Fig. 3. Experimental measurements of electron heating following 6 msec of 430-MHz heating on June 11, 1978. The SPS frequency-scaled power flux equals 25 mW/cm2 at 85 km. Theoretical predictions are from G. Meltz (1980). not very different from its heating counterpart. In the lower ionosphere, both heating and cooling equilibria are reached in tens of milliseconds or less. Relevant Observations. No present facilities can continuously irradiate the ionosphere at the SPS frequency of 2.45 GHz with the SPS power-flux density of 23 mW/cm2. To construct such a facility would be extremely costly. However, at least for the study of ionosphere-microwave interactions, this does not seem necessary. The SPS frequency is many times greater than normal plasma frequencies in the ionosphere, which are typically smaller than 10 MHz and usually do not exceed 20 MHz. Therefore, resonant ionosphere-microwave interactions are not expected to occur. Instead, thermal forces should drive any phenomena that develop. As can be seen in Eq. 9, this heating scales inversely as the square of frequency. Thus, by using lower experimental wave frequencies but still avoiding resonant interactions, the SPS microwave beam can be accurately simulated at much lower radiated powers. Initial tests of the enhanced electron heating theory were made in two series of experimental studies using the 430-MHz radar system of the Arecibo Observatory (National Astronomy and Ionosphere Center) (11). Preliminary results measured only about 100-K increases in electron temperature at 100-km altitude for SPS- equivalent power, compared to theoretical heating predictions of several times this. Experimental results are shown in Fig. 3. The heating pulse length for this experiment was limited to 10 msec. Although this is several times the normal heating time constant, we discovered that the enhanced heating time scale is nonlinearly depend-
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