On radio wave scattering in the outer heliosphere

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Abstract
Recently it has been proposed that scattering by density irregularities causes significant angular broadening of the 2–3 kHz radiation observed by the Voyager spacecraft beyond 10 AU. This proposal is based on analogies with scattered radio emissions within 1 AU, certain characteristics of the 2–3 kHz radiation, and preliminary calculations of angular broadening. This paper describes further angular broadening analyses. Predictions are presented for the scattered source size and the intensity modulation during Voyager rolls, based on the parabolic wave equation formalism and a density spectrum extrapolated from within 1 AU. Comparing the theory with data provides strong evidence that angular scattering dominates the intrinsic source size and produces the large source sizes observed, and that the source is located in the outer heliosphere. Subjects discussed include the level and radial dependence of density turbulence in the outer heliosphere, variations of the apparent source size and modulation index with radial distance and radiation frequency, the onset of modulation for radiation near 2 kHz, the formalism’s theoretical limitations, and future research.

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