Development of Ion Mode for Strofio, BepiColombo/MPO's Neutral Mass Spectrometer

Strofio is a novel neutral mass spectrometer and a member of the SERENA instrument suite aboard BepiColombo’s Mercury Planetary Orbiter (MPO), and is expected to perform the first in-situ measurements of Mercury’s neutral exosphere in early 2027. Strofio operates via ionizing incoming neutrals using an electron bombardment source, producing low-energy ions with roughly the same energy-per-charge (~15 eV/e), then determining mass via time-of-flight analysis. In principle, through disabling the ionization source, Strofio can observe Mercury’s ambient low-energy ion population. The characterization of this low-energy portion of the Hermean plasma environment is a key component of surface-exosphere-magnetosphere coupling. Recent results from BepiColombo flybys have shown a substantial flux of low-energy ions throughout the magnetosphere (Harada, 2022), including energies <100 eV (Harada, 2024; Williamson, 2026). However, BepiColombo’s other particle instruments are not designed for high-resolution, mass-resolving observations of ions in the 10s-eV range. Additionally, MESSENGER’s plasma instrument, FIPS, could observe ions only as low as 50 eV/e (Andrews, 2007). We present here the development and progress of Strofio’s Ion Mode, to perform in-situ measurements of ambient ions in the range 10-50 eV/e. This work has been carried out via SIMION simulations and electrode voltage optimization via Particle Swarm Optimization (PSO). Using a PSO algorithm that creates and tests electrode voltage configurations via SIMION simulations, voltage configurations that allow Strofio to measure ambient low-energy ions have been developed. The SIMION model used was developed by the Southwest Research Institute (SwRI) in San Antonio, TX, where this work will support laboratory calibration of Ion Mode using the Strofio Engineering Model (EM).