Flux-Rope Twist in Eruptive Flares and CMEs: due to Zipper and Main-Phase Reconnection

TL;DR

This paper models the three-dimensional magnetic reconnection processes during eruptive solar flares and CMEs, explaining how flux rope twist develops through zipper and main-phase reconnection, and comparing predictions with interplanetary observations.

Contribution

It introduces a simple model combining zipper and main-phase reconnection to predict flux rope twist, incorporating magnetic helicity conservation and initial flux fragmentation.

Findings

Zipper reconnection can produce flux ropes with up to two turns of twist.

Main phase reconnection adds uniform twist to the flux rope core.

Observed flux ropes' twist levels are consistent with model predictions.

Abstract

The nature of three-dimensional reconnection when a twisted flux tube erupts during an eruptive flare or coronal mass ejection is considered. The reconnection has two phases: first of all, 3D "zipper reconnection" propagates along the initial coronal arcade, parallel to the polarity inversion line (PIL), then subsequent quasi-2D "main phase reconnection" in the low corona around a flux rope during its eruption produces coronal loops and chromospheric ribbons that propagate away from the PIL in a direction normal to it. One scenario starts with a sheared arcade: the zipper reconnection creates a twisted flux rope of roughly one turn ($2\pi$ radians of twist), and then main phase reconnection builds up the bulk of the erupting flux rope with a relatively uniform twist of a few turns. A second scenario starts with a pre-existing flux rope under the arcade. Here the zipper phase can create a core with many turns that depend on the ratio of the magnetic fluxes in the newly formed flare ribbons and the new flux rope. Main phase reconnection then adds a layer of roughly uniform twist to the twisted central core. Both phases and scenarios are modeled in a simple way that assumes the initial magnetic flux is fragmented along the PIL. The model uses conservation of magnetic helicity and flux, together with equipartition of magnetic helicity, to deduce the twist of the erupting flux rope in terms the geometry of the initial configuration. Interplanetary observations show some flux ropes have a fairly uniform twist, which could be produced when the zipper phase and any pre-existing flux rope possess small or moderate twist (up to one or two turns). Other interplanetary flux ropes have highly twisted cores (up to five turns), which could be produced when there is a pre-existing flux rope and an active zipper phase that creates substantial extra twist.