Effects of cold electron emissions on thermal plasma measurements on board Solar Orbiter spacecraft

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Emissions of photo and secondary electrons influence thermal electron measurements on board spacecraft, typically below a threshold determined by the spacecraft's potential. We aim to examine and quantify this contamination of the observed low-energy electron fluxes. We seek to provide effective constraints for the correction methods used to accurately estimate unperturbed solar wind plasma parameters in the context of the Solar Orbiter mission. We performed a long-term statistical analysis of electron velocity distribution functions acquired by the Electron Analyser System experiment, which is part of the Solar Orbiter's Solar Wind Analyser suite of instruments. We employed analytical fits of time-averaged phase space density spectra to identify the energy break separating ambient solar wind electron populations from cold electron populations emitted by the spacecraft body. We analysed correlations between the observed energy break and the spacecraft potential, as well as other relevant plasma properties. Our analysis indicates that in contrast to other space missions, emitted electrons from the spacecraft are detected even above the spacecraft potential energy. The derived energy break is found to be uncorrelated with the measured spacecraft potential, but it is correlated strongly with the ambient electron temperature. We attribute this behaviour to the Solar Orbiter's geometric configuration, which can result in the detection of electrons emitted from spacecraft surfaces that are located far from the instrument's detector. We derived a theoretical expression for the energy break, assuming Maxwellian distribution functions for both the ambient and spacecraft electrons. This provides an effective constraint for the observed contamination by spacecraft electrons.