INERTIO-GRAVITY WAVES IN THE ATMOSPHERE OF NEPTUNE

A refined analysis of Voyager 2 radio occultation data has revealed distinct, quasi-periodic temperature variations in the troposphere and stratosphere of Neptune. Temperature oscillations with amplitudes of 0.1-1 K and vertical wavelengths of 1-10 km are present at latitudes 61°N (ingress) and 45°S (egress). We used techniques of spectral analysis to identify the predominant periodicities of the data and applied digital filters to isolate signal perturbations within selected wavelength passbands. Basic results include detailed vertical profiles of the temperature perturbations associated with these atmospheric waves. The data also provide a weak constraint on the horizontal structure-the horizontal wavelength probably exceeds the vertical wavelength by at least an order of magnitude. We confined our interpretation to two waves in the stratosphere whose vertical structure was characterized over 9 and 15 cycles of oscillation, respectively, spanning 2 to 3 pressure scale heights. In both cases, the observations are consistent with the characteristics of inertio-gravity waves, but cannot be reconciled with a Rossby-wave interpretation. We applied standard theory to model the wave behavior and to estimate the wave contributions to the eddy mixing and momentum budget of the middle atmosphere. The exponential growth of wave amplitude with height should result in overturning, or wave breaking, at pressures less than about 0.5 mbars for one wave and about 3 mbars for the other. The process of wave dissipation, though poorly understood, may be a significant source of "eddy mixing" on Neptune, with wave-generated eddy diffusivities as large as 50 m2 sec-1. The wave forcing of the mean flow that accompanies dissipation could alter the zonal winds by as much as about 9 m sec-1 per planet rotation. © 1993 by Academic Press, Inc.