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The plumes of Enceladus produce a cloud of neutral H2O molecules and, via dissociation, OH and O. These neutrals are ionized by charge exchange, solar UV, and electron impacts, producing the therma...

We examine long‐term suprathermal, singly charged heavy ion composition measured at three planets using functionally identical charge‐energy‐mass ion spectrometers, one on Geotail, orbiting Earth at ~9–30 Re, the other on Cassini, in interplanetary space, during Jupiter flyby, and then in orbit around Saturn. O+, a principal suprathermal (~80–220 keV/e) heavy ion in each magnetosphere, derives primarily from outflowing ionospheric O+ at Earth, but mostly from satellites and rings at Jupiter and Saturn. Comparable amounts of Iogenic O+ and S+ are present at Jupiter. Ions escaping the magnetospheres: O+ and S+ at Jupiter; C+, N+, O+, H2O+, 28M+ (possibly an aggregate of the molecular ions, MI, CO+, N2+, HCNH+, and/or C2H4+), and O2+ at Saturn; and N+, O+, N2+, NO+, O2+, and Fe+ at Earth. Generally, escaped atomic ions (MI) at Earth and Saturn have similar (higher) ratios to O+ compared to their magnetospheric ratios; Saturn's H2O+ and Fe+ ratios are lower. At Earth, after O+ and N+, ionospheric origin N2+, NO+, and O2+ (with proportions ~0.9:1.0:0.2) dominate magnetospheric heavy ions, consistent with recent high‐altitude/latitude ionospheric measurements and models; average ion count rates correlate positively with geomagnetic and solar activity. At ~27–33 amu/e, Earth's MIs dominate over lunar pickup ions (PUIs) in the magnetosphere; MIs are roughly comparable to lunar PUIs in the magnetosheath, and lunar PUIs dominate over MIs beyond Earth's bow shock. Lunar PUIs are detected at ~39–48 amu/e in the lobe and possibly in the plasma sheet at very low levels.

Previous observations have driven the prevailing assumption in the field that energetic ions measured by an instrument using a bare solid state detector (SSD) are predominantly protons. However, new near-equatorial energetic particle observations obtained between 7 and 12 RE during Phase 1 of the Magnetospheric Multiscale mission challenge the validity of this assumption. In particular, measurements by the Energetic Ion Spectrometer (EIS) instruments have revealed that the intensities of heavy ion species (specifically oxygen and helium) dominate those of protons at energies ≳150–220 keV in the middle to outer (>7 RE) magnetosphere. Given that relative composition measurements can drift as sensors degrade in gain, quality cross-calibration agreement between EIS observations and those from the SSD-based Fly's Eye Energetic Particle Spectrometer (FEEPS) sensors provides critical support to the veracity of the measurement. Similar observations from the Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) instruments aboard the Van Allen Probes spacecraft extend the ion composition measurements into the middle magnetosphere and reveal a strongly proton-dominated environment at L≲6 but decreasing proton intensities at L≳6. It is concluded that the intensity dominance of the heavy ions at higher energies (>150 keV) arises from the existence of significant populations of multiply-charged heavy ions, presumably of solar wind origin.

We analyze particle and magnetic field data obtained between July 2004 and December 2013 in the equatorial magnetosphere of Saturn, by the Cassini spacecraft. The radial and local time distribution of the total (thermal and suprathermal) particle pressure and total plasma beta (ratio of particle to magnetic pressure) over radial distances from 5 to 16 Saturn radii (RS = 60,258 km) is presented. The average azimuthal current density Jϕ and its separate components (inertial, pressure gradient, and anisotropy) are computed as a function of radial distance and local time and presented as equatorial maps. We explore the relative contribution of different physical mechanisms that drive the ring current at Saturn. Results show that (a) the particle pressure is controlled by thermal plasma inside of ~8 RS and by the hot ions beyond ~12 RS, exhibiting strong local time asymmetry with higher pressures measured at the dusk and night sectors; (b) the plasma beta increases with radial distance and remains >1 beyond 8–10 RS for all local times; (c) the ring current is asymmetric in local time and forms a maximum region between ~7 and ~13 RS, with values up to 100–115 pA/m2; and (d) the ring current is inertial everywhere inside of 7 RS, exhibits a mixed nature between 7 and 11 RS and is pressure gradient driven beyond 11 RS, with the exception of the noon sector where the mixed nature persists. In the dawn sector, it appears strongly pressure gradient driven for a wider range of radial distance, consistent with fast return flow of hot, tenuous magnetospheric plasma following tail reconnection.

Measurements in Saturn's equatorial magnetosphere from mid-2004 through 2013 made by Cassini's charge-energy-mass ion spectrometer indicate the presence of a rare, suprathermal (83–167 keV/e) ion species at Saturn with mass ~56 amu that is likely Fe+. The abundance of Fe+ is only ~10−4 relative to that of W+ (O+, OH+, H2O+, and H3O+), the water group ions which dominate Saturn's suprathermal and thermal ions along with H+ and H2+. The radial variation of the Fe+ partial number density (PND) is distinctly different from that of W+ and most ions that comprise Saturn's suprathermal ion populations which, unlike thermal energy plasma ions, typically have a prominent PND peak at ~8–9 Rs (1 Saturn radius, Rs = 60,268 km). In contrast, the Fe+ PND decreases more or less exponentially from ~4 to ~20 Rs, our study's inner and outer limits. Fe+ may originate from metal layers produced by meteoric ablation near Saturn's mesosphere-ionosphere boundary and/or possibly impacted interplanetary dust particles or the Saturn system's dark material in the main rings.

