Water Group Ions Research Articles

Electron energetics in the Enceladus torus

[1] The inner magnetosphere of Saturn contains both neutral gas and plasma whose composition is dominated by water group ion species. This gas is distributed in a torus-shaped region located near the orbit of Enceladus. The source of this gas is the predominately water vapor plume located in the south polar region of Saturn's icy satellite Enceladus. The electron distribution in the torus comprises two populations: a relatively cold ( 10 eV) suprathermal tail. This paper describes model calculations of the electron energy distribution and energetics in the torus in order to explore how the observed electron distributions can be explained. A thermal electron energy equation is numerically solved, and electron temperatures are calculated. A separate electron energy deposition model determines suprathermal electron fluxes as functions of energy. The main suprathermal electron population is due to photoelectron production from the absorption of solar radiation by water and other neutral species, and this is demonstrated by a comparison of the model results with recently published Cassini data. The model includes heating of thermalized electrons by fresh photoelectrons and by hot pickup ions and also includes cooling by collisions with water and other neutral species. Calculated electron temperatures for the thermal population are about 104 − 2 × 104 K (i.e., 1–2 eV), values which are somewhat lower than recently published Cassini Langmuir probe electron temperature values of 2–3 eV.

Solar wind contribution to surficial lunar water: Laboratory investigations

Remote infrared spectroscopic measurements have recently re-opened the possibility that water is present on the surface of the Moon. Analyses of infrared absorption spectra obtained by three independent space instruments have identified water and hydroxyl (–OH) absorption bands at ∼3 μm within the lunar surface. These reports are surprising since there are many mechanisms that can remove water but no clear mechanism for replenishment. One hypothesis, based on the spatial distribution of the –OH signal, is that water is formed by the interaction of the solar wind with silicates and other oxides in the lunar basalt. To test this hypothesis, we have performed a series of laboratory simulations that examine the effect of proton irradiation on two minerals: anorthite and ilmenite. Bi-directional infrared reflection absorption spectra do not show any discernable enhancement of infrared absorption in the 3 μm spectral region following 1 or 100 keV proton irradiation at fluences between 10 16 and 10 18 ions cm −2. In fact, the post-irradiation spectra are characterized by a decrease in the residual O–H band within both minerals. Similarly, secondary ion mass spectrometry shows a decrease rather than an increase of the water group ions following proton bombardment of ilmenite. The absence of significant formation of either –OH or H 2O is ascribed to the preferential depletion of oxygen by sputtering during proton irradiation, which is confirmed by post-irradiation surface analysis using X-ray photoelectron spectroscopy measurements. Our results provide no evidence to support the formation of H 2O in the lunar regolith via implantation of solar wind protons as a mechanism responsible for the significant O–H absorption in recent spacecraft data. We determine an upper limit for the production of surficial –OH on the lunar surface by solar wind irradiation to be 0.5% (absorption depth).

Survey of ion plasma parameters in Saturn's magnetosphere

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 H2+. While the survey extends to radial distances of 30 RS 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 RS. 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 H2+ in the outer magnetosphere; (3) water group ions are the dominant population in the inner magnetosphere, but only within ∼3 RS 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 RS; 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.

Nature of the ring current in Saturn's dayside magnetosphere

We employ magnetic field and plasma particle data from two equatorial passes of the Cassini spacecraft spanning the radial range between ∼3 and ∼20 RS to investigate the nature of the ring current in Saturn's dayside magnetosphere (RS is Saturn's equatorial radius, equal to 60,268 km). We examine plasma parameters obtained by Cassini for each pass and compare the azimuthal current density profiles deduced with those obtained from current disk modeling of the magnetic field perturbations. We show that the current associated with the P⊥ > P∥ pressure anisotropy of the warm water group ions is important inside ∼10 RS, canceling a significant fraction of the otherwise dominant inertia current inside ∼6 RS. The overall total current density profile is then found to be similar to that produced by the pressure gradient current but augmented in strength by factors of ∼1.5–2.0 by the difference between the inertia and anisotropy currents. The total current density rises from small values near ∼6 RS, peaks at ∼100 pA m−2 near ∼8 RS, and reduces to values below ∼25 pA m−2 at distances beyond ∼15 RS, up to the 20 RS limit of our study. The deduced current density profiles are in good agreement with the gross features of the profiles deduced from current disk modeling, with peak values occurring just outside the inner edge of the best fit models at ∼7 RS, though with the current density then falling more steeply with radial distance than the 1/r dependence assumed in the model.

