RADIATIVE-TRANSFER ANALYSIS OF FAR-ULTRAVIOLET BACKGROUND OBSERVATIONS OBTAINED WITH THE FAR-ULTRAVIOLET SPACE TELESCOPE

In 1992, the Far Ultraviolet Space Telescope provided measurements of the ultraviolet (140-180 nm) diffuse sky background at high, medium, a nd low Galactic latitudes. A significant fraction of the detected radi ation was found to be of Galactic origin, resulting from scattering by dust in the diffuse interstellar medium. To simulate the radiative tr ansfer in the Galaxy, we employed a Monte Carlo model that utilized a realistic, nonisotropic radiation field based on the measured fluxes ( at 156 nm) and positions of 58,000 TD-I stars, and a cloud structure f or the interstellar medium. The comparison of the model predictions wi th the observations led to a separation of the Galactic scattered radi ation from an approximately constant background, attributed to airglow and extragalactic radiation, and to a well-constrained determination of the dust scattering properties. The derived dust albedo a = 0.45 +/ - 0.05 is substantially lower than albedos derived for dust in dense r eflection nebulae and star-forming regions, while the phase function a symmetry g = 0.68 +/- 0.10 is indicative of a strongly forward-directe d phase function. We show the highly nonisotropic phase function to be responsible, in conjunction with the nonisotropic UV radiation field, for the wide range of observed correlations between the diffusely sca ttered Galactic radiation and the column densities of neutral atomic h ydrogen. The low dust albedo is attributed to a size distribution of g rains in the diffuse medium with average sizes smaller than those in d ense reflection nebulae.