LONGITUDE STRUCTURE OF THE IONOSPHERE F-REGION FROM TOPEX POSEIDON AND GROUND-BASED DATA DURING JANUARY 20-30, 1993, INCLUDING THE QUASI 2-DAY OSCILLATION/

The global structure of the F region ionosphere during January 20-30, 1993, is examined with emphasis on longitudinal variations. The databa se consists of total electron content (TEC) measurements from the TOPE X/Poseidon dual-frequency altimeter experiment from -66 degrees to +66 degrees geographic latitude for prenoon and premidnight conditions, a nd hourly f(o)F(2) and hmF(2) values from eight ionosonde stations bet ween -42 degrees and +26 degrees geographic latitude in the Pacific se ctor. The 10-day averaged daytime TEC data reveal latitude versus long itude structures whose salient features are captured (within similar t o 10 TEC units) by the 1990 International Reference Ionosphere (IRI-90 ), and which can be explained by dynamically induced neutral compositi on changes due to a combination of solar and magnetically driven circu lation systems. The premidnight TEC data at low latitudes reveal the d ouble-banded structure of the equatorial ionization anomaly with, howe ver, 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 captu red by IRI-90. Time series analyses of the f(o)F(2) and hmF(2) data, a nd 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 satellit e-based QTD results are subject to significant caveats described herei n. Nevertheless, the TOPEX/Poseidon TEC data set clearly represents an important resource for global studies of ionospheric structure and pl asma-neutral interactions.