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

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^-6 μbar using the EUV wavelengths 520<=λ<=1700 Å. H₂ dominates the atmosphere (the approximately 15% He content deduced by the Voyager 2 infrared spectrometer cannot be seen in occultation at these wavelengths) up to the vicinity of the exobase near 1.25 R_U, where atomic H becomes the major constituent. Apparently because of weak eddy mixing, the hydrocarbon mixing ratios are quite small in the measured pressure range, so that the atmosphere is more transparent than those of Jupiter and Saturn. Thus 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; the best fitting of several photochemical models which were matched to the observations assumed a value of 10⁴ cm²s^-1. This may represent an upper limit. In addition, the two high-latitude occultations indicate little difference in upper atmospheric structure between the day and night hemispheres, despite the constancy of the illumination geometry over recent decades. The atmospheric temperature above about 0.01 to 0.001 μbar is 800+/-100 K. 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 R_U. This high gas density has important implications for ring dynamics, possibly being responsible for the extreme narrowness and isolation of the visible Uranian rings. The extent and density of the H exosphere and the nonthermal corona (which has an even larger scale height) with also strongly affect the origin and maintenance of the unusual plasma populations observed at Uranus by the Voyager 2 plasma science and low-energy charged particle experiments.