2.3 Other Effects Although the preceding sections have discussed phenomena believed to be of most importance to the SPS microwave beam interactions study, several other potential processes have been considered. Either because of frequency constraints or high instability thresholds, these effects are not expected to be excited by the SPS microwave beam. Several of these effects are described briefly in the following sections. 2.3.1 Parametric ami Resonant Plasma Interactions. Parametric excitation of ionospheric plasma waves can develop for multiple-frequency electromagnetic radiation. In this case, parametric interactions occur not only at the wave frequency, but also at resonances of the difference frequencies. This process has been treated theoretically (16) and observed in recent HF experiments at Arecibo (17). These interactions may be important because the SPS microwave power beam does not operate at a single frequency. Parametric interactions might be excited by beat waves generated within the finite bandwidth of the downcoming beam. In addition, the upgoing pilot beam operates at a frequency slightly separated from the downcoming beam. Again, parametric instabilities might be produced if the difference frequency of these waves coincides with a multiple of a resonant ionospheric plasma frequency. Stimulated scattering of electromagnetic waves in the ionosphere is another process that can significantly disturb the ambient plasma. Stimulated diffusion scattering probably will not occur for the SPS microwave beam, but may be generated in HF experiments designed to simulate ionosphere-microwave interactions. Plasma heating and density modifications and a decrease in HF power flux reaching the higher altitudes would be associated with this phenomena. Careful experimental studies of multiple-frequency parametric interactions and stimulated scattering should resolve any questions about potential SPS microwave beam effects and application of the HF experimental results. 2.3.2 Thermal Self-Focusing in the Lower Ionosphere. In addition to thermal selffocusing of electromagnetic waves in the F region of the ionosphere, an analogous phenomenon may occur in the collision-dominated lower ionosphere. In this process, the nonlinear radio wave focusing is driven by changes in the electron collision frequency (18). In a sense, this is thermal self-focusing driven by enhanced electron heating. This process is more rapid than its F-region counterpart, developing on a time scale of tens of milliseconds. This effect may have been detected in a previous HF experiment; a detailed study is planned as part of the future HF experimental research program. 2.3.3 Electric Breakdown of the Neutral Gas. A radio wave propagating through a partially ionized plasma accelerates free electrons in its electric field. Under normal conditions, collisions of these electrons with neutrals of the background gas leads to ohmic heating of the plasma. However, for a sufficiently intense electric field, the ionization frequency of the accelerated electrons may increase rapidly, causing electrical breakdown of the neutral gas (19). This may lead to runaway ionization of the 20 to 100 km region, affecting communications systems up to UHF frequencies. The threshold field required to initiate electric breakdown has been estimated to be above 4 x 10B mV/m; the maximum SPS electric field is approximately 4 x 105 mV/m.
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