Threshold of non-potential magnetic helicity ratios at the onset of solar eruptions

TL;DR

This study investigates how different photospheric flows influence magnetic helicity accumulation in the solar corona and identifies a potential helicity ratio threshold that signals the onset of solar eruptions.

Contribution

It demonstrates that a specific ratio of non-potential to total magnetic helicity serves as a universal threshold for eruption onset across various flow conditions.

Findings

Magnetic energy and helicity are mainly injected by shearing flows.

Helicity associated with coronal currents increases regardless of flow type.

A consistent helicity ratio threshold predicts eruption onset.

Abstract

The relative magnetic helicity is a quantity that is often used to describe the level of entanglement of non-isolated magnetic fields, such as the magnetic field of solar active regions.The aim of this paper is to investigate how different kinds of photospheric boundary flows accumulate relative magnetic helicity in the corona and if and how-well magnetic helicity related quantities identify the onset of an eruption. We use a series of three-dimensional, parametric magnetohydrodynamic simulations of the formation and eruption of magnetic flux ropes. All the simulations are performed on the same grid, using the same parameters, but they are characterized by different driving photospheric flows, i.e., shearing, convergence, stretching, peripheral- and central- dispersion flows. For each of the simulations, the instant of the onset of the eruption is carefully identified by using a series of relaxation runs. We find that magnetic energy and total relative helicity are mostly injected when shearing flows are applied at the boundary, while the magnetic energy and helicity associated with the coronal electric currents increase regardless of the kind of photospheric flows. We also find that, at the onset of the eruptions, the ratio between the non-potential magnetic helicity and the total relative magnetic helicity has the same value for all the simulations, suggesting the existence of a threshold in this quantity. Such threshold is not observed for other quantities as, for example, those related to the magnetic energy.