Coronal mass ejection plasma diagnostics using Metis coronagraph

Deriving physical parameters of an erupting prominence from simultaneous visible-light and UV observations

¹ Faculty of Education, University of Ljubljana, Kardeljeva ploščad 16, 1000 Ljubljana, Slovenia
² Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia
³ Astronomical Institute, The Czech Academy of Sciences, 25165 Ondřejov, Czech Republic
⁴ Center of Excellence – Solar and Stellar Activity, University of Wroclaw, Kopernika 11, 51622 Wroclaw, Poland
⁵ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁶ INAF – Turin Astrophysical Observatory, Via Osservatorio 20, 10025 Pino Torinese, TO, Italy
⁷ University of Firenze, Firenze, Italy
⁸ INAF – Osservatorio Astrofisico di Arcetri, Firenze, Italy
⁹ INAF – Osservatorio Astrofisico di Catania, Via Santa Sofia 78, 95123 Catania, Italy
¹⁰ Institute of Physics, University of Graz, Universitätsplatz 5, 8010 Graz, Austria
¹¹ INAF – Osservatorio Astronomico di Trieste, Via G.B. Tiepolo, 11, I-34143 Trieste, Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 16 September 2025
Accepted: 16 March 2026

Abstract

Context. We investigate the physical conditions of erupting prominences embedded in coronal mass ejection (CME) cores.

Aims. The physical parameters of interest were derived by combining the hydrogen Lyman α (Lα) 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.

Methods. 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 Lα line (E_Lα), 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 Lα intensity with the electron column density (N_e) obtained from VL Stokes I and Q data. This approach determines the electron density (n_e) and the effective thickness (D_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.

Results. 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.

Conclusions. 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.