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ELECTRICAL STRUCTURE OF THE MIDDLE ATMOSPHERE 187 surements (Arnold and Henschen, 1978) showed the dominance of heavy ions. Laboratory measurements by Viggiano et al. (1980) showed that in the presence of sulfuric acid the species HSO4- would become an important core ion. Sulfuric acid is known to be the major component of the stratospheric aerosol layer and is a byproduct of volcanic activity, as discussed in Chapter 12 (this volume). There thus appear to be three fairly distinct negative-ion strata in the middle atmosphere. The central region between about 55 and 80 km is formed mainly by ion-molecule reactions involving the commoner minor constituents, following initial attachment to O2. This central layer has layers of heavier ions both above and belowâthe upper one probably a result of reactions involving meteoric species and the lower one built by clustering around HSO4 -. At present this is a very sketchy and incomplete picture, and many more observations are needed to clarify it. Figure 13.4 Schematic diagram of the principal negative-ion reactions in the mesosphere. The chain leading to the terminal species HCO3- is probably of minor importance. Figure 13.5 Model calculations of the steady-state positive-ion composition of the middle atmosphere, omitting the reactions leading to nonproton hydrates in the stratosphere. The left-hand panel represents quiet conditions, and the right-hand panelis for the case of an intense solar proton event. Figure 13.6 Model calculations of the steady-state negative-ion composition of the middle atmosphere, omitting reactions involving meteoric species at the higher altitudes and reactions involving sulfur species in the stratosphere. The left-hand panel represents quiet conditions, and the right-hand panel is for the case of an intense solar proton event. Model Calculations If the rates of production of the various ion species and the rates of the important chemical reactions are known, it is possible to calculate the steady-state ion composition. Many such calculations have been made, and examples are shown in Figures 13.5 and 13.6. Figure 13.5 illustrates the positive-ion composition calculated for an intense solar-particle event (righthand panel) and for undisturbed daytime conditions