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| Titel |
Evidence for four- and three-wave interactions in solar type III radio emissions |
| VerfasserIn |
G. Thejappa, R. J. MacDowall, M. Bergamo |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 31, no. 8 ; Nr. 31, no. 8 (2013-08-09), S.1417-1428 |
| Datensatznummer |
250086094
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| Publikation (Nr.) |
 copernicus.org/angeo-31-1417-2013.pdf |
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| Zusammenfassung |
| 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. |
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