Constraints on Jupiter's hydrogen corona from Galileo UVS observations

During Galileo orbits Cl 0 and El 1, the Ultraviolet Spectrometer (UVS) obtained H Lyalpha scans at low latitudes across the disk of Jupiter from a phase angle of 90-109degrees and a distance of 18-21 jovian radii. Jupiter's nightside Lyalpha brightness was about 0.5 kR, approximately half as bright as interplanetary space. From the nightside to the dayside, the Lyalpha brightness rose to a peak of similar to 15 kR near the sub-solar limb and then dropped sharply to interplanetary brightness levels. On orbit C10 three disk scans were obtained (near 30degreesN latitude, 20degreesS latitude, and along the equator), while a single equatorial scan was made on orbit E11. The steep falloff in Lyalpha brightness at the sub-solar point provides a strong constraint on the distribution of atomic hydrogen in the upper atmosphere. All disk scans are reasonably well fit by a resonance line radiative transfer model which assumes a total H column density of similar to 10(17) cm(-2) and a temperature profile consistent with the Galileo probe results with an exospheric temperature of 940 K. However, we find that the inclusion of a small amount of "hot" hydrogen, with an abundance described by a Chapman profile, can substantially improve the fit to the UVS limb scan data. Exactly such a hot hydrogen component is expected in the upper atmosphere of Jupiter, and has been previously invoked to explain observations of the jovian Lyalpha line profile made from Earth orbit. The best fits to the UVS data occur with a hot H column density that is 0.1% of the ambient H column, with an effective temperature of 25, 000 K and a topside scale height of 1000 km (in our empirical representation, the hot H scale height is unrelated to the hot H temperature). Other combinations of column abundance and the effective temperature can also provide good fits to the data. As long as the hot H is located above the homopause, there is no strong dependence on the height of the Chapman profile. (C) 2003 Elsevier Ltd. All rights reserved.