Analysis of the Pioneer 10 and rocket observations of disk averaged emission from the sunlit atmosphere of Jupiter indicates that the spectrally integrated EUV brightness was reduced by at least a factor of 2 relative to Voyager spacecraft observations in 1979. Most of the variation is caused by the HLy&agr; component in the spectrum, which was reduced ≈1 order magnitude near the time of solar minimum in 1972--1973. Although the analysis of the data does not produce entirely consistent results, the weight of evidence points to a factor of order ≈2 lower abundance of H in Jupiter's atmosphere in 1972--1973 relative to 1979. The low emission rate in HLy&agr; near the time of solar minimum in this proposed scenario is caused by an electroglow energy deposition rate reduced by a factor of ≈3. The apparent reduced abundance of H implies a reduced thermospheric temperature, even under the assumption of a constant electroglow deposition rate.