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Sigmoidal Active Region on the Sun: Comparison of a Magnetohydrodynamical Simulation and a Non-Linear Force-Free Field Mode
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Antonia Savcheva Submitted: 2012-03-05 12:44
In this paper we show that when accurate non-linear force free field (NLFFF) models are analyzed together with high resolution magnetohydrodynamic (MHD) simulations, we can determine the physical causes for the CME eruption on 12-Feb-2007. We compare the geometrical and topological properties of the three-dimensional magnetic fields given by both methods in their pre-eruptive phases. We arrive at a consistent picture for the evolution and eruption of the sigmoid. Both the MHD simulation and the observed magnetic field evolution show that flux cancellation plays an important role in building the flux rope. We compute the squashing factor, Q, in different horizontal maps in the domains. The main shape of the quasi-separatrix layers (QSLs), are very similar between the NLFFF and MHD models. The main QSLs lie on the edge of the flux rope. While the QSLs in the NLFFF model are more complex due to the intrinsic large complexity in the field, the QSLs in the MHD model are smooth and possess lower maximum value of Q. In addition, we demonstrate the existence of hyperbolic flux tubes (HFTs) in both models in vertical cross sections of Q. The main HFT, located under the twisted flux rope in both models, is identified as the most probable site for reconnection. We also show that there are electric current concentrations coinciding with the main QSLs. Finally, we perform torus instability analysis and show that a combination between reconnection at the HFT and the resulting expansion of the flux rope into the torus instability domain is the cause of the CME in both models.
Authors: A. Savcheva, E. Pariat, A. van Ballegooijen, G. Aulanier, E. DeLuca
Projects: Hinode/XRT
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Publication Status: accepted
Last Modified: 2012-03-05 14:19
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Non-Linear Force-Free Modeling of a Long-Lasting Coronal Sigmoid
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Antonia Savcheva Submitted: 2009-09-04 10:27
A study of the magnetic configuration and evolution of a long-lasting
quiescent coronal sigmoid is presented. The sigmoid was observed by
{it Hinode}/XRT and TRACE between 2007 Feb 6 and Feb 12 when it finally
erupted. We construct non-linear force-free field models for several
observations during this period, using the flux rope insertion method.
The high spatial and temporal resolution of XRT allows us to finely
select best-fit models that match the observations. The modeling shows
that a highly sheared field, consisting of a weakly twisted flux rope
inbedded in a potential field, very well describes the structure of
the X-ray sigmoid. The flux rope reaches a stable equilibrium,
but its axial flux is close to the stability limit of about 5x 1020 Mx.
The relative magnetic helicity increases with time from Feb 8 until
just prior to the eruption on Feb 12. We study the spatial distribution
of the torsion parameter α in the vicinity of the flux rope,
and find that it has a hollow-core distribution, i.e., electric currents
are concentrated in a current layer at the boundary between the flux rope
and its surroundings. The current layer is located near the Bald Patch
Separatrix Surface (BPSS) of the magnetic configuration, and the X-ray
emission appears to come from this current layer/BPSS, consistent with
the Titov & D'{e}moulin model. We find that the twist angle Phi
of the magnetic field increases with time to about 2 pi just prior
to the eruption, but never reaches the value necessary for the kink instability.
Authors: Antonia Savcheva and Adrian van Ballegooijen
Projects: Hinode/XRT
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Publication Status: accepted
Last Modified: 2009-09-04 14:38
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