Sources of Neutrals and Plasma in Saturn's Magnetosphere
Abstract
The surfaces of icy satellites, particles in the main-rings and the tenuous rings, and the atmospheres of Titan and Saturn are all potential sources of neutrals in Saturn's magnetosphere. These atoms and molecules are typically long-lived forming the so-called neutral tori. They are eventually ionized, by solar-UV, by plasma electrons or by charge exchange, populating Saturn's thermal plasma. The ions in the thermal plasma are lost by a number of processes or can be accelerated populating the hot plasma in Saturn's magnetosphere. Therefore, the composition of both the thermal and hot plasma is determined in part by the composition of the objects orbiting in Saturn's magnetosphere. A considerable modeling effort describing the neutral sources and ion formation rates occurred prior to the arrival of Cassini at Saturn. In that effort the principal constraints came from Voyager data and from the HST observations of the OH torus in Saturn's magnetosphere. Models for the ion source rate will be described here and evaluated and updated using the recent observations for Saturn's thermal plasma composition, densities and temperatures from the Cassini Plasma Spectrometer (CAPS). Such data is now available for a number of passes through Saturn's magnetosphere and for one pass over Saturn's main rings. At the writing of this abstract the thermal plasma measurements indicate that the icy ring particles in Saturn's main rings are a much larger source of molecular oxygen than expected and, therefore, an important source of molecular and atomic oxygen ions, not only over the main rings, but also in the region just beyond this ring system (Young et al., 2005; Waite et al. 2005; Tokar el al. 2005). In the region occupied by the tenuous rings and the icy satellites, ions from water molecules and their fragments are observed, including molecular oxygen produced by the decomposition of ice. Inside the orbit of Enceladus, molecular ion clusters may be present and the low relative velocities in ion-molecule collisions will lead to reactions. Such collisions also redistribute the water products, including molecular oxygen, to the outer magnetosphere. Therefore, the heavy ion plasma in the outer magnetosphere appears to be dominated by water related ions. Nitrogen ions have not yet been detected in the proposed Titan torus at energies less than 50keV, likely due to their rapid loss. They have been detected in the thermal plasma very close to Titan (Szego et al. 2005) and as a small fraction of the thermal plasma in the inner magnetosphere, indicative of the presence of nitrogen in the surfaces of the icy satellites or an inward extension of the Titan torus (Smith et al. 2005). The available ion and electron data from Cassini will be summarized and used to place new constraints on the neutral source rates. Smith, H.T, et al. GRL submitted 2005; Szego, K. et al. GRL submitted 2005; Tokar, R.L. et al. GRL submitted 2005; Waite, J. H. et al. Science in press 2005; Young, D.T. et al. Science in press 2005
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2005
- Bibcode:
- 2005AGUSMSM11A..04J
- Keywords:
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- 2768 Plasmasphere;
- 6008 Composition;
- 6275 Saturn