The first extensive phase scintillation measurements during both overdense and underdense heating at Platteville, Colorado, are described. The data were collected from an aircraft making repeated scans in the vicinity of the heated volume, to establish the overall spatial distribution of intermediate-scale irregularities. During overdense heating, the irregularities maximize in strength near the HF reflection height, and map downward at least 100 km with only moderate weakening. From the relative frequency shifts of the phase spectra measured for different directions of aircraft motion, the irregularity anisotropy and drift have also been estimated. The change in phase spectral shape between 6.2-MHz and 9.9-MHz overdense heating is consistent with thermal self-focusing irregularity generation. Underdense heating produces a different phase spectral signature than is theoretically predicted, although enhanced energy does occur at scintillation-producing scale sizes.
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