WAVE-LIKE PERTURBATIONS OBSERVED AT LOW ALTITUDES BY THE PIONEER VENUS ORBITER NEUTRAL MASS-SPECTROMETER DURING ORBITER ENTRY

Wave-like perturbations have been observed in the nightside neutral de nsity data acquired for He, N, O, N2 and CO2 by the Pioneer Venus Orbi ter Neutral Mass Spectrometer (ONMS) during entry in late 1992. The da ta cover an altitude range of 133-200 km from 0.5-4.5 hours local sola r time and occur at medium solar activity (F10.7=120). The perturbatio ns, with an effective wavelength along the orbit of about 100 to 600 k m, have similar amplitudes for the various species and helium is out o f phase with respect to the heavier mass species. The measurements are comparable to those observed in 1978-80 at solar maximum activity (F1 0.7=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) x 10(-3) km-1 fo r N, O, N2, and CO2, respectively. The average rms amplitudes above 14 5 km of 0.08 for N and 0 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 re spectively. CO2 is an exception for which the entry value is 0.17 comp ared to 0.09 earlier. By combining the two overlapping data sets there is a suggestion that the CO2 amplitudes grow in value with altitude u p to about 140-170 km and then decrease in amplitude. Like the earlier data, the entry data are consistent with die interpretation that the neutral density perturbations are due to gravity waves propagating upw ard from the lower thermosphere. Earlier data sampled a higher altitud e region where dissipation is important in limiting and ultimately rev ersing the growth of gravity waves while the lower altitude entry data probed the region where growth of these waves can be observed.