the computations. We made a considerable effort to arrive at a reasonable model for the winds (see Appendix B). Our current results with respect to wind fields on the Skylab launch trajectory are quite different, and probably more reliable, than our previous published results in Ref. 6. We still conclude, as in Ref. 6, that the ionospheric hole blew out of the observational lines of sight and did not actually recover in 4 h as the TEC records seem to imply. However, it is fair to ask what other evidence exists to support our current belief that the dominant F-layer wind direction was magnetic southward, rather than some other direction. Some supporting evidence comes from the general character of the diurnal TEC variations observed during the month of May 1973, which are shown by Mendillo et al. (1). The steady increase in electron column density from morning through afternoon implies an F-layer wind system that is dominantly northward in the morning and southward in the afternoon, according to our computer model. Also, the similar character of the post-launch TEC records for the several different lines of sight (1) is further evidence of a southward wind. It is noted by Mendillo et al. (1) that a geomagnetic storm occurred on the afternoon of May 13, and that somewhat depressed TEC levels were therefore to be expected on 14 May, the day of the Skylab launch. The depressed electron column densities would probably be associated with enhancements in ionospheric east-west E fields and vertical plasma drifts. E-field effects are not included in this set of computations. 4. HLLV SECOND-STAGE BURN AT 75- TO 124-km ALTITUDE The current Solar Power Satellite HLLV launch scenario, designed to minimize F-layer problems, calls for burning the second-stage engines only up through 124-km altitude. This leads to an eccentric initial orbit, which is later circularized by a brief bum near the apogee point at 477 km. With respect to F-layer depletion problems, this launch scenario is definitely preferable to the “direct insertion” alternative where the second-stage engines are burned all the way to the circular-orbit altitude. The ion chemistry in the 75- to 124-km altitude range (D and E layers) is very different from that which occurs in the F-layer. The normally occurring ion species are NO+ and O2+ instead of O+. These ions rapidly recombine with electrons, but they are continually replenished in the daytime by solar photoionization. At night they largely disappear. Addition of water vapor or H2 does not have a very drastic effect on these processes. Diffusion and gravitational settling are both much slower at these altitudes than they are in the F-layer. Horizontal spreading of the exhaust products would be caused primarily by fluctuating horizontal wind shears, and might amount to 1000 km per day. A gradual upward diffusion would also occur, bringing some H2 and H2O molecules into the F-layer in the course of 1 or 2 days. We ran a computation that represents a portion of the HLLV second-stage bum trajectory, covering a ground distance of 300 km and an altitude range between 118 and 123 km, with deposition of 2.54x 1031 molecules of H2O and 8.5x 1030 molecules of H2. The launch was assumed to occur at Cape Canaveral at noon. The computed results showed, as expected, that exhaust-cloud diffusion and settling rates are quite slow. In the course of 24 h, some water and H2 does diffuse up to the F2 layer, where it reacts with O+ ions. Because the molecular diffusion rate is very slow, the O+ ions are replaced by solar photoionization about as fast as they are
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