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The long-anticipated encounter by Voyager 2 (V2) of the region between the heliosphere and the very local interstellar medium (VLISM) occurred toward the end of 2018. Here, we report measurements of energetic (>28 keV) charged particles on V2 from the interface region between the heliosheath, dominated by heated solar wind plasma, and the VLISM, expected to contain cold non-solar plasma and the Galactic magnetic field. The number of particles of solar origin began a gradual decrease on 7 August 2018 (118.2 au), while those of Galactic origin (Galactic cosmic rays) increased ~20% in number over a period of a few weeks. An abrupt change occurred on 5 November when V2 was located at 119 au, with a decrease in the number of particles at energies of >28 keV and a corresponding increase in the number of Galactic cosmic rays of energy E > 213 MeV. This signature of the transition to the VLISM resembles, but is very different from, that observed on Voyager 1 at ~121.6 au, associated with the putative crossing of the heliopause some six years earlier. Measurements of energetic ions and electrons with the Low-Energy Charged Particle instrument on Voyager 2 are presented from the boundary of the heliosphere and from the interstellar medium. Voyager 2’s heliopause crossing bears some similarity to that of Voyager 1, despite differing solar wind conditions.

On 10 January 2001, Cassini briefly entered into the magnetosphere of Jupiter, en route to Saturn. During this excursion into the Jovian magnetosphere, the Cassini Magnetosphere Imaging Instrument/Charge‐Energy‐Mass Spectrometer detected oxygen and sulfur ions. While Charge‐Energy‐Mass Spectrometer can distinguish between oxygen and sulfur charge states directly, only 95.9 ± 2.9 keV/e ions were sampled during this interval, allowing for a long time integration of the tenuous outer magnetospheric (~200 RJ) plasma at one energy. For this brief interval for the 95.9 keV/e ions, 96% of oxygen ions were O+, with the other 4% as O2+, while 25% of the energetic sulfur ions were S+, 42% S2+, and 33% S3+. The S2+/O+ flux ratio was observed to be 0.35 (±0.06 Poisson error)., Key Points Cassini measured the relative charge state abundances of 95.9 keV/e oxygen and sulfur in the outer magnetosphere (~200 RJ) of JupiterThe flux of 95.9 keV/e O+ was higher than that of S2+, with S2+ being the most abundant charge state among sulfur ionsThe relative abundances of 95.9 keV/e heavy ions are compared to thermal ions in the inner to middle magnetosphere

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.

Between April and the end of its mission on 15 September, Cassini executed a series of 22 very similar 6.5‐day‐period proximal orbits, covering the mid‐latitude region of the nightside magnetosphere. These passes provided us with the opportunity to examine the variability of the nightside plasma sheet within this time scale for the first time. We use Cassini particle and magnetic field data to quantify the magnetospheric dynamics along these orbits, as reflected in the variability of certain relevant plasma parameters, including the energetic ion pressure and partial (hot) plasma beta. We use the University College London/Achilleos‐Guio‐Arridge magnetodisk model to map these quantities to the conjugate magnetospheric equator, thus providing an equivalent equatorial radial profile for these parameters. By quantifying the variation in the plasma parameters, we further identify the different states of the nightside ring current (quiescent and disturbed) in order to confirm and add to the context previously established by analogous studies based on long‐term, near‐equatorial measurements.

Enceladus is the main source of neutral and charged particles in the Saturnian magnetosphere. The particles originate at more than 100 active geysers forming a plume above the south pole of the moon and are continuously released into Saturn’s magnetosphere. Therefore the understanding of the interaction of those particles and the local magnetospheric environment of the moon is very important. One technique to study that interaction is to study the typical motion of charged particles in the perturbed plasma flow and the associated magnetic field lines in the vicinity of the moon especially during close flybys. The Cassini spacecraft flew by Enceladus 23 times between 2005 and 2015 at distances between 25 and 5000 km. During some of the flybys Cassini went directly through the south polar plume. Other flybys happened north of the moon or on high-latitude trajectories with respect to the moon. In this paper we present the energetic electron measurements during those flybys obtained by the Low Energy Magnetosphere Measurement System LEMMS, part of the Magnetosphere Imaging Instrument MIMI onboard Cassini (Krimigis et al., 2004). As already shown in Krupp et al. (2012) for the first 14 flybys MIMI/LEMMS typically observes dropouts in the particle intensities in the region of disturbed field lines and in the presence of the moon itself or dense material blocking the bounce and drift motions of the particles. We present in this paper a continuation of the Krupp et al. (2012) results and add a full classification for all 23 flybys using the full data set of energetic electron measurements of MIMI/LEMMS. We distinguish the observed absorption and dust signatures into four different categories: (1) full absorption signatures when all the particles within a fluxtube connecting the spacecraft with the moon are lost onto the moon during one of the particle motions; (2) partial dropouts (ramp-like feature) when not all the particles inside the fluxtube are lost; (3) short dropouts in the fluxes when particles are suddenly lost for a short period in time; and interpret those features as full or partial losses onto the moon or its environment as a result of different plasma and dust regimes in the vicinity of Enceladus. We compare the results with those of Meier et al. (2014) and Engelhardt et al. (2015); (4) In addition we also show dust-related “false electron” measurements for those flybys when Cassini directly went through the dense regions of the south polar plume. Those “dust-peaks” can be interpreted as the result of impacting dust particles inside the LEMMS aperture or nearby creating a plasma cloud.

