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 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 SE2 is not a contributor to the wave-4 longitude variation.