Gas torus

Abstract: The global hydrogen Lyman alpha, helium (584 angstroms), and molecular hydrogen band emissions from Saturn are qualitatively similar to those of Jupiter, but the Saturn observations emphasize that the H(2) band excitation mechanism is closely related to the solar flux. Auroras occur near 80 degrees latitude, suggesting Earth-like magnetotail activity, quite different from the dominant Io plasma torus mechanism at Jupiter. No ion emissions have been detected from the magnetosphere of Saturn, but the rings have a hydrogen atmosphere; atomic hydrogen is also present in a torus between 8 and 25 Saturn radii. Nitrogen emission excited by particles has been detected in the Titan dayglow and bright limb scans. Enhancement of the nitrogen emission is observed in the region of interaction between Titan's atmosphere and the corotating plasma in Saturn's plasmasphere. No particle-excited emission has been detected from the dark atmosphere of Titan. The absorption profile of the atmosphere determined by the solar occultation experiment, combined with constraints from the dayglow observations and temperature information, indicate that N(2) is the dominant species. A double layer structure has been detected above Titan's limb. One of the layers may be related to visible layers in the images of Titan.

Abstract: The Cassini spacecraft passed within 168.2 kilometers of the surface above the southern hemisphere at 19:55:22 universal time coordinated on 14 July 2005 during its closest approach to Enceladus. Before and after this time, a substantial atmospheric plume and coma were observed, detectable in the Ion and Neutral Mass Spectrometer (INMS) data set out to a distance of over 4000 kilometers from Enceladus. INMS data indicate that the atmospheric plume and coma are dominated by water, with significant amounts of carbon dioxide, an unidentified species with a mass-to-charge ratio of 28 daltons (either carbon monoxide or molecular nitrogen), and methane. Trace quantities (<1%) of acetylene and propane also appear to be present. Ammonia is present at a level that does not exceed 0.5%. The radial and angular distributions of the gas density near the closest approach, as well as other independent evidence, suggest a significant contribution to the plume from a source centered near the south polar cap, as distinct from a separately measured more uniform and possibly global source observed on the outbound leg of the flyby.

Abstract: The Cassini magnetometer has detected the interaction of the magnetospheric plasma of Saturn with an atmospheric plume at the icy moon Enceladus. This unanticipated finding, made on a distant flyby, was subsequently confirmed during two follow-on flybys, one very close to Enceladus. The magnetometer data are consistent with local outgassing activity via a plume from the surface of the moon near its south pole, as confirmed by other Cassini instruments.

Abstract: We present a description of plasma conditions in the Io plasma torus, between 5 and 10 RJ, based on Voyager 1 observations obtained in March 1979. The model includes updated analyses of Plasma Science (PLS) data obtained along the spacecraft trajectory as well as Ultraviolet Spectrometer (UVS) observations of composition made remotely from Jupiter. The plasma characteristics observed along the spacecraft trajectory have been extrapolated along magnetic field lines by numerically solving the equations of diffusive equilibrium to produce radial profiles of plasma properties at the centrifugal equator as well as maps of the densities of the major ionic species in a meridian plane. The diffusive equilibrium distribution of plasma along magnetic field lines depends mainly on T∥. Unfortunately, we only have measurements of T⊥ and must make assumptions about the thermal anisotropy of the plasma. We assume the thermal populations and the suprathermal electrons to be isotropic. The suprathermal ions have probably been recently picked-up and are expected to be highly anisotropic. Varying the thermal anisotropy of the hot ions between A=T⊥/T∥=1 to 5 has a minor effect on the plasma maps but makes a significant difference to the fraction of hot ions in the plasma when integrated over a complete shell of magnetic flux. We have found that the vertical extrapolation of plasma density is insensitive to the geometry of different magnetic field models except inside 5 RJ (where the plasma scale height is comparable to uncertainties in the location of the centrifugal equator) and outside 8 RJ (where the magnetospheric current sheet significantly perturbs the magnetic field). The radial profile of flux tube content (N L²) exhibits the same “precipice”, “ledge,” and “ramp” features as previous studies as well as confirming small - scale features which indicate local sources of plasma in the cold torus and near the orbit of Europa. The observations of O++ and molecular (SO2+ or S2+) ions inside 5.4 RJ, far from Io, in a region of cold dense plasma, remain difficult to explain, indicating either strong temporal variability in the Io plasma source or a strong source of plasma, possibly from the dissociation of dust, inside Io's orbit. Further evidence of a Europa source are the decrease in the ratio of sulfur to oxygen ions and the increase in plasma temperature outside 8 RJ.