Implosive Collapse about Magnetic Null Points: A Quantitative Comparison between 2D and 3D Nulls

Jonathan O Thurgood,James A Mclaughlin,David I Pontin

doi:10.3847/1538-4357/aab0a0

Abstract

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 favorable 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 favorable in terms of magnetic energy release via null collapse than their more symmetric counterparts.

Highlights

Magnetic reconnection is an important process for energy conversion throughout astrophysical plasmas, in the Sun and planetary magnetospheres as well as farther afield—for example in γ-ray bursts (e.g., Zweibel & Yamada 2009; Pontin 2012)
We have considered a detailed parameter study of collapsing 3D magnetic null points of variable eccentricity (k = 0.25, 0.5, 1), alongside 2D nulls with variable resistivity and variable initial perturbation amplitude, for both low and moderate plasma β
We find that the implosion proceeds to increasingly small length scales as resistivity is decreased

Summary

Introduction

Magnetic reconnection is an important process for energy conversion throughout astrophysical plasmas, in the Sun and planetary magnetospheres as well as farther afield—for example in γ-ray bursts (e.g., Zweibel & Yamada 2009; Pontin 2012). Recently, kinetic particle-in-cell (PIC) simulations of 2D null collapse find fast rates occur due to collisionless effects (Tsiklauri & Haruki 2007, 2008), and that null collapse seems to be able to efficiently accelerate particles and has been proposed as a source of γ-ray flares in the Crab Nebula (Lyutikov et al 2016) Since these simulations begin with a magnetic field in which the null is “precollapsed” at a kinetic scale, the question remains as to whether an external perturbation would initiate a collapse that forms a thin-enough current sheet to promote collisionless reconnection before being pressure-limited at an MHD scale.

Simulation Setup

Current Sheet Geometry

Accessing Fast Reconnection through Collapse in Astrophysical Plasmas

Conclusions and Discussion

Findings

D Dt p r