SMALL-SCALE STRUCTURE OF O2+ AND PROTON HYDRATES IN A NOCTILUCENT CLOUD AND POLAR MESOSPHERIC SUMMER ECHO OF AUGUST 9 10 1991 ABOVE KIRUNA/

A novel mass spectrometer designed to measure simultaneously positive ion composition in the mesosphere, was successfully launched during th e NLC-91 project. Instruments supporting the mass spectrometer were a probe to measure both electrons and positive ions as well as a wave pr opagation experiment. The location of the Noctilucent Clouds (NLC) was determined by a particle impact sensor to detect secondary electrons and ions from the impact of NLC particle. The density of proton hydrat es and of the related total ions is depleted in the NLC region at 83 k m. An improved detection limit of 5 x 10(4) m-3 for positive ions and improved height resolution revealed for the first time large gradients in the O2+, H+(H2O)2 and H(H2O)6 densities within a small height rang e of the order of 50 m. Such gradients at the altitude of NLC and Pola r Mesospheric Summer Echoes (PMSE) are associated with strong variabil ity of mesospheric water vapor, temperature and neutral air density.