Enabling Ion-Mode Measurements at Mercury: Calibration of the Strofio Mass Spectrometer with a Low-Energy Ion Beam Facility

The Strofio mass spectrometer, part of the SERENA instrument suite aboard the BepiColombo Mercury Planetary Orbiter (MPO), is designed to characterize Mercury’s exosphere. Strofio employs a time-of-flight (TOF) technique to determine the mass-per-charge (m/q) of neutral species entering its aperture. The instrument can also be configured for direct detection of ambient low-energy ions by disabling the ionization source and adjusting internal electrostatic potentials. Recent BepiColombo Mercury flybys have revealed a substantial population of low-energy ions throughout the Hermean magnetosphere [Harada, 2024], highlighting the importance of this measurement capability. These ions provide important constraints on magnetospheric dynamics and surface-exosphere-magnetosphere coupling, and Strofio has the potential to deliver composition-resolved measurements during nominal orbital operations at Mercury. To support this effort, we have developed a low-energy ion beam facility for ion-mode calibration of the Strofio engineering model (EM). The facility produces stable, well-characterized ion beams spanning energies representative of Mercury’s magnetospheric ion population (< 50 eV). The ion source, electrostatic optics, and beamline are housed in a dedicated vacuum chamber, with the Strofio EM mounted on translation stages to scan across the entrance aperture and map its angular and spatial response. We describe the facility design, calibration approach, and initial commissioning results, including first detection of the ion beam in ion mode and early characterization of the energy-dependent response.

Harada, Y., Saito, Y., Hadid, L. Z., Delcourt, D., Aizawa, S., Rojo, M., et al. (2024). Deep entry of low-energy ions into Mercury’s magnetosphere: BepiColombo Mio’s third flyby observations. Journal of Geophysical Research: Space Physics, 129, e2024JA032751. https://doi.org/10.1029/2024JA032751