x coordinate is horizontal, z is vertical, and the xz plane coincides with a geomagnetic meridian plane. In a computation designed to model some aspects of the Skylab launch, we chose a magnetic meridian (x,z) plane that intersects the launch trajectory plane 1300-km downrange (at an angle of 70°). The particular meridian plane was chosen because it includes the Sagamore Hill-ATS-3 observational line-of-sight. Initial conditions for the computations were taken from the 270-km footprint contours in Fig. 2, but with the following additional adjustments: (a) It was assumed that ten percent of the water molecules, upon their arrival on the 270-km horizontal plane, were in the form of 0.1-/zm diameter ice crystals (percentage estimated by Bernhardt (14)). These ice crystals subsequently fell under gravity while evaporating, with the falling rates and evaporation rates calculated by Zinn et al. (6). This process led to a fairly rapid redistribution of some of the water molecules from 270 km down to about 150 km. The computer code contains automatic algorithms to generate this redistribution. (b) The initial mass density of H2 and H2O molecules at 270-km altitude was comparable with, and in some cases larger than, the local density of the air. This would have led to Taylor-unstable mixing and hydrodynamic settling. The mixing would have proceeded rapidly until the excess density was reduced to about 10% over ambient, or perhaps less. The computer code contains automatic algorithms to simulate the effects of this mixing process. After these preliminary manipulations, the computer model enters its free-running mode. The model includes chemistry, photochemistry, and diffusion with gravitational and electromagnetic terms, and horizontal winds, under assumed conditions of quasi-steady flow. Further details are described in Appendices A and B. The code output includes two-dimensional contours of concentrations of H2O, H2, H, and electrons at predetermined intervals of time. It also produces vertical profiles of concentrations of ions and of neutral molecules, and vertical integrals of electron concentrations (TEC) for comparisons with Faraday-rotation data. Figure 3 shows some results. The figure consists of computed contours of H2O, H2, and electron concentrations one hour after the launch. By this time, according to the model, the exhaust gases have diffused 1000- to 2000-km laterally. The spreading is fastest at 300- to 500-km altitude, and it is faster for H2 than for H2O. The peak electron concentration directly under the trajectory has been reduced to about onetwentieth of its original value. However, along the Sagamore Hill-ATS-3 line-of- sight, the reduction is less severe. (The approximate line-of-sight is shown in the figure.) The influence of the winds is evident. The centers of the H2 and H2O clouds have been displaced about 200 km in the magnetic southward direction. The plasma hole moves downwind with the exhaust cloud, influenced only slightly by the geomagnetic field. The Sagamore Hill-ATS-5 line-of-sight is also represented schematically in Fig. 3 as a vertical line on the magnetic northward side of the trajectory plane. The computations show the ionospheric hole blowing rapidly away from this line of sight. The same is indicated by the data (1). Figures 4a, b, and c contain similar contour plots from the same computation, but for a time 3 h after the launch. By this time, according to the computation, the center of the exhaust product cloud has blown out of the Sagamore Hill-ATS-3 line-of- sight, and the electron density integral is beginning to return to normal. It also blows out of the Sagamore Hill-ATS-5 line of sight. Figures 5a and b are plots of total electron content vs time for the Sagamore
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