Eruptivity Criteria for Solar Coronal Flux Ropes in Magnetohydrodynamic and Magnetofrictional Models

TL;DR

This study compares MHD and magnetofrictional models to identify scalar quantities that best predict solar flux rope eruptions, finding that axial current squared normalized by helicity is a strong predictor across models.

Contribution

It introduces a comprehensive parameter study of flux rope eruptions using both MHD and magnetofrictional models, identifying effective eruptivity criteria.

Findings

Squared axial current normalized by helicity predicts eruptions well.

Similar flux rope behavior observed in both MHD and magnetofrictional models.

Eruptivity index variations also perform effectively as eruption predictors.

Abstract

We investigate which scalar quantity or quantities can best predict the loss of equilibrium and subsequent eruption of magnetic flux ropes in the solar corona. Our models are initialized with a potential magnetic arcade, which is then evolved by means of two effects on the lower boundary: firstly a gradual shearing of the arcade, modelling differential rotation on the solar surface, and secondly supergranular diffusion. These result in flux cancellation at the polarity inversion line and the formation of a twisted flux rope. We use three model setups: full magnetohydrodynamics (MHD) in cartesian coordinates, and the magnetofrictional model in both cartesian and polar coordinates. The flux ropes are translationally-invariant, allowing for very fast computational times and thus a comprehensive parameter study, comprising hundreds of simulations and thousands of eruptions. Similar flux rope behavior is observed using either magnetofriction or MHD, and there are several scalar criteria that could be used as proxies for eruptivity. The most consistent predictor of eruptions in either model is the squared current in the axial direction of the rope, normalised by the relative helicity, although a variation on the previously proposed 'eruptivity index' is also found to perform well in both the magnetofrictional and MHD simulations.