DYNAMICS OF LANGMUIR AND ION-SOUND WAVES IN TYPE-III SOLAR RADIO-SOURCES

The evolution of Langmuir and ion-sound waves in type III sources is i nvestigated, incorporating linear growth, linear damping, and nonlinea r electrostatic decay. Improved estimates are obtained for the wavenum ber range of growing waves and the nonlinear coupling coefficient for the decay process. The resulting prediction for the electrostatic deca y threshold is consistent with the observed high-field cutoff in the L angmuir field distribution. It is shown that the conditions in the sol ar wind do not allow a steady state to be attained; rather, bursty lin ear and nonlinear interactions take place, consistent with the highly inhomogeneous and impulsive waves actually observed. Nonlinear growth is found to be fast enough to saturate the growth of the parent Langmu ir waves in the available interaction time. The resulting levels of pr oduct Langmuir and ion-sound waves are estimated theoretically and sho wn to be consistent with in situ ISEE 3 observations of type Ill event s at 1 AU. Nonlinear interactions slave the growth and decay of produc t sound waves to that of the product Langmuir waves. The resulting pro bability distribution of ion-sound field strengths is predicted to hav e a flat tail extending to a high-field cutoff. This prediction is con sistent with statistics derived here from ISEE 3 observations. Agreeme nt is also found between the frequencies of the observed waves and pre dictions for the product S waves. The competing processes of nonlinear wave collapse and quasilinear relaxation are discussed, and it is con cluded that neither is responsible for the saturation of Langmuir grow th. When wave and beam inhomogeneities are accounted for, arguments fr om quasi-linear relaxation yield an upper bound on the Langmuir fields that is too high to be relevant. Nor are the criteria for direct wave collapse of the beam-driven waves met, consistent with earlier simula tion results that imply that this process is not responsible for satur ation of the beam instability. Indeed, even if the highest observed La ngmuir fields are assumed to be part of a long-wavelength ''condensate '' produced via electrostatic decay, they still fall short of the rele vant requirements for wave collapse. The most stringent requirement fo r collapse is that collapsing wave packets not be disrupted by ambient density fluctuations in the solar wind. Fields of several mV m-1 exte nding over several hundred km would be needed to satisfy this requirem ent; at 1 AU such fields are rare at best.