Spectrogram showing an example of semi-symmetric dispersive signals over the South China Sea (D₁ and D₂) bordering the magnetic dip equator observed in Swarm Alpha burst-mode data on 12-Aug-2022. The in-situ plasma density (dashed line), shows the equatorial ionisation anomaly as two distinct crests. Each signal is enhanced in the bottom panel by adjusting spectrogram parameters to increase temporal resolution. A lightning-generated whistler (W) is also marked.