Search

On rare occasions Titan has been observed in the magnetosheath, where the solar wind interferes with the regular magnetic field generated by Saturn. This particular orbital position allows for a de ...

The Cassini-Huygens mission turned the moons of Saturn into tangible worlds. Although the discoveries from the spacecraft have been compiled in various review articles (e. g., Dougherty et al., 2009), there is no single publication that summarizes the remaining outstanding questions. Drawing on a workshop sponsored by the Cassini Project, we summarize the unanswered questions for the main icy moons of Saturn – Mimas, Enceladus, Tethys, Dione, Rhea, Hyperion, Iapetus, and Phoebe - for the disciplines of surface composition, geology, thermal properties, Enceladus's plume activity, interiors, and the interactions between Saturn's magnetosphere and the moons.

The mission of the 2000 kg Cassini spacecraft concluded on 2017 September 15, by its deliberate entry into Saturn’s atmosphere at some 31.1 km s−1. Observations, using Hubble and groundbased observatories, to attempt optical detection of this 0.25 kT “artificial meteor” are summarized. No signatures were identified. A challenge with observing the event is that due to atmospheric drag, its timing was not completely deterministic months or even days in advance, a particular problem for space observatories. While imaging observations needed no geometric specification more than “Saturn,” observations with spectrometers required pointing the instrument aperture or slit at the specific impact site. Since giant planet longitude systems are not always familiar, distribution of an unambiguous “finder chart” showing the location of the predicted entry site on the disk is essential, as is clarity on whether stated times are spacecraft event time, or Earth received time (light-travel time, 83 minutes, later).

The Saturn auroral campaign carried out in the spring of 2013 used multiple Earth-based observations, remote-sensing observations from Cassini, and in situ-observations from Cassini to further our understanding of auroras at Saturn. Most of the remote sensing and Earth-based measurements are, by nature, not continuous. And, even the in situ measurements, while continuously obtained, are not always obtained in regions relevant to the study of the aurora. Saturn kilometric radiation, however, is remotely monitored nearly continuously by the Radio and Plasma Wave Science instrument on Cassini. This radio emission, produced by the cyclotron maser instability, is tightly tied to auroral processes at Saturn as are auroral radio emissions at other planets, most notably Jupiter and Earth. This paper provides the time history of the intensity of the radio emissions through the auroral campaign as a means of understanding the temporal relationships between the sometimes widely spaced observations of the auroral activity. While beaming characteristics of the radio emissions are known to prevent single spacecraft observations of this emission from being a perfect auroral activity indicator, we demonstrate a good correlation between the radio emission intensity and the level of UV auroral activity, when both measurements are available.

We model the interaction of the jovian magnetospheric plasma with the atmosphere of Europa using a multi-species chemistry model where the atmospheric distributions of H2 and O2 are prescribed. The plasma flow is idealized as an incompressible flow around a conducting obstacle. We compute changes in plasma composition resulting from this interaction as well as the reaction rates integrated over the simulation domain for several upstream plasma conditions (ion density, ion temperature and flow velocity). We show that for all cases, the main atmospheric loss process is a cascade of symmetrical charge exchanges on O2, which results in the ejection of neutrals. The production rate of ejected neutrals is about an order of magnitude larger than the production of ions. This conclusion is relevant to future missions to Europa that aim to detect fast neutrals. The neutral ejection resulting from this charge exchange creates an oxygen cloud around the orbit of the moon that is very extended radially but also very tenuous, and has not yet been directly detected.

International audience; Observations of the Mars upper atmosphere made from the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft have been used to determine the loss rates of gas from the upper atmosphere to space for a complete Mars year (16 Nov 2014 – 3 Oct 2016). Loss rates for H and O are sufficient to remove ∼2-3 kg/s to space. By itself, this loss would be significant over the history of the planet. In addition, loss rates would have been greater early in history due to the enhanced solar EUV and more-active Sun. Integrated loss, based on current processes whose escape rates in the past are adjusted according to expected solar evolution, would have been as much as 0.8 bar CO2 or 23 m global equivalent layer of H2O; these losses are likely to be lower limits due to the nature of the extrapolation of loss rates to the earliest times. Combined with the lack of surface or subsurface reservoirs for CO2 that could hold remnants of an early, thick atmosphere, these results suggest that loss of gas to space has been the dominant process responsible for changing the climate of Mars from an early, warmer environment to the cold, dry one that we see today.

With attention turned to Europa as a target for exploration, we focus on the space environment in which Europa is embedded. We review remote and in situ observations of plasma properties at Europa’s orbit, between Io’s dense, UV-emitting plasma torus and Jupiter’s dynamic plasma sheet. Where observations are limited (e.g. in plasma composition), we supplement our analysis with models of the neutral and plasma populations from Io to Europa. We evaluate variations and uncertainties in plasma properties with radial distance, latitude, longitude and time.

We present a technique to gather ion composition information in the form of relative abundances of the water group ion species in Saturn's inner magnetosphere, utilizing the Cassini Plasma Spectrometer's Straight-Through Time-of-Flight data from two orbits in 2011. We show that between 4.75 and 8 Saturn radii H2O+ ions dominate the water group species, and from 8 to 10 Saturn radii it is OH+ ions that dominate. Our results show that the relative proportion of H3O+ falls fastest with increasing distance, while the proportion of H2O+ decreases slowly. However, O+ and OH+ increase with distance, and O+ is the least dominant ion species out to eight Saturn radii outside of which it is comparable to H3O+. The relative abundance of H2O+ found here matches theoretical work based on Herschel telescopic data very well. These results are compared with other published work, and further improvements to the technique are discussed.