The short, 7.2-day orbital period of Cassini’s Ring Grazing Orbits (RGO) provided an opportunity to monitor how fast the effects of an intense magnetospheric storm-time period (days 336–343/2016) propagated into Saturn’s electron radiation belts. Following the storms, Cassini’s MIMI/LEMMS instrument detected a transient extension of the electron radiation belts that in subsequent orbits moved towards the inner belts, intensifying them in the process. This intensification was followed by an equally fast decay, possibly due to the rapid absorption of MeV electrons by the planet’s main rings. Surprisingly, all this cycle was completed within four RGOs, effectively in less than a month. That is considerably faster than the year-long time scales of Saturn’s proton radiation belt evolution. In order to explain this difference, we propose that electron radial transport is partly controlled by the variability of global scale electric fields which have a fixed local time pointing. Such electric fields may distort significantly the orbits of a particular class of energetic electrons that cancel out magnetospheric corotation due to their westward gradient and curvature drifts (termed “corotation-resonant” or “local-time stationary” electrons) and transport them radially between the ring current and the radiation belts within several days and few weeks. The significance of the proposed process is highlighted by the fact that corotation resonance at Saturn occurs for electrons of few hundred keV to several MeV. These are the characteristic energies of seed electrons from the ring current that sustain the radiation belts of the planet. Our model’s feasibility is demonstrated through the use of a simple test-particle simulation, where we estimate that uniform but variable electric fields with magnitudes lower that 1.0 mV/m can lead to a very efficient transport of corotation resonant electrons. Such electric fields have been consistently measured in the magnetosphere, and here we provide additional evidence showing that they may be constantly present all the way down to the outer edge of Saturn’s main rings, further supporting our model. The implications of our findings are not limited to Saturn. Corotation resonance at Jupiter occurs for electrons with energies above about 10 MeV throughout the quasi-dipolar, energetic particle-trapping region of the magnetosphere. The proposed process could in principle then lead to rapid transport and adiabatic acceleration electrons into ultra-relativistic energies. The observation by Galileo’s EPD/LEMMS instrument of an intense Jovian acceleration event at the orbital distance of Ganymede during the mission’s C22 orbit, when > 11 MeV electron fluxes were preferentially enhanced, provides additional support to our transport model and insights on the origin of that orbit’s extreme energetic electron environment. Finally, if the mode of radial transport that we describe here is a dominant one, radial diffusion coefficients (DLL) would be subject to strong energy, pitch angle and species dependencies.

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.

This article reviews the different sources of plasma for Saturn’s magnetosphere, as they are known essentially from the scientific results of the Cassini-Huygens mission to Saturn and Titan. At low and medium energies, the main plasma source is the \(\mathrm{H}_{2}\mathrm{O}\) cloud produced by the “geyser” activity of the small satellite Enceladus. Impact ionization of this cloud occurs to produce on the order of 100 kg/s of fresh plasma, a source which dominates all the other ones: Titan (which produces much less plasma than anticipated before the Cassini mission), the rings, the solar wind (a poorly known source due to the lack of quantitative knowledge of the degree of coupling between the solar wind and Saturn’s magnetosphere), and the ionosphere. At higher energies, energetic particles are produced by energy diffusion and acceleration of lower energy plasma produced by the interchange instabilities induced by the rapid rotation of Saturn, and possibly, for the highest energy range, by contributions from the CRAND process acting inside Saturn’s magnetosphere. Discussion of the transport and acceleration processes acting on these plasma sources shows the importance of rotation-induced radial transport and energization of the plasma, and also shows how much the unexpected planetary modulation of essentially all plasma parameters of Saturn’s magnetosphere remains an unexplained mystery.

