Helicity shedding by flux rope ejection

TL;DR

This study uses simulations to demonstrate that solar eruptions shed significant magnetic helicity, with the amount shed depending on the flux rope's properties, supporting the idea that CMEs regulate solar magnetic helicity.

Contribution

It provides the first parametric analysis of helicity shedding during flux rope ejections, quantifying how different configurations influence the amount of helicity expelled.

Findings

Self helicity is largely or completely shed during eruptions.

Mutual helicity is shed only partly, often less than self helicity.

Eruptions can shed up to 65% of initial helicity, supporting the helicity shedding conjecture.

Abstract

We quantitatively address the conjecture that magnetic helicity must be shed from the Sun by eruptions launching coronal mass ejections in order to limit its accumulation in each hemisphere. By varying the ratio of guide and strapping field and the flux rope twist in a parametric simulation study of flux rope ejection from approximately marginally stable force-free equilibria, different ratios of self- and mutual helicity are set and the onset of the torus or helical kink instability is obtained. The helicity shed is found to vary over a broad range from a minor to a major part of the initial helicity, with self helicity being largely or completely shed and mutual helicity, which makes up the larger part of the initial helicity, being shed only partly. Torus-unstable configurations with subcritical twist and without a guide field shed up to about two-thirds of the initial helicity, while a highly twisted, kink-unstable configuration sheds only about one-quarter. The parametric study also yields stable force-free flux rope equilibria up to a total flux-normalized helicity of 0.25, with a ratio of self- to total helicity of 0.32 and a ratio of flux rope to external poloidal flux of 0.94. These results numerically demonstrate the conjecture of helicity shedding by coronal mass ejections and provide a first account of its parametric dependence. Both self- and mutual helicity are shed significantly; this reduces the total initial helicity by a fraction of $\sim\!0.4\mbox{--}0.65$ for typical source region parameters.