There is no consensus on the mechanisms producing the intense bursty Langmuir wave packets routinely observed in source regions of interplanetary type III radio bursts. Using data from the Ulysses Fast Envelope Sampler (FES) near 4 AU, new analyses of the distributions of wavefields clearly separate the Langmuir waves into “intense localized structures” (ILSs) and “other waves”. The wave amplitude distributions also show that the observed ILSs, but not the other waves, are inconsistent with pure stochastic growth theory (SGT). Furthermore, it is shown that the ILSs are unlikely to be driven thermal waves or generated by pure exponential growth and damping. The wave distributions, structure, and spacing of the ILSs are compared to predictions for collapsing Langmuir wave packets described by the Zakharov equations, in isolated density depressions and in fully developed strong turbulence, either beam driven or driven at long wavelengths. The results show that the observed ILSs are very unlikely to be collapsing wave packets in any of the cases considered. The observed ILSs are inconsistent with the predictions of self-organized criticality, electron holes, and refractive focusing of waves. Most are also inconsistent with the predictions of kinetic localization, but this mechanism cannot be eliminated for all ILSs. Other mechanisms not eliminated include trapping of Langmuir waves in density depressions at levels below the collapse threshold, superposition of such trapped waves and “free” Langmuir waves, and superposition of trapped waves with a wave packet formed by beam-driven Langmuir waves beating with products of electrostatic decay.
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