Simulations of 3D Magnetic Merging: Resistive Scalings for Null Point and QSL Reconnection

TL;DR

This study uses high-resolution 3D MHD simulations to analyze how resistive current layers form and scale in magnetic reconnection scenarios involving null points and QSLs, revealing different resistive scalings.

Contribution

It provides the first detailed comparison of resistive scalings for current layers at null points versus QSLs using full MHD simulations.

Findings

Null point current density scales as J ~ η^{-1}.

QSL current density scales as J ~ η^{-0.4}.

Null points are crucial for fast reconnection despite QSLs' strong field gradients.

Abstract

Starting from an exact, steady-state, force-free solution of the magnetohydrodynamic (MHD) equations, we investigate how resistive current layers are induced by perturbing line-tied three-dimensional magnetic equilibria. This is achieved by the superposition of a weak perturbation field in the domain, in contrast to studies where the boundary is driven by slow motions, like those present in photospheric active regions. Our aim is to quantify how the current structures are altered by the contribution of so called quasi-separatrix layers (QSLs) as the null point is shifted outside the computational domain. Previous studies based on magneto-frictional relaxation have indicated that, despite the severe field line gradients of the QSL, the presence of a null is vital in maintaining fast reconnection. Here, we explore this notion using highly resolved simulations of the full MHD evolution. We show that for the null-point configuration, the resistive scaling of the peak current density is close to $J\sim\eta^{-1}$, while the scaling is much weaker, i.e. $J\sim\eta^{-0.4}$, when only the QSL connectivity gradients provide a site for the current accumulation.