A mass spectrometer probe for composition and structure analysis of the middle atmosphere plasma and neutral gas

An instrument for composition and structure analysis of the atmospheric plasma and neutral gas at altitudes below 130 km is described. The probe consists of a quadrupole mass filter which can be operated in three modes allowing composition analysis of atmospheric positive ions, negative ions, and neutral gases. The pressure in the spectrometer is kept low by a liquid helium-cooled cryopump. An electrostatic probe for the measurement of total positive ion densities and an ionization gage for the measurement of the total gas density and thereby also the gas temperature are also integrated in the instrument. Some results obtained by two flights of the instrument during the Western Europe Winter Anomaly Campaign 1975/76 are presented. The measurements indicate drastic enhancements of plasma densities in the D-region as well as characteristic changes compared to normal conditions of the positive ion composition, namely low fractional abundances of cluster ions and high fractional abundances of molecular and atomic ions