Fig. 5. Comparison of Arecibo measurements with theory. Source: Reference 30. Pulsed heating experiments, using the 430-MHz radar system at Puerto Rico's Arecibo Observatory, have shown a typical increase in electron temperatures of 100 K, which corresponds to predictions based on the modified electron heating theory (Fig. 5) (30). Future ionospheric heating studies to simulate the SPS ionospheremicrowave interactions with longer pulsed heating are planned at the Arecibo, Puerto Rico, and Platteville, Colorado facilities. Thermal self-focusing is considered to be the most important effect of ionosphere-microwave interactions to be investigated by future experiments. Several other potential plasma phenomena — i.e. parametric and resonant plasma interactions, thermal self-focusing in the lower ionosphere, electric breakdown of the neutral gas, and atypical ionsopheric conditions — are unlikely to lead to SPS ionosphere-microwave interactions (31). To date, no significant telecommunications or climate effects have been experimentally demonstrated; however, research to determine the ionospheric effects of microwave beams is continuing. Results of this research could have a significant effect on the design of the microwave power transmission system, particularly if it were shown that the threshold where ionospheric heating would occur is above the presently assumed level of 23 mW/cm2, occurring at the center of the microwave beam. Higher microwave power densities would lead to a reduction in the receiving antenna area. 8.2.7. Stratospheric pollution by space vehicle exhaust products. The potentially harmful effects of supersonic and space shuttle transport exhaust in the stratosphere are already receiving considerable attention. Injections of water vapor and NOX (which are involved in the complex sequence of chemical reactions governing the abundance of ozone in the region from 20 to 35 k) are projected to result in a reduction of the mean abundance of ozone, although there is still uncertainty regarding the roles of each of these components. The actual effects of any given rate of injection of either of these two components are difficult to determine because of uncertainties regarding the vertical and horizontal movements in the stratosphere which govern the rate at which they are injected, distributed and ultimately removed from it, the lack of experimental observations on space vehicle emissions, the composition of the stratosphere as a function of altitude, location over the surface of the globe, and the nature of the chemical and photochemical reactions
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