The Composition of the Mesosphere and Lower Thermosphere

L. Thomas


The experimental information available on the neutral and ion composition of the 50-130 km height range is reviewed and, wherever possible, comparisons are made with predictions based on theoretical models. The height distributions for various neutral constituents have been deduced from rocket-borne remote-sensing measurements and in situ sampling experiments, in which recent improvements in mass spectrometer techniques have been effective. The results obtained for oxygen, hydrogen, nitrogen and carbon species are described in turn. In addition, satellite-borne remote-sensing measurements have demonstrated the importance of temporal and spatial variations of molecular oxygen and nitric oxide in the lower thermosphere. In situ sampling with rocket-borne mass spectrometers has established that molecular oxygen and nitric oxide ions produced during or following photoionization represent the major ions down to about 85 km, with layers of metal ions also being present. Below this height, water cluster ions, H$^{+}$.(H$_{2}$O)$_{n}$, predominate. At lower mesospheric levels, negative ions are known to be important but the negative-ion composition has not been established.

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