Morphology of the Saturn Magnetospheric Neutral gas
Although it has been known that Saturn's magnetospheric volume is filled with neutral gas, from the time of the Voyager encounters and subsequent HST observations, the Cassini Mission was essential for revealing the depth of complexity in the source processes and structure of this system. The state of the magnetosphere is unique, containing a plasma environment quenched by neutral gas from the top of the atmosphere to beyond the bow shock with neutral/plasma mixing ratios in the range 100 to ∼ 3000. The dominant neutral species identified in the magnetosphere by remote sensing are atomic hydrogen and oxygen, OH and H2O . Atomic hydrogen was mapped using the Voyager UVS and found to have an asymmetric distribution in local time, filling the entire magnetosphere, with a broad latitudinal distribution. These observations were followed by the measurement of the OH spectrum using the HST FOS. The definition of the HST distribution was limited to a few points in the system, showing a peak near 3. Saturn radii (RS ) from system center. Atomic oxygen was detected and mapped using the Cassini UVIS system, showing orbital asymmetry and temporal variation, with a substantially broader distribution than OH. All of the observed species emissions from the magnetosphere are produced by solar photon fluorescence, the ambient plasma volume being too low in density and temperature to generate measurable particle excited emission. H2O has been measured in Cassini UVIS stellar occultations at the south polar plumes at Enceladus, with a total mass injection rate that is the same order needed to maintain the oxygen population. The oxygen distribution, however, indicates that sources other than Enceladus may be contributing. Virtually all of the atomic hydrogen in the system is attributed to escape from the top of the Saturn atmosphere. The complexity of this process was graphically revealed in the Cassini UVIS system higher resolution images showing a plume of atoms in ballistic and escaping orbits emerging from the sub-solar atmosphere at about -13 deg latitude, with a FWHM of about 20 deg. The total flux in H atoms is high enough to account for the heating required to maintain the temperature at the top of the atmosphere. There is only a crude understanding of this phenomenon, that evidently requires electrodynamic forcing in hydrogen physical chemistry in the vicinity of the exobase.
Ion Sources over Saturn's Rings, Within the Neutral Cloud, and Close to Enceladus
Since Cassini arrived at Saturn in July, 2004, the Cassini plasma spectrometer (CAPS) has detected a thermal water-group ion population throughout the inner magnetosphere including O+, OH+, H2O+, and H3O+. In addition, CAPS has also detected H+, H2+, and smaller concentrations of N+ and O2+. These data extend Voyager observations that established the presence of two primary ion populations, a light ion group and water-group. Some of the most striking CAPS data have been obtained when CAPS observed locally-produced ions within their source regions: near the main rings during orbit insertion (O+ and O2+ source), during transit through dense regions of the neutral cloud (W+ pick-up ion source), and close to Enceladus during flybys. In this talk, we review these measurements including ion composition, density, temperature and flow. Particular attention is given to the new measurements obtained during the close flybys of Enceladus in March and October of 2008, directly within the south polar plume where CAPS detected freshly-produced water-group ions together with heavier water dimer (HxO2)+ (x: 1-4) ions and a nearly-stagnant plasma. Data from other Cassini instrumentation for these encounters are also reviewed (RPWS/INMS), confirming the CAPS observations of ion species detected and the spatial scale of enhanced ion production.
Morphology of Saturn's Neutral Clouds: Interactions with the Plasma
Unlike at Jupiter, Saturn's magnetosphere is dominated by neutrals with a neutral to ion ratio near the magnetic equator ranging from ~102 to 104. These neutrals in turn are the principal source of the plasma for the magnetosphere. Embedded sources for hydrogen, likely the dominant species throughout the magnetosphere, are escape from Saturn and Titan, as well as the dissociation of ejected water molecules and the decomposition of icy surfaces throughout the system. The icy ring particles and satellite surfaces are significant source of oxygen-containing molecules which, on ionization, dominate the mass loading in the inner magnetosphere. The two principal oxygen sources are H2O from Enceladus and O2 from the main rings. In addition, the icy moon Enceladus and Titan's atmosphere are sources of nitrogen and carbon containing molecules. After describing our present understanding of the relative source strengths, we will focus on the morphology of the neutral clouds, which is determined by their source speeds and by their interactions with the plasma and UV radiation environment. Due to ionization, dissociation and reactive collisions, the radiation limits the neutral lifetimes but also scatters and redistributes neutrals throughout the magnetosphere. The spatial distribution of neutrals is also affected by interactions with surfaces and neutral- neutral collisions. Because the morphology of the neutral cloud determines the spatial distribution of the plasma source and loss processes, results of recent simulations of the morphology of the clouds will be presented.
