Fig. 4. Contours of theoretically calculated electron temperature over Boulder, Colorado for microwave heating by the solar power satellite power beam. The dashed lines give the beam direction and the light solid lines indicate the geomagnetic field direction (8). ent on the heating power-flux density. New time-dependent heating calculations yield predictions in general agreement with the experimental observations. In addition, unexpected natural variability in the ionospheric behavior was observed to affect the heating balance. No explanation for this variability is yet available. A detailed research program is planned using high-frequency (hf) ionospheric modification facilities at Arecibo, Puerto Rico, and Platteville, Colorado. These facilities currently produce SPS-equivalent heating only in the D-region ionosphere, but anticipated upgrading will extend this to cover all ionospheric heights. Studies to determine the threshold and magnitude of ionosphere-microwave interactions and to demonstrate the validity of extrapolating results of these frequency- and power- scaled experiments to the SPS microwave beam will be complemented by representative generic telecommunication tests designed to directly demonstrate the ionospheric interactions impacts on communication systems. If necessary, this research will also investigate possible impact mitigation strategies. Predictions for SPS. At the SPS frequency of 2.45 GHz, the threshold power-flux density for producing significant nonlinear heating is now thought to be approximately 40 mW/cm2. However, even for small power fluxes, enhanced electron heating is predicted to occur, as shown in Fig. 4. This heating is restricted to the neighborhood of the microwave beam. Large electron temperature increases in the ionosphere may produce several important environmental effects. Enhanced electron temperatures will increase hf radio-wave absorption in the lower ionosphere. The associated thermal forces can
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