Coronal mass ejection plasma diagnostics using Metis coronagraph. Deriving physical parameters of an erupting prominence from simultaneous visible-light and UV observations

We investigate the physical conditions of erupting prominences embedded in coronal mass ejection (CME) cores. The physical parameters of interest were derived by combining the hydrogen Lyman α () and visible-light (VL) images simultaneously observed by Solar Orbiter/Metis. In particular, we focus on the bright CME event that occurred on April 25-26, 2021. Our method is based on 2D non-LTE (i. e. non-local thermodynamic equilibrium) modeling of moving structures to derive the integrated intensity of the line (), together with VL emission. Our method is based on a novel diagnostic tool that combines the emission measure (EM) at a given temperature derived from the observed intensity with the electron column density (N_̊m e) obtained from VL Stokes I and Q data. This approach determines the electron density (n_̊m e) and the effective thickness (D_̊m eff) inside the prominence structure. Here, we use a similar diagnostic tool to the one that we proposed for solar eclipses by combining hydrogen Balmer α (Hα) and VL data in previous studies. We analyzed 32 spatial points within the northern part of the prominence. We ran a 2D non-LTE transfer code for these points by assuming a uniform prominence temperature. The results are presented as 2D maps of the electron density and effective thickness at a given temperature. For the brightest pixel, we also estimated the temperature by assuming collisional ionization equilibrium and compared the result with our value obtained using the emission-measure method. We demonstrate how n_ e and D_ eff inferences depend on the temperature of the structure. The higher the temperature, the lower the effective thickness, and the higher the electron density. This study creates foundation for future diagnostics of eruptive prominences with the Solar Orbiter/Metis coronagraph. It shows that combined UV and VL diagnostics provides a powerful tool for analyzing such events. But it also confirms that the consideration of the helium D₃ line emission within the VL channel is essential to obtain accurate results.