3D Solar Null Point Reconnection MHD Simulations

TL;DR

This study uses 3D MHD simulations to analyze magnetic reconnection near null points in the solar corona, comparing stratified and non-stratified atmospheres, and examining effects of driving speeds on plasma behavior.

Contribution

It provides a detailed comparison of stratified versus non-stratified solar atmosphere models and offers quantitative formulas linking simulation electric fields to solar observations.

Findings

Boundary motions build up current sheets near null points.

Nearly independent of driving speed for general behavior.

No flare-like energy release observed in simulations.

Abstract

Numerical MHD simulations of 3D reconnection events in the solar corona have improved enormously over the last few years, not only in resolution, but also in their complexity, enabling more and more realistic modeling. Various ways to obtain the initial magnetic field, different forms of solar atmospheric models as well as diverse driving speeds and patterns have been employed. This study considers differences between simulations with stratified and non-stratified solar atmospheres, addresses the influence of the driving speed on the plasma flow and energetics, and provides quantitative formulas for mapping electric fields and dissipation levels obtained in numerical simulations to the corresponding solar quantities. The simulations start out from a potential magnetic field containing a null-point, obtained from a Solar and Heliospheric Observatory (SOHO) magnetogram extrapolation approximately 8 hours before a C-class flare was observed. The magnetic field is stressed with a boundary motion pattern similar to - although simpler than - horizontal motions observed by SOHO during the period preceding the flare. The general behavior is nearly independent of the driving speed, and is also very similar in stratified and non-stratified models, provided only that the boundary motions are slow enough. The boundary motions cause a build-up of current sheets, mainly in the fan-plane of the magnetic null-point, but do not result in a flare-like energy release. The additional free energy required for the flare could have been partly present in non-potential form in the initial state, with subsequent additions from magnetic flux emergence or from components of the boundary motion that were not represented by the idealized driving pattern.