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Sigmoidal Active Region on the Sun: Comparison of a Magnetohydrodynamical Simulation and a Non-Linear Force-Free Field Mode  

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

Publication Status: accepted
Last Modified: 2012-03-05 14:19
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Field Topology Analysis of a Long-Lasting Coronal Sigmoid  

Antonia Savcheva   Submitted: 2012-03-05 12:42

We present the first field topology analysis based on Non-Linear Force-Free Field (NLFFF) models of a long-lasting coronal sigmoid observed in February 2007 with the X-ray telescope on Hinode. The NLFFF models are built with the flux rope insertion method and give the 3D coronal magnetic field as constrained by observed coronal loop structures and photospheric magnetograms. Based on these models, we have computed horizontal maps of the current and the squashing factor Q for 25 different heights in the corona for all 6 days of the evolution of the region. We use the squashing factor to quantify the degree of change of the field line linkage and to identify prominent Quasi-Separatrix Layers (QSLs). We discuss the major properties of these QSL maps and devise a way to pick out important QSLs since our calculation cannot reach high values of Q. The complexity in the QSL maps reflects the high degree of fragmentation of the photospheric field. We find main QSLs and current concentrations that outline the flux rope cavity and that become characteristically S-shaped during the evolution of the sigmoid. We note that although intermittent bald patches exist along the length of the sigmoid during its whole evolution, the flux rope remains stable for several days. However, shortly after the topology of the field exhibits hyperbolic flux tubes (HFT) on Feb 7 and Feb 12 the sigmoid looses equilibrium and produces two B-class flares and associated CMEs. The location of the most elevated part of the HFT in our model coincides with the inferred locations of the two flares. Therefore, we suggest that the presence of an HFT in a coronal magnetic configuration may be an indication that the system is ready to erupt. We offer a scenario in which magnetic reconnection at the HFT drives the system towards the marginally stable state. Once this state is reached, loss of equilibrium occurs via the torus instability, producing a CME.

Authors: A. Savcheva, A. van Ballegooijen, E. DeLuca
Projects: Hinode/XRT

Publication Status: published
Last Modified: 2012-03-05 14:19
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Non-Linear Force-Free Modeling of a Long-Lasting Coronal Sigmoid  

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

Publication Status: accepted
Last Modified: 2009-09-04 14:38
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Abstracts by Author
Sigmoidal Active Region on the Sun: Comparison of a Magnetohydrodynamical Simulation and a Non-Linear Force-Free Field Mode
Field Topology Analysis of a Long-Lasting Coronal Sigmoid
Non-Linear Force-Free Modeling of a Long-Lasting Coronal Sigmoid

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