After 9 years in the Saturn system, the Cassini spacecraft finally observed Titan in the supersonic and super-Alfvenic solar wind. These unique observations reveal that Titan's interaction with the solar wind is in many ways similar to unmagnetized planets Mars and Venus and active comets in spite of the differences in the properties of the solar plasma in the outer solar system. In particular, Cassini detected a collisionless, supercritical bow shock and a well-defined induced magnetosphere filled with mass-loaded interplanetary magnetic field lines, which drape around Titan's ionosphere. Although the flyby altitude may not allow the detection of an ionopause, Cassini reports enhancements of plasma density compatible with plasma clouds or streamers in the flanks of its induced magnetosphere or due to an expansion of the induced magnetosphere. Because of the upstream conditions, these observations may be also relevant to other bodies in the outer solar system such as Pluto, where kinetic processes are expected to dominate.

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.

[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] 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.

This article reviews the different sources of plasma for Saturn’s magnetosphere, as they are known essentially from the scientific results of the Cassini-Huygens mission to Saturn and Titan. At low and medium energies, the main plasma source is the \(\mathrm{H}_{2}\mathrm{O}\) cloud produced by the “geyser” activity of the small satellite Enceladus. Impact ionization of this cloud occurs to produce on the order of 100 kg/s of fresh plasma, a source which dominates all the other ones: Titan (which produces much less plasma than anticipated before the Cassini mission), the rings, the solar wind (a poorly known source due to the lack of quantitative knowledge of the degree of coupling between the solar wind and Saturn’s magnetosphere), and the ionosphere. At higher energies, energetic particles are produced by energy diffusion and acceleration of lower energy plasma produced by the interchange instabilities induced by the rapid rotation of Saturn, and possibly, for the highest energy range, by contributions from the CRAND process acting inside Saturn’s magnetosphere. Discussion of the transport and acceleration processes acting on these plasma sources shows the importance of rotation-induced radial transport and energization of the plasma, and also shows how much the unexpected planetary modulation of essentially all plasma parameters of Saturn’s magnetosphere remains an unexplained mystery.

The MIMI experiment (Magnetosphere Imaging Instrument) onboard Cassini is dedicated to the study of energetic particles, with in particular LEMMS analyzing charged particles, or the INCA detector which can image the Energetic Neutral Atoms produced by charge exchange collisions between cold neutrals and energetic ions. The MIMI experiment is thus well adapted to the study of the interaction between the Titan nitrogen rich atmosphere and the energetic Saturnian magnetospheric plasma. We analyze here the energetic protons at the Titan orbit crossings before January 2008 (MIMI-LEMMS data; 27–255 keV), which are very dynamic, with tri-modal flux spectra and probably quasi-isotropic pitch angle distributions. We provide statistical parameters for the proton fluxes, leading to estimates of the average energy deposition into Titan's atmosphere, before we discuss the possible influence of Titan on the magnetopause. We then analyze the H ENA images (24–55 keV) during the Titan flybys before June 2006 to obtain a better diagnostic of the Titan interaction: the ENAs variability is mostly related to the magnetospheric variability (the exosphere being roughly stable) or the distance from the moon, the ENAs halo around Titan is very stable (corresponding to a lower limit for ENAs emission at the exobase), and strong asymmetries are observed, due to finite gyroradii effects for the parent ions.

The existence of energetic particle events to ∼200 RS upstream and ∼1300 RS downstream of Saturn was established during the Voyager 1, 2 flybys in 1980 and 1981, respectively. The origin of the events could not be determined with certainty because of lack of particle charge state and species measurements at lower ( 80 RS. The events exhibited the following characteristics: (1) hydrogen ion bursts are observed in the energy range 3–220 keV (and occasionally to E>220 keV) and oxygen ion bursts in the energy range 32 to ∼700 keV. (2) Pitch angle distributions are initially anisotropic with ions moving away from the bow shock along the IMF, but tend to isotropize as the event progresses in time. (3) The duration of the ion bursts is several minutes up to 4 h. (4) The event examined in this study contains significant fluxes of singly charged oxygen. (5) Ion bursts are accompanied by distinct diamagnetic field depressions with β>10, and exhibit wave structures consistent with ion cyclotron waves for O+ and O++. Given the magnetic field configuration during the detection of the events and that energetic ions trapped within the magnetosphere of Saturn are H+, H2+ and various water products including O+, O++, we conclude that O-rich upstream events must be particles leaking from Saturn's magnetosphere under favorable IMF conditions. The spectral evolution of the upstream events and their anisotropy characteristics are discussed in the context of current models.

