A review of D-region processes in non-polar latitudes

Abstract

This paper presents the results of a re-examination of three major areas of D-region aeronomy, namely: (a) the height distribution of nitric oxide, (b) the distribution of the effective recombination coefficient and (c) the nature and distribution of negative ions. For nitric oxide, a previous estimate by the author from the solar cycle variation in the D-region electron density is supplemented by some new estimates from: (a) observed diurnal variation of the electron density and (b) from recent measurements of ion composition. These, and a few other ionospheric estimates currently available, as also considerations of the photo-chemistry of nitric oxide, indicate that a distribution consistent with most of these estimates is of the form n(NO) = 4 X 10^-1exp (-3700/T) n(O₂) + 5 X 10^-7 n(O). Values of the effective daytime loss coefficient φ= (1 + λ)d_eff have been obtained from simultaneous use of information on the ionizing radiation and several electron density profiles derived with rocket and ground-based experiments. Examination of these, as also the host of determinations on the effective recombination coefficient α' that have in the past been made from the variations in electron density or absorption with the time of the day or during eclipses and solar flares indicate that: (i) α' has a value around 2 × 10^-7 cm³/sec near 100 km, (ii) the dissociative recombination coefficient for NO⁺ is about the same as that for O₂⁺ (both being around 2 × 10^-7 cm³/sec), and (iii) negative ions are not important above 70 km during day and above 85 km during night. The observational results appear to agree with an empirical relationship of the form α' = 1.5 X 10^-7 + 2 X 10^-6n²(O₂)/n₀²(O₂) A new negative ion theory is presented, based on some recent laboratory measurements of Feerguson et al., in which it is shown that, while during the day the negative ions are predominantly O₂^-, there is a drastic chang immediately after sunset. With the rapid decrease in the concentration of atomic oxygen and increase in that of NO₂, O₂^- is rapidly transformed into NO₂^- which becomes the predominant negative ion at night at all heights below about 70 km.