The 430 MHz radar system also serves as the principal ionospheric diagnostic in this experiment. Performing as an incoherent scatter radar, a sensitive receiver system detects the radar signal power backscattered by free electrons in the ionosphere, recorded as a function of time. In this manner, ionospheric electron density altitude profiles were measured before and after a long (0.4 to 9 ms) radar heating pulse. On these short time scales electron number density variations are very small. As a result, differences in these “hot” and “cold” profiles can be interpreted as due to changes in the electron scattering cross section. Because of the known temperature dependence of this cross section, the effective electron heating averaged across the beam could be unambiguously determined. The heating and short diagnostic radar pulses were separated slightly in frequency to avoid signal contamination, with an altitude resolution of 3 km. Figure 5 shows results of this experiment. Although more than 100 °K increases in electron temperature were typically observed, this was much less than the predictions of enhanced electron heating theory at the time. This discrepancy was reduced by explicitly computing the heating time dependence as a function of power, and accurately treating the radar power distribu-
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