netic field. The instability threshold depends on the cube of the incident wave frequency, so that once again SPS-equivalent experiments can be conducted at lower heating frequencies with much less radiated power. Relevant Observations. Thermal self-focusing of high-frequency electromagnetic radiation has been observed in overdense ionospheric modification experiments. These observations will be extended to the appropriate underdense regime in similar experiments scheduled for the near future. The overdense self-focusing instabilities are observed to develop with the theoretically predicted threshold fields, scale lengths, and growth rates (18). Associated density irregularities form on time scales of seconds to tens of seconds and decay on time scales of minutes. A schematic of a self-focusing experiment conducted at the Arecibo Observatory is shown in Fig. 6. Results of this study, presented in Fig. 7, indicate that the measured striation widths agree with the theoretical predictions of X ~ 1 km for the experimental parameters used (19). However, the thermal self-focusing theory predicts that all irregularities with widths of 1 km and greater should be amplified. Apparently the saturation state of the self-focusing instability preferentially selects the smallest striation width compatible with a given threshold power for growth. In the plane perpendicular to the magnetic field, the striation widths are predicted to be approximately 500 m (20), again in relative agreement with the observations (19). Electron-density perturbations due to the field self-focusing are experimentally estimated at about 5% of the natural background density. In addition to these large-scale electron-density irregularities, overdense ionospheric heating is observed to produce short-scale (meter-size) plasma striations (12). However, they are believed to be produced through some type of resonant interaction (21), and thus are not predicted to be excited by the SPS microwave radiation. Predictions for SPS. Thermal self-focusing is expected to occur for the SPS microwave power beam. The environmental and system impacts of this process will depend on the degree of beam focusing, the size of the resulting large-scale density striations, and the magnitude of the density fluctuations within these irregularities. Best estimates are that the solar power satellite microwave beam should excite ionospheric striations with scale sizes of about 100 m and density fluctuations of Snln — 10% (17). These irregularities may cause scintillation of radio waves using transionospheric propagation, including the SPS pilot beam. In addition, hf radio waves will undergo multiple reflections in the striated region. The modified ionosphere will resemble a natural spread-F environment. If short-scale plasma striations develop in this region, much more serious and wide-ranging communications effects may occur. Coherent scattering of hf, vhf, and uhf radio waves can produce interference in many telecommunication systems, including television and radio. Detailed experimental studies scaled to SPS equivalence are planned to verify the initial predictions that such short-scale plasma striations will not be generated in the SPS microwave beam. CONCLUSIONS Although we have presented a simplistic review of a number of distinct ionospheric phenomena, it is clear that for a complete evaluation of the solar power satellite impacts, we must investigate a complex, coupled set of problems. HLLV propellant emissions may produce large depletions in the ionospheric electron den-
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