Heavy ion formation in Titan's ionosphere: Magnetospheric introduction of free oxygen and a source of Titan's aerosols?

Discovery by Cassini's plasma instrument of heavy positive and negative ions within Titan's upper atmosphere and ionosphere has advanced our understanding of ion neutral chemistry within Titan's upper atmosphere, primarily composed of molecular nitrogen, with ~2.5% methane. The external energy flux transforms Titan's upper atmosphere and ionosphere into a medium rich in complex hydrocarbons, nitriles and haze particles extending from the surface to 1200 km altitudes. The energy sources are solar UV, solar X-rays, Saturn's magnetospheric ions and electrons, solar wind and shocked magnetosheath ions and electrons, galactic cosmic rays (GCR) and the ablation of incident meteoritic dust from Enceladus’ E-ring and interplanetary medium. Here it is proposed that the heavy atmospheric ions detected in situ by Cassini for heights >950 km, are the likely seed particles for aerosols detected by the Huygens probe for altitudes <100 km. These seed particles may be in the form of polycyclic aromatic hydrocarbons (PAH) containing both carbon and hydrogen atoms C n H x . There could also be hollow shells of carbon atoms, such as C 60, called fullerenes which contain no hydrogen. The fullerenes may compose a significant fraction of the seed particles with PAHs contributing the rest. As shown by Cassini, the upper atmosphere is bombarded by magnetospheric plasma composed of protons, H 2 + and water group ions. The latter provide keV oxygen, hydroxyl and water ions to Titan's upper atmosphere and can become trapped within the fullerene molecules and ions. Pickup keV N 2 +, N + and CH 4 + can also be implanted inside of fullerenes. Attachment of oxygen ions to PAH molecules is uncertain, but following thermalization O+ can interact with abundant CH 4 contributing to the CO and CO 2 observed in Titan's atmosphere. If an exogenic keV O+ ion is implanted into the haze particles, it could become free oxygen within those aerosols that eventually fall onto Titan's surface. The process of freeing oxygen within aerosols could be driven by cosmic ray interactions with aerosols at all heights. This process could drive pre-biotic chemistry within the descending aerosols. Cosmic ray interactions with grains at the surface, including water frost depositing on grains from cryovolcanism, would further add to abundance of trapped free oxygen. Pre-biotic chemistry could arise within surface microcosms of the composite organic-ice grains, in part driven by free oxygen in the presence of organics and any heat sources, thereby raising the astrobiological potential for microscopic equivalents of Darwin's “warm ponds” on Titan.

A diffusive equilibrium model for the plasma density in Saturn's magnetosphere

Electron density measurements have been obtained by the Cassini Radio and Plasma Wave Science (RPWS) instrument for more than 50 passes through Saturn's inner magnetosphere from 30 June 2004 to 30 September 2007. The electron densities are derived from RPWS measurements of the upper hybrid resonance frequency and span latitudes up to 35° and L values from 3.6 to 10. The electron density measurements are combined with ion anisotropy measurements from the Cassini Plasma Spectrometer (CAPS) and electron temperature measurements from the RPWS and CAPS to develop a diffusive equilibrium model for the distribution of water group ions, hydrogen ions, and electrons in the inner region of Saturn's magnetosphere. The model uses an analytical solution of the field‐aligned force equation, including the ambipolar electric field, to determine the equatorial ion densities and scale heights as a function of L. Density contour plots for water group ions, hydrogen ions, and electrons are presented.

Plume ionosphere of Enceladus as seen by the Cassini ion and neutral mass spectrometer

The Cassini spacecraft passed within 168 km of the surface of Enceladus on 14 July 2005 during the E2 flyby and passed closer (50 km) during the E3 encounter on 13 March 2008. During both flybys the ion and neutral mass spectrometer (INMS) detected a plume atmosphere mainly composed of water. During the E3 flyby, the INMS measured an ion mass spectrum with a large peak at mass number of 19 Daltons (interpreted as H3O+ ions) but not at the mass numbers of other water group ion species (including H2O+, OH+, O+). In addition, the INMS observed ion species at mass numbers 36 and 37 ‐ possibly the water cluster ion species H2O+ ‐ H2O and H3O+ ‐ H2O. The INMS detection of cold H3O+ in the plume combined with the almost complete absence of cold H2O+ ions is attributed to an active ion‐neutral chemistry operating in a plume “ionosphere.”

