MSX contamination experiment ion mass spectrometer observations during early operations

Initial results from the midcourse space experiment (MSX) ion mass spectrometer (IMS) is presented. The IMS is designed to measure the concentration of contaminant and ambient ionospheric ions from 1 - 56 amu. In addition, the IMS is used to infer the floating potential of the satellite. Ambient ions observed in the IMS data include H+, He+, N+, and O+. The instrument was operated in a 'contaminant ion' mode during the early-operation phase (first seven days) in which the instrument was placed in its most sensitive state. This was done to search for contaminant ions, produced through charge exchange reactions between neutral water vapor and ionospheric O+ ions moving at 7.5 km/s relative to the satellite. Mass peaks near those expected for H2O+ were observed. Finally, the initial results on the satellite charge inferred from changes in the ion current collected as a function of the stopping potential are presented. It is found that the satellite potential floats between minus 6 to minus 10 V with respect to the plasma potential.