Probing the solar corona with the Solar Orbiter/Metis coronagraph: from instrumentation to CME diagnostics
The Solar Orbiter mission has significantly advanced the study of the solar corona and its connection to the heliosphere through its unique orbital configuration and state-of-the-art instrumentation. Among its remote-sensing payload, the SolO/Metis coronagraph provides, for the first time, simultaneous two-dimensional imaging of the extended solar corona in visible light (VL) and ultraviolet (UV) H I Ly-α emission. This dual-channel capability enables novel diagnostics of coronal plasma, bridging observations from the inner to the middle corona and offering new insights into the origin and evolution of coronal mass ejections (CMEs) and eruptive prominences.
The quantitative exploitation of Metis observations relies on a robust radiometric calibration of its dual channels. In-flight calibration strategies based on stellar observations enable the determination of absolute coronal brightness and the monitoring of instrumental stability over time. This calibration framework provides the foundation for reliable plasma diagnostics and ensures consistency with complementary observations from other instruments.
Metis data enable the application of advanced three-dimensional CME reconstruction techniques, including tie-point triangulation, the Graduated Cylindrical Shell model, and the polarization ratio technique. The combination of these methods allows the reconstruction of CME morphology and propagation in three dimensions, as well as the derivation of key plasma parameters such as electron density and mass, which are essential for characterizing CME dynamics and energetics.
A representative example is provided by the analysis of two CMEs observed on 28 October 2021. These events, characterized by exceptionally bright ultraviolet emission from erupting prominences, were investigated through a comprehensive multi-instrument dataset including STEREO-A, SDO, SOHO,PROBA-2, and ground-based H-α observations. The analysis revealed distinct eruption scenarios, ranging from a slow three-part CME associated with acquiescent filament to a fast halo CME originating from an active region. These events provided a testbed for validating the combined use of 3D reconstruction techniques and plasma diagnostics, demonstrating the capability of Metis to track prominence material to large heliocentric distances and to constrain CME physical properties.
A complementary approach exploits the synergy between Metis and the EUI/FSI instrument to achieve seamless tracking of eruptive prominences across the transition from the low to the middle corona. The development of the EUIMET tool enables the construction of continuous mosaics combining extreme-ultraviolet and coronagraphic observations, incorporating advanced image enhancement techniques and configurable opacity levels. This methodology has been applied to the polar crown eruption of 20 April 2023, providing a unified morphological and kinematic characterization of the event through triangulation and time–distance analysis, and demonstrating the potential for systematic multi-wavelength investigations of CME initiation and early propagation.
Overall, the combination of calibrated multi-wavelength observations, advanced three-dimensional reconstruction techniques, and dedicated analysis tools establishes Solar Orbiter Metis as a powerful diagnostic platform for investigating CME evolution. In particular, its unique capability to simultaneously observe the coronal plasma in VL and UV emission, at high spatial and temporal resolutions, provides unprecedented access to the fine structure of CMEs, including the detailed morphology and thermodynamic properties of erupting prominence material and internal substructures. These capabilities contribute to bridging the gap between the low corona and the heliosphere, improving the understanding of the physical processes governing coronal transients, and supporting future developments in heliospheric research and space weather forecasting.
Yara De Leo is an early-career postdoc and a member of the Metis Core Team, currently based at the Astrophysical Observatory of Catania and a guest researcher at the University of Graz, Austria. Originally from Florence, Italy, she has built her scientific career almost entirely around the Metis coronagraph aboard Solar Orbiter. From her early studies through her Ph.D., her work has consistently focused on Metis—from optical design during her undergraduate and master’s research, to the development of operational tools and radiometric calibration strategies. She completed her Ph.D. at the Max Planck Institute for Solar System Research and the University of Catania, where her research was largely dedicated to the in-flight radiometric calibration of Metis’ channels, while also beginning to investigate the first transient and eruptive events observed by the instrument. Building on this work, her current research focuses on the analysis of eruptive solar events, particularly CMEs, as observed by Metis. Her work includes the study of CME morphology, their 3D reconstruction using multiple techniques (triangulation, GCS reconstruction, and the polarization ratio technique), and the investigation of their plasma parameters. She has recently been awarded a research grant in Austria to lead, as PI, a three-year project focused on CMEs observed by Metis.