Chemistry of the thermosphere and ionosphere

The ionosphere offers an excellent laboratory for the study of atomic and molecular processes. Densities are low, permitting highly reactive species to accumulate in measurable quantities. Temporal and spatial scales are large, and the solar energy source causes substantial departures from thermodynamic equilibrium. This laboratory has been exploited by the Atmosphere Explorer Program of NASA. Simultaneous measurements of a large number of interrelated atmospheric parameters to altitudes as low as 150 km have provided data that can be analyzed in a quantitative manner to derive precise rates, as functions of temperature, for many important chemical reactions, including the reactions of electronically or vibrationally excited metastable species. Analysis of AE results has provided new information on the photochemical role of N in the thermosphere; the rate coefficients for dissociative recombination of NO+, O2+ and N2+; reactions of O+ with N2, N2+ with O, and O2+ with O; and reactions of metastable ions, atoms, and molecules. The progress in our understanding of ionospheric chemistry during the last few years shows the power of space science measurement programs carefully designed to provide rigorous tests of quantitative theoretical predictions.