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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.

The concentrations of atomic oxygen, [O], at heights from 60 to 140 km have been studied by a technique involving the use of a rocket-borne lamp to radiate the Oi triplet at 130 nm. The measurements of both absorption and resonance fluorescence in the atmospheric atomic oxygen have enabled absolute values of [O] to be determined with good relative accuracy over this height range. The method of analysis is described in detail. On some of the flights, attempts were made to measure [O] by oxidation of silver strips, but this technique did not give reliable results. Measurements of electron density, N e , were obtained in association with each flight of a u.v. lamp. The maximum value of [O] was found to occur near 95 km, and its value was usually in the range 1-3 x 10 12 cm -3 , rather larger than predicted by theory or found in the few previous measurements. Its value at 120 km was also large, though in agreement with the suggestion of von Zahn that [O]/[O 2 ] > 1 at that height. On a day of intense winter anomaly, however, a very low [O] peak of 3 x 10 11 cm -3 was measured. The distribution of atomic oxygen above 120 km in winter showed a gradient smaller than that expected for diffusive equilibrium; no summer measurements are re­ported. Often there was a similarity between features observed in the [O] and N e profiles, and some reasons for this are considered. One such feature was the abrupt fall to undetectable levels in [O] with decreasing height around 83 km at night; there was a similar fall in N e in the same region. Certain irregularities in the [O] distributions have scale sizes characteristic of gravity waves.