We investigate the complex interaction between Saturn's magnetosphere and Titan's upper ionosphere using ion data acquired by the Cassini Plasma Spectrometer (CAPS) during the T40 encounter. Bounds on ion-group abundances at altitudes between ~2733 and ~12,541 km are determined by fitting mass spectra with model functions derived from instrument calibration data. The spectra are dominated by H+, H2+, H3+, and two hydrocarbon groups with mass ranges 12–19 and 24–32 amu, respectively. Notably, this constitutes the first reported observation of H3+ in Titan's exosphere. These measurements are discussed in the context of data from the CAPS electron spectrometer (ELS) and the Ion and Neutral Mass Spectrometer (INMS), which fortuitously sampled the ionospheric outflow during the T40 encounter at altitudes between ~2225 and ~3034 km. The CAPS data reveal a composition that is constitutively similar to that sampled by INMS, with hydrocarbon ions first observed as far as ~11,000 km from Titan and increasing in density by more than an order of magnitude along Cassini's inbound trajectory. In addition, we juxtapose the CAPS ion data with numerical results from three different interaction models and show that it is consistent with the location of the field-draping boundary described by Ulusen et al. (2012) and the Saturnward ion tail predicted by Sillanpaa et al. (2006).

NASA implemented a Participating Scientist Program and released a solicitation for the Mars Atmosphere and Volatile EvolutioN mission (MAVEN) proposals on February 14, 2013. After a NASA peer review panel evaluated the proposals, NASA Headquarters selected nine on June 12, 2013. The program's intent is to enhance the science return from the mission by including new investigations that broaden and/or complement the baseline investigations, while still addressing key science goals. The selections cover a broad range of science investigations. Included are: a patching of a 3D exosphere model to an improved global ionosphere-thermosphere model to study the generation of the exosphere and calculate the escape rates; the addition of a focused study of upper atmosphere variability and waves; improvement of a multi-fluid magnetohydrodynamic model that will be adjusted according to MAVEN observations to enhance the understanding of the solar-wind plasma interaction; a global study of the state of the ionosphere; folding MAVEN measurements into the Mars International Reference Ionosphere under development; quantification of atmospheric loss by pick-up using ion cyclotron wave observations; the reconciliation of remote and in situ observations of the upper atmosphere; the application of precise orbit determination of the spacecraft to measure upper atmospheric density and in conjunction with other Mars missions improve the static gravity field model of Mars; and an integrated ion/neutral study of ionospheric flows and resultant heavy ion escape. Descriptions of each of these investigations are given showing how each adds to and fits seamlessly into MAVEN mission science design.

We use a forward modeling program to derive one-dimensional isotropic plasma characteristics in Saturn's inner, equatorial magnetosphere using a novel correction for the spacecraft potential and penetrating background radiation. The advantage of this fitting routine is the simultaneous modeling of plasma data and systematic errors when operating on large data sets, which greatly reduces the computation time and accurately quantifies instrument noise. The data set consists of particle measurements from the electron spectrometer (ELS) and the ion mass spectrometer (IMS), which are part of the Cassini Plasma Spectrometer (CAPS) instrument suite on board the data are limited to peak ion flux measurements within ±10°magnetic latitude and 3–15 geocentric equatorial radial distance (RS). Systematic errors such as spacecraft charging and penetrating background radiation are parameterized individually in the modeling and are automatically addressed during the fitting procedure. The resulting values are in turn used as cross calibration between IMS and ELS, where we show a significant improvement in magnetospheric electron densities and minor changes in the ion characteristics due to the error adjustments. adjustments. Preliminary results show ion and electron densities in close agreement, consistent with charge neutrality throughout Saturn's inner magnetosphere and confirming the spacecraft potential to be a common influence on IMS and ELS. Comparison of derived plasma parameters with results from previous studies using CAPS data and the Radio and Plasma Wave Science investigation yields good agreement.

The Cassini Langmuir Probe (LP) onboard the Radio and Plasma Wave Science experiment has provided much information about the Saturnian cold plasma environment since the Saturn Orbit Insertion in 2004. A recent analysis revealed that the LP is also sensitive to the energetic electrons (250–450 eV) for negative potentials. These electrons impact the surface of the probe and generate a current of secondary electrons, inducing an energetic contribution to the DC level of the current-voltage (I-V) curve measured by the LP. In this paper, we further investigated this influence of the energetic electrons and (1) showed how the secondary electrons impact not only the DC level but also the slope of the (I-V) curve with unexpected positive values of the slope, (2) explained how the slope of the (I-V) curve can be used to identify where the influence of the energetic electrons is strong, (3) showed that this influence may be interpreted in terms of the critical and anticritical temperatures concept detailed by Lai and Tautz (2008), thus providing the first observational evidence for the existence of the anticritical temperature, (4) derived estimations of the maximum secondary yield value for the LP surface without using laboratory measurements, and (5) showed how to model the energetic contributions to the DC level and slope of the (I-V) curve via several methods (empirically and theoretically). This work will allow, for the whole Cassini mission, to clean the measurements influenced by such electrons. Furthermore, the understanding of this influence may be used for other missions using Langmuir probes, such as the future missions Jupiter Icy Moons Explorer at Jupiter, BepiColombo at Mercury, Rosetta at the comet Churyumov-Gerasimenko, and even the probes onboard spacecrafts in the Earth magnetosphere.

