The standard flare model in three dimensions I. Strong-to-weak shear transition in post-flare loops
The standard flare model in three dimensions I. Strong-to-weak shear transition in post-flare loops
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DOI：
10.1051/0004-6361/201219311
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发表时间：
2012-07-01
影响因子：
6.5
通讯作者：
Schmieder, B.
中科院分区：
文献类型：
--
作者：
Aulanier, G.;Janvier, M.;Schmieder, B.
语境。喷发耀斑的标准 CSHKP 模型是二维的。然而，对爆发前 S 状曲线中的光球流、耀斑后环路中的剪切以及耀斑带的相对位置和形状的观测解释，所有这些都需要对模型进行三维扩展。我们重点关注耀斑后环路中强到弱的剪切转变，以及爆发耀斑发展过程中发生的光球电流几何形状的时间演化。目的是了解引起这些过程的三维物理过程，并了解耀斑后和喷发前的切变分布相互依赖的程度。方法。在 STEREO 观察到的耀斑以及使用 OHM 代码执行的 CME 启动的磁流体动力学模拟中，识别并量化了耀斑后环路中强到弱的剪切转变。在这两种方法中，磁剪切都是通过磁力线足点来评估的。在模拟中，还根据磁场分量之间的比率来估计剪切力。结果。后耀斑环路中模拟的强到弱剪切转变来自两种效应。首先，重新连接驱动的差动磁剪切力从喷发前配置转移到喷发后配置。其次，CME 内腿的垂直拉直会导致外部剪切减弱。该模型还预测 J 形耀斑带内会出现窄电流层，其中以直流电流为主。最后，当应用年轻和衰变太阳活动区域的典型尺度时，模拟自然地考虑了弱耀斑和强耀斑的能量和时间尺度。结论。结果为标准耀斑模型提供了三维扩展。这些扩展涉及应通过观察进行测试的 MHD 过程。
Context. The standard CSHKP model for eruptive flares is two-dimensional. Yet observational interpretations of photospheric currents in pre-eruptive sigmoids, shear in post-flare loops, and relative positioning and shapes of flare ribbons, all together require three-dimensional extensions to the model.Aims. We focus on the strong-to-weak shear transition in post-flare loops, and on the time-evolution of the geometry of photospheric electric currents, which occur during the development of eruptive flares. The objective is to understand the three-dimensional physical processes, which cause them, and to know how much the post-flare and the pre-eruptive distributions of shear depend on each other.Methods. The strong-to-weak shear transition in post-flare loops is identified and quantified in a flare observed by STEREO, as well as in a magnetohydrodynamic simulation of CME initiation performed with the OHM code. In both approaches, the magnetic shear is evaluated with field line footpoints. In the simulation, the shear is also estimated from ratios between magnetic field components.Results. The modeled strong-to-weak shear transition in post-flare loops comes from two effects. Firstly, a reconnection-driven transfer of the differential magnetic shear, from the pre- to the post-eruptive configuration. Secondly, a vertical straightening of the inner legs of the CME, which induces an outer shear weakening. The model also predicts the occurrence of narrow electric current layers inside J-shaped flare ribbons, which are dominated by direct currents. Finally, the simulation naturally accounts for energetics and time-scales for weak and strong flares, when typical scalings for young and decaying solar active regions are applied.Conclusions. The results provide three-dimensional extensions to the standard flare model. These extensions involve MHD processes that should be tested with observations.