Nighttime wavenumber-four and plasma depletion bays observed by FORMOSAT-5/AIP, ICON/IVM, and COSMIC-2/RO data

We examine nighttime longitudinal structures of ion density and ion velocity in wavenumber-four (WN4) and plasma depletion bays (PDBs) observed by FORMOSAT-5 (F5) and ICON, as well as electron density by FORMOSAT-7/COSMIC-2 (F7C2) in 2020. Three PDBs significantly occur in June and December solstices, while WN4 prominently appear in March and September equinox. F5/AIP (advanced ionospheric probe) ion velocities tend to be westward (eastward) at the PDB (WN4) longitudes, which shows that the ion density maximum and minimum are coincident with the eastward ion velocity maximum and minimum, respectively. ICON/IVM (ion drift meter) field-aligned velocities further reveal that the summer to winter trans-equatorial flow is essential to form PDBs, which further results in upward and downward ion drifts in the upstream and downstream areas (summer and winter hemisphere), respectively. Furthermore, F7C2 radio occultation electron density profiles reveal that three PDBs could prominently appear in June and December solstice in the upper ionosphere. F5/AIP and ICON/IVM ion velocities show that the zonal plasma drift is essential to form nighttime WN4 and PDBs, and the summer to winter trans-equatorial flow is important to PDBs.