Fig. 1. Measured total electron content (4) vs time following Skylab launch on line-of-sight from Sagamore Hill Observatory to ATS-3 model computation of TEC vs time for same line-of-sight for the 24 preceding and 4 h following the Skylab launch (3). 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 £>-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 (10G cm-3), the local ionization should be replaced by sunlight-induced photoionization of atomic oxygen in about four hours. Relevant Observations The launch of Skylab I (Saturn V rocket, 1230 EST, 14 May 1973) involved an unusually long second-stage burn through the ionospheric F-region. Nearly simultaneous observations of the formation of a large ionospheric hole were reported (4). The ionospheric total-electron column density was observed to be reduced by 50% or
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