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

TL;DR

This study reveals that during collisionless magnetic reconnection in a stressed X-point collapse, an octupolar out-of-plane magnetic field structure forms due to ion currents, providing new insights into magnetic field topology changes.

Contribution

It is the first to identify the formation of an octupolar magnetic field structure during X-point collapse, expanding understanding beyond the traditional quadrupolar pattern.

Findings

Octupolar magnetic structure emerges near the simulation box corners.

Ion currents are responsible for the outer magnetic polarity regions.

Octupolar structure is characteristic of X-point collapse, not tearing mode.

Abstract

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.