Do Langmuir wave packets in the solar wind collapse?

Intense localized Langmuir waves in the solar wind are often associated with type III solar radio bursts. Wave packets with a Gaussian fall-off of electric field with distance are shown to be consistent with strong turbulence simulations. The collapse threshold for these Gaussian wave packets is calculated analytically and shown to be consistent with previous estimates and simulations. We then assess whether intense Langmuir events detected by the SWAVES instrument on the twin STEREO spacecraft during type III bursts are consistent with known conditions for wave packet collapse. Eight different type III events are selected and a total of 167 wave packets analyzed. An approximate analysis of the observed spatial scales and electric fields shows that none of the wave packets are consistent with collapse. The electric field structures predicted for collapsing wave packets, based on the nucleation mechanism and extensive three-dimensional electrostatic and electromagnetic Zakharov simulations, are also fitted to the observed data: about a third of the packets are well fitted by a potential with a Gaussian radial function. Where good fits can be found, the wave packets do not meet the requirements for collapse. Good eigenmode fits are found for wave packets with waveforms well fitted by the structural form of collapsing wave packets. This is because the electric field envelopes of collapsing wave packets and trapped Langmuir eigenmodes are both Hermite-Gauss functions. This rules out the possibility of wave packet collapse and strong turbulence being important processes in type III source regions. Citation: Graham, D. B., I. H. Cairns, D. R. Prabhakar, R. E. Ergun, D. M. Malaspina, S. D. Bale, K. Goetz, and P. J. Kellogg (2012), Do Langmuir wave packets in the solar wind collapse?, J. Geophys. Res., 117, A09107, doi: 10.1029/2012JA018033.