A Sweeping Neutral and Positive Ion Mass Spectrometer for Atmospheric Composition at Satellite Altitudes

Accurate in situ measurements of atmospheric composition are difficult because of the locally disturbed environment resulting from the presence of the vehicle and its instruments. The quadrupole mass spectrometer described here was designed to be immersed directly into the atmosphere with minimal modification of the local environment from ambient conditions. This design represents a first attempt to develop a sweeping mass spectrometer which performs both neutral particle and positive ion concentration measurements aboard an earth satellite. An open ion source design is utilized to adapt, with minimum degradation, both kinds of particles for the filtering action of the quadrupole field. Six linear ranges of sensitivity are provided to permit complete compositional analysis between altitudes of 300 and 1000 km. The mass range is 0-50 u, and maximum sensitivities are 105 neutral particles per cm3 per volt output and 10 positive ions per cm3 per volt output. Neutral particle and positive ion measurements are made sequentially each requiring approximately 36 seconds to complete using a sweep duration of approximately six seconds. On OGO-IV this sweep duration provided a spatial resolution of 0.8 km per mass unit of sweep. Preliminary results show that the spectrometer is operating within design specifications and the signal-to-noise ratio is excellent. Geophysical interpretations of these data should aid in the formation of more definitive model atmospheres which are important in such diverse fields as weather prediction, manned space flights, and radio propagation.