Radio wave scattering in the outer heliosphere: Preliminary calculations

Detailed first estimates are presented of angular broadening in the outer heliosphere due to scattering of radio waves by density irregularities. The application is to the 2-3 kHz radiation observed by Voyager. Two plausible turbulence models, which account very well for scattering within 1 AU, are extrapolated beyond 10 AU. Both models predict significant angular broadening in the outer heliosphere, accounting semiquantitatively alone for the source sizes inferred from roll modulation data. Predictions are presented for radial variations in the apparent source size if scattering is important. Comparisons with available data argue that scattering is important (and indeed is the dominant contributor to the apparent source size) and that the radiation source is located in the outer heliosphere. Other evidence that scattering is important, such as the fluctuations in apparent source direction and intensity, are also identified. The effects of scattering should be included in future analyses of the 2-3 kHz emissions.