Suprathermal (87–212 keV/e) singly charged iron, Fe⁺, has been discovered in and near Earth's ~9–30 RE equatorial magnetosphere using ~21 years of Geotail STICS (suprathermal ion composition spectrometer) data. Its detection is enhanced during higher geomagnetic and solar activity levels. Fe⁺, rare compared to dominant suprathermal solar wind and ionospheric origin heavy ions, might derive from one or all three candidate lower‐energy sources: (a) ionospheric outflow of Fe⁺ escaped from ion layers near ~100 km altitude, (b) charge exchange of nominal solar wind iron, Fe⁺≥⁷, in Earth's exosphere, or (c) inner source pickup Fe⁺ carried by the solar wind, likely formed by solar wind Fe interaction with near‐Sun interplanetary dust particles. Earth's semipermanent ionospheric Fe⁺ layers derive from tons of interplanetary dust particles entering Earth's atmosphere daily, and Fe⁺ scattered from these layers is observed up to ~1000 km altitude, likely escaping in strong ionospheric outflows. Using ~26% of STICS's magnetosphere‐dominated data when possible Fe⁺² ions are not masked by other ions, we demonstrate that solar wind Fe charge exchange secondaries are not an obvious Fe⁺ source. Contemporaneous Earth flyby and cruise data from charge‐energy‐mass spectrometer on the Cassini spacecraft, a functionally identical instrument, show that inner source pickup Fe⁺ is likely not important at suprathermal energies. Consequently, we suggest that ionospheric Fe⁺ constitutes at least a significant portion of Earth's suprathermal Fe⁺, comparable to the situation at Saturn where suprathermal Fe⁺ is also likely of ionospheric origin.

Water group ions W+ (O+, OH+, H2O+, and H3O+), along with H+ and H2+, dominate Saturn's near-equatorial magnetospheric suprathermal ion populations. The singly charged, minor heavy ions O2+ and 28M+ were also observed in the suprathermal energy range, but at much lower densities, having ≤10−2 the abundance of W+. From 2004 through 2013, Cassini's charge-energy-mass ion spectrometer has measured suprathermal 83–167 keV/e heavy ions at ~4–20 Rs (1 Saturn radius, Rs = 60,268 km). Christon et al. (2013) found apparent O2+/W+ transient and seasonal responses to variable insolation of Saturn's ring atmosphere prior to mid-2012. A similar seasonal variation in 28M+/W+ (28M+ ~27–30 amu/e molecular minor ions) was suggested but inconclusive. Now with data from mid-2012 through 2013, we find that both O2+ and 28M+ clearly exhibit seasonal recoveries from mid-2012 onward. Prominent radial partial number density peaks at ~9 Rs identify W+, O2+, and 28M+ as clear ring current participants. It is presently unclear which part of Saturn's magnetosphere produces the seasonally varying 28M+ component. Dissimilar 28M+/W+ and O2+/W+ responses to a strong late 2011 solar UV burst suggest different seasonal ring-based photolytic processes.

[1] We show that the neutral gas vertical distribution at Saturn must be ~3–4 times more extended than previously thought for the >5 RSregions, while the neutral H distribution is consistent with H densities that reach up to ~150/cm3close to the orbit of Titan. We utilize a technique to retrieve the global neutral gas distribution in Saturn's magnetosphere, using energetic ion and energetic neutral atom (ENA) measurements, obtained by the Magnetospheric Imaging Instrument (MIMI) onboard the Cassini spacecraft. Our ENA measurements are consistent with a neutral cloud that consists of H2O, OH, H, and O, while the overall shapes and densities numbers concerning the neutral gas distributions are constrained according to already existing models as well as recent observations. The neutral gas distribution at Saturn is determined by simulating a 24–55 keV hydrogen image of the Saturnian magnetosphere, measured by the Ion and Neutral Camera (INCA), averaged over the time period from 1 July 2004 to 23 August 2005. The ionic input of the model includes a proton distribution of combined Charge Energy Mass Spectrometer (CHEMS, 3–230 keV/e), Low Energy Magnetospheric Measurements System (LEMMS, 30.7 keV to 2.3 MeV), and INCA (5–300 keV) in situ measurements. These measurements cover several passes from 1 July 2004 to 10 April 2007, at various local times over the dipole L range 5

