Evidence for wave coupling in type III emissions

Using new capabilities of waveform analyses provided by the S/WAVES instruments onboard the two STEREO spacecraft, we present for the first time a complete set of direct evidence for three-wave coupling occurring during a type III emission and involving two Langmuir waves and an ion acoustic wave. Information on the Doppler-shifted frequencies and especially the phases of the waves are used in order to check first the conservation of momentum and energy, through Fourier analyses, and second the phase locking between the waves, through bicoherence analyses. Wavelet analyses allow us to resolve for the first time the coupling regions, in which spatial length is estimated to be 18 +/- 5 km. The wave packets travel at comparable speed, and the characteristic available interaction time is about 1 s. Interpretations of the phase coupling and evaluation of the growth rate of the waves tend to favor the parametric decay, at least in the observational events considered in this work.