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J. Graf von der Pahlen and D. Tsiklauri, Phys. Plas. 21, 060705 (2014),established that the generation of octupolar out-of-plane magnetic fieldstructure in a stressed $X$-point collapse is due to ion currents. The fieldhas a central region, comprising of the well-known qaudrupolar field(quadrupolar components), as well as four additional poles of reversed polaritycloser to the corners of the domain (octupolar components). In this extendedwork, the dependence of the octupolar structure on domain size and ion massvariation is investigated. Simulations show that the strength and spatialstructure of the generated octupolar magnetic field is independent of ion toelectron mass ratio. Thus showing that ion currents play a significant role inout-of-plane magnetic structure generation in physically realistic scenarios.Simulations of different system sizes show that the width of the octupolarstructure remains the same and has a spacial extent of the order of the ioninertial length. The width of the structure thus appears to be independent onboundary condition effects. The length of the octupolar structure howeverincreases for greater domain sizes, prescribed by the external system size.This was found to be a consequence of the structure of the in-plane magneticfield in the outflow region halting the particle flow and thus terminating thein-plane currents that generate the out-of-plane field. The generation ofoctupolar magnetic field structure is also established in a tearing-modereconnection scenario. The differences in the generation of the octupolar fieldand resulting qualitative differences between $X$-point collapse andtearing-mode are discussed.

The effects of ion mass variation and domain size on octupolar out-of-plane magnetic field generation in collisionless magnetic reconnection