Spectrograms of the high-cadence electric field data from Fig. 1e. Panel a: Fourier spectrogram of electric field data, with frequencies normalized to the local f_ce. Panel b: sum of the power spectral density at each frequency over the ∼3.5 s burst interval. Horizontal dashed lines show integer multiples of f_ce. Panels c,d: same data as panels a,b, but spanning a narrower frequency range.