The atomic hydrogen in the Saturnian magnetosphere

The Voyager flyby observation have revealed that, in the Saturnian magnetosphere, a very broad distribution of the hydrogen atoms existed in a doughnut-shape region (Broadfoot et al., 1981). Shemansky and Hall (1992) showed that this atomic hydrogen cloud had an azimuthal asymmetry dependent on local time with higher intensity on the dusk side. Smyth and Marconi (1993) suggested that the accumulative effect of solar radiation pressure is important for the long-term orbital motion of the hydrogen atoms escaping from Titan and would explain this complex 3D morphology. From the modeling results, Ip (1996) also pointed out that, in addition to the Titan's hydrogen torus, the sun-lit hemisphere of Saturn's atmosphere and/or the ring system could be major sources of the hydrogen atoms in the inner magnetosphere (<~10RS). Recent Cassini UVIS observations confirm local-time asymmetry but also show the hydrogen cloud density increases with decreasing distance to Saturn's upper atmosphere (Shemansky et al., 2009) (Peak density is in Saturn itself). They suggested that there could be hydrogen plumes flowing outward from the Saturn's sun-lit hemisphere due to electron-impact dissociation of H2. The Saturnian system is also immersed in a vast gas cloud of H2O, O2 and H2 and their dissociative products like OH, O, and H, which are originated from Enceladus' plumes, the rings as well as the inner icy satellites, and Titan's exosphere (e.g. Johnson et al., 2006; Cassidy and Johnson, 2010; Tseng et al., 2010; 2011). In addition, the neutral hydrogen cloud is an important source of H+ for the magnetosphere. Therefore, in this paper, we will work on a global investigation of the atomic hydrogen cloud taking into account all possible sources: 1) Saturn's atmosphere, 2) the H2 atmosphere of main rings, 3) the Enceladus' H2O and OH torus, 4) Titan's H2 torus and 5) the atomic hydrogen directly escaping from Titan. We have found that the ejection velocity and angle distribution are modified by collisions of the hot hydrogen with the ambient atmospheric H2 affecting the morphology of Saturn's hydrogen plume. Preliminary modeling results also show the hydrogen atoms from the dissociation of H2 of the ring atmosphere could populate ~15% of the total number inside 5 RS observed by UVIS. Furthermore, dissociation of Enceladus' H2O and OH torus could also make a contribution inside ~10 RS, but dissociation of Titan's H2 torus could not. The atomic hydrogen escaping from Titan (with an exospheric temperature T>150K) shaped by the solar radiation pressure and Saturn's oblateness might account for the observation in the outer magnetosphere.