Studying CMEs with Metis on-board Solar Orbiter: constraints from synthetic data analysis

Abstract:

The Metis instrument on-board Solar Orbiter will provide the first ever multi-channel coronagraphic images of Coronal Mass Ejections (CMEs). This unprecedented combination of visible-light (VL) and UV HI Lyman-α images will allow to investigate the thermodynamic evolution of CMEs in their expansion and propagation phase, that is crucially associated with plasma heating and cooling processes. However, the UV HI Lyman-α emissivity of the coronal plasma depends on its density, temperature, outflow velocity, and scattering of chromospheric radiation, making the inversion of future images a complex and undetermined problem. To disentangle the problem, synergies between different analysis tools are required, where VL images will play a pivotal role. A few years ago, we started an effort to develop a complete set of diagnostic techniques and our main results are presented here. In our approach, we heavily relied on realistic 3D MHD simulations of CMEs and used these to derive VL and UV 2D synthetic images. The images are then analyzed and inverted using several techniques that are tested and verified by comparing the diagnostic with the MHD evolution. This effort provides a pathway to accurate estimates of plasma parameters with Metis and the errors associated with these measures. In the latest segment of this work, we are assessing the impact of non-equilibrium ionization effect on Lyman-α emission from the solar corona and, crucially, how much the ionization equilibrium approximation affects the diagnostic. These results will impact the analysis of future data acquired by multi-channel VL and UV Lyman-α coronagraphs such as Metis on-board Solar Orbiter, but also LST on ASO-S mission, and any other future VL+UV coronagraph.