whistler vortex transmitted through a three-dimensional magnetic null point

The transmission of a magnetic vortex through a magnetic null point on a se paratrix surface is studied experimentally in a large laboratory plasma. Th e plasma is in the electron magnetohydrodynamic parameter regime and the vo rtex is an antenna-produced magnetic field perturbation propagating in the whistler mode. Topologically, the background field is separated into two re gions; a closed field line region and an open field line region. The two re gions are separated by a surface of magnetic field lines with two cusp null points referred to as the separatrix. The vortex propagates into one of th e null points. Its energy is partially transmitted through the separatrix a nd partially spreads away from the null along curving field lines. The self and mutual-helicity of the transmitted vortex reverses, thus the total mag netic helicity is not conserved. Helicity conservation breaks down because the field lines are not frozen to electron flows in the unmagnetized plasma region around the magnetic null point. (C) 2001 American Institute of Phys ics.