Fig. 7. Experimental observations of wave self-focusing. The top figure shows the backscattered signal modulation induced by the natural drift of selffocusing striations through the fixed radar beam. The lower figure presents a series of striations as observed from rapid scanning of the radar beam across the interaction region immediately after the drift measurements. The dashed curve estimates the unstriated beam. sity; collisional damping of the SPS microwave beam will significantly enhance electron temperatures in the lower ionosphere; beam self-focusing can generate large-scale density irregularities in the upper ionosphere. In addition to their independently assessed effects, the interactive nature of these phenomena must be considered. A comprehensive research program to perform the SPS environmental impact assessment has been organized and is being administered by the US Department of Energy. Historically in the study of ionospheric physics, theory has been relatively poor at predicting experimental results, although quick to explain observations once they have been made. For this reason, it is premature to draw conclusions based on the ionospheric studies to date. Although theoretical investigations have identified sev-
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