The overall observational situation concerning ultraviolet background radiation has been reviewed recently by Henry (1991), and also by Bowyer (1991). Here we will survey progress since 1991, and try to resolve differences in interpretation of the existing data which have occurred.
Galactic sources that have been posited for diffuse background include fluorescence of interstellar molecular hydrogen (Martin, Hurwitz, and Bowyer 1990), atomic emission lines from hot gas in the interstellar medium and/or galactic halo (Feldman et al. 1981; Martin and Bowyer 1990), two-photon emission from the ionized component of the interstellar medium (Deharveng et al. 1982), and the light of hot stars scattering from interstellar dust grains (many authors).
There has been no recent change in the situation regarding molecular hydrogen fluorescence. The spectrum (Jakobsen 1986; Sternberg 1989) is sufficiently structured that there is no possibility that the signal at high galactic latitudes could be dominated by this source, which in any case continues strongly shortward of Lyman .
There has also been no change in the situation regarding two-photon emission. The expected intensity from galactic sources can be accurately predicted by using the H measurements of Reynolds (1986). No sharp break is expected at 1216 Å of course, and the average intensity is predicted to be well below the observed level longward of 1216 Å.
For the other two sources there is more to say on developments since 1991, and we devote separate sections, below, to each.