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

We present observational evidence for the generation of Langmuir envelope solirons in the source regions of solar type III radio bursts. The solirons 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 (627 kin). These broad field structures have the properties expected of Langmuir envelope solirons, namely, the normalized peak energy densities, WL/n•T• • 10 -• are well above the modulational instability threshold; the spatial scales L, which range from 1 to 5 Langmuir wavelengths, •how • high d•g• of inv• •o•l•ion wi•h (W•/•r•)•/•; •d •h• ob•v•d widths of these broad peaks agree well with the predicted widths of envelope solirons. We show that the orientation of the Langmuir field structures is random with respect to the ambient magnetic field, indicating that they are probably isotropic structures that have evolved from initially pancake-like solirons. These observations suggest that strong turbulence processes, such as the modulational instability or the OTSI, stabilize the electxon beams that produce type III bursts.