more over a period commencing within 10 min after the launch and persisting for about 4 h, as shown in Fig. 1. 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. Because of the potential impacts of these depletions on telecommunications systems, Los Alamos National Scientific Laboratory and Sandia Laboratories jointly sponsored two rocket experiments (code-named Lagopedo) to investigate the expected chemistry modifications produced by H2O and CO2 (5). The experiments generated ionospheric depletions by many of the same processes that are active following the deposition of rocket-propellant emission products; consequently, the Lagopedo data are directly pertinent to the SPS problem. The rockets’ explosive payloads deposited detonation products H2O, CO2, and N2 into the F-region ionosphere. Both experiments successfully produced ionospheric depletions. The first experiment, fired in local daylight, produced a visible ice cloud as a result of adiabatic expansion of the H2O, which expanded to a diameter of over 100 km within 30 sec. The cloud was visible due to scattered sunlight. No cloud was seen in the second shot, fired after local sunset. Both releases were accompanied by measurable airglow emissions at 6300 A [O('£>)] and 5577 A [OfS)]. It is also probable that the launch of large spacecraft produces similar global disturbances of electron density in the ionosphere. At observation points approximately 2000 km from the launching site, fluctuations in electron density of 5% to 10% were detected following the launch of Soyuz 19 and Apollo spacecraft (6). A decrease in density was recorded several minutes after each launch, followed by a quasiperiodic recovery with a period of about 90 min and persisting for several hours. Multiple reflections of hf radio waves were observed during the ionospheric disturbance. These disturbances resemble the ionospheric response following sudden commencement of a magnetic storm. The High-Energy Astrophysical Observatory satellite HEAO-C was launched from Cape Canaveral at 0530 GMT (0030 local time) on September 20, 1979, aboard an Atlas-Centaur rocket. The launch trajectory, which was almost due east, was unusual in that the Centaur second-stage engines burned to an altitude of 466 km, depositing large quantities of exhaust gases directly into the F-layer. Experimental observations confirmed that, as predicted by computer models, a significant F-layer depletion did occur over a region roughly 600 km in north-south extent. The depletion persisted until dawn. High-altitude density profiles measured using the Arecibo Observatory incoherent-scatter radar indicated that electron density in magnetic-flux tubes directly connected to the depleted region showed no apparent rocket-induced changes. Predictions for SPS Details of rocket-exhaust induced ionospheric depletion processes depend on many factors, including vertical and horizontal diffusion and convection, sunlight-
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