irregularities in the F region of the ionosphere and could cause large amounts of excess heat to be deposited in the D region. The F region irregularities could induce fading or scintillation on satellite-to-ground communications links. Radio waves propagated between various locations on the earth that pass through, or are reflected by, the heated D region will suffer increased attenuation that could result in the signal strength falling below the ambient noise level or the receiver sensitivity. The number and the frequency of launches of heavy lift launch vehicles needed to establish and maintain the SPS also may lead to significant changes in the ionosphere and magnetosphere. These changes could result from the exhaust from the launch vehicle engines and it is expected that they would be characterized by large areas of depleted electron concentration in the upper (250-450 km) regions of the ionosphere. These changes could lead as well to profound changes in the performance of telecommunications systems whose electromagnetic energy is propagated through the ionosphere. Considerable knowledge exists concerning the effects on the ionosphere of large amounts of heat propagated into it from below. Most of the knowledge results from ionospheric heating studies in the early 1970s sponsored by the Department of Defense Advanced Research Projects Agency. The experimental efforts and supporting theoretical studies were directed toward understanding the heating of the ionosphere using high frequency electromagnetic waves reflected from the ionosphere. The heating in this case results from parametric interactions between the ionosphere and the heating radio wave. In the case of heating by the SPS power beam, the radio wave will not be reflected by the ionosphere but will pass through. In this case, the heating is due to an ohmic-type interaction between the ionosphere and the SPS power beam. Information regarding launch effluent effects on ionospheric characteristics is primarily limited to that observed (and consequently explained theoretically) during the launch of Skylab. The data pertaining to launch effluent effects preclude inferring detailed temporal and spatial changes in ionospheric structure resulting from the launch activity. Experimental results are just becoming available on the effects on the ionosphere of high-powered radio waves whose frequencies exceed the F-region critical frequency, giving rise to underdense heating. Further theoretical and experimental studies are needed to assess the impact of
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