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

Abstract

We present, observational evidence for the generation of Langmuir envelope solitons 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 structures have the properties expected of Langmuir envelope solitons, namely, the normalized peak energy densities, WL/nₑTₑ ∼ 10⁻⁵, are well above the modulational instability threshold; the spatial scales L, which range from 1 to 5 Langmuir wavelengths, show a high degree of inverse correlation with (WL/nₑTₑ)¹/²; and the observed widths of these broad peaks agree well with the predicted widths of envelope solitons. 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 solitons. These observations suggest that strong turbulence processes, such as the modulational instability or the OTSI, stabilize the electron beams that produce type III bursts.