[1] Suprathermal singly charged molecular ions, O2+ (at ~32 Da/e) and the Mass-28 ion group 28M+ (ions at ~28 Da/e, with possible contributions from C2H5+, HCNH+, N2+, and/or CO+), are present throughout Saturn's ~4–20 Rs (1 Saturn radius, Rs = 60,268 km) near-equatorial magnetosphere from mid-2004 until mid-2012. These ~83–167 keV/e heavy ions measured by Cassini's CHarge-Energy-Mass Spectrometer have long-term temporal profiles that differ from each other and differ relative to the dominant water group ions, W+ (O+, OH+, H2O+, and H3O+). O2+/W+, initially ~0.05, declined steadily until equinox in mid-2009 by a factor of ~6, and 28M+/W+, initially ~0.007, declined similarly until early-2007 by a factor of ~2. The O2+/W+ decline is consistent with Cassini's in situ ring-ionosphere thermal ion measurements, and with proposed and modeled seasonal photolysis of Saturn's rings for thermal O2 and O2+. The water ice-dominated main rings and Enceladus plume depositions thereon are the two most likely O2+ sources. Enceladus' dynamic plumes, though, have no known long-term dependence. After declining, O2+/W+ and 28M+/W+ levels remained low until late-2011 when O2+/W+ increased, but 28M+/W+ did not. The O2+/W+ increase was steady and became statistically significant by mid-2012, indicating a clear increase after a decline, that is, a possibly delayed O2+ “seasonal” recovery. Ring insolation is driven by solar UV flux which itself varies with the sun's 11 year activity cycle. The O2+/W+ and 28M+/W+ declines are consistent with seasonal ring insolation. No O2+/W+ response to the late-2008 solar-cycle UV minimum and recovery is evident. However, the O2+/W+ recovery from the postequinox baseline levels in late-2011 coincided with a strong solar UV enhancement. We suggest a scenario/framework in which the O2+ observations can be understood.

[1] Energetic protons (2.8–78 keV) and water group ions (8.8–78 keV) observed in Saturn's magnetosphere using the Magnetospheric Imaging Instrument/Charge-Energy-Mass Spectrometer instrument from 2005 through 2012 were subjected to a Lomb periodogram analysis with the period window extending from 0.5 to 50 days, and the data constrained to the spatial region between 10 RS (1 RS = 60,268 km) and the magnetopause. Both the protons and water group ions exhibited solar periodicity at ~26 days and harmonics thereof. For all ions, the 26 day periodicities were strong on the dayside and duskside, weak at dusk, and virtually nonexistent on the nightside. The solar periodicity was evident during both the first and second halves of the 7 year time span, but strongest in the first half from January 2004 to July 2008. Some of the ion spectral peaks, especially for the water group ions, can be associated with orbital motion of the spacecraft. The 26 day periodicities are likely caused by corotating interaction regions in the solar wind that periodically sweep past Saturn, regularly compressing its dayside magnetosphere, but not its nightside.

[1] We analyze plasma, energetic particle, and magnetic field data from all available Cassini passes through the Saturnian magnetosheath between July 2004 and July 2011 and provide a statistical overview of particle and field properties. The results show that magnetosheath plasma has an average number density of ~0.1 cm−3 and a temperature of ~300 eV. The measured magnetic field strength is ~1 nT, and the plasma beta is in the range of 10 to 100. The prevailing flow and magnetic field configuration is close to that theoretically expected, with flow speed values of ~200 km/s. Compositional data reveal that although at low energies ( few keV) there is a strong presence of water group ions (W+) forming localized structures we refer to as W+ “islands” that travel downstream convected in the plasma flow. Under average magnetic field conditions in the Saturnian magnetosheath, the kinetic properties of these hot W+ ions can enable escape upstream from the bow shock. Based on the measured particle and field distributions and the modeled bow shock and magnetopause positions, we describe the energetic ion escape as a function of energy and pitch angle and successfully predict the energy distribution of the escaping W+ ions. Comparison with the ion spectra measured in the nearby solar wind confirms that the suggested escape mechanism due to large ion gyroradii is sufficient to explain the observed leakage of heavy energetic ions upstream from the Saturnian bow shock.

We report observations of neutral atoms from the solar wind in the Earth's vicinity with the low-energy neutral atom (LENA) imager on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft. This instrument was designed to be capable of looking at and in the direction of the Sun. Enhancements in the hydrogen count rate in the solar direction are not correlated with either solar ultraviolet emission or suprathermal ions and are deduced to be due to neutral particles from the solar wind. LENA observes these particles from the direction closest to that of the Sun even when the Sun is not directly in LENA's 90° field of view. Simulations show that these neutrals are the result of solar wind ions charge exchanging with exospheric neutral hydrogen atoms in the postshock flow of the solar wind in the magnetosheath. Their energy is inferred to exceed 300 eV, consistent with solar wind energies, based on simulation results and on the observation of oxygen ions, sputtered from the conversion surface in the time-of-flight spectra. In addition, the sputtered oxygen abundance tracks the solar wind speed, even when IMAGE is deep inside the magnetosphere. These results show that low-energy neutral atom imaging provides the capability to directly monitor the solar wind flow in the magnetosheath from inside the magnetosphere because there is a continuous and significant flux of neutral atoms originating from the solar wind that permeates the magnetosphere.

