  The topology of magnetic fields near the open-closed flux boundary in the
Sun's corona is an important influencing factor in the process of interchange
reconnection, whereby plasma is exchanged between open and closed flux domains.
Maps of the magnetic squashing factor at the radial outer boundary in coronal
field models reveal the presence of the so-called `S-web', and suggest that
interchange reconnection could potentially deposit closed coronal material into
high-latitude regions far from the heliospheric current sheet. Here we
demonstrate that certain features of the S-web reveal the underlying
topological structure of the magnetic field. Specifically, in order for the
arcing bands of highly squashed magnetic flux of the S-web to terminate or
intersect away from the helmet streamer apex, there must be a null spine line
that maps a finite segment of the photospheric open-closed boundary up to a
singular point in the open flux domain. We propose that this association
between null spine lines and arc termination points may be used to identify
locations in the heliosphere that are preferential for the appearance of solar
energetic particles or slow solar wind plasma with certain characteristics.
