Evolution of Magnetic Helicity During Eruptive Flares and Coronal Mass Ejections

TL;DR

This paper investigates how magnetic helicity evolves during solar eruptions, proposing models to determine the twist of erupting flux ropes based on magnetic flux conservation and helicity principles.

Contribution

It introduces a theoretical framework for calculating the twist of erupting flux ropes considering initial magnetic configurations and reconnection effects.

Findings

Magnetic helicity is conserved during eruptions.

Flux rope twist depends on initial magnetic structure and reconnection.

Two cases of flux rope formation and evolution are analyzed.

Abstract

During eruptive solar flares and coronal mass ejections, a non-pot{\-}ential magnetic arcade with much excess magnetic energy goes unstable and reconnects. It produces a twisted erupting flux rope and leaves behind a sheared arcade of hot coronal loops. We suggest that: the twist of the erupting flux rope can be determined from conservation of magnetic flux and magnetic helicity and equipartition of magnetic helicity. It depends on the geometry of the initial pre-eruptive structure. Two cases are considered, in the first of which a flux rope is not present initially but is created during the eruption by the reconnection. In the second case, a flux rope is present under the arcade in the pre-eruptive state, and the effect of the eruption and reconnection is to add an amount of magnetic helicity that depends on the fluxes of the rope and arcade and the geometry.