We present an in-depth characterization of the polarimetric channel of the Large-Angle Spectrometric COronagraph/LASCO-C2 onboard the Solar and Heliospheric Observatory (SOHO). The polarimetric analysis of the white-light images makes use of polarized sequences composed of three images obtained though three polarizers oriented at + 60 ∘ $+60^{\circ }$ , 0 ∘ $0^{\circ }$ , and − 60 ∘ $-60^{\circ }$ , complemented by a neighboring unpolarized image, and relies on the formalism of Mueller. The Mueller matrix characterizing the C2 instrument was obtained through extensive ground-based calibrations of the optical components and global laboratory tests. Additional critical corrections were derived from in-flight tests relying prominently on roll sequences of SOHO, on the basis of consistency criteria (e.g. the “tangential” direction of polarization), and from several applications, notably the time-dependent tomographic reconstruction of the coronal electron density. This took several years of effort, but resulted in the quasi-uninterrupted photopolarimetric analysis of the corona over two complete Solar Cycles 23 and 24, and the comparison with a variety of eclipse data obtained at different phases of these cycles. Our final results encompass the characterization of the polarization of the white-light corona, of its polarized radiance, of the two-dimensional electron density, and of the K-corona. The agreement with the eclipse data is excellent except for slight discrepancies affecting the innermost part of the C2 field of view. The present work leaves the two unpolarized components, F-corona and stray light, entangled and their complex separation will be dealt with in a follow-on article.

中文翻译：

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24 年来使用 LASCO-C2 日冕仪进行日冕光偏振测量 [1996 – 2019]

我们对太阳和日光层天文台 (SOHO) 上的大角度光谱 COronagraph/LASCO-C2 的偏振通道进行了深入表征。白光图像的偏振分析利用了由三个图像组成的偏振序列，这些图像通过三个偏振器获得，方向为 + 60 ∘ $+60^{\circ }$ 、0 ∘ $0^{\circ }$ 和 − 60 ∘ $-60^{\circ }$ ，辅以相邻的非极化图像，并依赖于 Mueller 的形式主义。表征 C2 仪器的穆勒矩阵是通过光学组件的广泛地面校准和全球实验室测试获得的。额外的关键修正来自飞行测试，主要依赖于 SOHO 的滚动序列，基于一致性标准（例如极化的“切向”方向），以及一些应用，特别是日冕电子密度的时间相关断层扫描重建。这花了几年的努力，但最终对两个完整的太阳周期 23 和 24 的日冕进行了准不间断的光偏振分析，并与在这些周期的不同阶段获得的各种日食数据进行了比较。我们的最终结果包括对白光日冕的极化、其极化辐射、二维电子密度和 K 日冕的表征。除了影响 C2 视场最内侧部分的轻微差异外，与日食数据的一致性非常好。目前的工作使两个非偏振分量 F 电晕和杂散光纠缠在一起，它们的复杂分离将在后续文章中讨论。