Determining ion production rates near Saturn's extended neutral cloud from ion cyclotron wave amplitudes

Recent Cassini observations of active venting of water molecules from Enceladus indicate that the moon is the primary source of Saturn's extended neutral cloud. Ionization of the neutrals through charge exchange creates a population of newborn ions with a velocity space distribution, which is highly unstable to the generation of electromagnetic ion cyclotron waves. Cassini observed such ion cyclotron waves, finding spatial and temporal variability in the wave amplitudes throughout the extended neutral cloud region. Since the amount of energy in the ion cyclotron waves is proportional to the number of newborn ions generating them, it is possible to infer the ion production rate in the region. To do so, we use two‐dimensional electromagnetic hybrid (kinetic ions, fluid electrons) simulations to investigate the growth and nonlinear evolution of ion cyclotron waves. We focus on conditions near Enceladus' L shell and compare the simulated and observed ion cyclotron wave amplitudes to estimate the neutral densities and ion production rates. Our simulation results find a relatively linear relation between ion production rate and quasisteady wave energy level (δB2). For conditions near Enceladus' L shell, we find that water group ion production rates of 0.007–0.014/cc/s (which yield wave amplitudes of ∼0.1–0.3 nT) are appropriate. For ion production within an annulus volume from 3.9 to 4 RS, we obtain ion production rates of 3.8 × 1026 to 7.6 × 1026 ions/s or 10.2–20.4 kg/s.

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

Plasma data from the Cassini Plasma Spectrometer experiment are analyzed using a robust forward modeling technique for dayside equatorial orbits within the range 5.5 to 11 Saturn radii (1 RS = 60,268 km). It is assumed the measured ion data may be 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⊥/T∥ 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.

Observations of molecular oxygen ions in Saturn's inner magnetosphere

We present an analysis of molecular oxygen ion (O2+) abundance in Saturn's inner magnetosphere based on observations made with the Cassini Plasma Spectrometer (CAPS). Using data summed over 23 orbits, we resolve and isolate O2+ counts from the tail of the water group ion mass distribution (W+ [O+, OH+, H2O+, H3O+]) and from background sources. O2+ was initially estimated at ∼1–2% of the total ion population in the inner magnetosphere based on CAPS data from the Saturn orbital insertion pass. Through refined analysis techniques, we have found O2+ to account for just above 0.3% of all W+ ions at L = 4.5, with only minor fluctuations in relative density out to L = 7.5 − 8.0. Beyond L = 7.5, the relative O2+/W+ abundance exhibits a statistically significant increase through L = 10.5, which may indicate a neutral O2 source in the vicinity of Rhea.

MESSENGER Observations of the Composition of Mercury's Ionized Exosphere and Plasma Environment

The region around Mercury is filled with ions that originate from interactions of the solar wind with Mercury's space environment and through ionization of its exosphere. The MESSENGER spacecraft's observations of Mercury's ionized exosphere during its first flyby yielded Na+, O+, and K+ abundances, consistent with expectations from observations of neutral species. There are increases in ions at a mass per charge (m/q) = 32 to 35, which we interpret to be S+ and H2S+, with (S+ + H2S+)/(Na+ + Mg+) = 0.67 +/- 0.06, and from water-group ions around m/q = 18, at an abundance of 0.20 +/- 0.03 relative to Na+ plus Mg+. The fluxes of Na+, O+, and heavier ions are largest near the planet, but these Mercury-derived ions fill the magnetosphere. Doubly ionized ions originating from Mercury imply that electrons with energies less than 1 kiloelectron volt are substantially energized in Mercury's magnetosphere.

