Abstract
Abstract. The high time resolution observations obtained by the STEREO/WAVES experiment show that in the source regions of solar type III radio bursts, Langmuir waves often occur as intense localized wave packets with short durations of only few ms. One of these wave packets shows that it is a three-dimensional field structure with WLneTe ~ 10−3, where WL is the peak energy density, and ne and Te are the electron density and temperature, respectively. For this wave packet, the conditions of the oscillating two-stream instability (OTSI) and supersonic collapse are satisfied within the error range of determination of main parameters. The density cavity, observed during this wave packet indicates that its depth, width and temporal coincidence are consistent with those of a caviton, generated by the ponderomotive force of the collapsing wave packet. The spectrum of each of the parallel and perpendicular components of the wave packet 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 wavelet-based 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. 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.
Highlights
10−3, welhecetrreonWdLenissitthyeapnedatkemenpeerrgaytudreen, sreitsyp,eacntidvenley.aFnodrTthe iasCrwelaitmhvee ateKdieoyewmoirsdsiso.nSs)olar packet, the conditions of the oscillating two-streaomf itnhsteabiPl- ast physics, astrophysics, and aCstrloimnomayte(Raof the Past ity (OTSI) and supersonic collapse are satisfied within the Discussions error range of determination of main parameters
The purpose of this paper is to present the results of (1) the proper analysis of three components of this wave packet, which uses the fast Fourier transform (FFT), as well as the trispectral and bispectral analysis techniques, and (2) the analysis of the density fluctuations measured during the wave packet
Time (UTC − Hours) electromagnetic wave excited as a result of merging of oppositely propagating sidebands excited by the oscillating two-stream instability (OTSI). These findings confirm that the observed wave packet provides unambiguous evidence for OTSI and spatial collapse as correctly concluded by Thejappa et al (2012a,b). These findings suggest that (1) the OTSI probably is responsible for the beam stabilization, (2) the coalescence of oppositely propagating up- and down-shifted Langmuir waves excited by OTSI probably is the excitation mechanism of the second harmonic emission, and (3) Langmuir collapse takes the route of OTSI in at least some type III bursts
Summary
Assuming that the intense peak (L) in the spectra of parallel and perpendicular components corresponds to Langmuir waves excited at the local electron plasma frequency fpe ∼ 30 kHz (Fig. 4), we estimate ne ∼ 1.1 × 107 m−3. These values yield Debye length, λDe = 69Te1/2n−e 1/2 ∼ 6.6 m. The peak electric field strengths of the parallel and perpendicular components are ∼ 25.2, and ∼ 50.1 and ∼ 31.3 mV m−1, respectively,
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