Local time variation of equatorial thermospheric composition determined by the San Marco 3 Nace. [Neutral atmospheric composition experiment]

The daily variations of the densities of O, N2, Ar, and He and mass density ρ in the equatorial lower thermosphere have been measured by the Neutral Atmospheric Composition Experiment (Nace) on the San Marco 3 satellite. The data, from the time interval April 29 through November 29, 1971, have been analyzed at the altitudes of 220, 250, and 280 km. The O and Ar densities at 220-km altitude exhibit two nearly equal daily maxima, one in the morning, which is the largest for O, and one in the afternoon. The N2 has a daily maximum in the afternoon, and He has a maximum in the morning at all altitudes examined. At 220 km, ρ shows double maxima, one in the morning and one in the afternoon, reflecting the O and N2 variations. Harmonic analysis of the measurements has revealed variations with 24-, 12-, and 8- hour periods. The diurnal modes in all gases are dominant at all altitudes, and their relative amplitudes increase with altitude more rapidly than the amplitudes of the semidiurnal and terdiurnal modes. The phases of the semidiurnal modes appear to move toward earlier local times between 220- and 250-km altitudes, while the phases of the diurnal modes are more nearly constant with altitude. The phases of the diurnal modes at all altitudes move toward earlier times as the molecular weight of the gas decreases. These results strongly imply that transport processes, possibly occurring elsewhere in the atmosphere, play an important role in determining the daily variation of thermospheric composition in this altitude range.