Probability Distributions of Langmuir Waves in Type II and III sources

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 non-linear 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. In general events for which spectral analyses provide evidence of ES
decay also show probability distributions consistent with the combination of
SGT and a nonlinear process like ES decay (∼ 86%), while those without
spectral evidence for ES decay are well fitted by pure SGT (∼ 79%).
Moreover, better fits with stronger statistical significance are obtained for pure
and nonlinear SGT than for Pearson distributions in the majority of events
(∼ 81%). These results provide strong evidence for SGT and ES decay
proceeding in type II and III source regions.