Direct measurements of neutral wave characteristics in the thermosphere

Abstract

Waves in the neutral upper atmosphere have been measured by the open source neutral mass spectrometer (Oss) during both the elliptical and the circular phases of the Atmosphere Explorer-C mission. Typical peak-to-peak wave amplitudes seen in [N•! are 30%, although amplitudes of 55% have been recorded. The amplitudes are mass dependent, Ar showing the largest perturbation. Helium is typically found to be out of phase with the heavier constituents. A survey of Oss data from 338 circular orbits shows that the highest wave amplitudes and the greatest number of occurrences are found in both the northern and the southern high-latitude regions. The near-simultaneous measurements of waves in both hemispheres at high latitudes also point toward a source related to conjugate phenomena. A possible second class of lower-amplitude waves peaks at high latitudes; however, a secondary peak near +--25 ø magnetic latitude may be due to waves generated near the equatorial region. A study of wave amplitudes and occurrences during a magnetic storm shows a possible transpolar propagation of waves from a magnetically active region near local midnight. The assumption of horizontal propagation would explain the observed high-latitude distribution of large-amplitude waves. The results of this study are compared with previous measurements of neutral wave structure. INTRO DU CTION In the last few years, in situ measurements of wave structure in the thermospheric neutral density have been made from earth-orbiting satellites. Early data obtained by means of ionization gages [Newton et al., 1969; Dyson et al., 1970! and accelerometers [Marcos and Champion, 1972; Forbes and Marcos, 1973! have indicated the existence of waves that appear to be propagating in the northsouth direction over a large range of latitudes. Recently, neutral mass spectrometers on the Atmosphere Explorer satellites have enabled high-resolution observations of wave structure in the individual neutral constituents to be made. Preliminary reports of the wave characteristics have been given by Reber et al. [1975! and Potter et al. [1975!. Mayr and Trinks [1975! also reported evidence of long-wavelength perturbations measured by a gas analyzer on the Esro 4 satellite. The unique elliptical and circular phases of the Atmosphere Explorer-C (AE-C) mission have provided the basis for a comprehensive wave study utilizing the large amount of data accumulated by the open source mass spectrometer (Oss) during the 2 years following launch. This paper discusses the phase relationships between the constituents and presents the results of a wave occurrence and amplitude survey covering 338 despun circular orbits in which the local time and latitude characteristics of the waves are presented. Conclusions based on this survey are then tested in a study of waves measured at high latitudes during a geomagnetic storm. INSTRUMENTATION The AE-C satellite was launched from the Western Test Range on December 16, 1973, into an elliptical orbit with 68.1 ø inclination. The initial perigee and apogee...

Compositional effects in Titan's thermospheric gravity waves

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