Detection of collapsing Langmuir wave packets in solar type III radio bursts

Abstract

We report the STEREO/WAVES high time resolution observations of an intense Langmuir wave packet associated with a solar type III radio burst. The transformation of the electric field measurements from the spacecraft into magnetic field aligned coordinate system indicates that this is a field aligned one-dimensional structure. The peak intensity and short duration of this wave packet well satisfy the criterion for it to be the collapsing envelope soliton trapped in a self-generated density cavity. The spectrum of this 1D wave packet consists of an intense peak and two sidebands, corresponding to beam-resonant Langmuir waves, and down- and up-shifted daughter Langmuir waves, respectively, and an ion sound associated low-frequency enhancement. The frequencies and wave numbers of these spectral components satisfy the resonance conditions of the four wave interaction, called the oscillating two stream instability (OTSI). Furthermore, trispectral analysis indicates that these spectral components are coupled to each other with a high degree of phase coherency (high tricoherence). These findings provide a strong indication that the observed wave packet is a collapsing Langmuir envelope soliton formed as a result of OTSI.