A partially collisional model of the Titan hydrogen torus

Abstract

A numerical model has been developed for atomic hydrogen densities in the Titan hydrogen torus which includes the effects of occasional collisions. The torus is found to be azimuthally symmetric with the density sharply peaked at Titan's orbit and decreasing rapidly in the radial outward and perpendicular directions and more gradually inward from 17 to 5 R s. The energetic hydrogen atoms from Saturn's upper atmosphere, first predicted by Shemansky and Smith ( D.E. Shemansky and G.R. Smith, 1982, EOS 63, 1019), were also investigated. Collisions of these Saturnian atoms with the torus population do not significantly affect the torus density and in fact will lead to a net loss of bound torus atoms if their launch speeds from Saturn extend above roughly 40 km/sec. The Saturnian atoms produce a corona which has a density at Saturn's exobase of 200 to 300 cm −3, decreasing to 3 or 4 cm −3 at 20 R S. Without the coronal population, the torus model does not reproduce the Voyager 2 UVS Lyman α intensities because the hydrogen atoms are too closely confined toward Titan's orbital plane. However, these observations can be reproduced by a model that includes the corona and has central plane maxima of 62 cm −3 at Titan's orbit and 318 cm −3 at Saturn's exobase. The torus plus corona model also reconciles the Pioneer 11 observations of the torus with those of the two Voyager spacecraft. H 2 and N are expected to be present as well, with H 2 densities being similar to those of H, and N densities being lower by a factor of 5 to 10. The exact density variations are diagnostic of launch speeds from Titan and thus the relative importance of thermal and nonthermal loss mechanisms in the satellite's exosphere.