Modulated Langmuir waves: Observations from Freja and SCIFER

Modulated Langmuir waves were commonly observed using the HF waveform‐capture instruments on the Freja satellite and SCIFER sounding rocket in the terrestrial auroral zone. The modulation frequency of the Langmuir waves during several hundred events was estimated, and found to extend routinely to several tens of kilohertz. Using the linear dispersion relation for oblique Langmuir waves in the ƒpe < ƒce domain, these modulation band widths were found to correspond to interactions between waves at significantly larger angles to each other, and to B0, than typically treated in theoretical analyses of Langmuir wave modulations. A model of the modulation based on scattering via electrostatic whistler/lower hybrid waves is proposed as a viable explanation of the broad modulation bandwidth.