The Jovian Auroral Distributions Experiment (JADE) on Juno provides the critical in situ measurements of electrons and ions needed to understand the plasma energy particles and processes that fill the Jovian magnetosphere and ultimately produce its strong aurora. JADE is an instrument suite that includes three essentially identical electron sensors (JADE-Es), a single ion sensor (JADE-I), and a highly capable Electronics Box (EBox) that resides in the Juno Radiation Vault and provides all necessary control, low and high voltages, and computing support for the four sensors. The three JADE-Es are arrayed 120∘ apart around the Juno spacecraft to measure complete electron distributions from ∼0.1 to 100 keV and provide detailed electron pitch-angle distributions at a 1 s cadence, independent of spacecraft spin phase. JADE-I measures ions from ∼5 eV to ∼50 keV over an instantaneous field of view of 270∘×90∘ in 4 s and makes observations over all directions in space each 30 s rotation of the Juno spacecraft. JADE-I also provides ion composition measurements from 1 to 50 amu with m/Δm∼2.5, which is sufficient to separate the heavy and light ions, as well as O+ vs S+, in the Jovian magnetosphere. All four sensors were extensively tested and calibrated in specialized facilities, ensuring excellent on-orbit observations at Jupiter. This paper documents the JADE design, construction, calibration, and planned science operations, data processing, and data products. Finally, the Appendix describes the Southwest Research Institute [SwRI] electron calibration facility, which was developed and used for all JADE-E calibrations. Collectively, JADE provides remarkably broad and detailed measurements of the Jovian auroral region and magnetospheric plasmas, which will surely revolutionize our understanding of these important and complex regions.

The formation of Titan’s induced magnetosphere is a unique and important example in the solar system of a plasma-moon interaction where the moon has a substantial atmosphere. The field and particle conditions upstream of Titan are important in controlling the interaction and also play a strong role in modulating the chemistry of the ionosphere. In this paper we review Titan’s plasma interaction to identify important upstream parameters and review the physics of Saturn’s magnetosphere near Titan’s orbit to highlight how these upstream parameters may vary. We discuss the conditions upstream of Saturn in the solar wind and the conditions found in Saturn’s magnetosheath. Statistical work on Titan’s upstream magnetospheric fields and particles are discussed. Finally, various classification schemes are presented and combined into a single list of Cassini Titan encounter classes which is also used to highlight differences between these classification schemes.

The sunlit portion of planetary ionospheres is sustained by photoionization. This was first confirmed using measurements and modelling at Earth, but recently the Mars Express, Venus Express and Cassini-Huygens missions have revealed the importance of this process at Mars, Venus and Titan, respectively. The primary neutral atmospheric constituents involved (O and CO2 in the case of Venus and Mars, O and N-2 in the case of Earth and N-2 in the case of Titan) are ionized at each object by EUV solar photons. This process produces photoelectrons with particular spectral characteristics. The electron spectrometers on Venus Express and Mars Express (part of ASPERA-3 and 4, respectively) were designed with excellent energy resolution (Delta E/E-8%) specifically in order to examine the photoelectron spectrum. In addition, the Cassini CAPS electron spectrometer at Saturn also has adequate resolution (Delta E/E=16.7%) to study this population at Titan. At Earth, photoelectrons are well established by in situ measurements, and are even seen in the magnetosphere at up to 7R(E). At Mars, photoelectrons are seen in situ in the ionosphere, but also in the tail at distances out to the Mars Express apoapsis (similar to 3R(M)). At both Venus and Titan, photoelectrons are seen in situ in the ionosphere and in the tail (at up to 1.45R(V) and 6.8R(T), respectively). Here, we compare photoelectron measurements at Earth, Venus, Mars and Titan, and in particular show examples of their observation at remote locations from their production point in the dayside ionosphere. This process is found to be common between magnetized and unmagnetized objects. We discuss the role of photoelectrons as tracers of the magnetic connection to the dayside ionosphere, and their possible role in enhancing ion escape. (C) 2010 Elsevier Ltd. All rights reserved.

During Cassini’s Enceladus encounter on 12th March 2008, the Cassini Electron Spectrometer, part of the CAPS instrument, detected fluxes of negative ions in the plumes from Enceladus. It is thought that these ions include negatively charged water group cluster ions associated with the plume and forming part of the ‘plume ionosphere’. In this paper we present our observations, argue that these are negative ions, and present preliminary mass identifications. We also suggest mechanisms for production and loss of the ions as constrained by the observations. Due to their short lifetime, we suggest that the ions are produced in or near the water vapour plume, or from the extended source of ice grains in the plume. We suggest that Enceladus now joins the Earth, Comet Halley and Titan as locations in the Solar System where negative ions have been directly observed although the ions observed in each case have distinctly different characteristics.

Multiple Titan encounters by the Cassini spacecraft have shown that ion chemistry in Titan's upper atmosphere is much more complex than previously thought. As well as showing a great variety of species present below 100 amu, they also include the detection of negative ions and of large abundances of ions above 100 amu. Here, we use data from two Cassini instruments, the Cassini plasma spectrometer's ion beam sensor (CAPS/IBS) and the ion and neutral mass spectrometer (INMS) during fourteen Cassini encounters with Titan's upper atmosphere. By simultaneous analysis of the combined data, we are able to determine the ion temperature, one component of the wind speed and spacecraft potential. Using these derived quantities, we are also able to extend the analysis of CAPS/IBS data to quantify the abundance of ions above 100 amu and to statistically estimate their composition.

