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A surprisingly strong influx of organic‐rich material into Saturn's upper atmosphere from its rings was observed during the proximal obits of the Grand Finale of the Cassini mission. Measurements by the Ion and Neutral Mass Spectrometer (INMS) gave insights into the composition of the material, but it remains to be resolved what fraction of the inferred heavy volatiles should be attributed as originating from the fragmentation of dust particles in the instrument versus natural ablation of grains in the atmosphere. In the present study, we utilize measured light ion and neutral densities to further constrain the abundances of heavy volatiles in Saturn's ionosphere through a steady‐state model focusing on helium ion chemistry. We first show that the principal loss mechanism of He+ in Saturn's equatorial ionosphere is through reactions with species other than H2. Based on the assumption of photochemical equilibrium at altitudes below 2,500 km, we then proceed by estimating the mixing ratio of heavier volatiles down to the closest approaches for Cassini's proximal orbits 288 and 292. Our derived mixing ratios for the inbound part of both orbits fall below those reported from direct measurements by the INMS, with values of ∼2 × 10−4 at closest approaches and order‐of‐magnitude variations in either direction over the orbits. This aligns with previous suggestions that a large fraction of the neutrals measured by the INMS stems from the fragmentation of infalling dust particles that do not significantly ablate in the considered part of Saturn's atmosphere and are thus unavailable for reactions. Plain Language Summary: During the final orbits of the Cassini mission, the spacecraft flew between Saturn's rings and the planets upper atmosphere. The onboard plasma instruments detected a large amount of ring particles falling toward the planet, but direct measurements of the composition of these grains are complicated due to the high spacecraft speed and instrumental effects. In this study, we present an independent method to estimate the abundance of heavier neutral species entering the atmosphere from infalling ring material. This method relies on helium ion chemistry and the measured light ion and neutral densities. Our results generally fall below those inferred from direct measurements. Together with comparisons to other studies, this potentially suggests that a large fraction of the infalling neutral species do not significantly ablate in the considered part of Saturn's atmosphere (and remain bound to the dust grains instead) and are thus unavailable for reactions. Key Points: The dominant loss mechanism for He+ ions in Saturn's equatorial ionosphere are reactions with heavier neutral species, not H2The mixing ratio of volatiles in Saturn's ionosphere can be estimated from light ion and neutral measurements, using helium ion chemistryComparing with other studies potentially suggests that only the most volatile species (CO, N2 and CH4) enter the atmosphere as vapor [ABSTRACT FROM AUTHOR]

The Cassini spacecraft's final orbits sampled Saturn's atmosphere and returned surprisingly complex mass spectra from the Ion and Neutral Mass Spectrometer. Signal returned from the instrument included native Saturn species, as expected, as well as a significant amount of signal attributed to vaporized ices and higher mass organics believed to be flowing into Saturn's atmosphere from the rings. In this paper, we present an in‐depth compositional analysis of the mass spectra returned from Cassini's last few orbits. We use a mass spectral deconvolution algorithm designed specifically to handle the complexities involved with unit resolution spaceflight mass spectrometry data to determine the relative abundance of species detected in the observations. We calculate the downward external flux and mass deposition rates of ring volatile species into Saturn's atmosphere and conclude that during these observations ring material was being deposited into Saturn's equatorial region at a rate on the order of 104 kg/s. Plain Language Summary: The mass spectrometer aboard the Cassini spacecraft at Saturn directly sampled the region between the planet and its rings. These measurements allow us to infer the chemical composition of the sampled region to better understand what material is present in the region and how Saturn's upper atmosphere and innermost rings interact. The mass spectra returned by the instrument are surprisingly complex and includes signal from vaporized ices and organics, which we attribute to ring material falling into the atmosphere. This material was entering Saturn's atmosphere at a rate on the order of 104 kg/s, revealing that the effect of the rings on the atmosphere is more extensive than previously thought. We use this information to infer the composition and amount of material flowing into the Saturn's atmosphere from the rings. Key Points: Ion and Neutral Mass Spectrometer returned surprisingly complex mass spectra during final orbits, indicating strong compositional interactions between Saturn and D ringWe have developed a deconvolution algorithm to handle the complexities of unit resolution mass spectra when calibration data are unavailableWe attribute a large amount of signal to vaporized ices and organics thought to be flowing into Saturn's atmosphere from the rings [ABSTRACT FROM AUTHOR]

