The magnetic field is the main driver of the activity in the solar upper atmosphere, but its measurement is notoriously difficult. In order to determine the magnetic field in the chromosphere, transition region, and corona, we need to measure and interpret the polarization signals that the scattering of anisotropic radiation and the Hanle and Zeeman effects introduce in the emitted spectral line radiation. A number of recent advances have activated the development of this research field. ▪ The quantum theory of the generation and transfer of polarized radiation allows us to explain the polarization signals observed in chromospheric and coronal lines and to make successful predictions in unexplored spectral regions. ▪ The development of diagnostic techniques for the solar upper atmosphere has served to improve our empirical knowledge of the magnetic field in a variety of plasma structures, as well as to pave the way for their application to the unprecedented data that the new generation of solar telescopes are expected to provide. However, further improvements are required. ▪ The CLASP suborbital experiments have opened a new diagnostic window, namely ultraviolet (UV) spectropolarimetry as a tool for probing the magnetism and geometry of the upper chromosphere and transition region. A space telescope equipped with a UV spectropolarimeter would lead to major advances in our empirical understanding of solar magnetism.

中文翻译：

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太阳高层大气的磁场诊断

磁场是太阳高层大气活动的主要驱动力，但其测量却非常困难。为了确定色球层、过渡区和日冕中的磁场，我们需要测量和解释各向异性辐射的散射以及汉勒和塞曼效应在发射的谱线辐射中引入的偏振信号。最近的一些进展激活了这一研究领域的发展。 ▪ 偏振辐射产生和传输的量子理论使我们能够解释在色球线和日冕线中观察到的偏振信号，并在未探索的光谱区域中做出成功的预测。 ▪ 太阳高层大气诊断技术的发展有助于提高我们对各种等离子体结构中磁场的经验知识，并为其应用新一代太阳望远镜所获得的前所未有的数据铺平道路预计将提供。然而，还需要进一步改进。 ▪ CLASP 亚轨道实验打开了一个新的诊断窗口，即紫外 (UV) 分光偏振法作为探测上色球层和过渡区的磁性和几何形状的工具。配备紫外分光偏振计的太空望远镜将导致我们对太阳磁性的经验理解取得重大进展。