Mc. Griskey et Rl. Stenzel, Magnetic helicity reversal of a whistler vortex transmitted through a three-dimensional magnetic null point, PHYS PLASMA, 8(11), 2001, pp. 4810-4815
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.