[1] The equatorial ionization anomaly (EIA) structures and evolutions are imaged using radio occultation observation of the newly launched FORMOSAT-3/COSMIC (F3/C) satellite constellation. Three-dimensional ionospheric images provide unprecedented detail of the EIA structure globally. This paper presents images of the EIA structures during July -August 2006 and discusses the development and subsidence of the EIA. Clear seasonal asymmetries in both ionospheric electron density and layer height are observed. Two-dimensional (cross section) maps at a meridian provide dynamic variations and motions of the northern and southern EIA crests. Results suggest that in addition to the asymmetric neutral composition effect, interactions between the summer-to-winter (transequatorial) neutral winds and strength of the equatorial plasma fountain effect play important roles in producing asymmetric development of the EIA crests as imaged by the F3/C.

The plausible effect of atmospheric tides on the longitudinal structure of the equatorial ionosphere is observed by the FORMOSAT‐3/COSMIC (F3/C) constellation during September Equinox, 2006, near solar minimum. The longitudinal structure was first reported in IMAGE satellite airglow observations at the far‐ultraviolet (FUV) 135.6‐nm wavelength during March Equinox, 2002, near solar maximum. The global three‐dimensional ionospheric electron density observed by F3/C shows a prominent four‐peaked wave‐like longitudinal enhancement in the equatorial ionization anomaly (EIA). The vertical electron density structures observed by F3/C reveal that the feature exists mainly above 250 km altitude indicating that the feature is an F‐region phenomenon. The four longitudinal F‐region enhancements of the EIA peaks may result from a stronger equatorial plasma fountain at each longitude region produced by a stronger F‐region eastward electric field transmitted along the magnetic field lines from E‐region where longitudinal variations in atmospheric tides affect the ionospheric dynamo process.