The OH radical formed in Reaction (2) can react further with O+. The dominant reaction path is before Reactions (6) and (7) can occur to a significant extent. The result of the two cycles, Reactions (1) and (2) and Reactions (8), (4), and (7) in sequence, is the destruction of two electron-ion pairs by each H2O molecule. Similar processes occur with other common rocket-exhaust products, such as H2 or CO2. With H2, the sequence is (The O('D) is a metastable, electronically excited oxygen atom that decays primarily by emission of 630.0-nm radiation.) The severity, geographic extent, and duration of the F-layer depletions produced by the exhaust product molecules are determined by a combination of interacting processes including chemistry, diffusion, gravitational settling, and advection by prevailing winds. The molecular diffusion rates and settling rates are both rapidly increasing functions of altitude. For a quantitative description of these coupled processes, we built a two-dimensional computer model. Some numerical results from the model are detailed in the following sections. Details of the model itself are amplified in Appendices A and B. The strong ion-removal effects induced by exhaust product molecules are confined to the ionospheric F2 layer above 200 km, where the normally occurring ion species is predominantly monatomic O+. Below 180 km, the dominant ion species are polyatomic (principally NO+ and O2+), and the effective rates of recombination of these ions with electrons are not affected much by the addition of contaminant molecules such as H2O or H2. The fastest process affecting the removal of exhaust product molecules from the F-layer is gravitational settling (regulated, of course, by molecular collisions). The time required for an isolated water molecule to fall across the F-layer from 400- to 200-km altitude is about 2 h. For an H2 molecule, the time is about 6 h.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==