Equatorial waves in the stratosphere of Uranus

Abstract

We have identified and characterized an equatorial wave in the stratosphere of Uranus through analysis of radio occultation data from Voyager 2. Our methodology relies on two physical phenomena: (1) atmospheric waves modulate the ambient background structure of a planetary atmosphere, producing a distinctive pattern of spatial variations in temperature and density; and (2) this type of spatial modulation in turn causes fluctuations in the phase and amplitude of radio signals received from an occulted spacecraft. We compared predictions of the linear theory for equatorial waves on a β-plane with observations of the stratosphere at both immersion (2.0°N, 342.2°E) and emersion (6.3°N, 197.5°E). (Note that the latitude system used here is the reverse of the IAU convention.) The observed quasi-periodic vertical variations in atmospheric density agree closely with theoretical predictions for a wave propagating vertically through the observed background structure of the stratosphere, providing compelling evidence for the presence of an atmospheric wave. The wave has an equivalent depth of 53 ± 12 m, a vertical wavelength of about one-third of a pressure scale height, and an amplitude at the 1-mbar pressure level at immersion (emersion) of 1.1 ± 0.4 K (0.5 ± 0.2 K). Within the framework of the linear wave theory, quantitative comparisons between the measurements at immersion and emersion yielded constraints on the meridional and zonal structure of the wave. The ratio of amplitudes at the two locations, 2.1 ± 0.3, is consistent with a wave confined in latitude near the equator. Moreover, the observations at immersion and emersion are correlated, suggesting a wave of planetary scale. These constraints allowed us to identify two equatorial wave modes as equally likely alternatives for explaining the observations; each retains a relatively simple spatial structure while accounting for the principal features of the data. The first is an inertia-gravity wave with westward zonal phase speed (relative to the mean zonal winds), a meridional index j = 1, and a zonal wavenumber n = 6. The second is an eastward inertia-gravity wave with j = 2 and n = 14. All Kelvin, Yanai, and Rossby wave modes are inconsistent with the observations. We expect the wave to begin breaking near the 200-μbar pressure level, resulting in an eddy diffusion coefficient of about 0.6 to 1.0 × 10 4 cm 2 sec −1, consistent with independent estimates of this parameter. Vertical transfer of momentum by the wave could accelerate the mean zonal wind at pressures less than 200 μbar by about ±10–20 cm sec −1 per planet rotation.