ION-ACOUSTIC-WAVE FREQUENCIES AND ONSET TIMES DURING TYPE-III SOLAR RADIO-BURSTS

Conflicting interpretations exist for the low-frequency ion acoustic ( S) waves often observed by ISEE 3 in association with intense Langmuir (L) waves in the source regions of type III solar radio bursts near 1 AU. Two indirect lines of observational evidence, as well as plasma t heory, suggest they are produced by the electrostatic (ES) decay L --> L' + S. However, contrary to theoretical predictions, an existing ana lysis of the wave frequencies instead favors the electromagnetic (EM) decays L --> T + S, where T denotes an EM wave near the plasma frequen cy. This conflict is addressed here by comparing the observed wave fre quencies and onset times with theoretical predictions for the ES and E M decays, calculated using the time-variable electron beam and magneti c field orientation data, rather than the nominal values used previous ly. Field orientation effects and beam speed variations are shown anal ytically to produce factor-of-three effects, greater than the differen ce in wave frequencies predicted for the ES and EM decays; effects of similar magnitude occur in the events analyzed here. The S-wave signal s are extracted by hand from a sawtooth noise background, greatly impr oving the association between S waves and intense L waves. Very good a greement exists between the time-varying predictions for the ES decay and the frequencies of most (but not all) wave bursts. The waves occur only after the ES decay becomes kinematically allowed, which is consi stent with the ES decay proceeding and producing most of the observed signals. Good agreement exists between the EM decay's predictions and a significant fraction of the S-wave observations while the EM decay i s kinematically allowed. The wave data are not consistent, however, wi th the EM decay being the dominant nonlinear process. Often the observ ed waves are sufficiently broadband to overlap simultaneously the freq uency ranges predicted for the ES and EM decays. Coupling the dominanc e of the ES decay with this frequency overlap provides support for a p revious suggestion that fundamental emission occurs when the EM decay is stimulated by the ES decay product waves. The periods in which the ES and EM decays produce observable S waves are consistent with the ob served and (independently) predicted times of fundamental and harmonic radio emission. This supports interpretation of fundamental emission as stimulated EM decay and harmonic emission as the coalescence L + L' --> T of beam-generated L waves and L' waves produced by the ES decay , where T denotes an electromagnetic wave at twice the plasma frequenc y. Analysis of the electron beam data reveals that the time-varying be am speed is consistent with ballistic beam propagation with minimal en ergy loss, implying that the beam propagates in a state close to time- and volume-averaged marginal stability. This confirms a central tenet of the stochastic growth theory for type III bursts. The observationa l data are thus shown to be uniformly consistent with the ES decay bei ng the dominant nonlinear process for type III bursts near 1 AU, as ex pected theoretically. Further work is required to determine whether th e ES decay appreciably limits the beam's energy loss over and above th e effects of linear stochastic growth theory.