One of the unexpected results of the Cassini mission was the discovery of negative ions at altitudes between 950 and 1400 km in Titan's ionosphere with masses up to 10,000 amu/q [Coates, A.J., Crary, F.J., Lewis, G.R., Young, D.T., Waite Jr., J.H., Sittler Jr., E.C., 2007. Discovery of heavy negative ions in Titan's ionosphere. Geophys. Res. Lett., 34, L22103, doi:10.1029/2007GL030978; Waite Jr., J.H., Young, D. T., Coates, A. J., Crary, F. J., Magee, B. A., Mandt, K. E., Westlake, J. H., 2008. The Source of Heavy Organics and Aerosols in Titan's Atmosphere, submitted to Organic Matter in Space, Proceedings IAU Symposium no. 251]. These ions are detected at low altitudes during Cassini's closest Titan encounters by the Cassini plasma spectrometer (CAPS) electron spectrometer. This result is important as it is indicative of complex hydrocarbon and nitrile chemical processes at work in Titan's high atmosphere. They may play a role in haze formation and ultimately in the formation of heavy particles (tholins), which fall through Titan's atmosphere and build up on the surface. During Cassini's prime mission negative ions were observed on 23 Titan encounters, including 7 in addition to those reported by Coates et al. [Coates, A.J., Crary, F.J., Lewis, G.R., Young, D.T., Waite Jr., J.H., Sittler Jr., E.C., 2007. Discovery of heavy negative ions in Titan's ionosphere. Geophys. Res. Lett., 34, L22103, doi:10.1029/2007GL030978]. Here, we also examine the altitude and latitude dependence of the high-mass negative ions observed in Titan's ionosphere, and we examine the implications of these results. We find that the maximum negative ion mass is higher at low altitude and at high latitudes. We also find a weaker dependence of the maximum mass on solar zenith angle.

We present observational evidence that the ionosphere of Titan below an altitude of 1150 km is a significant source of heavy (4100 amu) molecular organic species. This study is based on measurements by five instruments (RPWS/LP, RPWS/E, INMS, CAPS/ELS, CAPS/IBS) onboard the Cassini spacecraft during three flybys (T17, T18, T32) of Titan. The ionospheric peaks encountered at altitudes of 950–1300 km had densities in the range 900–3000 cm � 3 . Below these peaks the number densities of heavy positively charged ions reached 100–2000 cm � 3 and approached 50–70% of the total ionospheric density with an increasing trend toward lowest measured altitudes. Simultaneously measured negatively charged ion densities were in the range 50–150 cm � 3 . These results imply that � 10 5 –10 6 heavy positively charged ions/m 3 /s are continuously recombining into heavy neutrals and supply the atmosphere of Titan. The ionosphere may in this way produce 0.1–1 Mt/yr of heavy organic compounds and is therefore a sizable source for aerosol formation. We also predict that Titan’s ionosphere is dominated by heavy (4100 amu) molecular ions below 950 km.

著者人数：156名, Accepted: 2008-05-27, 資料番号: SA1000998000

The Deep Space 1 (DS1) spacecraft passed the sunward side of Comet 19P/Borrelly in 2001. Along its relatively north–south orbit, a set of plasma density and velocity measurement revealed a northward shift of the plasma boundaries and the mass loading peak. Both onboard and ground based telescopes found evidence for asymmetric distribution of the dust and neutrals. In this paper, five mass-loading patterns are studied to present the first study of the effect of non-spherical neutral distribution profiles on the solar wind-cometary plasma interaction environment. Using magnetohydrodynamic simulations, it is found that a combination of Gaussian and cosine neutral jet distribution, with cosine being the major part, can fit the DS1 general plasma measurement well, with a total gas production rate of around 5 × 10 28 s −1 . These model-data comparisons indicate that the general plasma distribution around Comet Borrelly can be explained with its aspherical neutral jet distribution. However, such neutral jets by themselves are insufficient to produce the density offset in the central peak. Kinetic effects, such as finite gyroradius may be required to create the offset plasma peak.

Ion-neutral chemistry in Titan's upper atmosphere (~ 1000 km altitude) is an unexpectedly prodigious source of hydrocarbon-nitrile compounds. We report observations from the Cassini Ion Neutral Mass Spectrometer (INMS; Waite et al. 2004) and Cassini Plasma Spectrometer (CAPS; Young et al. 2004) that allow us to follow the formation of the organic material from the initial ionization and dissociation of nitrogen and methane driven by several free energy sources (extreme ultraviolet radiation and energetic ions and electrons) to the formation of negative ions with masses exceeding 10,000 amu.

Jupiter's magnetotail is the largest cohesive structure in the solar system and marks the loss of vast numbers of heavy ions from the Jupiter system. The New Horizons spacecraft traversed the magnetotail to distances exceeding 2500 jovian radii ( R J ) and revealed a remarkable diversity of plasma populations and structures throughout its length. Ions evolve from a hot plasma disk distribution at ∼100 R J to slower, persistent flows down the tail that become increasingly variable in flux and mean energy. The plasma is highly structured—exhibiting sharp breaks, smooth variations, and apparent plasmoids—and contains ions from both Io and Jupiter's ionosphere with intense bursts of H + and H + 3 . Quasi-periodic changes were seen in flux at ∼450 and ∼1500 R J with a 10-hour period. Other variations in flow speed at ∼600 to 1000 R J with a 3- to 4-day period may be attributable to plasmoids moving down the tail.

