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Coronal Photopolarimetry with the LASCO-C2 Coronagraph over 24 Years [1996 – 2019]

Abstract : 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∘, 0∘, and −60∘, 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.
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https://hal-insu.archives-ouvertes.fr/insu-02886812
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Submitted on : Wednesday, July 1, 2020 - 6:36:12 PM
Last modification on : Saturday, October 10, 2020 - 3:35:46 AM

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Philippe Lamy, Antoine Llebaria, Brice Boclet, Hugo Gilardy, Michael Burtin, et al.. Coronal Photopolarimetry with the LASCO-C2 Coronagraph over 24 Years [1996 – 2019]. Solar Physics, Springer Verlag, 2020, 295 (7), pp.89. ⟨10.1007/s11207-020-01650-y⟩. ⟨insu-02886812⟩

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