Observational evidence for the collapsing Langmuir wave packet in a solar type III radio burst

Abstract

High time resolution observations from the STEREO spacecraft show that in solar type III radio bursts, Langmuir waves often occur as very intense one‐dimensional magnetic field aligned field structures. One of these events represents the most intense Langmuir wave packet with ever detected in a type III radio burst until now (WL is the peak energy density, and ne and Te are the electron density and temperature, respectively). The detailed analysis of this wave packet indicates that (1) its peak intensity is well above the threshold for the oscillating two‐stream instability (OTSI) and supersonic collapse; (2) its peak intensity and spatial scale satisfy the criterion for it to be a collapsing envelope soliton; (3) its low‐frequency components provide evidence for a density cavity, whose depth, width, and temporal coincidence indicate that probably it is the ponderomotive force generated density cavity; and (4) its spectrum contains harmonic peaks at 2fpe and 3fpe (in addition to the main Langmuir wave peak at the electron plasma frequency, fpe), which, as indicated by the bispectral analysis, probably are of the electromagnetic waves generated as a result of coalescence of two oppositely propagating Langmuir waves, and a Langmuir wave and a second harmonic electromagnetic wave, respectively. These characteristics strongly suggest that this wave packet and its associated density cavity represent the collapsing envelope soliton‐caviton pair formed as a result of OTSI, and in the present case, the strong turbulence processes probably play key roles in the beam stabilization as well as conversion of Langmuir waves into escaping radiation at 2fpe and 3fpe.