Neutral composition in the lower thermosphere

A photochemical model of the F1 region was set up from rocket-borne mass spectrometric measurements of the ionic and neutral composition, and from reaction rates determined in laboratory experiments. This model was applied to determine the seasonal variations of the neutral composition in the lower thermosphere from incoherent scatter ionic composition data. We find an atomic oxygen winter maximum, in agreement with satellite drag and energy balance studies. Moreover, we put in evidence a molecular oxygen winter minimum, which confirms results obtained from Lyman-α absorption measurements on O2, and from twilight 6300 Å airglow studies.