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L.A. Young (SwRI), G. Stark (Wellesley College), R.J. Vervack, Jr. (JHU/APL)
When Voyager 2 flew by Triton in 1989, it observed a warm (~100\ K) thermosphere near altitudes of ~450-700 km from N2 continnuum absorption during the UVS occultation (e.g., Broadfoot et al. 1989 Science 246, 1459), and a colder (~38 K) lower atmosphere below ~150 km from phase delay of the radio occultation (e.g., Gurrola 1995, PhD Thesis, Stanford). Models have attempted to bridge the missing 400 km, spanning four decades of pressure (e.g., Stevens et al. 1992 GRL 19, 669; Yelle et al. 1991, Icarus 89, 347). The observations needed to fill this 400-km altitude gap lie in the 80-100 nm region of the Voyager UVS occultation data, where N2 bands dominate the opacity. The line strengths were previously unavailable, but have been recently measured.
Using N2 ionization and dissociation cross sections (Heubner et al. 1991 A&SS 195, 1), N2 photoabsorption cross sections between 95.2 and 98.5 nm region (Stark et al. 1992 JCP 97, 4809; Stark et al. 2000 ApJ 531, 321; Ubachs et al. 1997 Chem. Phys. Lett. 268, 201), and the X-band phase delay of the radio occultation (Gurrola 1995), we can derive line-of-sight number densities from 0-20 km and 140-740 km. Four absorption bands were included in the photoabsorption cross section files: b(3) arrow X(0), b(4) arrow X(0), c3(0) arrow X(0), and c'4(0) arrow X(0). The cross section models are based on band f-value measurements, line width measurements, Voigt profiles, and Hönl-London factors.
A preliminary profile suggests that the heating source in Triton's middle and upper atmosphere occurs at a much lower altitude than assumed by previous models. Details of the modeling and its implications will be discussed.
This work was funded by NASA Planetary Atmosphere grant NAG5-12023.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.