Statistically significant variations have been observed in the differential flux of ∼27-MeV anomalous cosmic ray (ACR) oxygen, helium, and protons at the Voyager 1 spacecraft during 1998 and 1999 (at a helioradius of ∼73 AU). The quasiperiodic variations are in phase, with oxygen and helium having periods near 151 days, while protons exhibit a period of ∼146 days. The Voyager 1 ACRs vary by ∼30% with respect to the trend, and similar galactic cosmic ray variations, if they exist, must be less than ∼5%, probably much less. No similar, significant periodicities have been detected for these same ACR species at Voyager 2 (at 57 AU) during this period. We report on these and other periodicities in the Voyager Low Energy Charged Particle experiment measurements and address the possible connection between this ∼151-day ACR periodicity and the previously discovered ∼154-day periodicities in solar flares, the interplanetary magnetic field, and other phenomena.

We have measured H + , He ++ , and He + distribution functions over the solar wind through the suprathermal energy range during two corotating interaction region (CIR) events o b- served by the STICS, MASS, and STEP instruments on board the Wind spacecraft at 1 AU during April and May 1995. The major properties we find are as follows : In the suprathermal energy range (~10-500 keV/nucleon), the particle intensities peak inside the CIR itself, in the compressed and decelerated fast solar wind, in contrast to the situation at MeV energies, where the peak inte n- sities are observed outside the CIR in the fast solar wind. The distribution functions of solar wind H + and He ++ change smoothly from the core at solar wind speeds to a power law or exponential form at higher energies, with no turnover observed at intermediate energies. CIR He + is observed with an abundance ratio He + /He ++ ~ 16-17%, orders of magnitude higher than that in the bulk solar wind but nevertheless lower than that observed in CIRs at 4.5 AU. The H + , He ++ , and He + spectra have similar slopes above speeds of ~2.5-3 times the solar wind speed ( Vsw) in the space- craft frame. The ion speed at which the CIR He ++ /H + ratio changes from typical solar wind va l- ues of 4-5% to the higher (>10%) value typical of CIRs is ~1.5-1.7 Vsw, measured in the space- craft reference frame. Analyzing these observations in the context of previous global observations and simple models of CIR acceleration and transport ( Fisk and Lee, 1980), we conclude the fol- lowing: (1) Suprathermal CIR ions at 1 AU originated close (within ~0.5 AU) to the point of o b- servation, not in the outer heliosphere; (2) the injection/acceleration mechanism is not especially sensitive to charge-to-mass ratio over the range 0.25-1.0; (3) since the particles are locally accel - erated, the low-energy ion populations we observe contain the seed population; (4) the bulk solar wind itself is not the source of the energetic ions; rather, the source is in the suprathermal tail, with an injection threshold in the spacecraft frame of ~1.8-2.5 times the solar wind speed; and (5) in at least one of these CIRs, suprathermal particle acceleration is not shock associated and must therefore be associated with a statistical mechanism or co mpression in the solar wind.

The Dessler-Parker-Sckopke relation (DPS) predicts a linear dependence of the perturbation magnetic field at the surface of the Earth on the total ring current kinetic energy. In this paper, we test DPS by using measurements of the major ring current ion species made by the charge-energy-mass spectrometer on the Active Magnetospheric Particle Tracer Explorers CCE spacecraft. We use spectra from passes through the equatorial storm time ring current near the maximum phase of 80 magnetic storms between 1984 and 1989 to estimate the global ring current energy content E RC and compare it with the average value for Dst during each pass. Our work shows that DPS holds well on average. In particular, there is a strong linear correlation between ring current energy estimated from nightside ion measurements and the Dst index, and the slope of the least squares fit line giving Dst as a function of nightside E RC is in good agreement with the prediction of DPS. In contrast, dayside measurements of E RC do not yield a robust correlation with Dst. Although we cannot rule out the possibility that currents other than the ring current (for example, tail currents and the magnetopause current) may cause large magnetic perturbations, we conclude that these perturbations, if they exist, must be largely compensating. By examining how the ratio of Dst to E RC varies with the local time sector of the in situ ion measurements, we obtain statistical information on the anisotropy of the storm time ring current. We find that the largest values of E RC /Dst result from nightside measurements and the smallest values result from measurements in the 0600 to 1200 LT region, as would be expected for an ion population injected on the nightside that must drift westward around the Earth, undergoing losses, to reach the dayside morning sector.

Silicon is a common material in the solar system. For instance, Si accounts for about 10% of the material in primitive meteorites (CI chondrites). Since silicon is a refractory element, we expect the meteoritic isotopic composition to be very similar to that of the Sun. The isotopic composition of Si in meteorites is well known and varies little. Thus the three stable isotopes of Si may serve as powerful indicators to test fractionation of isotopes in the transition from the solar atmosphere into the solar wind. We present, for the first time, measurements of the isotopic composition of Si in the solar wind. The data were obtained with the MASS instrument aboard the WIND spacecraft and accumulated in exceedingly cold and slow wind. Such wind is often associated with large superradial expansion factors and with current sheet crossings which in turn are associated with the most efficient isotopic fractionation processes in the solar wind acceleration region. We detect little or no isotopic fractionation between the solar surface assumed to be of meteoritic composition and the solar wind. This constrains solar wind acceleration models and puts stringent limits on possible secular changes in the isotopic composition of the outer solar convective zone, the solar atmosphere, and the solar wind.

