A feedback model for the source of auroral kilometric radiation

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Abstract
The self‐excited oscillations which result from positive wave feedback are believed to produce the discrete spectrum of auroral kilometric radiation. This would require a closed wave feedback path inside the source, having a length equal to a multiple of the wavelength, and it yields an emission model which is quite analogous to an optical laser oscillator. The end reflections for this feedback path, which would be comparable to those produced by the laser mirrors, are attributed to partial wave reflections at the source boundaries. However, in order to compensate for the wave refraction inside the source, the boundary reflection surfaces must converge with altitude, and this implies that the most likely auroral kilometric radiation source would be a thin, local density enhancement, since the refractive index contours at its boundaries would be expected to slope inward. The ISEE observations of multiple spectral components, which are attributed to separate oscillations at different altitudes in the same enhancement, suggest a source thickness as small as 25 km and an internal wave growth threshold of roughly 40 dB, rather than the 70‐120 dB previously believed necessary to account for auroral kilometric radiation without feedback. But more significantly, the feedback model accounts for numerous aspects of the auroral kilometric radiation behavior, predicts emission at the wave growth saturation level, and leads to the conclusion that auroral kilometric radiation originates at many compact sites, each emitting a nearly monochromatic wave.

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