  While theoretical models and simulations of magnetic reconnection often
assume symmetry such that the magnetic null point when present is co-located
with a flow stagnation point, the introduction of asymmetry typically leads to
non-ideal flows across the null point. To understand this behavior, we present
exact expressions for the motion of three-dimensional linear null points. The
most general expression shows that linear null points move in the direction
along which the vector field and its time derivative are antiparallel. Null
point motion in resistive magnetohydrodynamics results from advection by the
bulk plasma flow and resistive diffusion of the magnetic field, which allows
non-ideal flows across topological boundaries. Null point motion is described
intrinsically by parameters evaluated locally; however, global dynamics help
set the local conditions at the null point. During a bifurcation of a
degenerate null point into a null-null pair or the reverse, the instantaneous
velocity of separation or convergence of the null-null pair will typically be
infinite along the null space of the Jacobian matrix of the magnetic field, but
with finite components in the directions orthogonal to the null space. Not all
bifurcating null-null pairs are connected by a separator. Furthermore, except
under special circumstances, there will not exist a straight line separator
connecting a bifurcating null-null pair. The motion of separators cannot be
described using solely local parameters, because the identification of a
particular field line as a separator may change as a result of non-ideal
behavior elsewhere along the field line.
