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Stationary planetary wave and nonmigrating tidal signatures in ionospheric wave 3 and wave 4 variations in 2007–2011 FORMOSAT‐3/COSMIC observations

The wave 3 and wave 4 modulations of the Equatorial Ionization Anomalies are a robust feature of the low‐latitude ionosphere, when viewed at constant local time. Although initially associated, respectively, with DE2 and DE3, nonmigrating diurnal tides in the mesosphere and lower thermosphere region, recent results have suggested that the wave 3 and wave 4 may also have significant contributions from other tidal and stationary planetary wave (SPW) signatures. We present observations of total electron content (TEC) variations associated with tidal and SPW signatures comprising the ionospheric wave 3 and wave 4 structures from FORMOSAT‐3/COSMIC from 2007 to 2011. We find that the wave 3 (wave 4) feature is comprised predominately by DE2 (DE3) and SPW3 (SPW4) signatures in TEC throughout all 5 years, with contributions from SE1 (SE2) being less significant. The wave 3 component also has recurring contributions from DW4 during December/January. The absolute amplitudes of all the aforementioned tidal and SPW signatures are directly related to the level of solar activity and the semiannual variation in zonal mean TEC. After normalizing by the zonal mean, the relative amplitudes of the wave 4 signatures are inversely related to solar activity through 2010, which is not seen with the wave 3‐related signatures. The seasonal variation and phases of the main constituents of wave 3 and wave 4 are consistent from year to year, as evidenced by the interannual recurrence in the peak and trough locations of wave 3 and wave 4.