Gravity wave (GW) signatures have been derived from temperature profiles observed by Cassini/Ultraviolet Imaging Spectrograph in the Saturnian thermosphere during the Grand Finale campaign. They demonstrate upward propagation of GW packets, their saturation, and breaking. We determined wave amplitudes, potential energy, and momentum fluxes and estimated the associated wave drag imposed by dissipating harmonics on the ambient flow. The data set of 18 profiles covers the middle and high latitudes of both hemispheres, which allows for exploring the global impact of waves. The diagnostics based on the Transformed Eulerian Mean and modified geostrophy approach reveal that the GW drag induces an equatorward flow in both hemispheres, facilitating transport of heat away from the auroral zones and redistributing energy across latitudes. Like all the outer planets, Saturn's thermosphere is hundreds of degrees hotter than what follows from radiative balance and these results help to explain the observed temperatures at all latitudes. Plain Language Summary: Gravity waves are small‐scale fluctuations of air density, temperature, and other atmospheric variables, which are dynamically important in the upper atmospheres. During Cassini's Grand Finale, observations with the Ultraviolet Imaging Spectrograph delivered a set of density profiles in the Saturnian thermosphere with a resolution sufficient to detect such waves. We derived various characteristics and estimated the forcing imposed by dissipating waves on the global circulation. It turned out that this "gravity wave drag" causes enhanced flow toward the equator in both hemispheres, transporting heat away from the auroral sources in high latitudes and distributing it over Saturn's thermosphere. Previous studies suggested that such redistribution would be prevented by fast westward jets (the "energy crisis"). We show that the waves observed on Saturn can successfully help winds to overcome this barrier. A similar mechanism can exist on other outer planets, whose thermospheres are also much hotter than what follows from radiative balance. Key Points: We present evidence for the omnipresence of gravity waves in the Saturnian thermosphere from Cassini/Ultraviolet Imaging Spectrograph Grand Finale measurementsWe determine wave amplitudes, energy, momentum fluxes, and forcing imposed on the mean flow along with their spatial distributionsGravity wave drag enhances pole‐to‐equator flow in both hemispheres, which helps to redistribute energy across latitudes [ABSTRACT FROM AUTHOR]

The Saturn Kilometric Radiation (SKR) was observed for the first time during the flyby of Saturn by the Voyager spacecraft in 1980. These radio emissions, in the range of a few kHz to 1 MHz, are emitted by electrons travelling around auroral magnetic field lines. Their study is useful to understand the variability of a magnetosphere and its coupling with the solar wind. Previous studies have shown a strong correlation between the solar wind dynamic pressure and the SKR intensity. However, up to now, the effect of an Interplanetary Coronal Mass Ejection (ICME) has never been examined in detail, due to the lack of SKR observations at the time when an ICME can be tracked and its different parts be clearly identified. In this study, we take advantage of a large ICME that reached Saturn mid-November 2014 (Witasse et al., J. Geophys. Res. Space Physics, 2017, 122, 7865–7890). At that time, the Cassini spacecraft was fortunately travelling within the solar wind for a few days, and provided a very accurate timing of the ICME structure. A survey of the Cassini data for the same period indicated a significant increase in the SKR emissions, showing a good correlation after the passage of the ICME shock with a delay of ∼13 h and after the magnetic cloud passage with a delay of 25–42 h.

While various source and loss processes have been proposed for ions in Saturn's magnetosphere, it is not yet well understood what role they play in different regions. In this study, we use a physical model of charge exchange to predict how proton and water group ion intensity profiles evolve over time and compare the results to MIMI/CHEMS measurements collected during the Cassini mission. First, we divide the CHEMS data into inbound and outbound half‐orbit segments, and create intensity profiles for 3–220 keV H+ and W+ ions between 5 and 15 Saturn radii, then using the inbound half‐orbits as initial conditions, we find qualitative similarities between measured and predicted outbound intensity profiles. This result is important because it provides strong evidence that charge exchange is the dominant loss process for these species in this region. The observed rate of charge exchange also presents information on the density of Saturn's neutral torus. We suggest that data‐model discrepancies in the water group ions may be an indication of a significant presence of ions with the water group mass that are multiply charged. Key Points: Survey of suprathermal proton and water group ions in Saturn's magnetosphere show strong variability at <1 MeVData comparisons to a physical model suggest that charge exchange plays a key role in removing suprathermal ions between 5 and 15 RSThis result is important for better understanding ion‐neutral interactions and loss processes in Saturn's magnetosphere [ABSTRACT FROM AUTHOR]

