Detection of Extreme and Exceptional Langmuir Wave Packets in Solar Type III Radio Bursts

Abstract

AbstractWe report the STEREO observations of two extreme and exceptional Langmuir wave packets, which are the most intense wave packets ever detected in solar type III bursts; their peak intensities Et∼ 215 and ∼ 161 mVm−1 beat out the previous 107 mVm−1 record, reported by Thejappa and MacDowall (2018a; https://doi.org/10.3847/1538-4357/aaca3b). These observations provide new evidence for (1) the four‐wave interaction called the oscillating two‐stream instability (OTSI), which is the coupling of two beam‐excited Langmuir waves with an ion sound wave yielding downshifted and upshifted side bands, and (2) Langmuir solitons formed as a result of OTSI, and (3) density cavities created by their ponderomotive forces. The fast Fourier transform spectra of these wave packets provide what is believed to be the first evidence for harmonics of the electron plasma frequency, fpe up to the order of 5 with peak intensities falling off with increasing harmonic number. The higher order spectral analysis indicates that these harmonics probably correspond to the electromagnetic waves excited as a result of various three‐wave interactions. We argue that although the observed characteristics also indicate that these wave packets could be collapsing wave packets formed as a result of nucleation instability, the observed evidence for OTSI and Langmuir solitons trapped inside the self‐generated density cavities strongly favor the OTSI as the route for the spatial collapse of Langmuir waves in the present case. The implication of these findings for theories of solar type III radio bursts is discussed.