Collisional Evolution of Saturn's Neutral Torus
We present a new DSMC (Direct Simulation Monte Carlo) model of Saturn's extended neutral torus. Previous models (e.g., Jurac and Richardson, 2005; Johnson et al. 2006) have attributed the cloud's observed breadth to plasma interaction. We found, instead, that neutral-neutral collisions could be as important in spreading the cloud. As suggested by Farmer (2008), a component of the cloud can spread like a stellar accretion disk: collisions impart a small fraction of the cloud with high energy and angular momentum, causing an expansion. This is compensated by an inward-moving component which, if not first ionized or accelerated to high energy by charge exchange, is lost to Saturn's rings or atmosphere. The substantial loss of H2O to Saturn's atmosphere, combined with the oxygen scattered from the ring atmosphere (Tseng et al., 2009) may provide the missing O source required by models of Saturn's stratosphere (Moses et al., 2002). The work may also help to explain new observations of the outer neutral cloud, which is broader than previously believed.
Numerical Simulations of Plasma Transport in Saturn's Inner Magnetosphere
The Rice Convection Model (RCM), a numerical simulation code developed over several decades for the study of plasma motion in Earth's inner magnetosphere, has been adapted for the study of centrifugally-driven plasma transport in the inner magnetosphere of Saturn (2 < L < 12). We have incorporated into the RCM code a continuously active plasma source deriving from the observed Enceladus neutral water vapor plume. The neutral water vapor source peaks at L=4 (the orbit of Enceladus), but the resulting plasma source peaks farther out (5.5 < L < 7.5) because that is where the ambient electron population has sufficient temperature to ionize water vapor. The inclusion of this continuously active distributed plasma source has elucidated many aspects of the resulting interchange plasma transport, including in particular the observed and heretofore unexplained fact that the longitudinal sectors of outflow are broader, and therefore slower, than the interspersed sectors of inflow.
Variability in Saturn's Suprathermal Ion Composition
Using over four and a half years of data from the Charge-Energy-Mass Spectrometer (CHEMS), we examine the variability in equatorial suprathermal ion composition in the inner (7-11.5 RS) and outer (11.5-16 RS) portions of Saturn's ring current. CHEMS, one of three sensors comprising the MIMI investigation on Cassini, determines the mass and charge state of ions in the energy per charge range 3-220 keV/e. The most abundant species are water group ions and H+. The water group ions originate mostly from the plumes of Enceladus while the H+ has several sources including Enceladus, the solar wind, and Saturn's ionosphere. H2+ is the third most abundant species, originating mostly from the radiolytic decomposition of ice with a possible contribution from Titan's atmosphere. Less abundant are He+ and He++, originating as interplanetary pickup ions or from the solar wind, respectively. We will quantify the variability in the relative abundances of these species on an orbit by orbit basis and look for any correlations with ion intensity, Saturn season, etc. that might be relevant to plasma source strengths and acceleration processes.
On How Plasma Transfers its Rotational Energy into Acceleration of Particles
Saturn's magnetosphere displays dramatic, large-scale (∼10 RS) injections of energetic ions and electrons (>10 keV) that span the inner magnetosphere out to distances beyond 40 RS. While the smaller-scale (≪1 RS) interchange injections appear seemingly randomly, the large-scale injections appear, on average, periodically in the midnight region with a direct relation to the onset and growth of Saturn Kilometric Radiation (SKR) and appear to be modulated by solar wind dynamic pressure. Beyond about 9 RS the plasma pressure of the energetic particle distributions is comparable to that of the low-energy particle population, but display clear azimuthal asymmetries. The associated azimuthally asymmetric pressure-driven current system therefore causes the periodic oscillations seen in the magnetic field measurements. In this paper we show how the rotational energy of the corotating plasma is transformed into the acceleration of particles up to several 100's keV. We present in-situ and global energetic neutral atom (ENA) observations and simulations showing that energetic particles achieve their energy from the rapid magnetic field configurations following the formation of plasmoids in the tail region. Differences in the temporal evolution of protons and O+ provide the strongest evidence that the particle energization is due to a large-scale reconfiguration of the magnetic field - similar to what takes place in terrestrial substorms. However, at Saturn we hypothesize that the centrifugal forces carried by the cold plasma is the main driver of processes that cause plasmoid release and associated field reconfigurations that result in energization ("injection") of energetic particles. These injections then create periodicities in other quantities such as the magnetic field components and SKR. We also discuss possible mechanisms responsible for the observed increase in energetic particle intensities with solar wind dynamic pressure.