Spectrogram of HF Doppler signal on (a) 11 July 2022, ~1500–2400 UT. Higher frequency wave structures appear to be embedded in a lower frequency wave in the post-sunset activity. (b) 16 September 2022, ~1400–2100 UT. A loop in the trace is highlighted by the black arrow. This can occur when shear flow is generated between waves propagating in opposite directions, thereby creating a vortex in the ionospheric plasma. Beyond the loop, periodic structures are visible in the post-sunset spread ~1900–2100 UT.