R. L. Tokar

The interaction of the atmosphere of Enceladus with Saturn's plasma.

During the 14 July 2005 encounter of Cassini with Enceladus, the Cassini Plasma Spectrometer measured strong deflections in the corotating ion flow, commencing at least 27 Enceladus radii (27 × 252.1 kilometers) from Enceladus. The Cassini Radio and Plasma Wave Science instrument inferred little plasma density increase near Enceladus. These data are consistent with ion formation via charge exchange and pickup by Saturns magnetic field. The charge exchange occurs between neutrals in the Enceladus atmosphere and corotating ions in Saturns inner magnetosphere. Pickup ions are observed near Enceladus, and a total mass loading rate of about 100 kilograms per second (3 × 1027 H2O molecules per second) is inferred.

The electron‐acoustic mode

This paper examines electrostatic modes in an unmagnetized, homogeneous, Vlasov plasma with three Maxwellian components: ions, hot electrons, and cool electrons. In such a plasma, the electron‐acoustic mode with frequencies between the ion and electron plasma frequencies may propagate with light damping. The conditions that allow propagation of this mode, which is distinct from the well‐known ion‐acoustic and Langmuir waves, are given in detail; approximate necessary conditions are 10≲Th/Tc and 0<nc<0.8ne, where the subscripts c, h, and e refer to the cool and hot electron components and the total electron population, respectively.

Cassini plasma spectrometer thermal ion measurements in Saturn's inner magnetosphere

represented by two anisotropic Maxwellian distributed species, H + and a water group ion, W + . Saturn’s magnetospheric plasma is shown to subcorotate by 15–30% below rigid corotation within this region, with a minimum in fractional lag between 7 and 9 RS. There is a suggestion of a small radial outflow, but the selection of data for this study precluded the inclusion of interchange injection events. Ion densities are in excellent agreement with the Cassini plasma wave instrument, giving confidence in the forward modeling technique. Plasma moments including density, temperatures, and velocities are presented, along with empirical models for density and azimuthal velocity. Water group temperature anisotropies T?/Tk have values between 3 and 8 near 5.5 RS, becoming less anisotropic as distance increases, but are still not isotropic by 10 RS. The implications of these results for mass loading in the Saturnian magnetosphere are discussed, with the conclusion that an important fraction of the plasma source is located inside of the 5.5 RS boundary of this study.

Soil Diversity and Hydration as Observed by ChemCam at Gale Crater, Mars

The ChemCam instrument, which provides insight into martian soil chemistry at the submillimeter scale, identified two principal soil types along the Curiosity rover traverse: a fine-grained mafic type and a locally derived, coarse-grained felsic type. The mafic soil component is representative of widespread martian soils and is similar in composition to the martian dust. It possesses a ubiquitous hydrogen signature in ChemCam spectra, corresponding to the hydration of the amorphous phases found in the soil by the CheMin instrument. This hydration likely accounts for an important fraction of the global hydration of the surface seen by previous orbital measurements. ChemCam analyses did not reveal any significant exchange of water vapor between the regolith and the atmosphere. These observations provide constraints on the nature of the amorphous phases and their hydration.

The Enceladus and OH Tori at Saturn

The remarkable observation that Enceladus, a small icy satellite of Saturn, is actively venting has led to the suggestion that ejected water molecules are the source of the toroidal atmosphere observed at Saturn for over a decade using the Hubble Space Telescope (HST). Here we show that the venting leads directly to a new feature, a narrow Enceladus neutral torus. The larger torus, observed using HST, is populated by charge exchange, the process that limits the lifetime of the neutrals in the Enceladus torus.

