  The out-of-plane magnetic field, generated by fast magnetic reconnection,
during collisionless, stressed $X$-point collapse, was studied with a kinetic,
2.5D, fully electromagnetic, relativistic particle-in-cell numerical code,
using both closed (flux conserving) and open boundary conditions on a square
grid. It was discovered that the well known quadrupolar structure in the
out-of-plane magnetic field gains four additional regions of opposite magnetic
polarity, emerging near the corners of the simulation box, moving towards the
$X$-point. The emerging, outer, magnetic field structure has opposite polarity
to the inner quadrupolar structure, leading to an overall octupolar structure.
Using Ampere's law and integrating electron and ion currents, defined at grid
cells, over the simulation domain, contributions to the out-of-plane magnetic
field from electron and ion currents were determined. The emerging regions of
opposite magnetic polarity were shown to be the result of ion currents.
Magnetic octupolar structure is found to be a signature of $X$-point collapse,
rather than tearing mode, and factors relating to potential discoveries in
experimental scenarios or space-craft observations are discussed.
