Local time variation of equatorial thermospheric composition determined by the San Marco 3 Nace

Abstract

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.

Tides and the midnight temperature anomaly in the thermosphere

“…The wa/,e from below has a dominant effect in the temperature and N 2 density'below 200km and 300km, respectively, and it shifts the semidiurnal phases to later local times (,--0800LT). The rapid phase shift at these altitudes is in qualitative agreement with satellite composition measurements (Newton et al, 1975;Hedin et al, 1979) and is the manifestation of a complex interference primarily between the tidal wave from the lower atmosphere and the one excited by ion drag. We note that the ion drag component (in relation to the wave from below) contributes more to temperature than to density.…”

“…This is one to two hours earlier than the time in the Brace and Theis model but presumably more representative of the shorter time constants at lower altitudes where ion drag is important. The energy sources for the absorption of solar radiation in water vapor and ozone were taken from Chapman and Lindzen (1970), except that the phase for 03 was taken to be one hour earlier; this produced somewhat better agreement between theory and mass spectrometer observations of the semidiurnal tide in the thermospheric composition (Newton et al, 1975). Lindzen and Hong (1974) have shown that mode coupling due to prevailing winds in the lower atmosphere is important for the semidiurnal tide.…”

A global thermospheric model based on mass spectrometer and incoherent scatter data MSIS, 1. N2density and temperature

“…The Aeros A neutral atmosphere temperature experiment N,, density data Chandra and Spencer, 1975] are averaged in a similar fashion. The N,, density data from San Marco 3 are interpolated 220-km and 250-km data [Newton et al, 1975] and 140-to 190-km data [Newton et al, 1974]. The 280-km San Marco data are not used because of probable systematic errors [Kasoerzak and Newton, 1976].…”

Comparisons of measured and theoretical thermospheric daily composition variations

“…The empirical evidence consists of three sources: 1. A three-term harmonic analysis of the daily variation in the neutral equatorial thermospheric composition measured by the Nace (Neutral Atmospheric Composition Experiment) mass spectrometer on the San Marco 3 satellite for the period April to November 1971 as discussed by Newton et al [1975]. Hedin et al [1974] presents the temporal and spatial variations in number density at 450 km for nitrogen, helium, atomic oxygen, and mass density measured between 400-and 600-km altitude by the mass spectrometer on the Ogo 6 satellite.…”