The global structure of the F region ionosphere during January 20--30, 1993, is examined with emphasis on longitudinal variations. The database consists of total electron content (TEC) measurements from the TOPEX/Poseidon dual-frequency altimeter experiment from -66¿ to +66¿ geographic latitude for prenoon and premidnight conditions, and hourly f0F2 and hmF2 values from eight ionosonde stations between -42¿ and +26¿ geographic latitude in the Pacific sector. The 10-day averaged daytime TEC data reveal latitude versus longitude structures whose salient features are captured (within ~10 TEC units) by the 1990 International Reference Ionosphere (IRI-90), and which can be explained by dynamically induced neutral composition changes due to a combination of solar and magnetically driven circulation systems. The premidnight TEC data at low latitudes reveal the double-banded structure of the equatorial ionization anomaly with, however, pronounced variations in amplitude and position as a function of geomagnetic longitude. None of these features, which probably reflect some combination of electric field and neutral wind effects, are captured by IRI-90. Time series analyses of the f0F2 and hmF2 data, and space-time Fourier spectra of the TEC data, reveal the presence of a quasi 2-day (QTD) oscillation in the ionosphere which is associated with QTD wind and temperature oscillations in the mesosphere and lower thermosphere previously documented for this time period. The satellite-based QTD results are subject to significant caveats described herein. Nevertheless, the TOPEX/Poseidon TEC data set clearly represents an important resource for global studies of ionospheric structure and plasma-neutral interactions.¿ 1997 American Geophysical Union