power fluxes, significant increases in electron temperature can occur, as seen in Fig. 2. Predicted electron temperatures and the electron heating rate at 90 km are shown in Fig. 4 for two different power fluxes of the SPS beam. Thermal runaway temporarily exists for the heating at 30 mW/cm2, but this has no significance for steadystate operation of the SPS. However, even for the proposed SPS power level of 23 mW/cm2 significant heating develops. In addition, these results show that the heating saturation limit is reached only after tens of milliseconds, much longer than normal heating time scales. Recent experimental tests of this enhanced electron heating theory are described in the next section. 2.1.2 Experimental Observations. Initial tests of the enhanced electron heating theory were made in two series of experimental studies using the 430 MHz radar system at the Arecibo Observatory (National Astronomy and Ionosphere Center) (8). The rapid electron heating and cooling initially predicted for the lower ionosphere suggested that pulsed heating experiments using high peak powers would be sufficient to initiate enhanced electron heating. The 430 MHz radar system at Arecibo operates with 2.5 MW peak pulse power and a maximum 10 ms pulse length with a 6% duty cycle. Coupled with the gain of the 305 m Arecibo reflector, this system delivers ~ 15 W/m2 (in the center of the radar beam) to 100 km altitude. This is roughly twice the current SPS design as well as twice the estimated threshold for exciting enhanced electron heating.
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