Metis: the Solar Orbiter Visible Light and Ultraviolet Coronal Imager Scientific Objectives

The Metis coronagraph onboard Solar Orbiter (Antonucci et al. 2019) will image the full off-limb corona, with the purpose of tracing its dynamics and evolution on a global-scale and at high temporal resolution. It will acquire for the first time simultaneous images of the full off-limb corona and inner heliosphere in broadband (580 - 640 nm) polarized visible light (VL) and narrow-band (121.6 ± 10 nm) ultraviolet (UV) H I Lyman-α line, with unprecedented temporal coverage and spatial resolution. For instance, a spatial scale of about 2000 km is achieved in visible light when the spacecraft is at the closest perihelion at 0.28 AU. The field of view (FoV) of the coronagraph spans over a wide range of heliocentric distances owing to the eccentric orbit of the spacecraft, thus permitting the study of the solar atmosphere from 1.7 to about 9 solar radii, during the nominal observation windows along the orbit. Metis measurements allow a complete characterization of the main properties and dynamics of the most important constituents of the coronal and solar wind plasma, i.e., electrons and protons. For instance, during the Remote Sensing Windows they will provide coronal maps of the solar wind outflow velocity in a continuous way, also giving the longitudinal distribution of the wind velocity in the out-of-ecliptic phases.

The Metis instrument versatility combined with the characteristics of the Solar Orbiter mission, addresses all the four key scientific questions of Solar Orbiter mission, by providing unique contributions to investigating the following scientific issues: Energy deposition and outflows in the expanding corona. Role of magnetic field lines in channeling the coronal wind. Coronal fluctuations and their role in the solar wind acceleration. Coronal mass ejection onset and early propagation. Eruption of prominences and their propagation in the corona. Global evolution of the streamer belt. Acceleration of the solar energetic particles.

Solar Orbiter remote sensing and in situ instruments, as well as observations obtained by simultaneous heliophysics space missions, will complement the Metis observations and facilitate our understanding of the scientific questions being addressed.