A Radiation Environmental Study for the Metis Coronagraph on board Solar Orbiter

The solar cycle 24 was the weakest of the last hundred years. All predictions available in the literature for the solar cycle 25 indicate a similar or even weaker period of solar activity. As a result, the highest galactic cosmic-ray flux of the last century will strike the Solar Orbiter spacecraft along its orbit. Conversely, only one solar energetic particle event (SEP) per year (average predictions) is expected during the cruise phase of the mission in the fluence range 106-107 protons cm-2 above 30 MeV.An instrument dedicated radiation environmental study will be carried out for Metis, the coronagraph on board Solar Orbiter. Pre-launch Monte Carlo simulations aiming to estimate the overall dose absorbed by the Cerium treated polarimeter lenses indicated an average dose absorption of 2000 Gy for the extended mission, while the lenses showed a few % transmittance loss with a 106 Gy of gamma radiation. Monte Carlo simulations will be also performed to study energetic particle single hits and tracks in the images of the visible light and ultraviolet detectors. On the other hand, dark images provide precious clues for cosmic-ray monitoring and images background estimates for both Metis and EUI (Extreme-Ultraviolet Imager) detectors. Collaboration with the EPD (Energetic Particle Detector) instrument scientists and data from cosmic-ray experiments in orbit during the Solar Orbiter mission will allow us to study cosmic-ray variations along the spacecraft orbit and their effects on the instrument performance.