instability threshold, the strength of enhanced plasma waves directly depends on the local power of the pump electric field. In addition, because of exact frequency and wave-number matching conditions for both the parametric wave-plasma interaction and the radar incoherent backscatter process, these enhanced waves are detected at only one altitude. As a result, systematic scanning of the narrow radar beam across the ionospheric interaction region of the enhanced plasma waves yields a two- dimensional cross section characteristic of the electric field intensity. These maps of electric-field strength clearly show self-focusing striations and large-scale structuring of the illuminated plasma (12). The results of a self-focusing experiment conducted at Arecibo from 2 June .through 17 June 1977 are shown in Fig. 8. When the radar beam is fixed (Fig. 8a), signal modulations are induced by the natural ExB drift of striations through the beam. Immediately afterward, rapid scanning of the radar beam (as shown in Fig. 8b) detected a series of striations in the interaction region. Typical striation dimensions deduced from observations over many such scans are 1.2 km in the north-south plane and 1.0 km in the east-west plane, in good agreement with thermal selffocusing theory. Striation velocities are on the order of 25 m/s, with components of 20 m/s to the east, and smaller than 15 m/s in the north-south direction. North-south velocity measurements are complicated by the strong spatial dependence of the striation width in the magnetic meridian plane. The enhanced plasma waves detected in these striations are observed approximately 5 km higher in altitude than photoelectron enhanced plasma oscillations seen outside the focused region (14). Interpreting this difference as a shift of the plasmadensity profile, the magnitude of the density fluctuations in the striation can be estimated by with Sne — 5%. Induced large-scale structuring of the ionosphere by thermal selffocusing, therefore, is much like a natural spread-F environment. Future experiments using upgraded HF ionospheric modification facilities at both Platteville and Arecibo are designed to simulate SPS underdense thermal selffocusing. Improved diagnostics will verify the instability threshold and scaling laws, determine the irregularity geometry and secondary phenomena, and directly measure the striation’s effect on telecommunication systems. 2.2.3 Short-Scale Striations. Short-scale (meter-size) striation generation has been observed during overdense HF ionospheric modification experiments (1,15). These irregularities can affect not only HF communications links, but also many VHF and UHF systems. Several theories have been proposed explaining the development of short-scale striations. All of these theories require resonant or parametric interactions between the exciting electromagnetic wave and the ionospheric plasma. As a result, short-scale plasma striations are not expected to be produced by the underdense SPS microwave beam. The proposed underdense HF ionospheric-heating experiments will demonstrate this effect, if it occurs. The following section describes other potential plasma phenomena that have been investigated, but are considered unlikely for SPS ionosphere-microwave interactions.
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