Ion chemistry of the lower thermosphere

Abstract

Three questions of ion photochemistry in the lower thermosphere (80–130 km) are discussed. First, a complete explanation of the behaviour of metal ions in the D- and E-regions still presents a problem. Experimental data show that the width of the metal ion layers decreases as altitude increases from around 90 to 130 km. There seems also to be a diurnal variability of this width at altitudes below 100 km. The Fe + Mg + ratio changes from 2.2 in the lower part of the region to 0.7 in the upper part, the latter value being close to the Fe + Mg + ratio in chondrites. Secondly, at D-region altitudes, below 90–95 km, a principal role is played by the photochemistry of the cluster ions, governing the ionization recombination cycle. The ratio of cluster to normal ion concentrations, ƒ +, shows strong variability, depending on local time, season, various disturbances, etc. The main aspects of that variability have been described in terms of a strong ƒ + dependence on atmospheric temperature, humidity and ionization rate. As a next step in the ion composition data systematization the variability of the parameter R (describing the proportion between proton hydrates and other clusters) is considered. It is shown that R is a maximum in summer during NLC events and minimum during WA events. Thirdly, the major ions above 90 km are NO + and O + 2. Their concentration ratio at a fixed height changes depending on local time and on various disturbances. The behaviour of this ratio is in principle described by ion chemistry (dissociative recombination and ion-molecular reactions), and it is possible to estimate neutral nitric oxide concentrations in the E-region from the ion composition data.