In July 2004 the Cassini spacecraft began its orbital tour in the Saturnian system and performed 74 orbits during the nominal mission (July 2004–July 2008) providing data from nearly all local times at various distances and latitudes relative to the planet. The particles and field instruments onboard the spacecraft were essentially operating continuously offering the possibility to study the global configuration and the dynamics of the second largest magnetosphere in our solar system extensively. One of those instruments aboard Cassini is the Low Energy Magnetospheric Measurement System (LEMMS), one of three particle detectors of the Magnetospheric Imaging Instrument (MIMI). MIMI/LEMMS measures the intensity, energy spectra and pitch angle distributions of energetic ions ( E > 30 keV ) and electrons ( E > 20 keV ) separately. The measured energetic particle distributions together with the measured magnetic field provide a very powerful tool to investigate the Saturnian magnetosphere in those regions covered by the Cassini orbits. This paper will give an overview of the energetic particle measurements of the MIMI/LEMMS sensor in the Saturnian system. In the first part of the paper synoptic maps will be shown where all the data are presented as a function of various trajectory parameters of the spacecraft. Secondly bi-directional electron distributions along the magnetic field direction will be described as a feature in the Saturnian system. Thirdly the particle parameters in the inner magnetosphere with absorption signatures of the various moons are presented. Fourthly it will be shown that the region around about 15 R S seems to be a characteristic region where depletion signatures in energetic particle distributions are very often observed. At the end of this work a 60 min intensity periodicity in the MIMI/LEMMS data is discussed.

We measured quiet-time differential intensities of ~2-60 keV nucleon–1 He+ and He++ during the 1999-2004, 1-9 AU portion of Cassini's interplanetary cruise to Saturn and found that the He+/He++ composition ratio grows as the distance from the Sun r increases. An increase in the ratio is expected from the theoretical pickup ion and solar wind intensities, but the absolute He+ intensity, counter to the predicted falling r –1 dependence of the density, is actually slightly increasing, and He++ falls off much more slowly than the r –2 dependence one might expect from a population with a solar source. With an approximately r 2.2 radial dependence, our rigorous numerical transport and acceleration model (with stochastic acceleration) matches the higher-energy (>13 keV nucleon–1) measured He+/He++ composition profiles well, as does our analytical theory. Two acceleration processes are likely needed: the composition ratios are explainable by stochastic acceleration while a velocity-dependent mechanism that acts beyond 1 AU equally on He+ and He++ is required to explain the spatial intensity profiles.

The idea that there is a ring current of trapped particles encircling the Earth at high altitudes first emerged in the early part of the twentieth century. The idea proved right, and measurements of the current's extent and composition were made in 1957. Ring currents of a different nature were later observed at Jupiter and inferred at Saturn. The magnetospheric imaging instrument on the Cassini probe has now obtained images of the ring current at Saturn. The current is highly variable, with strong longitudinal asymmetries that corotate nearly rigidly with the planet. This contrasts with Earth's ring current, where there is no rotational modulation and initial asymmetries depend on local effects. The concept of an electrical current encircling the Earth at high altitudes was first proposed in 1917 to explain the depression of the horizontal component of the Earth’s magnetic field during geomagnetic storms1,2,3,4. In situ measurements of the extent and composition of this current were made some 50 years later5 and an image was obtained in 2001 (ref. 6). Ring currents of a different nature were observed at Jupiter7,8 and their presence inferred at Saturn9,10. Here we report images of the ring current at Saturn, together with a day–night pressure asymmetry and tilt of the planet’s plasma sheet, based on measurements using the magnetospheric imaging instrument (MIMI) on board Cassini. The ring current can be highly variable with strong longitudinal asymmetries that corotate nearly rigidly with the planet. This contrasts with the Earth’s ring current, where there is no rotational modulation and initial asymmetries are organized by local time effects.

Titan's nitrogen-rich atmosphere is directly bombarded by energetic ions, due to its lack of a significant intrinsic magnetic field. Singly charged energetic ions from Saturn magnetosphere undergo charge exchange collisions with neutral atoms in Titan's exosphere, being transformed into energetic neutral atoms (ENAs). The ion and neutral camera (INCA), one of the three sensors that comprise the magnetosphere imaging instrument on the Cassini/Huygens mission to Saturn and Titan, images the ENA emissions from various ion/gas interaction regions in the Saturnian magnetosphere. During Cassinis second orbit around Saturn the spacecraft performed the Ta Titan flyby (26 October 2004), at an altitude of only 1174 km. INCA data acquired during this targeted close flyby not only confirm model predictions of dominant finite ion gyroradii effects, but also reveal a much more complex interaction. These observations are analyzed and compared to simulations. A new Titan exosphere model is then proposed.

