The comparative importance of DE3, SE2, and SPW4 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 (DE3), semidiurnal eastward propagating nonmigrating tide with zonal wavenumber 2 (SE2), and stationary planetary wave 4 (SPW4). From a fixed local time perspective, the DE3, SE2, and SPW4 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 DE3 and SPW4 are found to produce significant wave-4 variations in the equatorial vertical E×B drift velocity, and in the ionospheric peak density (NmF2) at 15N 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 SE2 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 DE3 and SPW4. We therefore conclude that, in addition to the DE3, the SPW4 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 SPW4 due to the nonlinear interaction between the migrating diurnal tide and the DE3, and demonstrate the impact of DE3 variability on the amplitude of the SPW4.