Theoretical vertical profiles of minor ions at the Equator

Abstract

A full solution to the problem of calculating equatorial vertical profiles of minor ion concentrations in the F-region requires a large amount of computing time. Full scale calculations by Moffett and Hanson to produce a vertical profile of He + concentration have shown that V(He +) ≏V(O +) in the daytime in the altitude region where He + diffusion is important. At mid latitudes similar results hold for O ++ and N +. This paper demonstrates that by assuming V( X) = V( O +), where X is the ion under consideration, the calculations can be made much simpler and an acceptable profile can be obtained. The method is tested by calculating an He + profile and comparing it with previous full scale results. Further calculations are then carried out for O ++ and N + in order to investigate certain production and loss processes. Comparison is made with the experimental profiles of Hanson and co-workers. It is suggested that the O ++ production rate at 300 km from photoionization of O + is such that the results of Hanson et al. require an O ++ loss rate of about 3 × 10 −2 s −1 at that altitude. Loss of O ++ by reaction with neutral atomic oxygen (with a rate coefficient of about 4 × 10 −17 m 3 s −1) gives better agreement with experiment in the photochemical equilibrium region than does loss by reaction with N 2. It is shown that, if the total production rate P(N +) of N + ions is assumed to vary with altitude z as P(N +) ∝ exp (−z/ H(N)) where H(N) is the scale height of neutral atomic nitrogen, then theory and experiment agree in that the N + concentrations are similar in the 200–300 km region and the heights of the N + peaks are around 450 km. The required P(N +) at 200 km is about 4 × 10 8 m −3 s −1.