The Structure of Titan's Stratosphere from the 28 Sgr Occultation

A dozen lightcurves obtrained during the ground-based observations of the occulation of 28 Sgr by Titan (3 July 1989) are reanalyzed. Profiles of density and temperature between altitude levels z of 290 and 500 km (pressures p from 110 to 1.4 μbar) are derived. A mean number- density scale height of 50.5±1.4 km is found with no significant difference between immersion and emrsion. Two in-version layers are observed at 425 and 450-455 km, respectively (p ~7 μbar and p ~4 μbar), with an increase in temperature of about 10 K in less than Δz=10 km. These layers are visible both at immersion and at emersion, at latitudes ranging from 46°S to 20°N, and are thus global features of the stratosphere. The profiles of temperature gradients exhibit a clear cutoff at the adiabatic lapse rate, indicating that fluctuations lead to marginal convective instabilities. Although ray crossing can also cause an apparent cut-off of the temperature gradients, we estimate it probably does not play an important role in the observed cutoff, at least for the larger structures under study. The vertical power spectra of fluctuations show a general power law behavior, with an exponent close to -3, between vertical wavelengths of ~5 and 50 km. The finite stellar diameter and ray crossings can distort the real spectra, and we can only conclude that the original power spectra have slopes between -2 and -3. The horizontal structure of the atmosphere exhibits typical aspect (horizontal-to-vertical) ratios of 15-45, with a tail in the distribution with values as high as 100-200 for some structures. Finally, the horizontal spectrum of fluxtuations is a power law with an exponent close to -4 (between horizontal wavelengths of ~25 and 250 km), if we assume it is separable from the vertical spectrum.