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B.C. Wolven, P.D. Feldman (The Johns Hopkins University)
Far-ultraviolet spectra of the Jovian equatorial dayglow in the range 820-1840 Åwere obtained at ~ 3.5 Å resolution with the Hopkins Ultraviolet Telescope (HUT), flown on the Space Shuttle Endeavour as part of the Astro-2 mission in March 1995. The Astro-2 data, taken near solar minimum, is compared with that obtained during the Astro-1 mission in December 1990, near solar maximum (Feldman et al., Ap. J. 406, p. 279, 1993). Changes in both the total flux (1.5 to 2.4 kilorayleighs) and the flux distribution with wavelength are observed. Detailed models have been constructed to determine the H2 fluorescence spectrum produced by a combination of resonant and fluorescent scattering of solar photons and photoelectron impact. A solar spectrum model covering the range 20-2000 Åis used to determine the flux incident on the top of the atmosphere. The vertical structure of the Jovian atmosphere is modeled, incorporating the effects of eddy and molecular diffusion and a thermal profile based on measurements obtained by the Atmospheric Structure Instrument (ASI) on the Galileo probe. Upward and downward fluxes of H, H2, and the dominant stable hydrocarbon species (CH4, C2H2, C2H4, C2H6) are introduced in order to reproduce density profiles resembling those of Gladstone et al. (Icarus 119, p. 1, 1996). Given the incident solar flux and the atmospheric temperature and density profiles, the radiative transfer equation is integrated to obtain the rates for photoelectron production and fluorescent scattering as a function of altitude. From the derived excitation rates, the emergent H2 fluorescence spectrum is computed, including the effects of H2 self-absorption and hydrocarbon attenuation. It appears that the solar EUV/FUV input is sufficient to create the observed Jovian dayglow emission at both high and low solar activity levels, eliminating the need to invoke any additional excitation processes. The observed temporal variations may also indicate long term changes in the upper atmosphere of Jupiter, manifested as a change in the eddy diffusion coefficient.
The author(s) of this abstract have provided an email address for comments about the abstract: wolven@pha.jhu.edu