Phase coupling in Langmuir wave packets: Evidence of four wave interactions in solar type III radio bursts

摘要

It is reported that one of the Langmuir wave packets in a type III solar radio burst is observed as a localized field structure with a short duration ～3.2 ms and high intensity exceeding the strong turbulence thresholds. The peak intensity and short time scale well satisfy the criterion for the observed wave packet to be the collapsing Langmuir envelope soliton. The spectrum of this wave packet contains a resonant peak at the local electron plasma frequency, f _(pe), a Stokes peak at a frequency slightly lower than f _(pe), anti-Stokes peak at a frequency slightly higher than f _(pe), and a low frequency enhancement below a few hundred Hz, which satisfy the frequency and wave number resonance conditions of the oscillating two stream instability (OTSI) type of four wave interaction. Here, for the first time, we apply the trispectral analysis technique, and show that the spectral components of this wave packet, namely, the beam-generated Langmuir wave, Stokes and anti-Stokes modes are coupled to each other with a high degree of phase coherency (high tricoherence). This conclusively shows that the observed characteristics of the wave packet provide evidence for the OTSI.