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Wave‐like perturbations observed at low altitudes by the Pioneer Venus Orbiter Neutral Mass Spectrometer during orbiter entry
Author(s) -
Kasprzak W. T.,
Niemann H. B.,
Hedin A. E.,
Bougher S. W.
Publication year - 1993
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/93gl02628
Subject(s) - orbiter , venus , amplitude , physics , altitude (triangle) , helium , atmosphere of venus , atmospheric sciences , astrophysics , geodesy , atomic physics , geology , astronomy , astrobiology , optics , geometry , mathematics
Wave‐like perturbations have been observed in the nightside neutral density data acquired for He, N, O, N 2 and CO 2 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 (F 10.7 =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 (F 10.7 =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, N 2 , and CO 2 , respectively. The average rms amplitudes above 145 km of 0.08 for N and O and 0.1 for N 2 are comparable in magnitude to those observed in the earlier 1978–80 data of 0.06, 0.08 and 0.095 respectively. CO 2 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 CO 2 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.