The 0(’D) is a metastable electronically excited oxygen atom that decays primarily by emission of 6300-A radiation. As a result of these two cycles, each H2O molecule can induce rapid recombination of two electron-ion pairs. Similar processes occur with other common rocket exhaust products, such as H2 and CO2. The spatial and temporal scale of the ionospheric depletion is influenced by diffusion and convection of the exhaust products from their initial point of deposition. A portion of the water deposited in the F region ionosphere will form ice and fall to D-region heights (65 to 90 km), where its interaction with the normal ionospheric chemistry is much less serious. When the H2O concentration along the rocket trajectory has decreased by diffusion to levels below the normal F-layer ion concentration (10® cm~3), the local ionization should be replaced by sunlight-induced photoionization of atomic oxygen in about four hours. 3.2 Relevant Observations The launch of Skylab I (Saturn V rocket, 1230 EST, 14 May 1973) involved an unusally long second-stage burn through the ionospheric F region. Nearly simultaneous observations of the formation of a large ionospheric hole were reported (21). The ionospheric total-electron column density was observed to be reduced by 50% or more over a period commencing within ten minutes after the launch and persisting for about four hours, as shown in Fig. 9. The depletion apparently extended over a region approximately 2000 km in diameter. These observations were made in the course of routine Faraday rotation measurements of VHF signals from geostationary satellites ATS-3 and ATS-5, as obtained from the Sagamore Hill Radio Observatory in Hamilton, Massachusetts. The ionosphere may have recovered from the depletion in the observed four-hour event duration, or the hole structure may simply have drifted beyond the observational line-of-sight. In either case, the large depletion in ionospheric electron density can be attributed directly to the rocket propellant emissions during launch. On 20 September 1979, an Atlas-Centaur rocket launch of the HEAO-III payload from Kennedy Space Center again involved a significant F-region bum. Although a strong depletion was observed near the launch trajectory, the depletion developed
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