Fig. 7: Illustrations of traced AKR frequency-drifting tones.
From: Radio emissions reveal Alfvénic activity and electron acceleration prior to substorm onset
a Spectrogram of the wave electric field from Polar PWI. The time range of AKR precursor emissions is indicated by the two vertical white lines. The traced frequency-drifting tones are indicated by white lines and corresponding numbers. b Traced frequency-drifting tones plotted as a function of time and frequency. Circles mark the selected frequency points from the original spectrogram, while colored lines represent the fitted results obtained using a second-order polynomial fit. Horizontal red lines indicate the tones used for the calculation of periods. c Derived periods between frequency-drifting tones plotted as a function of duration and frequency. The label “2–1” signifies the period calculated as the time difference between the second and first tones, as indicated by the numbers in (a, b). d Averaged frequency drift (df/dt) values plotted as a function of df/dt and frequency (with red label “2–1”). The frequency drift rate from each single frequency-drifting tone is plotted and marked by black label text. e–h present a second example in the same format as a-d. Source data are provided as a Source Data file.
