[1] A survey of the bulk plasma ion properties observed by the Cassini Plasma Spectrometer instrument over roughly the first 4.5 years of its mission in orbit around Saturn is presented. The moments (density, temperature, and flow velocity) of the plasma distributions below 50 keV have been computed by numerical integration of the observed counts in the "Singles" (non-mass-resolved) data, partitioned into species on the basis of concurrent determinations of the composition from the time-of-flight data. Moments are presented for three main species: H + , W + (water group ions), and ions with m/q = 2, which are presumed to be H 2 + . While the survey extends to radial distances of 30 R S and thus includes some solar wind or magnetosheath values, our principal interest is the large-scale spatial variation of the magnetospheric plasma properties, so we focus attention on radial distances inside of 17 R S . Principal findings include the following: (1) the densities of all three components are highly variable but are generally well organized by dipole L and magnetic latitude; (2) the density of ions with m/q = 2 varies from a few percentage of the H + density in the inner magnetosphere to a maximum of several tens of percentage near the orbit of Titan, suggesting that Titan is an important source for H 2 + in the outer magnetosphere; (3) water group ions are the dominant population in the inner magnetosphere, but only within ∼3 R S of the equatorial plane because of their strong centrifugal confinement; (4) derived latitudinal scale heights are largest for the light ions and generally increase with radial distance; (5) the L dependence of the calculated temperatures is not consistent with adiabatic transport but is in fair agreement with the expectations for plasma originating from ion pickup; (6) in agreement with the findings of Sergis et al. (2010), inside of L ∼ 11, the particle pressure is dominated by ions with energies below a few keV; (7) the derived flow velocities reveal the global circulation pattern of relatively dense populations in the magnetosphere, with no evidence for return circulation from the nightside to the dayside beyond ∼20 R S ; and (8) the azimuthal flow speeds are typically less than full corotation over the entire L range examined, varying from ∼50% to 70% of full corotation.

The Interstellar Boundary Explorer (IBEX) observes a remarkable feature, the IBEX ribbon, which has energetic neutral atom (ENA) flux over a narrow region ∼20 • wide, a factor of 2-3 higher than the more globally distributed ENA flux. Here, we separate ENA emissions in the ribbon from the distributed flux by applying a transparency mask over the ribbon and regions of high emissions, and then solve for the distributed flux using an interpolation scheme. Our analysis shows that the energy spectrum and spatial distribution of the ribbon are distinct from the surrounding globally distributed flux. The ribbon energy spectrum shows a knee between ∼1 and 4 keV, and the angular distribution is approximately independent of energy. In contrast, the distributed flux does not show a clear knee and more closely conforms to a power law over much of the sky. Consistent with previous analyses, the slope of the power law steepens from the nose to tail, suggesting a weaker termination shock toward the tail as compared to the nose. The knee in the energy spectrum of the ribbon suggests that its source plasma population is generated via a distinct physical process. Both the slope in the energy distribution of the distributed flux and the knee in the energy distribution of the ribbon are ordered by latitude. The heliotail may be identified in maps of globally distributed flux as a broad region of low flux centered ∼44 • W of the interstellar downwind direction, suggesting heliotail deflection by the interstellar magnetic field.

The Interstellar Boundary Explorer (IBEX) is a small explorer mission that launched on 19 October 2008 with the sole, focused science objective to discover the global interaction between the solar wind and the interstellar medium. IBEX is designed to achieve this objective by answering four fundamental science questions: (1) What is the global strength and structure of the termination shock, (2) How are energetic protons accelerated at the termination shock, (3) What are the global properties of the solar wind flow beyond the termination shock and in the heliotail, and (4) How does the interstellar flow interact with the heliosphere beyond the heliopause? The answers to these questions rely on energyresolved images of energetic neutral atoms (ENAs), which originate beyond the termination shock, in the inner heliosheath. To make these exploratory ENA observations IBEX carries two ultra-high sensitivity ENA cameras on a simple spinning spacecraft. IBEX's very high apogee Earth orbit was achieved using a new and significantly enhanced method for launching small satellites; this orbit allows viewing of the outer heliosphere from beyond the Earth's relatively bright magnetospheric ENA emissions. The combination of full-sky imaging and energy spectral measurements of ENAs over the range from ∼10 eV to 6 keV provides the critical information to allow us to achieve our science objective and understand this global interaction for the first time. The IBEX mission was developed to provide the first global views of the Sun's interstellar boundaries, unveiling the physics of the heliosphere's interstellar interaction, providing a deeper understanding of the heliosphere and thereby astrospheres throughout the galaxy, and creating the opportunity to make even greater unanticipated discoveries.

The Interstellar Boundary Explorer (IBEX) has obtained all-sky images of energetic neutral atoms emitted from the heliosheath, located between the solar wind termination shock and the local interstellar medium (LISM). These flux maps reveal distinct nonthermal (0.2 to 6 kilo-electron volts) heliosheath proton populations with spectral signatures ordered predominantly by ecliptic latitude. The maps show a globally distributed population of termination-shock-heated protons and a superimposed ribbonlike feature that forms a circular arc in the sky centered on ecliptic coordinate (longitude lambda, latitude beta) = (221 degrees, 39 degrees), probably near the direction of the LISM magnetic field. Over the IBEX energy range, the ribbon's nonthermal ion pressure multiplied by its radial thickness is in the range of 70 to 100 picodynes per square centimeter AU (AU, astronomical unit), which is significantly larger than the 30 to 60 picodynes per square centimeter AU of the globally distributed population.

No abstract