Magnetospheric Physics at Saturn: Is It Earth-Like, Jupiter-Like, or Even Like It Was 25 Years ago? I
Presiding: C Paranicas, Applied Physics Laboratory; F J Crary, Southwest Research Institute
SM11A-01 INVITED 08:30h
A comparative look at magnetospheric dynamics at Saturn, Jupiter, and Earth
With the discovery of energetic particle injections within Saturn's inner magnetosphere, substorm-like dynamics in Saturn's near-planet magnetotail, and solar-wind driven dynamics of Saturn's aurora, Saturn joins Jupiter and Earth as having a space environment that hosts an array of complex, seemingly familiar dynamical features. The lesson here may be that the transport of mass and energy within and throughout complex magnetized plasma systems proceeds by means of transient "fits and starts" rather than by means of smoothly continuous processes. It is of substantial interest to determine the extent to which the dynamic phenomena involve the same fundamental physical processes. For example, with injection processes observed within all three magnetospheres, to what extent does magnetic energy storage and release play a role, irrespective of whether the energy source is the interplanetary environment or the rotation of the planet? Here we review what is known about the dynamics of Saturn, Jupiter, and Earth to address the degree to which they involve fundamentally distinct or similar physical processes.
SM11A-02 08:45h
Initial Cassini Magnetometer Observations from Saturn's Magnetosphere
An overview of magnetic field observations from the first 9 months of the Cassini orbital tour at Saturn will be described. Observations within the magnetosphere will be focussed upon, including the dynamic nature of the magnetosphere, the various magnetospheric regions traversed and the variety of current systems observed. Comparisons between earlier Pioneer and Voyager observations at Saturn will be given as well as comparisons between magnetospheric dynamics at Jupiter and the Earth.
SM11A-03 09:00h
Magnetospheric Drift Resonance Effects on Local Time Asymmetry, Injection Events, and Moon Interactions for Saturn as Compared to Earth
The magnetospheres of Earth and Saturn have similarities in terms of the highest energy radiation belt components from Cosmic Ray Albedo Neutron Decay (CRAND) but have otherwise been expected to differ on the role of charged particle convection driven by solar wind interactions with these magnetospheres. Saturn's inner and middle magnetosphere has been assumed to be dominated by corotation with little direct penetration by solar wind and magnetotail plasma. Since Saturn's planetary magnetic field characterized by the Z3 model is axisymmetric, although slightly offset northward from the ring plane, it has been difficult to understand previous Pioneer and Voyager measurements of local time asymmetry in energetic particle populations, including just outside the main rings as found by Pioneer 11. Small scale features (microsignatures) of charged particle absorption by Saturn moons and possible 'ghost' clouds of co-orbiting debris show no consistent patterns in the context of symmetric models for longitudinal drift shells. Since the 100-MeV CRAND proton drift shells are highly symmetric, it is apparent that lower energy electrons and ions showing substantial local time asymmetry are influenced by forces other than simple corotation. Cassini Huygens neutral atom observations show clear evidence of substorm injections reaching into the middle magnetosphere of Saturn preferentially on the nightside. One model would be that hot magnetotail plasma is convecting sunward into regions of colder plasma previously observed by Voyager, consistent with a dusk-to-dawn convective electric field. Numerical simulations of keV to MeV electron motion under such conditions, and with variability of upstream solar wind speed and magnetic field, show large perturbations of electron drift shells maximizing at energies of drift resonance where retrograde gradient-cuvature drift exactly cancels corotation for electrons at energies above several hundred keV. Averaged over many drift periods these perturbations produce the previously observed local time asymmetries in electron flux while also providing alternatives to formation of the detected moon microsignatures. Analysis of electron and ion flux distributions in Saturn magnetospheric longitude may provide sensitive measures of large scale electric fields.
SM11A-04 INVITED 09:15h
Sources of Neutrals and Plasma in Saturn's Magnetosphere
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
http://www.people.virginia.edu/~rej
SM11A-05 09:30h
Saturn: An Inside-Out Magnetosphere
At the Earth, the dominant sources of magnetospheric plasma are the solar wind and the upper atmosphere, both in a sense "external" to the magnetosphere itself. The path from source to sink is largely a solar-wind-driven convection into the magnetospheric tail, inward through and around the inner magnetosphere, and out through the dayside magnetopause. By contrast, at Saturn the magnetospheric plasma appears to be dominantly produced in situ, by local ionization of neutral material within the magnetospheric volume. Relevant evidence includes the plasma composition observed so far, the thermal structure, and the strong inward gradient in the density. There is also considerable evidence that this material is transported outward from the source region, at least in part through a very active process of centrifugally-driven flux-tube interchange. Ultimately, the plasma must be lost from the magnetosphere, through the magnetopause or down the tail as a planetary wind. We present observations from the Cassini CAPS instrument that demonstrate this "inside-out" character of Saturn's magnetospheric plasma sources, transport, and loss.
SM11A-06 09:45h
Variations in Ion Composition in Saturn's Magnetosphere and a Comparison with Earth and Jupiter
The Charge-Energy-Mass Spectrometer (CHEMS), one of three sensors comprising the MIMI investigation on Cassini, measures the mass and charge state of ions in the energy per charge range 3--220 keV/e. The suprathermal ion composition determined by CHEMS has been generally consistent during the first four passes through Saturn's magnetosphere. The observed heavy ions are largely water products (O+, OH+, H2O+, and O2+) with a near absence of N+ ions. Nonetheless, both spatial and temporal variations have been observed in the heavy ion composition and in the ratio of heavy to light ions (H+, H2+, He+, and He++). We will discuss those variations and their causes and compare the average composition at Saturn with CHEMS' measurements during the flybys of Earth and Jupiter.