The Influence of Spacecraft Charging on Low‐Energy Ion Measurements Made by RPC‐ICA on Rosetta

Spacecraft charging is problematic for low‐energy plasma measurements. The charged particles are attracted to or repelled from the charged spacecraft, affecting both the energy and direction of travel of the particles. The Ion Composition Analyzer (RPC‐ICA) on board the Rosetta spacecraft is suffering from this effect. RPC‐ICA was measuring positive ions in the vicinity of comet 67P/Churyumov‐Gerasimenko, covering an energy range of a few eV/q to 40 keV/q. The low‐energy part of the data is, however, heavily distorted by the negatively charged spacecraft. In this study we use the Spacecraft Plasma Interaction Software to model the influence of the spacecraft potential on the ion trajectories and the corresponding distortion of the field of view (FOV) of the instrument. The results show that the measurements are not significantly distorted when the ion energy corresponds to at least twice the spacecraft potential. Below this energy the FOV is often heavily distorted, but the distortion differs between different viewing directions. Generally, ions entering the instrument close to the aperture plane are less affected than those entering with extreme elevation angles.