Detectability of Electrostatic Decay Products in [ITAL]Ulysses[/ITAL] and [ITAL]Galileo[/ITAL] Observations of Type III Solar Radio Sources

Recent in situ Ulysses and Galileo observations of the source regions of type III solar radio bursts appear to show an absence of ion acoustic waves S produced by nonlinear Langmuir wave processes such as the electrostatic (ES) decay, in contradiction with earlier ISEE 3 observations and analytic theory. This Letter resolves these apparent contradictions. Refined analyses of the maximum S -wave electric fields produced by ES decay and of the characteristics of the Ulysses Wave Form Analyzer (WFA) instrument show that the bursty S waves observed by ISEE 3 should be essentially undetectable by the Ulysses WFA. It is also shown that the maximum S -wave levels predicted for the Galileo event are ≲ the instrumental noise level, thereby confirming an earlier suggestion. Thus, no contradictions exist between the ISEE 3 and Ulysses / Galileo observations, and no evidence exists against ES decay in the published Ulysses and Galileo data. All available data are consistent with, or at worst not inconsistent with, the ES decay proceeding and being the dominant nonlinear process in type III bursts.