STEREO Observations of Non-linear Plasma Processes in Solar Type III Radio Bursts

Solar type III radio bursts are the fast drifting radio emissions excited by the flare accelerated electron beams in the corona and interplanetary medium. In this paper, we report the observations of several nonlinear processes, obtained by the time domain sampler (TDS) of the WAVES experiment of the STEREO spacecraft in the source region of one of these bursts. These include: (1) a four wave-wave interaction process called the oscillating two stream instability (OTSI) $L+L\stackrel{S}{\to }{L}_U+L_D$, where L is the beam excited Langmuir wave, S is the ion sound wave, and L_U and L_D are the up- and down-shifted Langmuir waves, respectively, and (2) two wave-wave merging processes L_U + L_D → T_2fpe and L + T_2fpe → T_3fpe, where T_2fpe and T_3fpe are the electromagnetic waves at the second and third harmonic of the electron plasma frequency, f_pe and L can be either a beam-excited Langmuir wave, or an up- or down-shifted sideband. The newly developed higher order spectral (HOS) techniques have been used to identify the signatures of these wave-wave interactions in the waveform data. The implication of these findings is that strong Langmuir turbulence processes play key roles in excitation of solar type III radio bursts, especially, in the beam stabilization and conversion of Langmuir waves into electromagnetic waves at the fundamental and higher harmonics of the electron plasma frequency, f_pe. (paper)