Longitude structure of ionospheric total electron content at low latitudesmeasured by the TOPEX/Poseidon satellite

The longitude structure of ionospheric total electron content (TEC) at low latitudes has been evaluated using the NASA/Centre Nationale d'Etudes Spati ales TOPEX/Poseidon satellite. The TEC data set is given by the ionospheric range correction, which is computed from TOPEX dual-frequency altimeter me asurements. The satellite's orbit allows analysis of vertically measured TE C values at approximately 30 degrees intervals of longitude across the worl d at local time differences of only 6-12 min. Patterns of longitudinal depe ndence of the equatorial anomaly were observed during the equinoxes, summer s, and winters of 1993, 1994, and 1995. TOPEX observations reveal occurrenc e of relative maximum anomaly TEC values in the Indian/Asian longitude sect or. This dominance in TEC is seen most consistently in the Asian Southern H emisphere. Also, a relative decrease in anomaly TEC values is evident in th e western American region, which is observed primarily during equinox and w inter. This configuration of the equatorial anomaly TEC is observed on a da y-to-day basis at particular periods of local time. Global theoretical iono spheric model results are presented in an attempt to reproduce the distinct ive longitude structure. Variability in E x B vertical drift velocity withi n specific longitude sectors is shown to be a primary factor in the longitu de dependence of equatorial anomaly TEC.