During their fall, the molecules diffuse laterally, and they react with O+ ions. An isolated H2O or H2 molecule falling across the daytime F-layer has a better-than-90% chance of reacting with an O+ ion, if the O+ concentration has not already been depleted by reactions with other H2O or H2 molecules. In most cases associated with large rocket exhausts, the descending H2O and H2 cloud is dense enough to react with all the O+ ions in its path. The concentration of molecules is overwhelmingly larger than the concentration of O+ ions, so effectively all the ions are removed. The excess H2O and H2 molecules fall through intact. The diameter of the cloud increases with vertical distance fallen, and the cloud continues to spread after the settling has largely ceased. The H2 cloud spreads about four times as fast as the water cloud. If, after destroying the existing ionization, the excess H2O and H2 settled completely out of the F-layer, the ionization would soon return due to the action of solar extreme ultraviolet radiation. In the daytime, the ion replenishment would take about 2 h. However, the H2O and H2 would not settle out completely. Instead, they would maintain a quasi-steady concentration profile, falling off exponentially with altitude (z) approximately as exp[-(z~z0)/H]. Here z0 is the altitude of maximum concentration and H is a scale height equal to about 50 km for H2O and 400 km for H2. The base altitude zo descends with time, but at an ever-decreasing rate. After 8 h it would be about 150 km, and after 24 h it would be 120 km. The H2O and H2 are gradually destroyed chemically, mainly by reactions with O+ ions. The rate of destruction is essentially equal to the rate of production of ions by sunlight — a number of the order 103 cm-3 s-1 at 150- to 200-km altitude. With this small destruction rate, the molecules can survive for several days. It is also notable that the reactions leading to destruction of H2O and H2 all lead invariably to production of H atoms (Reactions 2, 6, 8, 9, and 10). Repeated HLLV launches will lead to production of very large quantities of H atoms, possibly enough to modify the upper thermosphere. Dispersal of the exhaust products is hastened by the ionospheric winds. Typical wind speeds in the F-layer are 300 km h-1, and they exhibit considerable shear (7-12). This enhances the growth rate of the depleted region, but it also speeds up the recovery. Neutral winds in the ionosphere are believed to have a rather regular diurnal behavior. Therefore, to a limited extent, their effects may be regular and predictable. The size and severity of the ionospheric depletion are influenced, of course, by the initial conditions of the problem, that is, by processes that occur early in time and determine the “initial” concentration and spatial distribution of the exhaust products. One such process is the condensation of water vapor to form ice crystals. Such condensation is to be expected on theoretical grounds (6,13,14), and it was observed to occur, in fact, during the translunar injection bum of Apollo 8 (Molander and Wolfhard (15)). A similar ice cloud was observed in the Los Alamos-Sandia Lagopedo experiments (HE detonations in the F-layer) (16). The condensation has an important influence on the rate of gravitational settling of the water. The ice crystals have an evaporative lifetime of about 5 min (6), long enough to fall a great distance before they evaporate. The net effect is that the condensation leads to rapid transport of some of the water to much lower altitudes. The initial settling rates of the exhaust gases are also influenced by “hydrodynamic” processes (as opposed to “diffusive”). That is, at early times when molecular concentrations of those gases are large (i.e. comparable with the back-
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