用Metis探测氦D $_3$线偏振的可能性
On the Possibilities of Detecting Helium D$_3$ Line Polarization with Metis
摘要：
太阳轨道飞行器上的空间日冕物质抛射仪为我们研究爆发性日冕物质抛射和日冕物质抛射（CME）提供了新的能力。它的两个光谱通道，氢L $α$和可见光（VL）将首次提供共对齐和共时图像，以研究CME的动力学和等离子体特性。此外，我们发现，与VL通道（580 - 640 nm）的一个令人兴奋的可能性，以检测氦D $_3$线（587.73 nm）和它的线偏振。本研究的目的是预测这条线的诊断潜力有关的CME的热和磁结构。对于网格模型，我们首先计算的D $_3$线的强度连同VL连续强度由于汤姆逊散射的核心电子。我们表明，Metis VL通道将检测到两者的混合物，在30 - 50，000 K之间的中间温度下，氦发射占主导地位。然后，我们使用的代码HAZEL计算的VL通道中可检测的线偏振度。这是D $_3$散射偏振和连续偏振的混合。前一个在磁场的存在下降低，并且极化轴旋转（汉勒效应）。Metis具有测量$Q/I$和$U/I$偏振度的能力，我们显示了它们对温度和磁场的依赖性。在$T$ = 30，000 K时，我们发现$Q/I$的显著降低，这是由于D $_3$线发射的强烈增强，而在10 G时的去极化约为10%。
英文摘要：
Space coronagraph Metis on board of the Solar Orbiter offers us new capabilities for studying eruptive prominences and coronal mass ejections (CME). Its two spectral channels, hydrogen L$α$ and visible-light (VL) will provide, for the first time, co-aligned and co-temporal images to study dynamics and plasma properties of CMEs. Moreover, with the VL channel (580 - 640 nm) we find an exciting possibility to detect the helium D$_3$ line (587.73 nm) and its linear polarization. The aim of this study is to predict the diagnostics potential of this line regarding the CME thermal and magnetic structure. For a grid of models we first compute the intensity of the D$_3$ line together with VL continuum intensity due to Thomson scattering on core electrons. We show that the Metis VL channel will detect a mixture of both, with predominance of the helium emission at intermediate temperatures between 30 - 50,000 K. Then we use the code HAZEL to compute the degree of linear polarization detectable in the VL channel. This is a mixture of D$_3$ scattering polarization and continuum polarization. The former one is lowered in the presence of a magnetic field and the polarization axis is rotated (Hanle effect). Metis has the capability of measuring $Q/I$ and $U/I$ polarization degrees and we show their dependence on temperature and magnetic field. At $T$=30,000 K we find a significant lowering of $Q/I$ which is due to strongly enhanced D$_3$ line emission, while depolarization at 10 G amounts roughly to 10 \%.
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