SUMMARY In the context of reviewing the potential environmental impact of SPS construction and deployment on the upper atmosphere, the present study was designed to identify particular atmospheric experiments and theoretical studies which should be given high priority for support; it also served as a follow-up to the initial environmental impact workshops held at Argonne National Laboratory in August and September of 1978 (Brubaker, 1979; and Rote, 1978) *. The present study deals only with effects above 70 km where worldwide rather than localized effects are anticipated. The study was conducted by a panel of 27 scientists and engineers who met in La Jolla, CA, on 25-27 June 1979; the membership is listed in Appendix A. The approach used was to identify the injectants in different altitude regions and to review the anticipated impact of each (see Table S.l) in order to identify critical research requirements (see Table S.2). While emphasis was originally placed on the design of atmospheric experiments, it quickly became apparent that considerable theoretical study effort is needed before one designs dedicated atmospheric experiments. *References are listed in the bibliography, Appendix E. **See Appendix C for a definition of abbreviations, acronyms, and specific technical terms used here. The single most critical problem identified in the present study is the impact on global HF ** radio propagation of a band of permanently depleted ionization in the F-region as a result of launch operations. Some of the hydrogen and water emitted from the exhaust of the second stage of the HLLV rocket in the 70-120 km altitude region diffuses upward and leads to the replacement of atomic 0+ ions with molecular ions H2O and 0H+. Molecular ions recombine with electrons much more rapidly than do atomic ions, and thus replacing atomic with molecular ions leads to a reduction in effective ionization. This effect is significant above 160-180 km only, as at+lower altitudes the main natural atmospheric ions are molecular, NO and O2 The physical extent of the region could cover a band at the latitude of launch (28.5° for Cape Canaveral) of north-south extent of several thousand kilometers, extending around the globe at the latitude of injection. The effective ionization may be reduced by a factor of two at night and by 10-20% in the daytime. The critical consequence of such a reduction in ionization is that it drastically reduces the available HF band that can be used for long-range radio communication at a time when this frequency band is already heavily overcommitted internationally (see Section 3.2.6 for a discussion, Section 5.2 for research recommendations, and item F.3 of Appendix F for a brief account of this problem). Other problems associated with water and hydrogen releases involve the general enhancement in mesopheric humidity and cloudiness with, as opposed to without, SPS operations, including the possible production of long-lasting contrails. These changes may have some climatic impact and could impact
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