The magnetospheric imaging instrument (MIMI) is a neutral and charged particle detection system on the Cassini orbiter spacecraft designed to perform both global imaging and in-situ measurements to study the overall configuration and dynamics of Saturn’s magnetosphere and its interactions with the solar wind, Saturn’s atmosphere, Titan, and the icy satellites. The processes responsible for Saturn’s aurora will be investigated; a search will be performed for substorms at Saturn; and the origins of magnetospheric hot plasmas will be determined. Further, the Jovian magnetosphere and Io torus will be imaged during Jupiter flyby. The investigative approach is twofold. (1) Perform remote sensing of the magnetospheric energetic (E > 7 keV) ion plasmas by detecting and imaging charge-exchange neutrals, created when magnetospheric ions capture electrons from ambient neutral gas. Such escaping neutrals were detected by the Voyager 1 spacecraft outside Saturn’s magnetosphere and can be used like photons to form images of the emitting regions, as has been demonstrated at Earth. (2) Determine through in-situ measurements the 3-D particle distribution functions including ion composition and charge states (E > 3 keV/e). The combination of in-situ measurements with global images, together with analysis and interpretation techniques that include direct “forward modeling” and deconvolution by tomography, is expected to yield a global assessment of magnetospheric structure and dynamics, including (a) magnetospheric ring currents and hot plasma populations, (b) magnetic field distortions, (c) electric field configuration, (d) particle injection boundaries associated with magnetic storms and substorms, and (e) the connection of the magnetosphere to ionospheric altitudes. Titan and its torus will stand out in energetic neutral images throughout the Cassini orbit, and thus serve as a continuous remote probe of ion flux variations near 20R S (e.g., magnetopause crossings and substorm plasma injections). The Titan exosphere and its cometary interaction with magnetospheric plasmas will be imaged in detail on each flyby. The three principal sensors of MIMI consists of an ion and neutral camera (INCA), a charge-energy-mass-spectrometer (CHEMS) essentially identical to our instrument flown on the ISTP/Geotail spacecraft, and the low energy magnetospheric measurements system (LEMMS), an advanced design of one of our sensors flown on the Galileo spacecraft. The INCA head is a large geometry factor (G ~ 2.4 cm2 sr) foil time-of-flight (TOF) camera that separately registers the incident direction of either energetic neutral atoms (ENA) or ion species (≥5° full width half maximum) over the range 7 keV/nuc < E < 3 MeV/nuc. CHEMS uses electrostatic deflection, TOF, and energy measurement to determine ion energy, charge state, mass, and 3-D anisotropy in the range 3 ≤ E ≤ 220 keV/e with good (~0.05 cm2 sr) sensitivity. LEMMS is a two-ended telescope that measures ions in the range 0.03 ≤ E ≤ 18 MeV and electrons 0.015 ≤ E < 0.884 MeV in the forward direction (G ~ 0.02 cm2 sr), while high energy electrons (0.1–5 MeV) and ions (1.6–160 MeV) are measured from the back direction (G ~ 0.4 cm2 sr). The latter are relevant to inner magnetosphere studies of diffusion processes and satellite microsignatures as well as cosmic ray albedo neutron decay (CRAND). Our analyses of Voyager energetic neutral particle and Lyman-a measurements show that INCA will provide statistically significant global magnetospheric images from a distance of ~60 RS every 2–3 h (every ~10 min from ~20 RS). Moreover, during Titan flybys, INCA will provide images of the interaction of the Titan exosphere with the Saturn magnetosphere every 1.5 min. Time resolution for charged particle measurements can be

The outer limit of the Solar System is often considered to be at the distance from the Sun where the solar wind changes from supersonic to subsonic flow1. Theory predicts that a termination shock marks this boundary, with locations ranging2 from a few to over 100 au (1 au ≈ 1.5 × 108 km, the distance from Earth to the Sun). ‘Pick-up ions’ that originate3,4 as interstellar neutral atoms should be accelerated to tens of MeV at the termination shock, generating anomalous cosmic rays5,6,7. Here we report a large increase in the intensity of energetic particles in the outer heliosphere, as measured by an instrument on the Voyager 1 spacecraft. We argue that the spacecraft exited the supersonic solar wind and passed into the subsonic region (possibly beyond the termination shock) on about 1 August 2002 at a distance of ∼85 au (heliolatitude ∼34° N), then re-entered the supersonic solar wind about 200 days later at ∼87 au from the Sun. We show that the composition of the ions accelerated at the putative termination shock is that of anomalous cosmic rays and of interstellar pick-up ions.

