The comparative importance of DE 3, SE 2, and SPW 4 on the generation of wavenumber‐4 longitude structures in the low‐latitude ionosphere during September equinox

Numerical simulations are performed to investigate the generation of the wave‐4 longitude variation in the low‐latitude ionosphere due to the diurnal eastward propagating nonmigrating tide with zonal wavenumber 3 ( DE 3), semidiurnal eastward propagating nonmigrating tide with zonal wavenumber 2 ( SE 2), and stationary planetary wave 4 ( SPW 4). From a fixed local time perspective, the DE 3, SE 2, and SPW 4 all appear as wave‐4 structures in longitude, and thus each of these waves must be considered as a potential source of the wave‐4 variation in the ionosphere. Both the DE 3 and SPW 4 are found to produce significant wave‐4 variations in the equatorial vertical E  ×  B drift velocity, and in the ionospheric peak density (NmF2) at 15°N magnetic latitude. The daytime wave‐4 variation in NmF2 is driven by the combination of vertical E  ×  B drift variability and in‐situ effects due largely to meridional neutral winds. The simulation results indicate that the SE 2 is not a contributor to the wave‐4 longitude variation. Our results further demonstrate that the actual wave‐4 longitude variation is due to a combination of the DE 3 and SPW 4. We therefore conclude that, in addition to the DE 3, the SPW 4 also needs to be considered as an important driver of the wave‐4 longitude variation in the low‐latitude ionosphere. We additionally present evidence for the generation of the SPW 4 due to the nonlinear interaction between the migrating diurnal tide and the DE 3, and demonstrate the impact of DE 3 variability on the amplitude of the SPW 4.