Future experiments are designed to produce continuous, SPS equivalent heating within the same spot size through the ionosphere. As shown in Eq. 1, ionospheric heating scales inversely as the square of the radiowave frequency. As a result, ionospheric heating equivalent to that of the SPS microwave beam may be achieved at lower radiated powers by heating at lower wave frequencies. This scaling is valid as long as the radiowave does not excite resonant plasma interactions. Other high- frequency ionospheric studies have produced both resonant and ohmic plasma heating effects; application of these results to the SPS environmental assessment must distinguish the responsible mechanism producing the observed effects. The recent experiments at Platteville in which the SPS beam was simulated in the lower ionosphere show that the SPS as presently proposed with a peak power density of 23 mW/cm2 will not adversely impact the performance of VLF (3-30 kHz), LF (30-300 kHz), and MF (300-3000 kHz) telecommunications systems. Small amounts of cross modulation were observed on signals in the range 0.6-5.0 MHz that passed through the heated volume, but the impacts were insignificant. Further ionospheric heating studies to simulate SPS ionosphere-microwave interactions are planned using HF facilities at Arecibo, Puerto Rico and Platteville, Colorado. 2.2 Collective Plasma Phenomena Differential ohmic heating of the ionosphere produces electron-temperature gradients that can cause convective plasma motions and excite plasma instabilities. This large-scale plasma behavior is driven by macroscopic thermal forces, as opposed to the microscopic kinetics of resistive heating effects. The resulting changes in ion-
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