NONLINEAR FORCE-FREE MODELING OF A LONG-LASTING CORONAL SIGMOID
NONLINEAR FORCE-FREE MODELING OF A LONG-LASTING CORONAL SIGMOID
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DOI：
10.1088/0004-637x/703/2/1766
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发表时间：
2009-10
期刊：
影响因子：
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通讯作者：
A. Savcheva;A. van Ballegooijen
中科院分区：
文献类型：
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作者：
A. Savcheva;A. van Ballegooijen
本文研究了长时间静止的日冕Sigmoid的磁构型和演化。2007年2月6日至12日，由Hinode/XRT、过渡区和日冕探测器(TRACE)观测到乙状结肠最终喷发。我们使用磁绳插入法，为这一时期的几次观测建立了非线性无力场模型。X射线望远镜(XRT)的高空间和时间分辨率使我们能够精细地选择与观测相匹配的最佳模型。模拟表明，由嵌入势场中的弱扭曲磁绳组成的高剪切场很好地描述了X射线S型的结构。磁绳达到稳定平衡，但其轴向磁通已接近5×1020Mx的稳定极限。研究了磁绳附近扭转参数α的空间分布，发现其具有中空分布，即电流集中在磁绳与周围交界处的电流层中。电流层位于磁性构型的光秃片分隔线表面(BPSS)附近，X射线发射似乎来自该电流层/BPSS，与铁托夫和德莫林模型一致。我们发现，磁场的扭转角Φ在喷发前随时间增加到约2π，但从未达到扭结不稳定性所需的值。
A study of the magnetic configuration and evolution of a long-lasting quiescent coronal sigmoid is presented. The sigmoid was observed by Hinode/XRT and Transition Region and Coronal Explorer (TRACE) between 2007 February 6 and 12 when it finally erupted. We construct nonlinear force-free field models for several observations during this period, using the flux-rope insertion method. The high spatial and temporal resolution of the X-Ray Telescope (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 embedded 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 5 × 1020 Mx. The relative magnetic helicity increases with time from February 8 until just prior to the eruption on February 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 and Démoulin model. We find that the twist angle Φ of the magnetic field increases with time to about 2π just prior to the eruption, but never reaches the value necessary for the kink instability.