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

Vol. 680
9. The Sun and the Heliosphere

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

The continuous monitoring of the solar corona with space-based visible light (VL) coronagraphs over the last decades has led to the discovery of a multitude of small comets (with nuclei of a few tens of meters) with perihelia usually smaller than 2 solar radii. The origin of these comets called 'sungrazers' is not yet fully understood, but many originate from the breakup of a larger comet that passed just above the solar surface. Now for the first time, thanks to the availability of the multiband coronagraphic observations acquired by the Metis instrument on board the ESA Solar Orbiter mission, it is possible to study both the dust tail in visible light (VL) and the neutral Hydrogen coma in the UV Lyman-alpha emission. The Metis observations of a sungrazer on 24 and 25 December 2021 allowed us to estimate both the equivalent radius of the rocky component (about 10 meters) from VL photometry, and of the icy component (about 65 meters) from UV emission. The combination of multiple spacecraft observations permitted a reconstruction via triangulation of the dust tail shape in 3D, leading us to conclude that the tail followed the path expected from mechanics and solar radiation pressure (implying a rather homogeneous distribution of dust grain sizes and compositions), with no evidence of additional forces related with the electric charging of the grains. Moreover, the analysis of the shape of the UV coma allowed us to infer the physical properties of the solar wind plasma (outflow speed, electron density, and proton temperature) at heliocentric distances (8-15 solar radii) and heliolatitudes (31-37 degrees) that are usually hard to explore with remote sensing observations, and not covered by in situ instruments on board current missions exploring the heliosphere, showing that these comets can be considered as 'local probes' of the solar wind plasma.