On the Cascade Spectrum of Langmuir Waves Observed in Arecibo Heating Experiments

Abstract

Two parametric instabilities cascading Langmuir waves, which are the sideband of the parametric decay instability (PDI) excited by an O-mode HF heating wave, are studied. The first one decays a Langmuir pump wave into a Langmuir sideband and an ion acoustic wave. Both resonant and nonresonant cascade processes are considered. A nonresonant cascade of Langmuir waves proceeds at the same location and is severely hampered by the frequency mismatch effect. Resonant cascade takes place in different resonant locations to minimize the frequency mismatch effect, but it has to overcome the propagation loss of the mother Langmuir wave in each cascade step. Depending on the propagation loss, the resonant cascade could have a lower threshold, but cascade lines spread over a height region. The second one is a three-dimensional coupling process that employs the lower hybrid wave as the decay mode. Through this process, the nonresonant cascade of Langmuir waves produces a broad spectrum of frequency-downshifted plasma waves, similar to that of experimentally observed HFPLs, in a narrow altitude range, which is preferentially located near the matching height of the PDI sideband.