Characteristics of observed high-frequency wave events. (a) Peak wave power of wave is plotted as a function of peak spectral frequency. (b) Ratio of peak spectral frequency to upper-hybrid frequency (≈ electron plasma frequency) for all 19 events categorized as narrowband-type (category I), broadband-type (category II and III), and weak narrowband-type seen simultaneously with broadband-type (category III) waves.