We present annual 0.3-40 MeV nucleon-1 anomalous cosmic-ray (ACR) oxygen and helium energy spectra from the 1991-1999 cosmic-ray recovery phase of solar cycle 22. These observations were made with the Low Energy Charged Particle instruments aboard the Voyager 1 and Voyager 2 spacecraft while at helioradial positions ranging from 34 to 76 AU. The peak intensities of both species increased by 2 orders of magnitude during this period, while the energies of peak O and He intensity decreased from ~9 to 1.3 MeV nucleon-1 and from ~35 to 6 MeV nucleon-1, respectively. Using these observations along with published O measurements from the Solar Anomalous Magnetospheric Particle Explorer at 1 AU, we investigate ACR transport phenomena. We make estimates related to transport parameters such as the relative change in the scattering mean free path over time, the rigidity dependence of the mean free path, and the distance between the Sun and the solar wind termination 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.

The IMAGE mission is the first of its kind. It is designed to comprehensively image a variety of emissions from the Earth's magnetosphere with sufficient time resolution to follow the dynamics associated with the development of magnetospheric storms. This paper describes initial observations of two qualitatively different geomagnetic storms by the IMAGE High Energy Neutral Atom imager (HENA). HENA images formed at energies between 10 and 60 keV/nucleon reveal the distribution and the evolution of energetic ion distributions as they are injected into the ring current during geomagnetic storms, drift about the Earth on both open and closed drift paths, and decay through charge exchange to pre-storm levels. Substorm ion injections are also imaged, as are regions of low altitude, high latitude ion precipitation into the upper atmosphere. Two events are discussed: one a major magnetic storm (the “Bastille Day” storm of July 15 and 16, 2000, Dst=−300nT), and the other, a minor storm (June 10, 2000, Dst=−55nT). The larger storm is characterized by ion injection deep into the magnetosphere (L∼3 RE), while the images from the minor storm are consistent with injection to L∼7 RE.

We report observations of low energy neutral atoms (LENA) from the solar wind and the ionosphere, obtained by the LENA Imager on the IMAGE spacecraft. The LENA Imager detects and images LENAs arriving at the spacecraft from within a 90° field of view (8° × 8° pixels), swept through 360° every two minutes by spacecraft spin. Neutral atoms arriving at the sensor are converted to negative ions by a conversion surface. The resulting negative ions are separated in energy (3 bins, 10–250 eV) and arrival direction (±45°). They are then accelerated, detected, and time-of-flight mass analyzed. The solar wind and the ionosphere both emit measurable neutral atom fluxes, the latter responding rapidly to to variations of the former.

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.

November 1997 saw a series of very intense and unusual solar particle events that have been well studied [Cohen et al., 1999; Lario et al., 1998; Mason et al., 1999; Mazur et al., 1999; Mobius et al., 1999]. However, the composition of the solar wind has not previously been analyzed. We report the elemental and charge-state composition of the solar wind during these events. We find unusually broad and high charge-state distributions for O, Si, and Fe. Fe/O was enhanced by a factor >3–5 relative to the ambient solar wind in the magnetic cloud and in the following material. We speculate whether the observed compositional similarities in the solar wind and energetic particles might, in part, be explained by a common reservoir out of which both kinds of particles were accelerated.

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.

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.

A comparison of the heliocentric dependence of the relative abundance of low energy (~1 MeV nucleon-1) carbon and oxygen on the Ulysses (~3-4 AU) and Voyager 1 and 2 (~35 and ~47 AU) spacecraft shows that the C/O abundance ratio is significantly smaller at the larger radial distances. This finding is attributed to a more abundant seed population for anomalous cosmic-ray oxygen than for carbon in the outer solar system, where the seed population can be accelerated by shocks in global merged interaction regions. The energy spectra of carbon and oxygen at the different locations are approximately the same.

We present results from a two spacecraft study of the ratio of O 6+ and He 2+ kinetic temperatures as a function of solar wind speed. Data from the Wind/MASS and Ulysses/SWICS instruments both indicate the O 6+ /He 2+ kinetic temperature ratio increases with increasing solar wind speed, peaking near 500 km/s. Above 500 km/s, the ratio appears to decrease slightly. Values near unity were obtained for slow solar wind while fast wind yielded ratios substantially higher than what is expected from the general rule T i /T j ≃M i /M j . (1) Although averaging over all data would result in the nominal ratio of 4, deviations from equation (1) appear to be common and the assumption of equal thermal velocities should not be made a priori.

