H-I LYMAN ALPHA EMISSION FROM SATURN (1980-1990)

Observations of Saturn made with the International Ultraviolet Explorer (IUE) satellite over the past decade have been analyzed to determine the characteristics of the H I Ly alpha-dayglow and auroral emissions. The dayglow emission shows significant long- (years) and short-term (days-months) variability, with peak emissions in excess of 2 kR observed in 1980 and again in 1989; the disk brightness decreased after 1980 to its lowest value of approximately 1 kR in January 1984; it then rose steadily and also exhibited increased short-term variability between 1987 and 1989. The dayglow emission is strongly correlated (r > 0.7) with the solar Ly alpha-flux, the solar He 10,830 angstrom equivalent width (EQW), and the solar 10.7-cm radio flux. The amplitude of the long-term variation best matches that of the solar 10.7-cm radio flux, while the short-term variability is more closely proportional to the solar Ly alpha-flux and the He 10,830 angstrom EQW. Unless the interplanetary medium hydrogen density is larger than the majority of published values, a large discrepancy of more than a factor of 2 exists between the Ly alpha-brightnesses measured by IUE and Voyagers 1 and 2 on nearly the same dates that cannot be explained by solar variability. Current models suggest that resonant scattering can account for less than one-half of the observed dayglow emission. In addition, the ratio of solar maximum to solar minimum brightnesses is larger than can be explained by resonant scattering from hydrogen produced by solar extreme ultraviolet (EUV) processes. We conclude, based on current models, that the Saturn Ly alpha-dayglow is not dominated by resonant scattering but nonetheless varies in direct proportion to solar activity. Postulations of a significant collisional source for the emission therefore remain plausible. Intermittent outbursts of up to 1 kR in auroral emission above disk brightness have also been observed over the past decade. Neither the dayglow nor the auroral emission exhibits any dependence on magnetic longitude. Several attempts to observe Saturn aurora associated with a major solar flare that produced unprecedented geomagnetic activity in March 1989 were unsuccessful.