Evidence for Langmuir envelope solitons in solar type III burst source regions

We present observational evidence for the generation of Langmuir envelope s olitons in the source regions of solar type III radio bursts. The solitons appear to be formed by electron beams which excite either the modulational instability or oscillating two-stream instability (OTSI). Millisecond data from the Ulysses Unified Radio and Plasma Wave Experiment (URAP) show that Langmuir waves associated with type III bursts occur as broad intense peaks with timescales ranging from 15 to 90 ms (6 - 27 km). These broad field st ructures have the properties expected of Langmuir envelope solitons, namely , the normalized peak energy densities, W-L/n(e)T(e) similar to 10(-5), are well above the modulational instability threshold; the spatial scales L, w hich range from 1 to 5 Langmuir wavelengths, show a high degree of inverse correlation with (W-L/n(e)T(e))(1/2); and the observed widths of these broa d peaks agree well with the predicted widths of envelope solitons. We show that the orientation of the Langmuir field structures is random with respec t to the ambient magnetic field, indicating that they are probably isotropi c structures that have evolved from initially pancake-like solitons. These observations suggest that strong turbulence processes, such as the modulati onal instability or the OTSI, stabilize the electron beams that produce typ e III bursts.