Electrostatic decay of Langmuir/z-mode waves in type III solar radio bursts

摘要

About 40% of the waveforms observed by STEREO during type III solar radio bursts exhibits Langmuir beating and have split spectral peaks, suggestive of decay into product Langmuir and ion-acoustic waves. For lower electron beam speeds v_b/c?0.1, the spectra of Langmuir events with split spectral peaks are shown to be consistent with electrostatic (ES) decay into Langmuir-like waves with frequencies above the electron plasma frequency. For faster beam speeds vb/ca‰3;0.1, the spectra are consistent with one or more successive generations of ES decay and an end state of low wave number Langmuir/z-mode waves with strong electric fields perpendicular to the magnetic field. For many of the split spectral-peak events, an intense low-frequency response occurs that is consistent with ion-acoustic waves produced by ES decay, providing further evidence that these events are ES decay. An observed event is shown to be consistent with three successive backscatter decays, but such events are very rare in type III bursts. About 90% of the split spectral-peak events occur when T_i/T_e?0.6. Similarly, over 80% of the split peak events have energy densities above the theoretical nonlinear threshold for ES decay (for reasonable Langmuir damping rates ?10 ~(-3)ω_p). All events have beam speeds and energy densities below the maxima appropriate for ES decay of beam-driven Langmuir waves.