Analysis of the first coronagraphic multi-band observations of a sungrazing comet

Context. Between 24 and 25 December 2021 a sungrazing comet (SOHO-4341) approached the Sun, being observed by “classical” visible light (VL) coronagraphs on board the SOHO and STEREO missions, and also by the innovative Metis coronagraph on board the ESA-NASA Solar Orbiter mission in the VL and ultraviolet (UV H I Lyman-α) band. Aims. We show how VL data acquired by the Metis coronagraph can be combined with those provided by other space-based coronagraphs to reconstruct the comet orbit, but also to provide information on the dust composition from the polarized VL emission. Moreover, we show how the UV emission can be employed to measure local plasma parameters of the ambient solar wind. Methods. By using the comet positions tracked with VL Metis images (with spatial resolution that is four times better than UV), the UV images (with a time cadence that is five times faster than VL) have been coaligned to maximize the signal-to-noise ratio in the UV band. The local electron density ne was measured from the observed exponential decay of the UV Lyman-α intensity along the tail, while the solar wind speed vwind was measured from the UV Lyman-α tail inclination with respect to the cometary orbital path deprojected in 3D. Moreover, the proton kinetic temperature Tk was also obtained by the aperture angle of the UV Lyman-α tail. Results. When the comet was at an average heliocentric distance of 14.3 R⊙, the comet had a radial speed of 155 km s−1 and a tangential speed of 59 km s−1. The comet had a UV Lyman-α tail extending in the anti-solar direction over more than 1.5 R⊙. From the analysis of the tail shape in UV we obtained the local solar wind speed (vwind = 190 km s−1), electron density (ne = 1.5 × 104 cm−3), and proton temperature (Tk = 1.2 × 106 K). Moreover, theoretical analysis of the measured UV Lyman-α intensity allowed us to estimate the radius of the cometary nucleus (Rcom = 65 m) and the water outgassing rate (QH2O = 4.8 × 1028 molec s−1). Conclusions. These results show that sungrazing comets are unique “local probes” for the ambient coronal plasma, providing measurements that are not as affected by the line-of-sight integration effects as those provided by remote sensing instruments, in regions of the Heliosphere that are not explored in situ by the ongoing space missions.

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