From global soundings of ionospheric electron density made with FORMOSAT 3/COSMIC satellites for September 2006-August 2009, day-night variations in vertical and longitudinal structures of the electron densities in equatorial E-and F-regions for different seasons are investigated for the first time. The results reveal that the wavenumber-3 and wavenumber-4 patterns dominated the nighttime (22:00-04:00 LT) F-region longitudinal structures in solstice and in equinox seasons, respectively. In daytime (08:00-18:00 LT) F-region, the wavenumber-4 patterns governed the longitudinal structures in the September equinox and December solstice, and wavenumber-3 in March equinox and June solstice respectively. A comparison of the daytime and nighttime longitudinal electron density structures indicates that they are approximately 180 degrees out of phase with each other. It is believed that this out of phase relation is very likely the result of the opposite phase relation between daytime and nighttime nonmigrating diurnal tidal winds that modulate background E-region dynamo electric field at different places, leading to the day-night change in the locations of the equatorial plasma fountains that are responsible for the formation of the F-region longitudinal structures. Further, a good consistency between the locations of the density structures in the same seasons of the different years for both day-time and nighttime epochs has been noticed indicating that the source mechanism for these structures could be the same.