The Solar Wind and Suprathermal Ion Composition Experiment (SMS) on WIND is designed to determine uniquely the elemental, isotopic, and ionic-charge composition of the solar wind, the temperatures and mean speeds of all major solar-wind ions, from H through Fe, at solar wind speeds ranging from 175 kms−1 (protons) to 1280 kms−1 (Fe+8), and the composition, charge states as well as the 3-dimensional distribution functions of suprathermal ions, including interstellar pick-up He+, of energies up to 230 keV/e. The experiment consists of three instruments with a common Data Processing Unit. Each of the three instruments uses electrostatic analysis followed by a time-of-flight and, as required, an energy measurement. The observations made by SMS will make valuable contributions to the ISTP objectives by providing information regarding the composition and energy distribution of matter entering the magnetosphere. In addition SMS results will have an impact on many areas of solar and heliospheric physics, in particular providing important and unique information on: (i) conditions and processes in the region of the corona where the solar wind is accelerated; (ii) the location of the source regions of the solar wind in the corona; (iii) coronal heating processes; (iv) the extent and causes of variations in the composition of the solar atmosphere; (v) plasma processes in the solar wind; (vi) the acceleration of particles in the solar wind; and (vii) the physics of the pick-up process of interstellar He as well as lunar particles in the solar wind, and the isotopic composition of interstellar helium.

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.

The SAMPEX (Solar, Anomalous, and Magnetospheric Particle Explorer) LEICA instrument is designed to measure approximately 0.5-5-MeV/nucleon solar and magnetospheric ions over the range from He-Ni. The instrument is a time-of-flight (TOF) mass spectrometer, which measures particle TOF over an approximately 0.5-m path and the residual energy deposited in an array of Si solid state detectors. Large-area microchannel plates are used, resulting in a large geometrical factor for the instrument (0.6 cm/sup 2/sr), which is essential for accurate compositional measurements in small solar flares and in studies of precipitating magnetospheric heavy ions. >

[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.

We report initial results on the particle pressure distribution and its contribution to ring current density in the equatorial magnetosphere of Saturn, as measured by the Magnetospheric Imaging Instrument (MIMI) and the Cassini Plasma Spectrometer (CAPS) onboard the Cassini spacecraft. Data were obtained from September 2005 to May 2006, within +/- 0.5 R-S from the nominal magnetic equator in the range 6 to 15 RS. The analysis of particle and magnetic field measurements, the latter provided by the Cassini magnetometer (MAG), allows the calculation of average radial profiles for various pressure components in Saturn's magnetosphere. The radial gradient of the total particle pressure is compared to the inertial body force to determine their relative contribution to the Saturnian ring current, and an average radial profile of the azimuthal current intensity is deduced. The results show that: ( 1) Thermal pressure dominates from 6 to 9 RS, while thermal and suprathermal pressures are comparable outside 9 RS with the latter becoming larger outside 12 RS. ( 2) The plasma beta (particle/magnetic pressure) remains >= 1 outside 8 RS, maximizing (similar to 3 to similar to 10) between 11 and 14 RS. ( 3) The inertial body force and the pressure gradient are similar at 9-10 R-S, but the gradient becomes larger >= 11 R-S. ( 4) The azimuthal ring current intensity develops a maximum between approximately 8 and 12 RS, reaching values of 100-150 pA/m(2). Outside this region, it drops with radial distance faster than the 1/r rate assumed by typical disk current models even though the total current is not much different to the model results.

The ion composition in the Jovian environment was investigated with the Solar Wind Ion Composition Spectrometer on board Ulysses. A hot tenuous plasma was observed throughout the outer and middle magnetosphere. In some regions two thermally different components were identified. Oxygen and sulfur ions with several different charge states, from the volcanic satellite Io, make the largest contribution to the mass density of the hot plasma, even at high latitude. Solar wind particles were observed in all regions investigated. Ions from Jupiter's ionosphere were abundant in the middle magnetosphere, particularly in the high-latitude region on the dusk side, which was traversed for the first time.

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] 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).

In this chapter, we review selected fundamental plasma processes that control the extensive space environment, or magnetosphere, of Saturn (see Chapter 9, for the global context). This writing occurs at a point in time when some measure of maturity has been achieved in our understanding of the operations of Saturn's magnetosphere and its relationship to those of Earth and Jupiter. Our understanding of planetary magnetospheres has exploded in the past decade or so partly because of the presence of orbiting spacecraft (Galileo and Cassini) as well as remote sensing assets (e.g., Hubble Space Telescope). This book and chapter are intended to take stock of where we are in our understanding of Saturn's magnetosphere following the successful return and analysis of extensive sets of Cassini data. The end of the prime mission provides us with an opportunity to consolidate older and newer work to provide guidance for continuing investigations.

