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Temporal variation of the Jovian H I Lyman alpha emission (1979-1986)

Observations monitoring the long-term spatial and temporal behavior of the Jovian H I Ly α emission have been made with the International Ultraviolet Explorer (IUE) observatory beginning in December 1978, and extending through December 1986. The hydrogen bulge region centered near λIII = 100° has been observed to be a persistent feature of the Jovian upper atmosphere throughout the period of the observations, which includes the maximum of solar cycle 21 and the current minimum in solar activity. New and previously unpublished measurements made since early 1982 show that the average brightness of both the bulge and nonbulge regions has varied with the long-term solar Ly α output, which has decreased by a factor of ∼ 2 in this time frame. However, the bulge is ∼ 25% brighter on average and exhibits more short-term variability than the nonbulge region. The factor of ∼ 2 change in Jupiter's Ly α brightness during the descending phase of solar cycle 21 is significantly less than the order of magnitude variability reported for the ascending phase of the same cycle. The IUE results are consistent with pure resonant scattering from an H column of ∼1017 cm−2 and indicate that no observable changes have taken place in the Jovian atomic hydrogen abundance or the average atmospheric conditions during the 8-year period since December 1978. If the electroglow process recently proposed for the outer planets plays a significant role in the long-term behavior of the Jovian Ly α emission, the electroglow must also follow the solar cycle at Ly α. Finally, as a result of revisions in both the IUE instrumental sensitivity at Ly α and the dimensions of the spectrograph large aperture, there is no significant difference between Voyager and IUE measurements of the Jovian Ly α brightness made within the same time frame and with approximately the same field of view.