The spatial distribution of N + in Saturn's magnetosphere obtained from Cassini Plasma Spectrometer (CAPS) data can be used to determine the spatial distribution and relative importance of the nitrogen sources for Saturn's magnetosphere. We first summarize CAPS data from 15 orbits showing the spatial and energy distribution of the nitrogen component of the plasma. This analysis re-enforces our earlier discovery [Smith, H.T., Shappirio, M., Sittler, E.C., Reisenfeld, D., Johnson, R.E., Baragiola, R.A., Crary, F.J., McComas, D.J., Young, D.T., 2005. Geophys. Res. Lett. 32 (14). L14S03] that Enceladus is likely the dominant nitrogen source for Saturn's inner magnetosphere. We also find a sharp enhancement in the nitrogen ion to water ion ratio near the orbit of Enceladus which, we show, is consistent with the presence of a narrow Enceladus torus as described in [Johnson, R.E., Liu, M., Sittler Jr., E.C., 2005. Geophys. Res. Lett. 32. L24201]. The CAPS data and the model described below indicate that N + ions are a significant fraction of the plasma in this narrow torus. We then simulated the combined Enceladus and Titan nitrogen sources using the CAPS data as a constraint. This simulation is an extension of the model we employed earlier to describe the neutral tori produced by the loss of nitrogen from Titan [Smith, H.T., Johnson, R.E., Shematovich, V.I., 2004. Geophys. Res. Lett. 31 (16). L16804]. We show that Enceladus is the principal nitrogen source in the inner magnetosphere but Titan might account for a fraction of the observed nitrogen ions at the largest distances discussed. We also show that the CAPS data is consistent with Enceladus being a molecular nitrogen source with a nitrogen to water ratio roughly consistent with INMS [Waite, J.H., and 13 colleagues, 2006. Science 311 (5766), 1419–1422], but out-gassing of other nitrogen-containing species, such as ammonia, cannot be ruled out.

Titan's lower atmosphere has long been known to harbor organic aerosols (tholins) presumed to have been formed from simple molecules, such as methane and nitrogen (CH 4 and N 2 ). Up to now, it has been assumed that tholins were formed at altitudes of several hundred kilometers by processes as yet unobserved. Using measurements from a combination of mass/charge and energy/charge spectrometers on the Cassini spacecraft, we have obtained evidence for tholin formation at high altitudes (∼1000 kilometers) in Titan's atmosphere. The observed chemical mix strongly implies a series of chemical reactions and physical processes that lead from simple molecules (CH 4 and N 2 ) to larger, more complex molecules (80 to 350 daltons) to negatively charged massive molecules (∼8000 daltons), which we identify as tholins. That the process involves massive negatively charged molecules and aerosols is completely unexpected.

The Plasma Experiment for Planetary Exploration (PEPE) flown on Deep Space 1 combines an ion mass spectrometer and an electron spectrometer in a single, low-resource instrument. Among its novel features PEPE incorporates an electrostatically swept field-of-view and a linear electric field time-of-flight mass spectrometer. A significant amount of effort went into developing six novel technologies that helped reduce instrument mass to 5.5 kg and average power to 9.6 W. PEPE’s performance was demonstrated successfully by extensive measurements made in the solar wind and during the DS1 encounter with Comet 19P/Borrelly in September 2001.

International audience; Coupling between the lower and upper atmosphere, combined with loss of gas from the upper atmosphere to space, likely contributed to the thin, cold, dry atmosphere of modern Mars. To help understand ongoing ion loss to space, the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft made comprehensive measurements of the Mars upper atmosphere, ionosphere, and interactions with the Sun and solar wind during an interplanetary coronal mass ejection impact in March 2015. Responses include changes in the bow shock and magnetosheath, formation of widespread diffuse aurora, and enhancement of pick-up ions. Observations and models both show an enhancement in escape rate of ions to space during the event. Ion loss during solar events early in Mars history may have been a major contributor to the long-term evolution of the Mars atmosphere.

Dust is common close to the martian surface, but no known process can lift appreciable concentrations of particles to altitudes above ~150 kilometers. We present observations of dust at altitudes ranging from 150 to above 1000 kilometers by the Langmuir Probe and Wave instrument on the Mars Atmosphere and Volatile Evolution spacecraft. Based on its distribution, we interpret this dust to be interplanetary in origin. A comparison with laboratory measurements indicates that the dust grain size ranges from 1 to 12 micrometers, assuming a typical grain velocity of ~18 kilometers per second. These direct observations of dust entering the martian atmosphere improve our understanding of the sources, sinks, and transport of interplanetary dust throughout the inner solar system and the associated impacts on Mars’s atmosphere.

