The distribution of atomic hydrogen in the magnetosphere of Saturn

Abstract

Three sets of previously unpublished Voyager ultraviolet spectrometer (UVS) observations in H Ly α emission reveal a complex three‐dimensional distribution of atomic hydrogen in the Saturn system. Voyager 1 observations during the postencounter period have provided a map of the distribution looking down on the equatorial plane. The reduced data show a nonuniform distribution in local time with a preponderance of emission on the duskside. The emission extends radially inward to the top of the Saturn atmosphere with stronger signals appearing close to the planet strongly suggesting that the principal source is the sunlit Saturn atmosphere. In the subsolar sector of the magnetosphere no excess emission in the vicinity of Titan's orbit is detectable. But a peak in the emissions near 20 Saturn radii (RS) appears in the antisolar sector suggesting a significant contribution from Titan. Voyager 1 and 2 preencounter observations also show H Ly α emissions increasing monotonically toward the planet but with a distinctive dawnside excess. The preencounter and postencounter results may be reconciled if there exists a complex H gas distribution with significant radial and azimuthal variations. We conclude that the preliminary results reporting a toroidal hydrogen distribution with a cavity inside 8 RS (Broadfoot et al., 1981) are invalid because of limited spatial resolution and poor statistics. The fact that the hydrogen distribution joins continuously into the atmosphere of Saturn and shows significant azimuthal variations indicates that the dominant source inside 8 RS is in the Saturn exosphere. The local time character and magnitude of the distribution could be produced by ballistic and escaping atoms originating in the sunlit exosphere. The energies required for the source atoms can be obtained by electron excitation of H2 within a scale height of and above the exobase. The total rate of production of atomic hydrogen may be as much as 3 times larger than that inferred earlier from the observed brightness of H2 EUV radiation from the equatorial atmosphere. The deposition of heat at the top of the thermosphere by the atomic hydrogen source can account for the measured temperature. We have examined the impact of the conditions imposed by the measured content of neutrals and ions in the inner magnetosphere through model calculations. We predict in a preliminary analysis that the neutral gas in the low latitude region of the inner magnetosphere is dominated by water products. A typical calculation shows [O] ∼ 400 cm−3, [OH] ∼ 40 cm−3, [H] ∼ 130 cm−3, [O+] ∼ 30 cm−3, [H+] ∼3 cm−3, in the plasma sheet near 4.5 RS; a mix of neutrals and ions of this order is required to balance the energy budget.