Wavenumber-4 patterns of the total electron content over the low latitude ionosphere

The global ionospheric maps (GIMs) produced by JPL are used to investigate the longitudinal structure of the low latitude ionosphere. As a proxy of the ionization parameter at low latitudes, the latitudinally integrated total electron content (ITEC) is first extracted from low latitude GIMs and then Fourier filtered to obtain the wavenumber-4 components. We then study in detail the diurnal, seasonal and solar cycle variations of the wave patterns. It is found that the wavenumber-4 patterns are intense and well developed in boreal summer and early boreal autumn, but quite weak in boreal winter. This seasonal variation is consistent with that of the zonal wind of the non-migrating tide mode DE3. We also found that the wavenumber-4 patterns shift eastward with a shifting speed that is smaller in daytime than at night. This is attributed to the contribution of both the eastward propagation of DE3 in E-region and the zonal E x B ion drifts in F-region. Our results support the suggestion that the longitudinal wavenumber-4 structure of the low latitude ionosphere should be originated from the non-migrating tide mode DE3.