International audience; The Mars Atmosphere and Volatile Evolution (MAVEN) mission, during the second of its Deep Dip campaigns, made comprehensive measurements of martian thermosphere and ionosphere composition, structure, and variability at altitudes down to ~130 kilometers in the subsolar region. This altitude range contains the diffusively separated upper atmosphere just above the well-mixed atmosphere, the layer of peak extreme ultraviolet heating and primary reservoir for atmospheric escape. In situ measurements of the upper atmosphere reveal previously unmeasured populations of neutral and charged particles, the homopause altitude at approximately 130 kilometers, and an unexpected level of variability both on an orbit-to-orbit basis and within individual orbits. These observations help constrain volatile escape processes controlled by thermosphere and ionosphere structure and variability.

The Cassini plasma spectrometer (CAPS) instrument made measurements of Titan's plasma environment when the Cassini Orbiter flew through the moon's plasma wake October 26, 2004 (flyby TA). Initial CAPS ion and electron measurements from this encounter will be compared with measurements made by the Voyager 1 plasma science instrument (PLS). The comparisons will be used to evaluate previous interpretations and predictions of the Titan plasma environment that have been made using PLS measurements. The plasma wake trajectories of flyby TA and Voyager 1 are similar because they occurred when Titan was near Saturn's local noon. These similarities make possible direct, meaningful comparisons between the various plasma wake measurements. They lead to the following: (A) The light and heavy ions, H+and N+/O+, were observed by PLS in Saturn's magnetosphere in the vicinity of Titan while the higher mass resolution of CAPS yielded H+ and H2+as the light constituents and O+/CH4+ as the heavy ions. (B) Finite gyroradius effects were apparent in PLS and CAPS measurements of ambient O+ ions as a result of their absorption by Titan's extended atmosphere. (C) The principal pickup ions inferred from both PLS and CAPS measurements are H+, H2+, N+, CH4+ and N2+. (D) The inference that heavy pickup ions, observed by PLS, were in narrow beam distributions was empirically established by the CAPS measurements. (E) Slowing down of the ambient plasma due to pickup ion mass loading was observed by both instruments on the anti-Saturn side of Titan. (F) Strong mass loading just outside the ionotail by a heavy ion such as N2+ is apparent in PLS and CAPS measurements. (G) Except for the expected differences due to the differing trajectories, the magnitudes and structures of the electron densities and temperatures observed by both instruments are similar. The high-energy electron bite-out observed by PLS in the magnetotail is consistent with that observed by CAPS.

During the 14 July 2005 encounter of Cassini with Enceladus, the Cassini Plasma Spectrometer measured strong deflections in the corotating ion flow, commencing at least 27 Enceladus radii (27 × 252.1 kilometers) from Enceladus. The Cassini Radio and Plasma Wave Science instrument inferred little plasma density increase near Enceladus. These data are consistent with ion formation via charge exchange and pickup by Saturn's magnetic field. The charge exchange occurs between neutrals in the Enceladus atmosphere and corotating ions in Saturn's inner magnetosphere. Pickup ions are observed near Enceladus, and a total mass loading rate of about 100 kilograms per second (3 × 10 27 H 2 O molecules per second) is inferred.

Resume Le survol des anneaux A et B de Saturne par la sonde Cassini lors de son insertion sur son orbite autour de la planete a revele l'existence d'un halo ionospherique compose d'ions O+ et O 2 + . Ces observations peuvent s'expliquer par la presence d'une atmosphere d'oxygene moleculaire O2 autour des anneaux cree par decomposition photolytique de la glace constituant les grains. L'existence de cette atmosphere est rendue possible par la propriete des molecules d'oxygene de ne pas coller a la glace dans les conditions de temperature des grains et par consequent de ne pas etre perdues lors de leurs collisions avec les grains. Nous presentons un modele utilisant une approche particulaire pour traiter le transport des neutres et des ions et les differents processus physico-chimiques. Ce modele rend compte de facon tres satisfaisante des observations. Pour citer cet article : M. Bouhram et al., C. R. Physique 7 (2006).

Molecular oxygen produced by the decomposition of icy surfaces is ubiquitous in Saturn's magnetosphere. A model is described for the toroidal O2 atmosphere indicated by the detection of O 2 + and O+ over the main rings. The O2 ring atmosphere is produced primarily by UV photon-induced decomposition of ice on the sunlit side of the ring. Because O2 has a long lifetime and interacts frequently with the ring particles, equivalent columns of O2 exist above and below the ring plane with the scale height determined by the local ring temperature. Energetic particles also decompose ice, but estimates of their contribution over the main rings appear to be very low. In steady state, the O2 column density over the rings also depends on the relative efficiency of hydrogen to oxygen loss from the ring/atmosphere system with oxygen being recycled on the grain surfaces. Unlike the neutral density, the ion densities can differ on the sunlit and shaded sides due to differences in the ionization rate, the quenching of ions by the interaction with the ring particles, and the northward shift of the magnetic equator relative to the ring plane. Although O+ is produced with a significant excess energy, O 2 + is not. Therefore, O 2 + should mirror well below those altitudes at which ions were detected. However, scattering by ion–molecule collisions results in much larger mirror altitudes, in ion temperatures that go through a minimum over the B-ring, and in the redistribution of both molecular hydrogen and oxygen throughout the magnetosphere. The proposed model is used to describe the measured oxygen ion densities in Saturn's toroidal ring atmosphere and its hydrogen content. The oxygen ion densities over the B-ring appear to require either significant levels of UV light scattering or ion transmission through the ring plane.