Cassini Finds an Oxygen–Carbon Dioxide Atmosphere at Saturn’s Icy Moon Rhea

Extraterrestrial Atmosphere The detection of oxygen in the atmospheres of Jupiters icy moons, Europa and Ganymede, and the presence of this gas as the main constituent of the atmosphere that surrounds Saturns rings, has suggested the possibility of oxygen atmospheres around the icy moons that orbit inside Saturns magnetosphere. Using the Ion Neutral Mass Spectrometer onboard the Cassini spacecraft, Teolis et al. (p. 1813, published online 25 November; see the Perspective by Cruikshank) report the detection of a very tenuous oxygen and carbon dioxide atmosphere around Saturns icy moon Rhea. As with other icy satellites, this atmosphere is maintained through the dissociation of surface molecules and ejection into the atmosphere as a result of Saturns magnetospheric radiation. Rhea’s atmosphere is maintained by chemical decomposition of surface water ice under irradiation from Saturn’s magnetosphere. The flyby measurements of the Cassini spacecraft at Saturn’s moon Rhea reveal a tenuous oxygen (O2)–carbon dioxide (CO2) atmosphere. The atmosphere appears to be sustained by chemical decomposition of the surface water ice under irradiation from Saturn’s magnetospheric plasma. This in situ detection of an oxidizing atmosphere is consistent with remote observations of other icy bodies, such as Jupiter’s moons Europa and Ganymede, and suggestive of a reservoir of radiolytic O2 locked within Rhea’s ice. The presence of CO2 suggests radiolysis reactions between surface oxidants and organics or sputtering and/or outgassing of CO2 endogenic to Rhea’s ice. Observations of outflowing positive and negative ions give evidence for pickup ionization as a major atmospheric loss mechanism.

Thermal ion flow in Saturn's inner magnetosphere measured by the Cassini plasma spectrometer: A signature of the Enceladus torus?

For the middle and outer magnetosphere of Saturn (>5.5 R S ), Cassini plasma spectrometer ion counting data provides thermal ion moments by assuming Maxwellian phase space density distributions for each ion species. However, in the inner magnetosphere (<5.5 R S ) within Saturns extended neutral cloud and proposed Enceladus torus, there is fresh ion production via charge exchange, yielding a complex ion velocity distribution. In this study, ion flow velocities in the inner region are obtained using the assumption that the ion phase space distributions are gyrotropic. Significantly sub-corotating ion flow velocities (∼75% of corotation) are found in the vicinity of the Enceladus orbit and the radial extent of the sub-corotation is about 1.0 R S , in reasonable agreement with the simulated radial dimension of the Enceladus torus.

A prolonged He+ enhancement within a coronal mass ejection in the solar wind

A coronal mass ejection and magnetic cloud containing an unusually large enhancement of He+ was observed in the solar wind by the plasma and magnetic field instruments on the Advanced Composition Explorer (ACE) spacecraft on May 2–4, 1998. The He+/He++ ratio during this event exceeded 0.5% for a period of more than 24 hours, and reached values as high as 100%. The high He+/He++ ratio indicates the presence of prominence material, and in fact a disappearing filament and prominence were observed at the Sun in association with this event. The prolonged observation of He+ indicates that prominence material extended through much of this CME, the first such observation in a CME in the solar wind.

Cassini detection of Enceladus' cold water‐group plume ionosphere

This study reports direct detection by the Cassini plasma spectrometer of freshly-produced water-group ions (O{sup +}, OH{sup +}, H{sub 2}O{sup +}, H{sub 3}O{sup +}) and heavier water dimer ions (H{sub x}O{sub 2}{sup +}) very close to Enceladus and where the plasma begins to emerge from the Enceladus plume The data wcre obtained during two close (52 and 25 km) flybys of Enceladus in 2008, and are similar to ion data in cometary comas. The ions are observed in detectors looking in the Cassini ram direction at energies consistent with the Cassini speed, indicating a nearly stagnant plasma flow in the plume. North of Enceladus the plasma slowing commences about 4 to 6 Enceladus radii away, while south of Enccladus signatures ofthe interaction are detected as far as 22 Enceladus radii away.

The interplanetary shock of September 24, 1998: Arrival at Earth

At close to 2345 UT on September 24, 1998, the magnetosphere was suddenly compressed by the passage of an interplanetary shock. In order to properly interpret the magnetospheric events triggered by the arrival of this shock, we calculate the orientation of the shock, its velocity, and its estimated time of arrival at the nose of the magnetosphere. Our best fit shock normal has an orientation of (−0.981 −0.157 −0.112) in solar ecliptic coordinates, a speed of 769 km/s, and an arrival time of 2344:19 at the magnetopause at 10 RE. Since measurements of the solar wind and interplanetary magnetic field are available from multiple spacecraft, we can compare several different techniques of shock-normal determination. Of the single spacecraft techniques the magnetic coplanarity solution is most accurate and the mixed mode solution is of lesser accuracy. Uncertainty in the timing and location of the IMP 8 spacecraft limits the accuracy of solutions using the time of arrival at the position of IMP 8.