Combined evidence for four- and three-wave interactions in solar type III radio bursts

The high time resolution observations obtained by the STEREO/WAVES experiment show that in solar type III radio bursts, Langmuir waves often occur as intense one-dimensional or three-dimensional wave packets. We present the observations of one of the 1d wave packets as well as one of the 3d wave packets, whose short durations and peak intensities satisfy the threshold conditions for the oscillating two-stream instability (OTSI) and formation of collapsing envelope solitons. The depths, widths and temporal coincidences of the density cavities, observed during these wave packets indicate that they probably correspond to cavitons, generated by the ponderomotive force of the collapsing wave packets. The spectrum of each of the parallel and perpendicular components of the 3d wave packet as well as the spectrum of the parallel component of the 1d wave packet show similar characteristics, namely, each of them contains a primary peak at fpe, two secondary peaks at fpe×fS and a low-frequency enhancement below fS, which, as indicated by the frequency and wave number resonance conditions, and the fast Fourier transform (FFT)-based tricoherence spectral peak at (fpe, fpe, fpe+fS, fpe- fS) are coupled to each other by the OTSI type of four-wave interaction (fpe is the local electron plasma frequency and fS is the frequency of ion sound waves). In addition to the primary peak at fpe, each of these spectra also contains a peak at 2fpe, which as indicated by the frequency and wave number resonance conditions, and the waveletbased bicoherence spectral peak at (fpe, fpe), appears to correspond to the second harmonic electromagnetic waves generated as a result of coalescence of oppositely propagating sidebands excited by the OTSI. Additionally, the spectrum of the 1d wave packet also contains a peak at 3fpe, which as indicated by the bispectral analysis probably corresponds to electromagnetic waves excited as a result of merging of Langmuir waves with second harmonic electromagnetic waves. Thus, these observations for the first time provide combined evidence that (1) the OTSI and related strong turbulence processes play a significant role in the stabilization of the electron beam, (2) the coalescence of the oppositely propagating up- and down-shifted daughter Langmuir waves excited by the OTSI probably is the emission mechanism of the second harmonic radiation, and (3) the Langmuir collapse follows the route of OTSI in some of the type III radio bursts.