In September 1979, a large organized effort was mounted to make ionospheric measurements and optical airglow measurements in conjunction with the launch of satellite HEAO-C from Cape Canaveral. The Atl as-Centaur-powered flight trajectory passed through the F2 layer, and was predicted3 to result in a significant ionospheric depletion. Our own participation consisted of (1) furnishing detailed computer-model predictions, (2) performing incoherent- scatter radar measurements from Arecibo, and (3) optical airglow measurements from the Florida peninsula. We will describe the experimental results and compare these results with prelaunch and post-launch model computations. A set of computed electron-density profiles representing the ionosphere one hour after the HEAO-C launch is shown in Fig. 1. Figure 2 is an image- enhanced photograph of the HEAO-C airglow, as seen from Melbourne, Florida. For the case of an HLLV launch, where the second-stage burn is confined to altitudes below 124 km, the chemistry problem is quite different. Addition of HnO and H2 molecules at those altitudes does not appreciably accelerate ion/electron recombination. However, the molecules survive at those altitudes for weeks, and gradually diffuse into the F-layer where they lead to weak but widespread and persistent depletions of ionization. The calculated depletion amounts to about 10% in the daytime (24 hours after a single launch) and 30% at night. Large quantities of atomic H are produced at a rate that may be sufficient to substantially increase the density of the upper thermosphere (above 1000 km). A potentially serious problem is the formation of high-altitude (70 to 100 km) condensed water clouds. Since the water survives for weeks at these altitudes, it can accumulate over the course of many launches. The HLLV circularization maneuvers, which occur at the apogee near 480-km altitude, do produce F-layer depletions similar to those observed with Skylab or HEAO-C. The calculated depletion extends over a region 1000 by 2000 km and persists for four hours. The peak electron density is reduced to one-third of its normal value. REFERENCES 1. M. Mendillo, G. S. Hawkins, and J. A. Klobuchar, "A Sudden Vanishing of the Ionospheric F-Region due to the Launch of Skaylab," J. Geophys. Res. 80, 2217-2228 (1975). 2. J. Zinn and C. D. Sutherland, "Effects of Rocket Exhaust Products in the Thermosphere and Ionosphere," Los Alamos Scientific Laboratory report LA- 8233-MS (1980). 3. M. Mendillo, J. Baumgardner, and J. A. Klobuchar, "Opportunity to Observe a Large-Scale Hole in the Ionosphere," EOS 60, 513 (July 1979).
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