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FORMATION OF TORUS-UNSTABLE FLUX ROPES AND ELECTRIC CURRENTS IN ERUPTING SIGMOIDS
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Guillaume Aulanier Submitted: 2009-11-06 05:48
We analyze the physical mechanisms that form a three-dimensional coronal flux rope and later cause
its eruption. This is achieved by a zero-beta MHD simulation of an initially potential, asymmetric bipolar
field, which evolves by means of simultaneous slow magnetic field diffusion and sub-Alfvenic, line-tied
shearing motions in the photosphere. As in similar models, flux-cancellation driven photospheric
reconnection in a bald-patch separatrix transforms the sheared arcades into a slowly rising and stable
flux rope. A bifurcation from a bald-patch to a quasi-separatrix layer (QSL) topology occurs later on
in the evolution, while the flux rope keeps growing and slowly rising, now due to shear-driven coronal
slip-running reconnection, which is of tether-cutting type and takes place in the QSL. As the flux
rope reaches the altitude at which the decay index −d lnB/d ln z of the potential field exceeds ∼ 3/2,
it rapidly accelerates upward while the overlying arcade eventually develops an inverse tear-drop
shape, as observed in coronal mass ejections (CMEs). This transition to eruption is in accordance
with the onset criterion of the torus instability. Thus we find that photospheric flux-cancellation
and tether-cutting coronal reconnection do not trigger CMEs in bipolar magnetic fields, but are
key pre-eruptive mechanisms for flux ropes to build up and to rise to the critical height above the
photosphere at which the torus instability causes the eruption. In order to interpret recent Hinode X-
Ray Telescope observations of an erupting sigmoid, we produce simplified synthetic soft X-ray images
from the distribution of the electric currents in the simulation. We find that a bright sigmoidal
envelope is formed by pairs of J-shaped field lines in the pre-eruptive stage. These field lines form
through the bald-patch reconnection, and merge later on into S-shaped loops through the tethercutting
reconnection. During the eruption, the central part of the sigmoid brightens due to the
formation of a vertical current layer in the wake of the erupting flux rope. Slip-running reconnection
in this layer yields the formation of flare loops. A rapid decrease of currents due to field line expansion,
together with the increase of narrow currents in the reconnecting QSL, yields the sigmoid hooks to
thin in the early stages of the eruption. Finally, a slightly rotating erupting loop-like feature (ELLF)
detaches from the center of the sigmoid. Most of this ELLF is not associated with the erupting flux
rope, but with a current shell which develops within expanding field lines above the rope. Only the
short, curved end of the ELLF corresponds to a part of the flux rope. We argue that the features
found in the simulation are generic for the formation and eruption of soft X-ray sigmoids.
Authors: G. Aulanier, T. Torok, P. Demoulin, E.E. DeLuca
Projects: Hinode/XRT
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Publication Status: in press
Last Modified: 2009-11-06 09:59
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