The asymmetric distribution of titan's atomic hydrogen cloud as a function of local time

The detailed analysis of the Voyager ultraviolet spectrometer (UVS) data by Shemansky & Hall has led to a new view of the atomic hydrogen torus of Titan. Instead of an axially symmetric torus, the global distribution of the hydrogen atoms was found to have a strong local time dependence. We have followed up the suggestion by Smyth & Marconi that such complex morphology might be the result of orbital perturbations of the hydrogen atoms by the solar radiation pressure acceleration effect over the ionization lifetime of these neutral atoms. From Monte Carlo calculations, we show theoretically how the initial orbits of the hydrogen atoms would gradually be shaped into eccentric orbits with apoapses pointing to the predawn direction. The steady state distribution of the hydrogen atoms bears a certain resemblance to the three-dimensional structure as derived from the Voyager observations. For example, the density depletion in the predawn sector within 18 R_s could be reproduced from our model. However, the major observational result that the number density, inside of 10 planetary radii, should continuously increase inward until the planetary surface is reached has no counterpart in our present model. This might mean that other near-planet sources such as Saturn's dayside exosphere and/or the ring system could be major suppliers of the atomic hydrogen cloud in the inner region of the Saturnian system.