Eruptive events with exceptionally bright emission in H i Ly-α observed by the Metis coronagraph

Context. Ultraviolet (UV) emission from coronal mass ejections can provide information on the evolution of plasma dynamics, temperature, and elemental composition, as demonstrated by the UV Coronagraph Spectrometer (UVCS) on board the SOlar and Heliospheric Observatory (SOHO). Metis, the coronagraph on board Solar Orbiter, provides for the first time coronagraphic imaging in the UV H i Ly-α line and, simultaneously, in polarized visible light, thus providing a host of information on the properties of coronal mass ejections and solar eruptions such as their overall dynamics, time evolution, mass content, and outflow propagation velocity in the expanding corona. Aims. For this work, we analyzed six coronal mass ejections observed by Metis between April and October 2021, which are characterized by a very strong H i Ly-α emission. We studied in particular the morphology, kinematics, and the temporal and radial evolution of the emission of such events, focusing on the brightest UV features. Methods. The kinematics of the eruptive events under consideration were studied by determining the height-time profiles of the brightest parts on the Metis plane of the sky. Furthermore, the 3D positions in the heliosphere of the coronal mass ejections were determined by employing co-temporal images, when available, from two other coronagraphs: LASCO/C2 on board SOHO, and COR2 on board STEREO-A. In three cases, the most likely source region on the solar surface could be identified. Finally, the radiometrically calibrated Metis images of the bright UV features were analyzed to provide estimates of their volume and density. From the kinematics and radiometric analysis, we obtained indications of the temperatures of the bright UV cores of these events. These results were then compared with previous studies with the UVCS spectrocoronagraph. Results. The analysis of these strong UV-emitting features associated with coronal mass ejections demonstrates the capabilities of the current constellation of space coronagraphs, Metis, LASCO/C2, and COR2, in providing a complete characterization of the structure and dynamics of eruptive events in their propagation phase from their inception up to several solar radii. Furthermore, we show how the unique capabilities of the Metis instrument to observe these events in both the H i Ly-α line and polarized VL radiation allow plasma diagnostics on the thermal state of these events.