Data from the Atmosphere Explorer-C satellite clearly exhibit wavelike variations in neutral composition, ion density, and electron temperature, variations which appear to be a general feature of the atmosphere. The neutral constituents do not exhibit uniform wave characteristics, since the density variations of argon are approximately twice those of molecular nitrogen, which range between 5 and 15%. Helium structure, on the other hand, has about one half the amplitude of the N•. variation and is 180 ø out of phase with the two heavy gases most of the time. The waves evident in the ion density are nearly in phase with the heavy neutrals, while the electron temperature variations are predominantly out of phase with those in the ion density. Scale sizes for the structure range from a few tens of kilometers to several hundred kilometers. A simple model is suggested to explain the neutral composition results, wherein the enhancements in the major gas densities are in phase with the vertical component of the perturbation velocity of the gas. The vertical velocity modifies the composition by transporting parcels of air to higher or lower regions where the composition is different. The phase relationship between the density and velocity implies phase velocities (assuming that these are gravity waves) of the order of 500 m s -•
-The density gauges on the Explorer 32 satellite have measured local variations in the atmospheric density which confirm that waves propagate in the neutral atmosphere. The waves were observed in the northern-hemisphere over the altitude range from 286 km (satellite perigee) to at least 510 km. The waves are most prevalent at the higher latitudes near the auroral zone (orbit inclination is 659 and were observed most frequently in the late evening and early morning hours, but were not limited to these latitudes and times. The virtual vertical half-wavelengths of the waves increase with altitude from 1 km at 286 km altitude to 70 km at 510 km altitude. Four integrally related wavelengths have been observed, a "fundamental" and the second, third, and fourth harmonics.The wave half-amplitudes range from the limit of detectability to the largest observed of 50% of the smoothed density profile value. These waves are identified as free internal gravity waves propagating predominately north-south, or southnorth, with horizontal wavelengths between 130 and 520 km and periods between 10 and 100 min. This interpretation is consistent with observations of large scale traveling ionospheric disturbances by Thome and Georges .
The neutral atmosphere composition experiment on Atmosphere Explorer C measured N2, O, Ar, and He densities during a magnetic storm in February 1974 at altitudes down to about 160 km. At latitudes above 45øN, N2 and Ar densities generally increase during the storm, while He and O densities decrease. Below 45øN all densities tend to increase during the storm. The density increases at perigee indicate that density or temperature profile changes are taking place below 160 km. The return to prestorm conditions is very slow, demonstrating the integrating effect of the atmospheric response. A recent theoretical model incorporating thermospheric circulation and diffusion effects reproduces the longitudinally averaged data including latitude trends and the asymmetry about the storm maximum. Comparison with the mass spectrometer and incoherent scatter empirical model shows qualitative agreement with latitude trends but not with storm asymmetry, while the earlier J71 model based on total mass density is not in agreement with observed latitudinal trends. No significant correlation is found with the short-term variations of the ap index. At any fixed altitude and for latitudes above 45øN (perigee) the density variations are closely correlated with invariant (or magnetic) latitude, although invariant latitude alone is not adequate to order the data completely. A close correlation is found between in situ O/N• measurements and in situ and ground-based ionosonde measurements of electron density. 1, -40 LENINGRAD 60 ø N 30 ø E UPPSALA 59 ø N 17 ø E ß d• ß ß ßß ß ß •o ßß ß 41 42 43 44 45 46 47 DAY COUNT (1974)
The neutral‐atmosphere composition experiment instrumentation is designed to obtain in‐situ measurements of neutral thermosphere composition from Atmosphere Explorer‐C, ‐D, and ‐E. The system is based on previously flown OGO‐6 and San Marco‐3 composition instruments. The mass‐spectrometer sensor includes a gold‐plated thermalizing chamber and ion source, a hyperbolic rod quadrupole analyzer, and an off‐axis electron multiplier. Automatic ion‐source sensitivity control and pulse‐counting techniques provide density measurement capability from approximately 125 to 1000 km altitude. The normal operating mode includes measurement at all masses in the range 1 to 44 amu with emphasis on hydrogen, helium, oxygen, nitrogen, and argon. Additional operational modes are optimized for minor constituent studies of any individual gas in this mass range.
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.
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