太阳日冕磁场的非线性无力模拟
Nonlinear force-free modeling of the solar coronal magnetic field
摘要：
日冕磁场是一个重要的量，因为磁场控制着日冕的结构。不幸的是，通常无法直接测量日冕磁场。光球磁场是用矢量磁象仪进行常规测量的。这些光球层的测量值被外推到日冕中。外推的日冕磁场取决于关于日冕等离子体的假设，例如无力。无磁力意味着所有的非磁力如压力梯度和重力都被忽略。由于等离子体β较低，这种方法在日冕中是合理的。然而，人们必须注意光球层中的模糊性，噪音和非磁力，磁场矢量是在光球层测量的。本文综述了非线性无力日冕磁场外推的几种数值方法：Grad-Rubin程序、向上积分法、MHD松弛法、最优化法和边界元法。我们简要地讨论了不同的方法的主要特点，并主要集中在最近开发的新代码。
英文摘要：
The coronal magnetic field is an important quantity because the magnetic field dominates the structure of the solar corona. Unfortunately direct measurements of coronal magnetic fields are usually not available. The photospheric magnetic field is measured routinely with vector magnetographs. These photospheric measurements are extrapolated into the solar corona. The extrapolated coronal magnetic field depends on assumptions regarding the coronal plasma, e.g. force-freeness. Force-free means that all non-magnetic forces like pressure gradients and gravity are neglected. This approach is well justified in the solar corona due to the low plasma beta. One has to take care, however, about ambiguities, noise and non-magnetic forces in the photosphere, where the magnetic field vector is measured. Here we review different numerical methods for a nonlinear force-free coronal magnetic field extrapolation: Grad-Rubin codes, upward integration method, MHD-relaxation, optimization and the boundary element approach. We briefly discuss the main features of the different methods and concentrate mainly on recently developed new codes.
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