In this paper we analyze a pre-shock event that we observed in the foot region of the quasi-parallel bow shock (BS) that the Cassini spacecraft crossed on 30 January 2001, at about 1030 UT. Before crossing the BS, the incoming solar wind first decelerated, and then the bulk velocity both of the proton and α components increased, the flow accelerated and decelerated, heated and cooled several times. We characterize the plasma in the foot using the data measured by the magnetometer, the radio and plasma wave science (RPWS) instrument, and the Cassini plasma spectrometer (CAPS) being carried onboard the Cassini spacecraft, and analyze the observations. We argue that the velocity and temperature changes can be caused by firehose instabilities excited by ions reflected from the shock. We investigate another possibility, shocklet formation, to account for the observed features, but conclude that this explanation seems to be less likely. In the foot we also identified both backstreaming electrons and ions and electrostatic waves in the 100–1000 Hz range very likely excited by the backstreaming electrons.

We present Cassini magnetometer and plasma data for the January 2004 ‘solar wind campaign’ in which the particles and fields instruments monitored the solar wind and interplanetary magnetic field, while the Hubble Space Telescope (HST) simultaneously observed the UV aurora in Saturn’s southern ionosphere. Clear structuring is evident in the data which is associated with the highly developed nature of corotating interaction regions (CIRs) at this distance. The interplanetary medium during January consisted of four distinct types of behaviour. We see a ‘major’ compression region at the start of the interval followed by a rarefaction region, a ‘minor’ compression region, an ‘intermediate’ rarefaction region, and another major compression region at the end of the month. The highly dynamic nature of Saturn’s aurora revealed by the HST observations appears to relate directly to the concurrent solar wind activity measured by Cassini. Collectively these data provide a unique insight into the solar wind driving of Saturn’s magnetosphere and consequent auroral response.

Resume Le premier passage de la sonde Cassini dans l'environnement de Saturne, au dessus de l'anneau E, a mis en evidence l'existence d'un plasma compose d'un melange d'ions issus des produits de l'eau (H + , O + , OH + , H 2 O + ) avec une faible composante en ions N + (3 %). A partir d'un modele simple du transport des ions dans la magnetosphere, nous montrons que la source de ces ions N + coincide avec le satellite Encelade. Un tel resultat peut s'expliquer par la presence de composes volatiles tels que l'ammoniac NH 3 sur ce satellite de glace, suppose encore actif geologiquement, ou par la presence d'ions N + d'origine externe prealablement implantes sur sa surface. Pour citer cet article : M. Bouhram et al., C. R. Physique 6 (2005).

The Cassini spacecraft Ultraviolet Imaging Spectrograph (UVIS) obtained observations of Jupiter's auroral emissions in H-2 band systems and H Lyman-alpha from day 275 of 2000 (October 1), to day 81 of 2001 (March 22). Much of the globally integrated auroral variability measured with UVIS can be explained simply in terms of the rotation of Jupiter's main auroral arcs with the planet. These arcs were also imaged by the Space Telescope Imaging Spectrograph (STIS) on Hubble Space Telescope (HST). However, several brightening events were seen by UVIS in which the global auroral output increased by a factor of 2-4. These events persisted over a number of hours and in one case can clearly be tied to a large solar coronal mass ejection event. The auroral UV emissions from these bursts also correspond to hectometric radio emission (0.5-16 MHz) increases reported by the Galileo Plasma Wave Spectrometer (PWS) and Cassim Radio and Plasma Wave Spectrometer (RPWS) experiments. In general, the hectometric radio data vary differently with longitude than the UV data because of radio wave beaming effects. The 2 largest events in the UVIS data were on 2000 day 280 (October 6) and on 2000 days 325-326 (November 20-21). The global brightening events on November 20-21 are compared with corresponding data on the interplanetary magnetic field, solar wind conditions, and energetic particle environment. ACE (Advanced Composition Explorer) solar wind data was numerically propagated from the Earth to Jupiter with an MHD code and compared to the observed event. A second class of brief auroral brightening events seen in HST (and probably UVIS) data that last for similar to 2 min is associated with aurora] flares inside the main auroral ovals. On January 8, 2001, from 18:45-19:35 UT UVIS H-2 band emissions from the north polar region varied quasiperiodically. The varying emissions, probably due to amoral flares inside the main auroral oval, are correlated with low-frequency quasiperiodic radio bursts in the 0.6-5 kHz Galileo PWS data. (c) 2005 Elsevier Inc. All rights reserved.

Saturn is a source of intense kilometre-wavelength radio emissions that are believed to be associated with its polar aurorae, and which provide an important remote diagnostic of its magnetospheric activity. Previous observations implied that the radio emission originated in the polar regions, and indicated a strong correlation with solar wind dynamic pressure. The radio source also appeared to be fixed near local noon and at the latitude of the ultraviolet aurora. There have, however, been no observations relating the radio emissions to detailed auroral structures. Here we report measurements of the radio emissions, which, along with high-resolution images of Saturn's ultraviolet auroral emissions, suggest that although there are differences in the global morphology of the aurorae, Saturn's radio emissions exhibit an Earth-like correspondence between bright auroral features and the radio emissions. This demonstrates the universality of the mechanism that results in emissions near the electron cyclotron frequency narrowly beamed at large angles to the magnetic field.

