  For 3D magnetic reconnection to occur there must exist a volume within which
the electric field component parallel to the magnetic field is non-zero. In
numerical experiments, locations of non-zero parallel electric field indicate
sites of 3D magnetic reconnection. If these experiments contain all types of
topological feature (null points, separatrix surfaces, spines and separators)
then comparing topological features with the reconnection sites reveals that
all the reconnection sites are threaded by separators with the local
maxima/minima of the integrated parallel electric along fieldlines coinciding
with these separators. However, not all separators thread a reconnection site.
Furthermore, there are different types of separator. Cluster separators are
short arising within an individual weak magnetic field region and have
negligible parallel electric field along them so are not associated with much
reconnection. Intercluster separators connect a positive null point lying in
one weak-field region to a negative null point that lies in a different
weak-field region. Intercluster separators often thread enhanced regions of
parallel electric field and are long. Since separators form the boundary
between 4 globally-significant topologically-distinct domains, they are
important sites of reconnection which can results in the global restructuring
of the magnetic field. By considering the kinematic bifurcation models in which
separators form it is possible to understand the formation of cluster and
intercluster separators and explain their key properties.