The extended velocity distribution function for solar wind doubly charged helium (He+2) has been systematically studied for seven high-speed streams occurring in the first 7 months of 1995, near solar minimum. The two instruments, suprathermal ion composition spectrometer (STICS) and high mass resolution spectrometer (MASS), used in this study are both part of the SMS experiment on board the Wind spacecraft. The helium distributions were fit over the speed range v/vsw ≈ 0.8–1.6 constituting ∼7.5 thermal widths and 5 orders of magnitude in phase space density, where vsw is the proton solar wind speed. The distributions are reasonably well fit by kappa functions with values of kappa ranging from 3.4 to 5.8, although there may be systematic deviations from a perfect kappa function over the 5 orders of magnitude in phase space density. This is the first study that characterizes the extended helium distribution functions using kappa functions in the high-speed solar wind.

The possible role of precipitation losses in eroding stormtime ring current is subject to debate. To explore this controversy, the recovery phase of the February 6–10, 1986, great magnetic storm is examined, when intense ion precipitation was observed at midlatitudes by NOAA-6 and DMSP satellites. This storm period is particularly interesting because the ring current exhibits distinctive two-phase decay as seen in the Dst index, the early rapid timescale decay corresponding to the intense ion precipitation period described above. Hamilton et al. [1988] concluded, from close agreement between the observed timescale for ring current decay and the theoretical timescale for O+ charge exchange loss, that rapid early recovery phase of this storm resulted from the charge exchange loss of high energy O+; the second and longer decay phase was equated with H+ charge exchange loss. A model of the ring current evolution during this great magnetic storm [Fok et al., 1995] failed to reproduce the observed ring current decay rates, a puzzling result because charge exchange losses were well represented in the ring current model and initial and boundary conditions were taken from the same data set used in the Hamilton et al. [1988] study. A simple energy balance calculation for the global ring current is carried out using (1) either an energy input predicted from upstream solar wind parameters or one calculated from the drift-loss model output, (2) collisional loss timescales extracted from the drift-loss model, and (3) precipitation losses estimated from NOAA-6 and DMSP observations. The energy balance model replicates the evolution of the ring current energy content derived from Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) observations when ion precipitation losses are included and model energy input function is reduced to agree with predictions based upon upstream solar wind parameters. The O+ charge exchange losses and observed global precipitation losses were of equal magnitude in early recovery of the ring current during this great magnetic storm. Later longer decay timescales in the model resulted from a combination of O+ and H+ charge exchange losses; O+ charge exchange losses remained important throughout the model time interval. The present model produces agreement with the AMPTE/CCE estimates of ring current kinetic energy content versus time. Disagreement between the Dst* inferred from the AMPTE/CCE particle measurements and the observed Dst* is an interesting issue needing further explanation.

Measurements of the 16O and 18O distribution functions in the solar wind at low to average solar wind speeds from the MASS instrument on the Wind spacecraft are reported. The 16O/18O density ratio is 450 ± 130, a value consistent with terrestrial, solar photospheric, solar energetic particle, and galactic cosmic ray 16O/18O isotopic ratios. This study constitutes the first reported spacecraft measurement of the isotope 18O in the core solar wind and may represent the best determination of the solar 16O/18O density ratio to date.

Using data from the Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer spacecraft, we make a detailed comparison between the observed polarization properties of electromagnetic ion cyclotron (EMIC) waves and those predicted by theory. The polarization can be described by three parameters: the ellipticity e, the ratio of parallel (to the background magnetic field B0) magnetic fluctuations δBz to the major axis component of the elliptical perturbation in the perpendicular plane δBmajor, and the phase angle between δBz and δBmajor. On the basis of the plasma parameters observed during EMIC events, we have calculated the linear properties of the theoretical modes and compared these to the observations. The result is that two and in some cases, three of the observed polarization properties are inconsistent with the assumption that the waves result from a single linear mode. We use a simple model with two constituent waves in various azimuthal orientations (around B0) and temporal phase relations and show that the distribution of observed polarization properties can be understood as resulting from the superposition of more than one mode. When there is superposition, the instantaneous polarization characteristics of the fluctuations do not reliably reflect the constituent wave properties and the minimum variance direction cannot be associated with a wave vector direction. Nonetheless, we have shown that the constituent wave properties can be inferred from the distribution of observed properties. For superposition of two waves with only slightly dissimilar characteristics, the constituent wave e is approximately the median observed e, e, and the constituent θkB (angle between the wave vector k and B0) is approximately given by tan θkB = δBz/δBmajor/e, with the overbar on δBz/δBmajor again indicating a median value.

