much more rapidly and was smaller in extent than predicted. In addition to numerous optical diagnostics and total electron content measurements made near the rocket path, detailed ionospheric observations also were taken using the Arecibo incoherent scatter radar. No rocket-induced ionospheric effects were detected at this range of approximately 1000 km south of the trajectory, although as shown in Fig. 9 a strong natural collapse of the ionosphere occurred just prior to the rocket launch. This collapse may have enhanced the depletion observed at the closer sites. In addition, high altitude radar observations measured the electron density within magnetic flux tubes passing through the depleted region. No rocket-induced loss of electron density was measured, further suggesting that ionospheric depletions associated with large rocket launches are localized to the launch trajectory. Details of rocket-exhaust induced ionospheric depletion processes depend on many factors, including vertical and horizontal diffusion and convection, sunlight- 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 (20). Preliminary results indicate the potential for substantial reduction of the total ionosphere, as shown in Fig. 10. 4. IMPACTS ON COMMUNICATIONS Numerous telecommunications systems rely on ionospheric reflections or use transionospheric propagation as part of the communications signal path. Any system that can significantly modify the ionosphere has the potential to produce additional telecommunications paths that may be useful or a source of interference. The large-scale ionospheric depletions associated with launch vehicle emissions could have many consequences. Decreases in ionospheric electron density will directly affect high-frequency telecommunications systems that depend on radio waves reflecting from the ionosphere. If the depletions are localized near the launch site, as expected, only those radio links having the launch site near midpath are likely to be affected. The problem could be solved by changing the operating frequency or the route. Additional impacts are much more speculative. The ionospheric electron temperature profile, electrical conductivities, and wave-particle interactions will be affected to some extent. Furthermore, the argon plasma exhaust produced while boosting the satellite from its construction low-earth orbit is predicted to generate additional ionospheric heating (18). The ionosphere-magnetosphere coupling will be altered and satellite drag may increase. The modifications produced by the microwave beam are confined to the region at and near the intersection of the microwave beam and the ionosphere. If the modification produces radio scatterers in the ionosphere above about 110 km, they will be elongated and aligned along the Earth’s magnetic field. If radio scatterers are produced below about 110 km, they are likely to be isotropic (cf. sporadic-E scattering of VHF signals). If the scatterers are produced, effective transmission paths could be established with proper planning. Observations of field-aligned scattering at VHF and UHF display strict aspect sensitivity explainable as specular reflection from long linear scattering elements (1). Thus the pairs of radio terminals that could communicate are calculable, given the volume containing the field-aligned striations. It should be emphasized that these
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