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Spatiotemporal Variations of Quiet Time Equatorial Ionosphere Longitudinal Structure Under Low Solar Activity

The low‐latitude longitudinally periodic density structure with three or four peaks at a fixed local time is well known. Both harmonic analysis of observations and numerical simulations have been extensively performed to ascribe the periodic structures to different atmospheric tides. But to date, no research has been done to explain how the two structures transform between seasons. Besides, other different wave structures have also been reported. To clarify these uncertainties, the COSMIC electron density data during 2006–2019 are used to further examine the spatiotemporal variations of the longitudinal structure. The results show that a wavenumber‐4 structure is clearly distinguishable during equinoxes and the four peaks are basically the same size, but in two solstices, a wavenumber‐3 structure is finally observed and the three peaks vary greatly in size. The wavenumber‐4 is well established before noon while the stable wavenumber‐3 form cannot be reached until after noon. Due to the blurred boundary caused by plasma transportation, the density distribution can be characterized by other wave structures before reaching its stable form during solstices. Subdividing the data, we find that from vernal (autumn) equinox to June (December) solstice, the peak in American sector gradually merges with its west (east) side peak into a super‐large one, its east (west) side peak weakens into a small one, and the peak in Southeast Asian sector gradually shrinks (expands). We speculate that the transition from wavenumber‐4 to wavenumber‐3 is due to the modulation of transequatorial winds and magnetic declination.