Parametric interaction and spatial collapse of beam‐driven Langmuir waves in the solar wind

Abstract

This paper presents observations of the parametric decay and spatial collapse of Langmuir waves driven by an electron beam streaming into the solar wind from the Jovian bow shock. High‐resolution frequency‐time spectrograms from Voyager 1 and 2 show that long wavelength Langmuir waves upstream of the bow shock are very effectively converted into short wavelength Langmuir waves which are no longer in resonance with the beam. This conversion is shown to be the result of a nonlinear interaction involving the beam‐driven pump, a sideband emission and a low level of ion‐acoustic turbulence which always appears to be present in the solar wind. The onset of the interaction occurs at about the time that the amplitude of the pump wave saturates, which indicates that parametric processes are probably playing an important role in stabilizing the electron beam. Detailed examination of the electric field waveforms shows that the beam‐driven Langmuir wave emission breaks up into a very complex sideband structure with both positive and negative Doppler shifts. Positive frequency shifts correspond to waves propagating away from the sun and negative frequency shifts correspond to waves propagating toward the sun. In some cases the sideband emissions consist of isolated wave packets with very short durations, sometimes lasting only a few msec. These short duration bursts, which are usually very intense, are thought to consist of envelope solitons which have collapsed down to spatial scales of only a few Debye lengths.