Implosive collapse about magnetic null points: A quantitative comparison between 2D and 3D nulls

TL;DR

This study uses numerical simulations to compare the behavior of 2D and 3D magnetic null point collapses, revealing similarities in plasma enhancement but differences in reconnection scaling and effects of symmetry.

Contribution

First detailed numerical analysis of 3D null collapse in resistive MHD, comparing with 2D cases and exploring effects of symmetry and plasma pressure.

Findings

3D null collapse resembles 2D in plasma and current density enhancements

Reconnection rate scaling with resistivity is less favorable in symmetric 3D nulls

Higher plasma pressure can inhibit collapse, affecting reconnection efficiency

Abstract

Null collapse is an implosive process whereby MHD waves focus their energy in the vicinity of a null point, forming a current sheet and initiating magnetic reconnection. We consider, for the first time, the case of collapsing 3D magnetic null points in nonlinear, resistive MHD using numerical simulation, exploring key physical aspects of the system as well as performing a detailed parameter study. We find that within a particular plane containing the 3D null, the plasma and current density enhancements resulting from the collapse are quantitatively and qualitatively as per the 2D case in both the linear and nonlinear collapse regimes. However, the scaling with resistivity of the 3D reconnection rate - which is a global quantity - is found to be less favourable when the magnetic null point is more rotationally symmetric, due to the action of increased magnetic back-pressure. Furthermore, we find that with increasing ambient plasma pressure the collapse can be throttled, as is the case for 2D nulls. We discuss this pressure-limiting in the context of fast reconnection in the solar atmosphere and suggest mechanisms by which it may be overcome. We also discuss the implications of the results in the context of null collapse as a trigger mechanism of Oscillatory Reconnection, a time-dependent reconnection mechanism, and also within the wider subject of wave-null point interactions. We conclude that, in general, increasingly rotationally-asymmetric nulls will be more favourable in terms of magnetic energy release via null collapse than their more symmetric counterparts.