The influence of spacecraft charging on low-energy ion measurements – implications for Comet Interceptor

Low-energy ions play important roles in many plasma environments of the Solar System. In cometary environments, low-energy ions are dominating the inner part of the magnetosphere. Understanding these ions is therefore important for an accurate understanding of the interaction of comets with the solar wind. Unfortunately, measuring low-energy ions is difficult due to the influence by the spacecraft potential. The ions are attracted to or repelled from the charged spacecraft surface before detection, resulting in an energy shift and a change in travel direction of the ions. This results in complicated distortions of the sampled ion distribution which has to be corrected for, often using simulations of the spacecraft-plasma interactions.

In 2019, Comet Interceptor was selected by ESA as a new F-class mission. This mission will visit a long-period and, ideally, pristine comet that is yet-to-be discovered. For the first time, multipoint measurements will be made in the cometary environment using three spacecraft: spacecraft A, developed by ESA, and two sub-probes B1 and B2 developed by JAXA and ESA, respectively. Both spacecraft A and probe B1 will carry an ion spectrometer, but the ion measurements will likely suffer from spacecraft potential effects.

In this seminar, the general difficulty in measuring low-energy ions in the presence of charged spacecraft surfaces will be discussed. Recent simulation work for Comet Interceptor, targeting the spacecraft-plasma interactions, will also be presented.