Overview of the Voyager ultraviolet spectrometry results through Jupiter encounter

The Voyager ultraviolet spectrometers (UVS) have been making almost continuous observations, in the 500-A to 1700-A wavelength range, of sources in the solar system and galaxy since launch in 1977. Due to their sensitivity, stability, and dynamic range, the spectrometers have made a remarkable number of discoveries pertaining to the Jupiter system, the interstellar medium, astronomical, and astrophysical sources. The most surprising general aspect of these results has been the wide variety of emission processes and species which have been observed. On Jupiter's disc, the emissions detected to date are H Lyman α, H Lyman β, He (584 A), and the H2 Lyman and Werner bands. The atomic emissions are excited by resonance scattering of sunlight, while the H2 bands appear to be excited by particle precipitation. On the nightside disc, only H Lyman α is present. Jupiter's auroral region is clearly delineated by intense emissions of H and H2 bands on both dayside and nightside of the planet. Emission from He is also present in the auroral regions. At Jupiter, the atmosphere was also probed by means of solar and stellar occultation experiments. The solar occultation has revealed the distribution of H2 and H in the upper atmosphere, while the stellar occultation has probed the structure of the upper mesosphere and lower thermosphere. Current analysis indicates an eddy diffusion coefficient ∼106 cm² s−1 with a mesospheric temperature ∼200 K. The solar occultation analysis suggests an exospheric temperature of 1450±300 K. The thermospheric lapse rate appears to be ∼1 K/km. The Lyman α observations of the disc have revealed a longitudinal asymmetry in H which may reflect longitudinal asymmetries in Jupiter's magnetosphere. Strong EUV emission from a plasma torus at the orbit of Io has been observed in transitions of sulfur and oxygen ions with a possible small contribution from potassium. The effective electron temperature of the dense regions of the plasma is estimated to be 8 × 104 K. No localized EUV emission has been detected from Io, limiting mass loading at Io to ∼1027 ions s−1. The partitioning of ion sub-species shows deviation from pure collisional equilibrium, but preliminary analysis indicates a low diffusive loss rate. The radiative cooling rate of the torus is ∼3 × 1012 W. The appearance of the entire sky in the outer solar system has been mapped in the emission lines of He (584 A) and H Lyα (1216 A) arising from resonant scattering of the solar lines by neutral interstellar hydrogen and helium entering the solar system. Diffuse galactic EUV emission has been measured in a number of selected directions. Stellar photospheric emissions shortward of the Lyman limit of atomic hydrogen at 912 A have been measured. Finally, spectral images, in several emission lines, have been obtained of the Cygnus Loop supernova remnant. Sections of this article review the progress in the study of these subjects and the relationships of the EUV results to the other Voyager experiments. We include a discussion of the characteristics of the instrument and the methods of spectral analysis to verify the integrity of the reported results.