Low‐frequency radio emissions in the outer heliosphere

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Abstract
Low‐frequency radio emissions at 2 and 3 kHz were observed by the plasma wave receivers on both Voyager 1 and Voyager 2 during the interval 1983–1987 at radial distances from the Sun greater than 17 AU and 13 AU, respectively. We present a report on progress toward a model in which the emitted radio waves are generated near multiples of the plasma frequency ƒp by nonlinear processes involving electrostatic Langmuir waves at a source in the outer heliosphere. Constraints on the emission processes and source characteristics are discussed. The observed spectral flux density of ∼10−17 W m−2 Hz−1 corresponds to an inferred minimum brightness temperature of ∼1015 K, comparable to the most intense type III solar radio bursts and more intense than the 2ƒp radiation generated at the Earth's bow shock. Minimum Langmuir wave electric fields in the source region, based on the kinematics of the radiation processes at 2ƒp, lie in the range ∼3–30 µV/m for nominal source and electron beam parameters. This field strength is plausible based on the intensity of Langmuir waves observed upstream of planetary bow shocks in the solar wind.