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.

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