Wave-like perturbations observed at low altitudes by the Pioneer Venus Orbiter Neutral Mass Spectrometer during orbiter entry
Article first published online: 7 DEC 2012
Copyright 1993 by the American Geophysical Union.
Geophysical Research Letters
Volume 20, Issue 23, pages 2755–2758, 14 December 1993
How to Cite
- Issue published online: 7 DEC 2012
- Article first published online: 7 DEC 2012
- Manuscript Accepted: 9 SEP 1993
- Manuscript Received: 7 JUL 1993
Wave-like perturbations have been observed in the nightside neutral density data acquired for He, N, O, N2 and CO2 by the Pioneer Venus Orbiter Neutral Mass Spectrometer (ONMS) during entry in late 1992. The data cover an altitude range of 133–200 km from 0.5–4.5 hours local solar time and occur at medium solar activity (F10.7=120). The perturbations, with an effective wavelength along the orbit of about 100 to 600 km, have similar amplitudes for the various species and helium is out of phase with respect to the heavier mass species. The measurements are comparable to those observed in 1978–80 at solar maximum activity (F10.7=200) above 145 km. Between 133 and 160 km the rms amplitudes grow with altitude at a rate of about (1.6, 2.1, 2.7, 4.4) × 10−3 km−1 for N, O, N2, and CO2, respectively. The average rms amplitudes above 145 km of 0.08 for N and O and 0.1 for N2 are comparable in magnitude to those observed in the earlier 1978–80 data of 0.06, 0.08 and 0.095 respectively. CO2 is an exception for which the entry value is 0.17 compared to 0.09 earlier. By combining the two overlapping data sets there is a suggestion that the CO2 amplitudes grow in value with altitude up to about 140–170 km and then decrease in amplitude. Like the earlier data, the entry data are consistent with the interpretation that the neutral density perturbations are due to gravity waves propagating upward from the lower thermosphere. Earlier data sampled a higher altitude region where dissipation is important in limiting and ultimately reversing the growth of gravity waves while the lower altitude entry data probed the region where growth of these waves can be observed.