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Field Topology Analysis of a Long-Lasting Coronal Sigmoid View all abstracts by submitter

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
Go to main E-Print page  Sigmoidal Active Region on the Sun: Comparison of a Magnetohydrodynamical Simulation and a Non-Linear Force-Free Field Mode  Estimation of the squashing degree within a three-dimensional domain  Edit Entry  Download Preprint  Delete Entry 

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