The structure of Titan's stratosphere from the 28 Sgr occultation

Citation
B. Sicardy et al., The structure of Titan's stratosphere from the 28 Sgr occultation, ICARUS, 142(2), 1999, pp. 357-390
Citations number
35
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Space Sciences
Journal title
ICARUS
ISSN journal
0019-1035 → ACNP
Volume
142
Issue
2
Year of publication
1999
Pages
357 - 390
Database
ISI
SICI code
0019-1035(199912)142:2<357:TSOTSF>2.0.ZU;2-E
Abstract
A dozen lightcurves obtained during the ground-based observations of the oc cultation of 28 Sgr by Titan (3 July 1989) are reanalyzed. Profiles of dens ity and temperature between altitude levels z of 290 and 500 km (pressures p from 110 to 1.4 mu bar) are derived. A mean number-density scale height o f 50.5 +/- 1.4 km is found with no significant difference between immersion and emersion. Two inversion layers are observed at 425 and 450-455 km, res pectively (p similar to 7 mu bar and p similar to 4 mu bar), with an increa se in temperature of about 10 K in less than Delta z = 10 km, These layers are visible both at immersion and at emersion, at latitudes ranging from 46 degrees S to 20 degrees N, and are thus global features of the stratospher e. The profiles of temperature gradients exhibit a clear cutoff at the adia batic lapse rate, indicating that fluctuations lead to marginal convective instabilities. Although ray crossing can also cause an apparent cutoff of t he temperature gradients, we estimate it probably does not play. an importa nt role in the observed cutoff, at least for the larger structures under st udy. The vertical power spectra of fluctuations show a general power law be havior, with an exponent close to -3, between vertical wavelengths of simil ar to 5 and 50 km. The finite stellar diameter and ray crossings can distor t the real spectra, and we can only conclude that the original power spectr a have slopes between -2 and -3. The horizontal structure of the atmosphere exhibits typical aspect (horizontal-to-vertical) ratios of 15-45, with a t ail in the distribution with values as high as 100-200 for some structures. Finally, the horizontal spectrum of fluctuations is a power law with an ex ponent close to -4 (between horizontal wavelengths of similar to 25 and 250 km), if we assume it is separable from the vertical spectrum. (C) 1999 Aca demic Press.