Characterization of unexpected anomalies in the Metis UV channel on board Solar Orbiter

Metis is a solar coronograph, on board the ESA/NASA mission Solar Orbiter, based on a two-channel architecture for broadband polarization imaging in the visible range (580 to 640 nm) and narrow-band imaging in the UV (121.6 nm). The UV channel detector is constituted by an intensified camera equipped with a single-stage microchannel plate (MCP) intensifier coupled to a rad-hard CMOS detector. After the launch on 10 February 2020, some anomalies have been observed in the UV channel data, consisting of unexpected signal variations which are not originated from solar phenomena. In particular, the anomalies appear to be unrelated to the other payload elements, except for the detector itself. Post-processing algorithms have been studied to correct the issue, but a more radical solution is desirable also for the benefit of future missions based on similar MCP detectors. In this work, we investigate a possible origin of the anomalies by exploring the effects of external electric fields on an equivalent detector prototype in a controlled laboratory environment. The obtained results show a significative similarity between the observed variations and the effects of the electric fields on the internal gain of the prototype MCP. A mitigation strategy has been identified by acting on the high voltage bias of the intensifier, and will be soon deployed to the on-orbit instrument. An in-depth analysis of the new acquired data is therefore planned for the next months to verify the impact and the effectiveness of the proposed solution.