Jovian H₂ dayglow emission (1978–1989)

Observations of the Jovian equatorial region obtained with the International Ultraviolet Explorer (IUE) satellite over the past 10 years (December 1978 to March 1989) have been analyzed to determine the long‐term temporal variation of the H 2 dayglow emission. Lyman band emission in the region of 1500–1700 Å decreased by ≈40–50% between 1981 and 1986 and has increased by more than a factor of 2 between 1986 and March 1989. Observations in late 1988 and early 1989 show the highest levels of Jovian equatorial H 2 emission ever recorded by the IUE satellite. Comparison of the observations with simple models indicates a possible relation between the dayglow emission and 11‐year solar cycle activity. The magnitude of the observed variation is closer to that of the solar Ly α flux than that of the 10.7‐cm radio flux. The intensity of the short‐wavelength H 2 band emission (1240–1325 Å) is inconsistent with the amount of long‐wavelength emission (1500–1700 Å) but may be reconciled if relatively low‐energy electron excitation or fluorescence of solar radiation is invoked. It is not possible to unambiguously distinguish between these possibilities with the present data. Crude longitudinal binning of the observations reveals no persistent feature analogous to the H I Ly α “bulge,” although there may be enhanced emission at the approximate longitude of the Ly α bulge in the 1988 observations.