Statistical Investigation of Langmuir Waves in the Solar Wind

Abstract

Type II and III solar radio bursts involve streaming electrons that produce Langmuir waves that then couple wave energy into radio emission. These Langmuir waves can be driven to large enough amplitudes that they undergo electrostatic (ES) decay into a backward propagating Langmuir wave and forward propagating ion acoustic wave. The forward and backwards propagating Langmuir waves can then “beat” together to produce characteristic Langmuir waveforms and spectra, plus radio emission. Stochastic growth theory (SGT) predicts that the probability distribution of the Langmuir wave electric field strength should be lognormal, with known modifications if nonlinear processes like ES decay are occurring. Other analyses suggest Pearson distributions may be relevant. Here, previous work on Langmuir waves in type II and III source regions is generalized and tested by analysing the probability distributions of the waveforms of Langmuir waves observed by the STEREO spacecraft. The focus is on a set of published events identified using spectral analyses to have or not have spectral evidence for ES decay. For events for which spectral analyses provide evidence of ES decay, 86% of the probability distributions are consistent with the combination of SGT and a nonlinear process like ES decay, while of those without spectral evidence for ES decay 78% of the probability distributions are well fitted by pure SGT. Moreover, better fits with stronger statistical significance are obtained for pure and nonlinear SGT than for Pearson distributions in the majority of events (∼ 90%). These results provide strong evidence for SGT and ES decay proceeding in type II and III source regions.