Photometric Comparison of Metis and LASCO-C2 Polarized Brightness Images

Abstract

The Metis coronagraph onboard Solar Orbiter and the LASCO-C2 coronagraph onboard SoHO both acquire white light polarized brightness (pB) images of the solar corona. When the Sun–Solar Orbiter distance is less than 0.85 AU, i.e., outside orbital segments around aphelia, the range of elongations covered by the fields-of-view of the two instruments overlap significantly, allowing a quantitative comparison of their images. We report on such a comparison during September 2022, with images taken during a superior conjunction of the two spacecraft with the Sun, as well as close to that event. In each comparison, the two instruments observed the corona from opposite viewpoints, within ≈1∘ in both Carrington longitude and latitude, with Metis at a distance of about half an astronomical unit from the Sun. We find that the Metis measurements are systematically larger than those of LASCO-C2 throughout the corona, with the Metis-to-C2 ratio of pB exhibiting a median value of ≈1.6. The discrepancy is observed comparing essentially simultaneous observations, so it cannot be explained as an effect of coronal dynamics. Synthetic images of the solar corona computed from a stationary three-dimensional magneto-hydrodynamic model, replicating the geometry of the observations, are photometrically consistent. This rules out the small departure of the two instruments from observing from opposite viewpoints, or their different distance to the Sun, as the cause of their discrepant measurements. We conclude that the reported discrepancy has its root in the calibration methods of the two instruments, which should be further investigated.