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