A review of topside sounder studies of the equatorial ionosphere

The structure of the equatorial ionosphere as derived from topside sounder data is reviewed. Shortly after sunrise a single ionization crest occurs above the magnetic equator at all heights, but later during the day an equatorial anomaly develops in the form of an arch along which the ionization is enhanced and which lies along different magnetic field lines at different local times. The arch intersects the F2-layer peak at the latitudes where the well-known equatorial anomaly crests occur. The anomaly develops after about 0800 LMT in the Asian sector, but there is some evidence which suggests that the anomaly may start to develop several hours later in the American sector. The anomaly starts to decay after about 1500 LMT in the Asian sector, but only after about 2000 LMT in the American sector. The behavior during this decay phase, however, has not yet been fully established. During the night hours marked spread-F echoes are observed on the low-latitude topside sounder ionograms.