Periodicities observed in broadband-type waves. Panel a shows average spectrogram power 〈PSD〉 as a function of time for event no. 5 shown in Fig. 2 (broadband-type wave, category II). Here, time is in milliseconds after the start time 01:55:46:200 hh:mm:ss:mss (in UT); panel b shows Fourier transform of average spectrogram power as a function of periods; and panel c shows the distribution of dominant repetitive periods observed in 13 broadband-type events of category II and III. The dominant repetitive period is estimated to be 10.6±2 ms. The red dashed line in panel b indicate the 90% statistical significance level for the Fourier spectra.