We present a study of the proton cyclotron instability in the Earth's outer magnetosphere, L > 7, using Active Magnetosphere Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) magnetic field, ion, and plasma wave data. The analysis addresses the energy of protons that generate the waves, the ability of linear theory to predict both instability and stability, comparison of the predicted wave properties with the observed wave polarization and frequency, and the temperature anisotropy/parallel beta relation. The data were obtained during 24 intervals of electromagnetic ion cyclotron (EMIC) wave activity (active) and 24 intervals from orbits without EMIC waves (quiet). This is the same set of events used by Anderson and Fuselier [1994]. The active events are drawn from noon and dawn local times for which the wave properties are significantly different. For instability analysis, magnetospheric hot proton distributions required the use of multiple populations to analytically represent the data. Cyclotron waves are expected to limit the proton temperature anisotropy, Ap = T⊥p/T‖p − 1, according to Ap < aβ‖pc with a ∼ 1 and c ∼ 0.5, where T⊥p, T‖p, and β‖p are the perpendicular and parallel proton temperatures and the proton parallel beta, respectively. During cyclotron wave events, Ap should be close to aβ‖pc whereas in the absence of waves Ap should be below aβ‖pc. The active dawn cases yielded instability in 9 of 12 cases using the measured plasma data with an average growth rate γ/Ωp = 0.025 and followed the relation Ap = 0.85β‖p−0.52. The active noon events gave instability in 10 of 12 cases, but only when an additional ∼2 cm−3 cold plasma was assumed. The noon wave events fell well below the dawn events in Ap-β‖p space, slightly above the Ap = 0.2β‖p−0.5 curve. The lower Ap limit for the noon cases is attributed to the presence of unmeasured cold plasma. The quiet events were all stable even for additional assumed cold ion densities of up to 10 cm−3, the upper limit implied by the plasma wave data. The quiet events gave Ap < 0.2β‖p−0.5. At noon, the unstable component has T⊥p ∼ 20 keV and Ap ∼0.8. At dawn the unstable component has T⊥p ∼ 4 keV and Ap ∼ 2.3. Observed wave frequencies agree with the frequencies of positive growth, and the difference in frequency between noon and dawn is attributable to the combined effects of the different hot proton T⊥p and Ap and the inferred higher cold plasma density at noon. The dawn events had significant growth for highly oblique waves, suggesting that the linear polarization of the dawn waves may be due to domination of the wave spectrum by waves generated with oblique wave vectors.

We analyze electron plasma, energetic ion, and magnetic field data from four almost vertical Cassini passes through the nightside plasma sheet of Saturn (segments of the high-latitude orbits of the spacecraft) separated in two subsets: two passes of identical geometry from January 2007 with Cassini crossing the equatorial plane in the postmidnight sector at a distance of similar to 21 Saturn radii (R-S) and two passes from April 2009, also of identical geometry, with Cassini crossing the equatorial plane in the premidnight sector again at a distance of similar to 21 R-S. The vertical structure and variability of the plasma sheet is described for each individual pass, and its basic properties (scale height, vertical displacement, tilt angle, hinging distance) are computed. The plasma sheet presents an energy-dependent vertical structure, being thicker by a factor of similar to 2 in the energetic particle range than in the electron plasma. It further exhibits intense dynamical behavior, evident in the energetic neutral atom emission. In two of the four passes, we observe a clear north-south asymmetry, presumably a combined result of vertical plasma sheet motion and short time scale dynamics. Comparison between the 2007 and 2009 passes reveals a clear change in the tilt and vertical offset of the planetary nightside plasma sheet, which progressively becomes aligned to the solar wind direction as we approach Saturnian equinox (August 2009). Temperature, pressure, and number density in the center of the sheet remain relatively stable and essentially unaffected by the seasonal change.

A study of the transport and loss of ions in the earth's quiet time ring current, in which the standard radial diffusion model developed for the high-energy radiation belt particles is compared with the measurements of the lower-energy ring current ions, is presented. The data set provides ionic composition information in an energy range that includes the bulk of the ring current energy density, 1-300 keV/e. Protons are found to dominate the quiet time energy density at all altitudes, peaking near L of about 4 at 60 keV/cu cm, with much smaller contributions from O(+) (1-10 percent), He(+) (1-5 percent), and He(2+) (less than 1 percent). A minimization procedure is used to fit the amplitudes of the standard electric radial diffusion coefficient, yielding 5.8 x 10 exp -11 R(E-squared)/s. Fluctuation ionospheric electric fields are suggested as the source of the additional diffusion detected.

[1] The charge-energy-mass spectrometer instrument in the Cassini spacecraft measured differential fluxes of protons (2.8–236 keV) and oxygen ions (8.8–236 keV) from July 2004 to August 2007. The fluxes were bin-averaged in Saturn longitude system (SLS) longitude within ±5 RS of the equator and between 8 and 12 RS in radial distance (1 RS = 60,238 km) to determine their global morphology. The 3-year time period is the range of validity of the SLS, which is based on a variable period of Saturn kilometric radiation. Fluxes at all energies of H+ and O+ display an essentially sinusoidal variation in longitude, often with peak-to-trough ratios of 2:1. For E 77 keV; the maxima shift to ∼250°. The ion distributions closely resemble those of energetic neutral hydrogen and neutral oxygen atoms observed by the ion neutral camera onboard Cassini.

