Limits on the extent of Saturn's hydrogen cloud

Abstract

The 30‐day reports from the Voyager Ultraviolet Spectrometer team characterize the atomic hydrogen observed near Saturn as a torus with a half width of 7 Rs extending from about 8 to 25 Rs. The source of this torus was hypothesized to be escape of hydrogen from Titan's atmosphere. Several recent papers propose that the source is actually Saturn's exosphere and that the hydrogen cloud extends inwards to the planet. We model the atomic processes occurring in the inner magnetosphere, including sputtering, ionization, charge exchange, ion‐atom interchange, recombination, and transport, and add a neutral hydrogen source to test the proposal that a hydrogen cloud exists in this region. Plasma observations in the inner magnetosphere are found to be inconsistent with the presence of a dense (20 cm−3) atomic hydrogen cloud. The presence of such a cloud in the observed plasma environment would result in a much larger proton density than observed and would remove nearly all heavy ions from the magnetosphere. Thus we conclude that the hydrogen cloud must be a torus as originally reported, and that the cloud does not extend inward to the orbits of the inner satellites of Saturn.