The upper atmosphere of Uranus - EUV occultations observed by Voyager 2

Details The upper atmosphere of Uranus - EUV occultations observed by Voyager 2 The upper atmosphere of Uranus - EUV occultations observed by Voyager 2

Dec 1987

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987jgr....9215093h&link_type=abstract

Journal of Geophysical Research (ISSN 0148-0227), vol. 92, Dec. 30, 1987, p. 15093-15109.

Astronomy and Astrophysics

Astronomy

61

Atmospheric Chemistry, Extreme Ultraviolet Radiation, Occultation, Upper Atmosphere, Uranus Atmosphere, Voyager 2 Spacecraft, Acetylene, Ethane, Hydrocarbons, Hydrogen, Methane, Ultraviolet Astronomy, Ultraviolet Spectroscopy

Scientific paper

Occultation observations of the upper atmosphere of Uranus by the Voyager 2 ultraviolet spectrometer are analyzed. The measurements extend from 0.5 mbar to about 10-6μbar using the EUV wavelengths 520 ≤ λ ≤ 1700 Å. H2 dominates the atmosphere up to the vicinity of the exobase near 1.25 RU, where atomic H becomes the major constituent. H2 Rayleigh scattering is the dominant source of opacity in the lower portion of the observed pressure range. The mixing ratio of C2H2 is on the order of 10-8 there, while only an upper limit (≤10-7) is available for CH4. Also, some evidence exists for the possible presence of C2H6 at a mixing ratio of several ×10-8. The value of the eddy diffusion coefficient at the homopause is much lower than at Jupiter and Saturn. In addition, the two high-latitude occultations indicate little difference in upper atmospheric structure between the day and night hemispheres. The atmospheric temperature above about 0.01 to 0.001 μbar is 800±100K. Because of this high temperature the thermal component of the H exosphere extends to great altitude, with number densities of several hundred cm-3 at 2 RU. This high gas density has important implications for ring dynamics, possibly being responsible for the extreme narrowness and isolation of the visible Uranian rings.

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