Ion and neutral sources and sinks within Saturn's inner magnetosphere: Cassini results

Using ion–electron fluid parameters derived from Cassini Plasma Spectrometer (CAPS) observations within Saturn's inner magnetosphere as presented in Sittler et al. [2006a. Cassini observations of Saturn's inner plasmasphere: Saturn orbit insertion results. Planet. Space Sci., 54, 1197–1210], one can estimate the ion total flux tube content, N ION L 2, for protons, H +, and water group ions, W +, as a function of radial distance or dipole L shell. In Sittler et al. [2005. Preliminary results on Saturn's inner plasmasphere as observed by Cassini: comparison with Voyager. Geophys. Res. Lett. 32(14), L14S04), it was shown that protons and water group ions dominated the plasmasphere composition. Using the ion–electron fluid parameters as boundary condition for each L shell traversed by the Cassini spacecraft, we self-consistently solve for the ambipolar electric field and the ion distribution along each of those field lines. Temperature anisotropies from Voyager plasma observations are used with ( T ⊥ / T ∥ ) W + ∼ 5 and ( T ⊥ / T ∥ ) H + ∼ 2 . The radio and plasma wave science (RPWS) electron density observations from previous publications are used to indirectly confirm usage of the above temperature anisotropies for water group ions and protons. In the case of electrons we assume they are isotropic due to their short scattering time scales. When the above is done, our calculation show N ION L 2 for H + and W + peaking near Dione's L shell with values similar to that found from Voyager plasma observations. We are able to show that water molecules are the dominant source of ions within Saturn's inner magnetosphere. We estimate the ion production rate S ION∼10 27 ions/s as function of dipole L using N H + , N W + and the time scale for ion loss due to radial transport τ D and ion–electron recombination τ REC. The ion production shows localized peaks near the L shells of Tethys, Dione and Rhea, but not Enceladus. We then estimate the neutral production rate, S W, from our ion production rate, S ION, and the time scale for loss of neutrals by ionization, τ ION, and charge exchange, τ CH. The estimated source rate for water molecules shows a pronounced peak near Enceladus’ L shell L∼4, with a value S W∼2×10 28 mol/s.

Understanding the escape of water from Enceladus

On 14 July 2005, Cassini passed within 175 km of Enceladus’ surface enabling a direct in situ measurement of water escaping from the surface by the Ion and Neutral Mass Spectrometer (INMS) and the observation of a stellar occultation by the Ultraviolet Spectrometer (UVIS). We have developed a three‐dimensional, Monte Carlo neutral model to simultaneously model the INMS and UVIS measurements of water gas density and column density, respectively. The data are consistent with a two‐component atmosphere; the first with a weak, distributed source on the surface which, if global, has a source rate of ∼8 × 1025 H2O/s, and the second with a much larger source localized at the south pole with a source rate ∼1028 H2O/s. This latter source is possibly coincident with the “tiger stripe” series of fractures revealed by the Imaging Science Subsystem instrument where the ice was measured to be warmer than the surrounding regions by the Composite Infrared Spectrometer instrument. We estimate the plasma mass loading rate due to interaction between the plume and magnetospheric plasma is 2–3 kg/s for a plume source of 1028 H2O/s. Pickup of water group ions in the plume slows down the plasma to ∼10 km/s relative to Enceladus in the region of, and downstream of, the south polar plume. This is consistent with the mass loading rate inferred from magnetic field perturbations detected during the Cassini flyby and suggests an additional source may be needed to explain the plasma flow deflections detected by the Cassini Plasma Spectrometer.

Assessment of the magnetospheric contribution to the suprathermal ions in Saturn's foreshock region

Nineteen months of Cassini Plasma Spectrometer measurements are surveyed for episodes of suprathermal ions upstream from Saturn's bow shock. A total of 45 hours of mass‐resolved observations are obtained. Suprathermal ions (between 3 and 50 keV/q) in Saturn's foreshock are found to be dominantly comprised of H+ and ions with m/q = 2, presumably solar wind He++, with no detectable contribution from magnetospheric water group ions. In light of the dominant contribution of water group ions to the hot plasma of the outer magnetosphere, it thus appears that magnetospheric leakage is not a significant source of upstream ions in this energy range. One implication is that the more energetic O+ ions reported recently most likely originate from direct leakage of already‐energized magnetospheric particles, rather than from their upstream acceleration by bow shock‐related processes.