Data from the CHEM instrument on AMPTE CCE, data from the 3D plasma instrument and the SULEICA instrument on AMPTE IRM, and magnetometer data from both spacecraft are used to determine the particle pressure and total pressure as a function of radial distance in the plasma sheet for periods before and after the onset of substorm-associated ion enhancements over the range 7-19 RE. Events were chosen that occurred during times of increasing magnetospheric activity, as determined by an increasing AE index, in which a sudden increase, or 'injection', of energetic particle flux is observed. It is shown that the simultaneous appearance of energetic particles and changes in the magnetic field results naturally from pressure balance and does not necessarily indicate that the local changing field is accelerating the particles.

[1] Ion conics accompanied by electron beams are observed regularly in Saturn's magnetosphere. The beams and conics are seen throughout the outer magnetosphere, on field lines that nominally map from well into the polar cap (Ldipole > 50) to well into the closed field region (Ldipole < 10). The electron beams and ion conics are often observed together but also sometimes separately. Typically, the ion conics are prominent at energies between about 30 keV and 200 keV. The electron beams extend from below the ∼20 keV lower energy threshold for the instrument to sometimes as high as 1 MeV. The electrons may be either unidirectional (upward) or bidirectional; the ions are exclusively unidirectional upward. The ion conics are usually seen in conjunction with enhanced broadband electromagnetic noise in the 10 Hz to few kHz frequency range. Most of the wave energy appears below the local electron cyclotron frequency, hence, is propagating in the whistler mode, although some extension to higher frequencies is sometimes observed, suggesting an electrostatic mode. Sometimes the particle phenomena and the broadband noise occur in pulses of roughly 5 min duration, separated by tens of minutes. At other times they are relatively steady over an hour or more. Magnetic signatures associated with some of the pulsed events are consistent with field aligned current structures. The ions are almost exclusively light ions (H, H2, H3, and/or He) with only occasional hints of oxygen or other heavier species, suggesting an ionospheric source. Taken together, the observations are strikingly similar to those made at Earth in association with auroral zone downward sheet currents, except that in the case of Saturn the particle energies are 20 to 100 times higher.

[1] The Magnetospheric Imaging Instrument on board Cassini has been providing measurements of energetic ion intensities, energy spectra, and ion composition, combining the Charge Energy Mass Spectrometer over the range 3 to 236 keV/e, the Low Energy Magnetospheric Measurements System for ions in the range 0.024 to 18 MeV, and the Ion and Neutral Camera for ions and energetic neutral atoms in the range 3 to > 200 keV. Results of the energetic (E > 3 keV) particle pressure distribution throughout the Saturnian magnetosphere and comparison with in situ measurements of the magnetic pressure are presented. The study offers a comprehensive depiction of the average, steady state hot plasma environment of Saturn over the 3 years since orbit insertion on 1 July 2004, with emphasis on ring current characteristics. The results may be summarized as follows: (1) The Saturnian magnetosphere possesses a dynamic, high-beta ring current located approximately between 8 and ∼15 RS, primarily composed of O+ ions, and characterized by suprathermal (E > 3 keV) particle pressure, with typical values of 10−9 dyne/cm2. (2) The planetary plasma sheet shows significant asymmetries, with the dayside region being broadened in latitude (±50°) and extending to the magnetopause, and the nightside appearing well confined, with a thickness of ∼10 RS and a northward tilt of some 10° with respect to the equatorial plane beyond ∼20 RS. (3) The average radial suprathermal pressure gradient appears sufficient to modify the radial force balance and subsequently the azimuthal currents. (4) The magnetic perturbation due to the trapped energetic particle population is ∼7 nT, similar to values from magnetic field–based studies (9 to 13 nT).

[1] We report sample results on Saturn magnetospheric energetic ion spectral shapes using measurements obtained from the Magnetospheric Imaging Instrument (MIMI) suite onboard Cassini. The ion intensities are measured by the Charge Energy Mass Spectrometer (CHEMS) that covers the energy range of 3 to 236 keV/e, the Low Energy Magnetospheric Measurements System (LEMMS) covering the energy range of 0.024 220 keV for protons. The data used cover several passes from the period 1 July 2004 to 10 April 2007, at various latitudes over the dipole L range 5 < L < 20 RS. The spectra generally show a power law in energy form at larger L values but display a flattening/relative peak at lower (L < 10) values centered at ∼50 to ∼100 keV and can be fit by a κ distribution function with characteristic kT ranging from ∼10 to ∼100 keV. The results are consistent with the assumption that energetic protons are heated adiabatically as they move inward to stronger magnetic fields, in contrast to the singly ionized oxygen that seems to be heated locally at each L shell. The lack of any trend of the O+ temperature versus L shell implies that nonadiabatic energization mechanisms and charge exchange with Saturn's neutral gas cloud play an important role for ion energetics.

