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

摘要

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 theory, suggest they are produced by the electrostatic (ES) decay L → L′ + S. However, contrary to theoretical predictions, an existing analysis of the wave frequencies instead favors the electromagnetic (EM) decays L → T + S, where T denotes an EM wave near the plasma frequency. This conflict is addressed here by comparing the observed wave frequencies and onset times with theoretical predictions for the ES and EM decays, calculated using the time-variable electron beam and magnetic field orientation data, rather than the nominal values used previously. Field orientation effects and beam speed variations are shown analytically to produce factor-of-three effects, greater than the difference in wave frequencies predicted for the ES and EM decays; effects of similar magnitude occur in the events analyzed here. The S-wave signals are extracted by hand from a sawtooth noise background, greatly improving the association between S waves and intense L waves. Very good agreement 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 consistent 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 is kinematically allowed. The wave data are not consistent, however, with the EM decay being the dominant nonlinear process. Often the observed waves are sufficiently broadband to overlap simultaneously the frequency ranges predicted for the ES and EM decays. Coupling the dominance of the ES decay with this frequency overlap provides support for a previous 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 observed 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 frequency. Analysis of the electron beam data reveals that the time-varying beam speed is consistent with ballistic beam propagation with minimal energy 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 observational data are thus shown to be uniformly consistent with the ES decay being the dominant nonlinear process for type Ⅲ bursts near 1 AU, as expected theoretically. Further work is required to determine whether the ES decay appreciably limits the beam's energy loss over and above the effects of linear stochastic growth theory.