Positive ion composition and electron and total positive ion densities were measured on July 30 (S26/1) and August 13 (S26/2), above Kiruna as part of a multinational rocket campaign to study noctilucent clouds (NLC). Two magnetic ion mass spectrometers took measurements at altitudes of 65-126 km in twilight at times of very low magnetic activity and of NLC sightings at 82.5-83.4 km. The hydration order of the most abundant proton hydrates increased with height from n -3 below 76 km to n = 6 and n = 5 at 88 and 87 km, respectively. Temperatures inferred from ion composition revealed a minimum of 104 ø + 20øK at 90.5 km. At this altitude a special ion spectrum with heavy proton hydrates up to H+(H20)12 was measured, which seems to indicate the possibility for ice particle formation through ion nucleation in a narrow layer. The composition measurements were also used to infer mesospheric concentrations of nitric oxide, water vapor, and hydrogen peroxide. The water vapor mixing ratio in the NLC region was found to be around 3 parts per million by volume.

A positive ion composition measurement at the summer, high latitude mesopause in the presence of noctilucent clouds has revealed the existence of very massive positive ionospheric ions. The ions were assigned to be proton hydrates of which the heaviest ones contained up to 20 water molecules. These most massive ions were concentrated in a thin layer at an altitude of 90 km. A likely explantation of the existence of so heavy ionospheric ions is a strong local cooling within the layer in combination with a very low electron concentration. In spite of the high altitude of detection of these ions it is conceivable that they represented condensation nuclei eventually leading to the formation of visible notilucent clouds.

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Results from a multinational rocket experiment in noctilucent cloud (NLC) conditions are presented. Weak NLC was detected at 83 km by visible photometry. Two separate ion mass spectrometer experiments both detected a narrow layer of very heavy positive ions at 90 km. The mass distribution of the large ions showed an increase of “most abundant mass” with height up to 90 km, indicating a temperature decrease up to that altitude. At 90 km the ion size approached the critical size at which nucleation of ice particles can start. If water ice particles would be formed in such a layer, they would continue to grow, as long as the ambient temperature is low enough and the water supply sufficient. While sedimenting through the atmosphere, they would add to the existing population of aerosol particles and thereby increase the total surface area of particles. Such an increased surface area of small particles would increase the loss rate for electrons and cause a deficiency of electrons at heights below the nucleation layer. A pronounced deficiency of electrons below 90 km was found in the measurements. With a theoretically modeled mesospheric H2O concentration, ice particle growth at 90 km is impossible. This situation may, however, change completely under the influence of an upwelling in the mesosphere. In addition to lowering the sink rate of the particles, such an upwelling would increase the concentration of H2O by vertical transport, and lower the gas temperature, which would also enhance the probability of particle growth. The upwelling would have to terminate in a horizontal motion above 90 km. At this height the rocket measurements show marked changes in the atomic oxygen concentration, electron/positive ion ratio and inferred concentrations of nitric oxide and negatively charged aerosol particles in addition to abrupt changes in positive ion composition.

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