The Cassini Plasma Spectrometer (CAPS) will make comprehensive three-dimensional mass-resolved measurements of the full variety of plasma phenomena found in Saturn's magnetosphere. Our fundamental scientific goals are to understand the nature of saturnian plasmas primarily their sources of ionization, and the means by which they are accelerated, transported, and lost. In so doing the CAPS investigation will contribute to understanding Saturn's magnetosphere and its complex interactions with Titan, the icy satellites and rings, Saturn's ionosphere and aurora, and the solar wind. Our design approach meets these goals by emphasizing two complementary types of measurements: high-time resolution velocity distributions of electrons and all major ion species; and lower-time resolution, high-mass resolution spectra of all ion species. The CAPS instrument is made up of three sensors: the Electron Spectrometer (ELS), the Ion Beam Spectrometer (IBS), and the Ion Mass Spectrometer (IMS). The ELS measures the velocity distribution of electrons from 0.6 eV to 28,250 keV, a range that permits coverage of thermal electrons found at Titan and near the ring plane as well as more energetic trapped electrons and auroral particles. The IBS measures ion velocity distributions with very high angular and energy resolution from 1 eV to 49,800 keV. It is specially designed to measure sharply defined ion beams expected in the solar wind at 9.5 AU, highly directional rammed ion fluxes encountered in Titan's ionosphere, and anticipated field-aligned auroral fluxes. The IMS is designed to measure the composition of hot, diffuse magnetospheric plasmas and low-concentration ion species 1 eV to 50,280 eV with an atomic resolution M/ΔM ∼70 and, for certain molecules, (such asN 2 + and CO+), effective resolution as high as ∼2500. The three sensors are mounted on a motor-driven actuator that rotates the entire instrument over approximately one-half of the sky every 3 min.

The Plasma Experiment for Planetary Exploration (PEPE) made detailed observations of the plasma environment of Comet 19P/Borrelly during the Deep Space 1 (DS1) flyby on September 22, 2001. Several distinct regions and boundaries have been identified on both inbound and outbound trajectories, including an upstream region of decelerated solar wind plasma and cometary ion pickup, the cometary bow shock, a sheath of heated and mixed solar wind and cometary ions, and a collisional inner coma dominated by cometary ions. All of these features were significantly offset to the north of the nucleus–Sun line, suggesting that the coma itself produces this offset, possibly because of well-collimated large dayside jets directed 8°–10° northward from the nucleus as observed by the DS1 MICAS camera. The maximum observed ion density was 1640 ion/cm 3 at a distance of 2650 km from the nucleus while the flow speed dropped from 360 km/s in the solar wind to 8 km/s at closest approach. Preliminary analysis of PEPE mass spectra suggest that the ratio of CO + /H 2 O + is lower than that observed with Giotto at 1P/Halley.

Limited single-spacecraft observations of Jupiter's magnetopause have been used to infer that the boundary moves inward or outward in response to variations in the dynamic pressure of the solar wind. At Earth, multiple-spacecraft observations have been implemented to understand the physics of how this motion occurs, because they can provide a snapshot of a transient event in progress. Here we present a set of nearly simultaneous two-point measurements of the jovian magnetopause at a time when the jovian magnetopause was in a state of transition from a relatively larger to a relatively smaller size in response to an increase in solar-wind pressure. The response of Jupiter's magnetopause is very similar to that of the Earth, confirming that the understanding built on studies of the Earth's magnetosphere is valid. The data also reveal evidence for a well-developed boundary layer just inside the magnetopause.

After deriving from Ulysses-URAP measurements the low-frequency limit of the Jovian hectometer emission spectrum ( 250±50 kHz at the −20 dB level below the emission peak), and confirming the absence of Io's control on Jovian radio emission below ∼1 MHz , we propose a single common explanation for these low-frequency limits: both are well explained by the quenching of the generation mechanism (the cyclotron-maser instability) where the ratio fpe/fce reaches a critical value (about 0.14) along the source field lines. This occurs in the external part of Io's plasma torus for the hectometer component, and in Io's dense plasma wake discovered by Galileo for the Io-dependent (decameter) component. As a consequence, we infer new constraints on Jovian radio source locations, which are found to extend along the L≈6 field lines intersecting Io's wake (where the electron density is up to 10–20 times higher than in the average torus) for the Io-dependent decameter emission, and along L≈7–9 field lines (with apex at ≈7– 11 R J ) for the hectometer emission. We also infer that the broadband kilometer (auroral) emission originates from L>10 field lines, with apex well beyond 12 R J , closing in the distant Jovian magnetosphere or opened to the solar wind. These results confirm and precise the source locations obtained from direction-finding studies with Ulysses. Finally, we discuss implications in terms of source extent and of the origin of the accelerated electrons responsible for the emissions, and we derive lower limits on the proton concentration in Io's plasma wake.

In the downward current region of the auroral ionosphere, the FAST spacecraft has observed bipolar electrostatic structures on Debye length scales and waves at frequencies between the H+ ion cyclotron harmonics. Such bipolar structures have been previously identified with the nonlinearly evolved state of a two-stream electron instability. In this paper, we present the results of long-duration and large-scale particle-in-cell (PIC) simulations which produce, from one set of initial conditions, both bipolar electrostatic structures and, at later times, ion Bernstein waves with peak intensities between multiples of the ion cyclotron frequency. The ion Bernstein waves are driven by a weaker beam instability caused by a residual positive slope in the nonlinearly evolved (nonthermal) electron distribution. Although there are a variety of processes which can produce ion Bernstein modes, we show that a common source (an electron beam) can produce both of these observed phenomena in the downward current region.