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Observational Evidence for Beat Phenomenon in Complex Solar Type III Radio Bursts

We present new observational evidence for one of the most important three wave interactions, called the electrostatic decay instability (ESD) in the source regions of complex solar type III radio bursts (L is the electron beam-excited Langmuir wave, and and S are the ESD excited daughter Langmuir and ion sound waves, respectively). The STEREO in situ wave observations in the source regions of complex type III bursts show that Langmuir waves often occur as one-dimensional magnetic field aligned beat-type wave packets, with peak intensities well in excess of the threshold for excitation of ESD, and with spectra containing (a) two closely spaced narrow peaks (L and ) corresponding probably to the beating modes responsible for the beat patterns at frequencies very close to the local electron plasma frequency, f pe , and (b) narrow peaks at ion sound frequencies, f S , which are very close to beat frequencies. Using the FFT and higher order spectral techniques, we show that the frequency, wavevector and phase resonance conditions required for excitation of ESD are well satisfied for these wave packets, and the speeds of electron beams derived from the resonance conditions agree reasonably well with those derived from the drift rates of the associated type III events. We also show that the merging of (L) and ( ) most probably is the excitation mechanism of the second harmonic radio emission of these type III bursts.