  We report here, for the first time, an observed instability of a
Kelvin-Helmholtz (KH) nature occurring in a fully three-dimensional (3D)
current-vortex sheet at the fan plane of a 3D magnetic null point. The
current-vortex layer forms self-consistently in response to foot point driving
around the spine lines of the null. The layer first becomes unstable at an
intermediate distance from the null point, with the instability being
characterized by a rippling of the fan surface and a filamentation of the
current density and vorticity in the shear layer. Owing to the 3D geometry of
the shear layer, a branching of the current filaments and vortices is observed.
The instability results in a mixing of plasma between the two topologically
distinct regions of magnetic flux on either side of the fan separatrix surface,
as flux is reconnected across this surface. We make a preliminary investigation
of the scaling of the system with the dissipation parameters. Our results
indicate that the fan plane separatrix surface is an ideal candidate for the
formation of current-vortex sheets in complex magnetic fields and, therefore,
the enhanced heating and connectivity change associated with the instabilities
of such layers.
