Generation mechanism of the low-frequency radio emissions in the outer heliosphere

Abstract

We present a model for the generation of Langmuir waves near the inner heliospheric shock. We further invoke the nonlinear interaction between the Langmuir waves resulting in electromagnetic radiation at the second harmonic of the plasma frequency, which is able to leave the source region. This model provides a quantitative explanation of the remote observation of radio emissions at 2–3 kHz by the Voyager plasma wave instrument in the outer heliosphere. If these radio emissions are generated by Langmuir waves, the Langmuir wave electric fields at the remote source are estimated to be 100–200 μV m−1; the minimum value calculated assuming saturation of these emission processes is ∼10 μV m−1. The reality of the emissions detected by the Voyager plasma wave instrument, the location of the source, and expected in situ observation of the heliospheric shock are also discussed.