[1] Saturn's magnetosphere is currently studied from the microphysical to the global scale by the Cassini-Huygens mission. During the first half of 2004, in the approach phase, remote sensing observations of Saturn's magnetosphere gave access to its auroral, radio, UV, energetic neutral atom, and dust emissions. Then, on 1 July 2004, Cassini Saturn orbit insertion provided us with the first in situ exploration of Saturn's magnetosphere since Voyager. To date, Saturn orbit insertion is the only Cassini orbit to have been described in common by all field and particle instruments. We use the comprehensive suite of magnetospheric and plasma science instruments to give a unified description of the large-scale structure of the magnetosphere during this particular orbit, identifying the different regions and their boundaries. These regions consist of the Saturnian ring system (region 1, within 3 Saturn radii (RS)) and the cold plasma torus (region 2, within 5–6 RS) in the inner magnetosphere, a dynamic and extended plasma sheet (region 3), and an outer high-latitude magnetosphere (region 4, beyond 12–14 RS). We compare these observations to those made at the time of the Voyager encounters. Then, we identify some of the dominant chemical characteristics and dynamical phenomena in each of these regions. The inner magnetosphere is characterized by the presence of the dominant plasma and neutral sources of the Saturnian system, giving birth to a very special magnetosphere dominated by water products. The extended plasma sheet, where the ring current resides, is a variable region with stretched magnetic field lines and contains a mixture of cold and hot plasma populations resulting from plasma transport processes. The outer high-latitude magnetosphere is characterized by a quiet magnetic field and an absence of plasma. Saturn orbit insertion observations enabled us to capture a snapshot of the large-scale structure of the Saturnian magnetosphere and of some of the main plasma processes operating in this complex environment. The analysis of the broad diversity of these interaction processes will be one of the main themes of magnetospheric and plasma science during the Cassini mission.

[1] Radiation belts have been detected in situ at five planets. Only at Earth however has any variability in their intensity been heretofore observed, in indirect response to solar eruptions and high altitude nuclear explosions. The Cassini spacecraft's MIMI/LEMMS instrument has now detected systematic radiation belt variability elsewhere. We report three sudden increases in energetic ion intensity around Saturn, in the vicinity of the moons Dione and Tethys, each lasting for several weeks, in response to interplanetary events caused by solar eruptions. However, the intensifications, which could create temporary satellite atmospheres at the aforementioned moons, were sharply restricted outside the orbit of Tethys. Unlike Earth, Saturn has almost unchanging inner ion radiation belts: due to Saturn's near-symmetrical magnetic field, Tethys and Dione inhibit inward radial transport of energetic ions, shielding the planet's main, inner radiation belt from solar wind influences.

[1] A magnetic axis that is tilted with respect to the spin axis causes waves that generate spin-periodic effects in a planet's outer magnetosphere. Saturn's magnetic axis is aligned with its spin axis, and yet its outer magnetosphere exhibits spin-periodicities in all species of charged particles, just as expected from a wavy magnetodisk. In a spin-aligned geometry, however, a rotating anomaly (or “cam”) in the inner magnetosphere can also generate spin-periodic waves by shaking the plasma sheet of the outer magnetosphere. Like a wobbling magnetodisk, this model predicts oscillations of the plasma sheet at distances beyond ∼20 RS (1 RS = 60268 km). When viewed from a moving spacecraft, these oscillations cause charged particle periodicities near the spin period of the planet. The phase and polarity of these periodicities should change systematically with local time, and spin-periodic effects should appear along the magnetopause.

[1] The Magnetospheric Imaging Instrument (MIMI) on the Cassini spacecraft provides measurements of the energetic ion population within the magnetosphere of Saturn. Energetic ion directional intensities, energy spectra and ion composition, are measured by the Charge Energy Mass Spectrometer (CHEMS) over the range ∼3 to 236 keV per charge and by the Low Energy Magnetospheric Measurements System (LEMMS) for ions in the range 0.024 10 RS; (2) most particle pressure is contained in the range of 10 < E < 150 keV; and (3) in the high beta region 10 < L < 19, where the apparent ring current resides, oxygen generally contributes more than 50% of the total particle pressure. The results demonstrate that typical assumptions of MHD models, whereby particle pressure is presumed to reside with the cold plasma, are not supported by the data.

[1] Measurements from the Cassini Magnetospheric Imaging Instrument (MIMI) beginning in July 2004 reveal that sudden planetward injections of energetic ions (5–200 keV) and electrons (20–200 keV) over confined regions of azimuth are pervasive events in Saturn's magnetosphere over the radial range of 3.8 to 11.2 Saturn radii (RS). Saturn is thus similar to both Earth and Jupiter in the pervasiveness of such injections. At Saturn with Cassini, unlike Jupiter with Galileo, effects of the large radial gradients in the global azimuthal rotational flow pattern often dominates the dispersive particle injection signatures. Accurate knowledge of rotational flow patterns is required to calculate the age and formation positions of injections that are more than several hours old. Here we develop procedures to derive the needed flow profiles from the injections themselves.