induced photoionization and photodissociation, and a large number of chemical reactions. Large computer models that incorporate these many factors affecting ionospheric chemistry are being constructed to predict the effects of the HLLV launches envisioned as part of the SPS transportation system (3). Preliminary results indicate the potential for substantial reduction of the total ionosphere, as shown in Fig. 2. A single HLLV second stage would emit about 7.8 x 10" H2O molecules and 2.4 x 1031 H2 molecules. If all of these molecules went into the F-layer, they could recombine 1.8 x 1032 ion-electron pairs, or about twice the number present in the entire global ionosphere. However, before reaching any catastrophic conclusions, the atmospheric models must include accurate treatments of diffusion, gravitational settling, and convection effects, as well as a realistic description of the HLLV launch trajectory and associated vehicle effluents. These large-scale ionospheric depletions could have many consequences. Decreases in ionospheric electron density will directly affect high-frequency (hf) telecommunication systems that depend on radio waves reflecting from the ionosphere. Additional impacts are much more speculative. The ionospheric electron temperature profde, electric conductivities, and wave-particle interactions will be affected to some extent. The ionosphere-magnetosphere coupling may be altered and satellite drag may increase. Auroral behavior and airglow intensities could also change. Several potentially serious environmental impacts may accompany these modifications of ionospheric structure and behavior. In addition to direct hf radio-wave effects, changes in the ionospheric propagation properties of other electromagnetic waves may lead to impacts on many telecommunication systems. The role of solarterrestrial coupling in triggering climatic changes is not yet well-understood; however, several theories suggest that changes in the upper atmospheric conductivity and composition may lead to climate modifications. All of these possibilities, as well as many others, must be studied in much greater detail before we commit ourselves to constructing solar power satellites. IONOSPHERE-MICROWAVE INTERACTIONS Enhanced Electron Heating The collisional heating and cooling processes of the ionospheric plasma are all dependent on the electron temperature. For sufficiently strong radiation, the rate of heating may increase much faster than the normal cooling interactions, initiating a rapid increase in the electron temperature that continues until compensating processes set in that limit the temperature rise. The net result is a phenomenon originally described as an electron thermal runaway (7). We now understand that the compensating processes that saturate the heating develop quickly enough to preclude an actual runaway in electron temperature, although significantly enhanced electron heating can occur. This heating then can affect the electron-ion recombination rates, changing ionospheric densities, or drive secondary nonlinear ionosphere-microwave interactions, further disturbing the ambient plasma. These disturbances can produce potentially serious telecommunication impacts. Electron Heating Theory. Holway and Meltz (7), investigating the effects of strong radio-wave heating of free electrons in the lower ionosphere, first introduced the
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