BEAT-TYPE LANGMUIR WAVE EMISSIONS ASSOCIATED WITH A TYPE-III SOLAR RADIO-BURST - EVIDENCE OF PARAMETRIC DECAY

Recent measurements from the plasma wave instrument on the Galileo spa cecraft. have shown that Langmuir waves observed in conjunction with a type III solar radio burst contain many beat-type waveforms, with bea t frequencies ranging from about 150 to 650 Hz. Strong evidence exists that the beat pattern is produced by two closely spaced narrowband co mponents. The most likely candidates for these two waves are a beam-ge nerated Langmuir wave and an oppositely propagating Langmuir wave prod uced by parametric decay. In the parametric decay process, nonlinear i nteractions cause the beam-driven Langmuir wave to decay into a Langmu ir wave and a low-frequency ion sound wave. Comparisons of the observe d beat frequency are in good agreement with theoretical predictions fo r a three-wave parametric decay process. Weak low-frequency emissions are also sometimes observed at the predicted frequency of the ion soun d wave.