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Parametric Decay of Beam‐Generated Langmuir Waves and Three‐Wave Interaction in Plateau Plasmas: Implications for Type III Radiation

Inspired by the results of particle‐in‐cell simulations on beam‐plasma interaction, we present a mechanism for the generation of radio waves in solar type III bursts which provides a closed chain of the conversion processes from beam‐generated Langmuir waves into electromagnetic emission. The mechanism is characterized by two key steps. The first is a hitherto unknown parametric decay process in which the primary wave directly decays to low‐k Langmuir waves (plasma oscillations) and ion acoustic waves. This decay becomes possible due to changes in the dispersion properties of longitudinal waves initiated by the presence of a beam or a plateau in the electron velocity distribution. A resonant three‐wave interaction is established between the beam‐induced Langmuir wave, the plasma oscillations, and the ion acoustic wave. Due to weak damping of the waves involved, this interaction persists after the beam has relaxed to a plateau and lasts as long as the latter exists. In the second step, low‐k Langmuir waves in the range of “optical wavelengths” can linearly couple to obliquely propagating radio waves at the plasma frequency in a wave number region around the cross points where quasi‐longitudinal and quasi‐transverse modes approach each other. Our model does not need the persistence of the beam and the associated instability for radiation to be produced. Although the theory is local with respect to the plasma frequency, it may open a way to overcome Sturrock's ( ) dilemma of rapid beam relaxation on the one side and long‐lived radiation on the other. The observed frequency variation is suggested to arise by “adiabatic” motion of the beam‐created interaction region through the heliospheric plasma of decreasing electron density. Implications for the interpretation of in situ Langmuir waveforms and the diversity of their frequency spectra are discussed.