BepiColombo/Mio mass spectrum analyzer time-of-flight characteristics and noise removal during cruise and Mercury swing-bys

The mass spectrum analyzer (MSA) on board the ESA/JAXA BepiColombo mission collected some data during its cruise phase and during the different planetary swing-bys. The instrument measures the ions’ energy, and the time-of-flight (TOF) of the ions entering MSA–TOF chamber. A striking feature observed in the data is a large band of detection counts present at similar energies than protons, when a large number of protons are detected by the instrument. This feature was attributed to the noise from uncorrelated start–stop signals combined with energy straggling, causing a delay in the ions’ detection with respect to their theoretical TOF. Another characteristic feature of the data is the signal induced by secondary electrons emitted by positive ions deflected by the linear electric field (LEF) impacting MSA electrode, causing LEF echoes. As protons are the most abundant ionic species of the solar wind (SW), the straggler induced noise and the proton LEF echoes are nearly always present, and contaminate strongly the rest of the measurements. Using the data acquired by MSA in the SW, we derive a generic statistical fit of the noise induced by the proton stragglers, connecting the proton counts to the noise level. By applying our method to re-analyze some of the data collected by MSA during the Mercury swing-bys (MSB), we demonstrate that: (1) the stragglers play an important role in the determination of the spacecraft outgassing rate and confirm that the outgassing follows an exponential decrease and (2) the MSB3 data collected in Mercury’s magnetosphere are heavily affected by the stragglers’ noise. Using our noise subtraction method provides a refined constraint on the heavy ion populations during MSB3 and further strengthens the evidence for ion detections from the O + , Na + , and K + -groups. We also derive a scaling relation between the detection of LEF-H echoes and the detection of protons, allowing us to estimate MSA carbon foil efficiency in situ.