The Deep Space 1 encounter with comet 19P/Borrelly

The Deep Space 1 Mission successfully encountered comet 19P/Borrelly on 22 September 2001, returning a wealth of images and in situ plasma measurements. To enhance the scientific return of the mission, it is desired to gain knowledge of basic properties of the cometary environment through the interpretation and analysis of the spacecraft data. For this purpose, a coma model for 19P/Borrelly during encounter is presented that consistently represents the cometary environment from the nucleus surface to the solar wind interaction region with detailed chemistry. Neutral gas and plasma dynamics and spatial distributions of various cometary species from the model are presented for interpreting in situ plasma measurements obtained during the encounter. Whereas several features of the plasma environment were well modeled, such as, the standoff distance of the bow shock and the ion velocity profile; the abundances of water-group ions around closest approach are not easily understood in terms of the coma chemistry model. Further data analysis and interpretation is needed.

Nature of magnetic fluctuations in Saturn's middle magnetosphere

The saturnian magnetosphere contains a rich array of plasma wave and MHD wave phenomena. These waves are fed by mass loading in the region of the E ring starting with Enceladus at 3.9 Saturn radii (Rs). In this paper we examine the nature of these waves in what we term to be the middle magnetosphere from just outside the mass loading at Enceladus to just inside the region of the magnetodisk, roughly 5 Rs to 15 Rs. In the inner part of this region are ion‐cyclotron waves near the water group gyrofrequency associated with mass loading of the magnetosphere by the E ring neutral torus that produces a ring beam of water group ions in velocity space. At about 5.5 Rs, isolated flux tubes are seen that appear to be convecting outward containing cooler plasma. This is a region of high levels of compressional noise. The compressional fluctuations associated with isolated flux tubes never completely disappear but their amplitude diminishes with distance. Weak mirror mode waves are present even at 5 Rs. Those waves grow with increasing radial distance, become dominant, and then the ion cyclotron waves weaken. While both instabilities grow from the same pressure anisotropy, the mirror mode dominance at large distances may be due to the fact that the ion cyclotron waves propagate out of the wave growth region, while the mirror mode waves remain in it and convect with the plasma. Finally, around 12 Rs the magnetosphere switches to be turbulent, more strongly in the transverse component than in the compressional component.

First whistler observed in the magnetosphere of Saturn

[1] This paper describes and analyzes the first whistler observed in the magnetosphere of Saturn. The whistler was detected by the Radio and Plasma Wave Science (RPWS) instrument on the Cassini spacecraft during the inbound pass on October 28, 2004, at a radial distance of 6.18 RS (Saturn radii). Based on the measured dispersion, a travel time computation shows that the whistler originated from lightning in the northern hemisphere of Saturn. Using a simple centrifugal potential model for the plasma distribution along the magnetic field line, we have determined the scale height and fractional concentrations of water group ions and protons that give the best fit to the observed dispersion. In addition, a more complex diffusive equilibrium model was also analyzed. The inferred electron density profiles for the two models are very similar. Representative scale heights and equatorial ion fractional concentrations that provided good fits to the dispersion of the whistler were found to be = 0.89 RS and = 0.7 for water group ions, and = 2.67 RS and = 0.3 for protons.

Cassini observations of Saturn's inner plasmasphere: Saturn orbit insertion results

