Radio wave scattering in the outer heliosphere

The Voyager 1 and 2 plasma wave instruments have observed low-frequency (1. 5-4 kHz) radio waves apparently generated near the heliopause. The waves ar e found in two distinct bands. Power at the lower frequencies centered on 1 .78 kHz shows no modulation when the spacecraft is rolled about the Earth-s pacecraft line, indicating that the radiation is isotropic. Fewer in the hi gher band centered on 3.11 kHz shows roll modulation as high as 60%, indica ting that the source of the radiation is quite compact, subtending an angle <1 rad at the spacecraft. Simple estimates of the scattering of 3.11 kHz r adiation from electron density fluctuations indicate that, if the radiation had originated from the distance of the heliopause, the scattering would b e so large that no roll modulation should be observed. Here we show that th ese earlier scattering estimates were too high because they ignored the lat itude variation of scattering and the inner scale of the electron density f luctuation spectrum. With these two effects properly included, the expected scattering is consistent with the observations and the postulate that the radiation originates from near the nose of the heliosphere.