We present the discovery of seven new flux transfer events (FTEs) at Saturn's dayside magnetopause by the Cassini spacecraft and analyze the observations of all eight known FTEs. We investigate how FTEs may differ at Saturn where the magnetopause conditions are likely to diamagnetically suppress magnetic reconnection from occurring. The measured ion‐scale FTEs have diameters close to or above the ion inertial length di∼1–27 (median and mean values of 5 and 8), considerably lower than typical FTEs found at Earth. The FTEs magnetic flux contents are 4–461 kWb (median and mean values of 16 and 77 kWb), considerably smaller (<0.1%) than average flux opened during magnetopause compression events at Saturn. This is in contrast to Earth and Mercury where FTEs contribute significantly to magnetospheric flux transfer. FTEs therefore represent a negligible proportion of the amount of open magnetic flux transferred at Saturn. Due to the likely suppression of the two main growth‐mechanisms for FTEs (continuous multiple x‐line reconnection and FTE coalescence), we conclude that adiabatic expansion is the likely (if any) candidate to grow the size of FTEs at Saturn. Electron energization is observed inside the FTEs, due to either Fermi acceleration or parallel electric fields. Due to diamagnetic suppression of reconnection at Saturn's magnetopause, we suggest that the typical size of FTEs at Saturn is most likely very small, and that there may be more di∼1 FTEs present in the Cassini magnetometer data that have not been identified due to their brief and unremarkable magnetic signatures.Key Points: Eight Saturn ion‐scale flux transfer events (FTEs) are analyzed with diameters of di∼1–27FTEs at Saturn are found to transfer negligible amounts of flux at Saturn's magnetosphereEvidence for electron energization is observed inside some of the FTEs, due to either Fermi acceleration or parallel electric fields [ABSTRACT FROM AUTHOR]

Before Cassini, scientists viewed Saturn’s unique features only from Earth and from three spacecraft flying by. During more than a decade orbiting the gas giant, Cassini studied the planet from its interior to the top of the atmosphere. It observed the changing seasons, provided up-close observations of Saturn’s exotic storms and jet streams, and heard Saturn’s lightning, which cannot be detected from Earth. During the Grand Finale orbits, it dove through the gap between the planet and its rings and gathered valuable data on Saturn’s interior structure and rotation. Key discoveries and events include: watching the eruption of a planet-encircling storm, which is a 20- or 30-year event, detection of gravity perturbations from winds 9000 km below the tops of the clouds, demonstration that eddies are supplying energy to the zonal jets, which are remarkably steady over the 25-year interval since the Voyager encounters, re-discovery of the north polar hexagon after 25 years, determination of elemental abundance ratios He/H, C/H, N/H, P/H, and As/H, which are clues to planet formation and evolution, characterization of the semiannual oscillation of the equatorial stratosphere, documentation of the mysteriously high temperatures of the thermosphere outside the auroral zone, and seeing the strange intermittency of lightning, which typically ceases to exist on the planet between outbursts every 1–2 years. These results and results from the Jupiter flyby are all discussed in this review. [ABSTRACT FROM AUTHOR]

The nature and structure of the observed east-west flows on Jupiter and Saturn have been a long-standing mystery in planetary science. This mystery has been recently unraveled by the accurate gravity measurements provided by the Juno mission to Jupiter and the Grand Finale of the Cassini mission to Saturn. These two experiments, which coincidentally happened around the same time, allowed the determination of the overall vertical and meridional profiles of the zonal flows on both planets. This paper reviews the topic of zonal jets on the gas giants in light of the new data from these two experiments. The gravity measurements not only allow the depth of the jets to be constrained, yielding the inference that the jets extend to roughly 3000 and 9000 km below the observed clouds on Jupiter and Saturn, respectively, but also provide insights into the mechanisms controlling these zonal flows. Specifically, for both planets this depth corresponds to the depth where electrical conductivity is within an order of magnitude of 1 S m−1, implying that the magnetic field likely plays a key role in damping the zonal flows. An intrinsic characteristic of any gravity inversion, as discussed here, is that the solutions might not be unique. We analyze the robustness of the solutions and present several independent lines of evidence supporting the results presented here. [ABSTRACT FROM AUTHOR]

The Cassini spacecraft's last orbits directly sampled Saturn's thermosphere and revealed a much more chemically complex environment than previously believed. Observations from the Ion and Neutral Mass Spectrometer (INMS) aboard Cassini provided compositional measurements of this region and found an influx of material raining into Saturn's upper atmosphere from the rings. We present here an in‐depth analysis of the CH4, H2O, and NH3 signal from INMS and provide further evidence of external material entering Saturn's atmosphere from the rings. We use a new mass spectral deconvolution algorithm to determine the amount of each species observed in the spectrum and use these values to determine the influx and mass deposition rate for these species. Plain Language Summary: The Cassini spacecraft's last orbits around Saturn provided measurements to help us understand how the rings of Saturn interact with its upper atmosphere. Using measurements from the mass spectrometer aboard the spacecraft, we find that a lot of material from the rings is entering Saturn's atmosphere. We use a new method to determine the amount of water, methane, and ammonia that are entering the atmosphere from the rings and find that this large influx could deplete the ring system in a relatively short amount of time. Key Points: We measure the density profiles of H2, He, CH4, H2O, and NH3 in Saturn's thermosphere from Cassini INMSWe use a new mass spectral deconvolution algorithm to determine the relative abundances of different species found in the mass spectraWe report further evidence of CH4, H2O, and NH3 entering Saturn's atmosphere from the rings at a rate of at least 103 kg/s [ABSTRACT FROM AUTHOR]