We present new and definitive results of Cassini plasma spectrometer (CAPS) data acquired during passage through Saturn's inner plasmasphere by the Cassini spacecraft during the approach phase of the Saturn orbit insertion period. This analysis extends the original analysis of Sittler et al. [2005. Preliminary results on Saturn's inner plasmasphere as observed by Cassini: comparison with Voyager. Geophys. Res. Lett. 32, L14S07, doi:10.1029/2005GL022653] to L∼10 along with also providing a more comprehensive study of the interrelationship of the various fluid parameters. Coincidence data are sub-divided into protons and water group ions. Our revised analysis uses an improved convergence algorithm which provides a more definitive and independent estimate of the spacecraft potential Φ SC for which we enforce the protons and water group ions to co-move with each other. This has allowed us to include spacecraft charging corrections to our fluid parameter estimations and allow accurate estimations of fluctuations in the fluid parameters for future correlative studies. In the appendix we describe the ion moments algorithm, and minor corrections introduced by not weighting the moments with sin θ term in Sittler et al. [2005] (Correction offset by revisions to instruments geometric factor). Estimates of the spacecraft potential and revised proton densities are presented. Our total ion densities are in close agreement with the electron densities reported by Moncuquet et al. [2005. Quasi-thermal noise spectroscopy in the inner magnetosphere of Saturn with Cassini/RPWS: electron temperatures and density. Geophys. Res. Lett. 32, L20S02, doi:10.1029/2005GL022508] who used upper hybrid resonance (UHR) emission lines observed by the radio and plasma wave science (RPWS) instrument. We show a positive correlation between proton temperature and water group ion temperature. The proton and thermal electron temperatures track each with both having a positive radial gradient. These results are consistent with pickup ion energization via Saturn's rotational electric field. We see evidence for an anti-correlation between radial flow velocity V R and azimuthal velocity V φ , which is consistent with the magnetosphere tending to conserve angular momentum. Evidence for MHD waves is also present. We show clear evidence for outward transport of the plasma via flux tube interchange motions with the radial velocity of the flow showing positive radial gradient with V R ∼ 0.12 ( L / 4 ) 5.5 km / s functional dependence for 4< L<10 (i.e., if we assume to be diffusive transport then D LL ∼ D 0 L 11 for fixed stochastic time step δt). Previous models with centrifugal transport have used D LL ∼ D 0 L 3 dependence. The radial transport seems to begin at Enceladus’ L shell, L∼4, where we also see a minimum in the W + ion temperature T W ∼ 35 eV . For the first time, we are measuring the actual flux tube interchange motions in the magnetosphere and how it varies with radial distance. These observations can be used as a constraint with regard to future transport models for Saturn's magnetosphere. Finally, we evaluate the thermodynamic properties of the plasma, which are all consistent with the pickup process being the dominant energy source for the plasma.

Energetic nitrogen ions within the inner magnetosphere of Saturn

We investigate the importance of nitrogen ions within Saturn's magnetosphere and their contribution to the energetic charged particle population within Saturn's inner magnetosphere. This study is based on the Voyager observations of Saturn's magnetosphere and Cassini observations. The latter have shown that water group ions dominate both the plasma and energetic particle populations but that nitrogen ions over a broad range of energies were observed at ∼5% abundance level. In the outer magnetosphere, methane ions were predicted to be an important pickup ion at Titan and were detected at significant levels in the outer magnetosphere and at Titan. O+ ions were found to be the dominant heavy ion in the outer magnetosphere, ∼60%, with methane ions being ∼30% of the heavy ions and N+ being a few percent. The two major sources of nitrogen ions within Saturn's magnetosphere are Titan's atmosphere and primordial nitrogen trapped in the icy crust of Saturn's moons and its ring particles deep within the magnetosphere. It is important to understand the source, transport, and sinks of nitrogen in order to determine whether they have a primordial origin or are from Titan's atmosphere. The energetic component is important, since it can come from Titan, be implanted into the surfaces of the icy moons, and reappear at plasma energies via sputtering obfuscating the ultimate source. As we will show, such implantation of nitrogen ions can produce interesting chemistry within the ice of Saturn's moons. The emphasis will be on the nitrogen, but the oxygen and other water group ions are also considered. We argue that neutral clouds of heavy atoms and molecules within Saturn's outer magnetosphere may be the dominant source of energetic heavy ions observed within the inner magnetosphere. Pickup heavy ions in the outer magnetosphere have energies ∼1–4 keV when born. If they diffuse radially inward, while conserving the first and second adiabatic invariants, they can have energies greater than several hundred keV inside of Dione's L shell. We will show how observations relate to the various sources and acceleration processes such as ionization, collisions, wave‐particle interactions, and radial diffusion.

Le satellite Encelade source d'ions N + dans la magnétosphère de Saturne

The first pass of the Cassini probe in the vicinity of Saturn, above the E-ring, demonstrated a plasma consisting of water group ions (H +, O +, OH +, H 2O +) with a small N + ion component (3%). Using a simple model for the transport of magnetospheric ions, we show that the N + ions can be traced back to the Enceladus satellite. Such a result can be explained by the existence in this icy satellite, supposed to be still geologically active, of volatile components such as ammonia NH 3, or by the previous implantation of N + ions of external origin on its surface. To cite this article: M. Bouhram et al., C. R. Physique 6 (2005).