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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 squaregrid. It was discovered that the well known quadrupolar structure in theout-of-plane magnetic field gains four additional regions of opposite magneticpolarity, emerging near the corners of the simulation box, moving towards the$X$-point. The emerging, outer, magnetic field structure has opposite polarityto the inner quadrupolar structure, leading to an overall octupolar structure.Using Ampere's law and integrating electron and ion currents, defined at gridcells, over the simulation domain, contributions to the out-of-plane magneticfield from electron and ion currents were determined. The emerging regions ofopposite 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 inexperimental scenarios or space-craft observations are discussed.

Octupolar out-of-plane magnetic field structure generation during collisionless magnetic reconnection in a stressed X-point collapse