drive potentially serious nonlinear interactions, resulting in beam self-focusing and further development of ionospheric irregularities. If generated, these irregularities can scatter hf, vhf, and uhf radio waves, producing widespread communication interference on many systems using transionospheric propagation (12). In addition, accompanying changes in ionospheric density and conductivity may induce climate modifications through effects on upper-atmospheric solar-terrestrial coupling mechanisms (13). Despite identification of these potential environmental impacts, much more research is needed before any definitive statements can be made. At this point in our very limited studies, no significant environmental impacts associated with enhanced ionospheric heating have been experimentally demonstrated. On the basis of these preliminary experimental results, we believe the instability threshold for SPS excitation of nonlinear ionospheric interactions is greater than the current microwave beam power-flux density design limit of 23 mW/cm2. Collective Plasma Phenomena Differential ohmic heating of the ionosphere gives rise to electron temperature gradients, convective plasma motions, and macroscopic thermal forces capable of exciting plasma instabilities. The large-scale ionospheric responses to these induced heating effects can generally be described as collective plasma phenomena. This large-scale plasma behavior is driven by dynamic macroscopic thermal forces, as opposed to the microscopic kinetics of resistive heating effects. Beam Self-Focusing Theory. A considerable amount of attention has recently been directed at wave self-focusing, a macroscopic plasma phenomenon. Natural density fluctuations cause small variations in the index of refraction of a plasma, resulting in a slight focusing and defocusing of an electromagnetic wave propagating through the medium. The electric-field intensity increases as the incident wave refracts into regions of comparatively underdense plasma. Ohmic heating (14) and the electricfield ponderomotive force (15) then drive plasma from these focused regions, amplifying the initial perturbation. This self-focusing process continues until hydrodynamic equilibrium is reached, creating field-aligned striations within the plasma. This process is illustrated schematically in Fig. 5. Thermal self-focusing has been shown to develop at much lower power fluxes than those required for self-focusing driven by the ponderomotive force (16). Thermal self-focusing theories are currently limited to threshold calculations and usually involve geometric restrictions. In the magnetic meridian plane, the thermal selffocusing threshold power flux can be expressed as (17) where n is the ambient electron density, To is the ambient electron temperature,/ is the microwave frequency, and CF is roughly unity, depending on spatial and temporal growth rates. This expression is valid for underdense ionospheric heating (f>fp), which is the case for the SPS ionosphere-microwave interactions. It is not possible to determine a threshold power in the usual sense for thermal self-focusing because the instability threshold power depends nonlinearly on the excited striation width. As the incident power flux increases, smaller striations can become unstable. Also, the power flux necessary to amplify any particular striation size becomes very small when the self-focusing beam nearly parallels the geomag-
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