The Saturn magnetosphere interacts with the Titan atmosphere through various mechanisms. One of them leads, by charge exchange reactions between the energetic Saturnian ions and the exospheric neutrals of Titan, to the production of energetic neutral atoms (ENAs). The Ion and Neutral Camera (INCA), one of the three sensors that comprise the Magnetosphere Imaging Instrument (MIMI) on the Cassini/Huygens mission to Saturn and Titan, images the ENA emissions in the Saturnian magnetosphere. This study focuses on the ENA imaging of Titan (for 20–50 keV H ENAs), with the example of the Ta Titan flyby (26 October 2004): our objective is to understand the positioning of the ENA halo observed around Titan. Thus we investigate the main ENA loss mechanisms, such as the finite gyroradii effects for the parent ions, or the charge stripping with exospheric neutrals. We show that multiple stripping and charge exchange reactions have to be taken into account to understand the ENA dynamics. The use of an analytical approach, taking into account these reactions, combined with a reprocessing of the INCA data, allows us to reproduce the ENA images of the Ta flyby and indicates a lower limit for ENA emission around the exobase. However, the dynamics of energetic particles through the Titan atmosphere remains complex, with an inconsistency between the ENA imaging at low and high altitudes.

[1] A Lomb periodogram analysis is applied to charged particle data from the LEMMS/CHEMS instruments on the Cassini spacecraft. The data represent count rates, averaged within 30 min bins, from electrons (28-330 keV) and protons and oxygen ions (2.8-236 keV) during 350 days in 2005 and all 365 days in 2006. Sun effects, spacecraft maneuvers, and measurements within 20 R S of Saturn were removed from the data prior to analysis. The main peaks in the frequency periodograms (or power spectra) were found within a frequency window from 9.5 hours to 12.5 hours. For signal-to-noise ratios exceeding 8, the periodograms within this window reveal a consistent peak near 10.80 hours (10 hours 48 min 36 sec) for all the charged particles regardless of energy or species. Even for lower signal-to-noise ratios, a peak near this period is generally present. The Lomb analyses are consistent with an azimuthal anomaly that rotates with a period of 10.80 hours.

Voyager 1 and 2 sensor data are analyzed in order to derive the composition, energy spectra, and spatial distribution of energetic ions in the Saturn magnetosphere. In order of abundance, the major species are H, H2(+), He, H3(+), C, and O. The fluxes of all species decreased inside the orbit of Dione, and nearly vanished in the 'slot' region within the orbit of Tethys. Both satellite absorption and precipitation due to pitch angle scattering may be important loss processes in that region. In the outer magnetosphere, photodissociation rapidly destroys a large fraction of the H2(+) ions, but dissociation by impact with neutral H atoms is faster for H2(+) ions in the lowest vibrational state. The ground state lifetime of about 23 days places a limit of about 10-100 days on the mean overall residence time for energetic ions in Saturn's magnetosphere.

Short-lived and long-lived ion flux increases (E ≥30 keV) of Jovian origin have been observed by the low energy charged particle (LECP) instrument on the Voyager 1 and 2 spacecraft. The short-lived events are observed more than 860 RJ upstream and more than 1500 RJ downstream of Jupiter. Observations of long-lived events appear to be confined to ≲200 RJ upstream of Jupiter. The short-lived events last from a few minutes to a couple of hours, while the long-lived events last from 8 to 21 hours. Both types of events have sharp onsets and decays, are usually confined to energies below 1 MeV total energy, and show a large general enrichment of Z ≥6 particles relative to proton and helium particles when compared with energetic particle events of solar or interplanetary origin. Many of the events have a noticeable peak in the energy spectrum above 100 keV after the main portion of the event. In addition, the short-lived events (upstream and downstream) are (1) extremely anisotropic with a flow direction consistent with flow away from Jupiter, (2) display no noticeable velocity dispersion, and (3) display initially steep energy spectra that flatten with time. The peak flux level at the lowest energies in the magnetosheath is similar to that observed during long-lived events. We conclude that a significant fraction of the particles observed during Jovian ion events originate from within the magnetosphere of Jupiter and simply leak out into the magnetosheath. If the interplanetary magnetic field favorably connects to the bow shock, particles can leak out into the interplanetary medium. Together with the necessary leakage model, the observations presented here cannot rule out the existence of a wave particle acceleration region located immediately upstream of the bow shock.

A detailed account is given of the results of the Voyager 2 low-energy charged particle investigation of the Uranian magnetosphere. Data show that the encounter of the inbound bow shock was immediately preceded by intense upstream proton events characterized by bulk streaming pointing approximately tangentially to the magnetospheric boundaries. Observations are presented which suggest that substorm processes analogous to those occurring within the earth's magnetotail are occurring within the Uranian magnetotail.

Observations are presented from Voyager 1 and 2 of nonthermal ions from H through Fe in the Jovian magnetosphere using the low-energy particle telescope (LEPT), one of the two sensors of the low-energy charged particle (LECP) experiment. At approximately 1 MeV/nucleon, the principal constituents of the ion population were H, He, C, O, Na, S, and the hydrogen molecules H2 and H3. In relation to He, the abundance of H and H3 at equal energy/nucleon was highest in the outer magnetosphere, the abundance of O, Na, and S was highest in the inner magnetosphere, and the abundance of C was constant throughout the magnetosphere.