Titan's magnetic environment is a dynamic and unique place. We detect 85 flux ropes during all the Cassini flybys of Titan from 2005–2017. Analysis describing the location of flux ropes in Titan's ionosphere as well as where Titan is in Saturn's magnetosphere shows that the flux ropes are more often found when Titan is in the noon sector of Saturn's magnetosphere. A secondary peak of occurrence is found in the postmidnight area, where it is expected that Saturn's magnetosphere is highly dynamic. We also find that the flux rope occurrence is correlated with the average electron density profile of Titan's ionosphere. A force‐free model is utilized to estimate the radii and core magnetic field strength of the flux ropes. We find a large range of radii from 150–500 km with a small number of very large flux ropes (500–1,000 km) and a range of core field strengths of 1–15 nT, again with some much larger values(20–40 nT). The model also shows that more flux ropes are right‐handed than left‐handed in twist; however, we are unable to determine if there is a physical reason or if this is due to an observer bias. Additionally, we evaluate the goodness of fit for the model in each instance and conclude that, on average, the flux ropes may be better represented by a non‐force‐free model. Key Points: Eighty‐five flux ropes are detected in Titan's ionosphere using all of Cassini's flybys of TitanFlux ropes are present when Titan is in dynamic areas of Saturn's magnetosphereFitting a force‐free flux rope model leads to the conclusion that on average they are unlikely to be force free [ABSTRACT FROM AUTHOR]

Saturn's morningside auroras consist mainly of rotating, transient emission patches, following periodic reconnection in the magnetotail. Simultaneous responses in global energetic neutral atom (ENA) emissions have been observed at similar local times, suggesting a link between the auroras and large‐scale injections of hot ions in the outer magnetosphere. In this study, we use Cassini's remote sensing instruments to observe multiple plasma injection signatures within coincident auroral and ENA imagery, captured during 9 April 2014. Kilometric radio emissions also indicate clear injection activity. We track the motion of rotating signatures in the auroras and ENAs to test their local time relationship. Two successive auroral signatures—separated by ~4 hr UT—form postmidnight before rotating to the dayside while moving equatorward. The first has a clear ENA counterpart, maintaining a similar local time mapping throughout ~9 hr observation. Mapping of the ionospheric equatorward motion post‐dawn indicates a factor ~5 reduction of the magnetospheric source region's radial speed at a distance of ~14‐20 RS, possibly a plasma or magnetic boundary. The second auroral signature has no clear ENA counterpart; viewing geometry was relatively unchanged, so the ENAs were likely too weak to detect by this time. A third, older injection signature, seen in both auroral and ENA imagery on the nightside, may have been sustained by field‐aligned currents linked with the southern planetary period oscillation system, or the re‐energization of ENAs around midnight local times. The ENA injection signatures form near magnetic longitudes associated with magnetotail thinning. Key Points: Rotating enhancements in Saturn's morning auroras do not always have a counterpart in ENA emissions from the magnetosphereCounterpart signatures can maintain a near 1:1 local time mapping throughout at least a planetary rotationRemote sensing imagery, in conjunction with magnetic field mapping models, could provide characterization of plasma flow boundaries [ABSTRACT FROM AUTHOR]

We investigate the average configuration and structure of Saturn's magnetosphere in the nightside equatorial and high‐latitude regions. Electron data from the Cassini Plasma Spectrometer's Electron Spectrometer (CAPS‐ELS) is processed to produce a signal‐to‐noise ratio for the entire CAPS‐ELS time of operation at Saturn's magnetosphere. We investigate where the signal‐to‐noise ratio falls below 1 to identify regions in the magnetosphere where there is a significant depletion in the electron content. In the nightside equatorial region, we use this to find that the most planetward reconnection x‐line location is at 20–25 RS downtail from the planet in the midnight to dawn sector. We also find an equatorial dawn‐dusk asymmetry at a radial distance of >20 RS, which may indicate the presence of plasma‐depleted flux tubes returning to the dayside after reconnection in the tail. Furthermore, we find that the high‐latitude magnetosphere is predominantly in a state of constant plasma depletion and located on open field lines. We map the region of high‐latitude magnetosphere that is depleted of electrons to the polar cap to estimate the size and open flux content within the polar caps. The mean open flux content for the northern and southern polar caps are found to be 25 ± 5 and 32 ± 5 GWb, respectively. The average location of the open‐closed field boundary is found at invariant colatitudes of 12.7 ± 0.6° and 14.5 ± 0.6°. The northern boundary is modulated by planetary period oscillations more than the southern boundary. Key Points: The high‐latitude magnetosphere is predominantly in a state of constant plasma depletion and located on open field linesThe reconnection x‐line is located at 20–25 RS downtail from the planet on the midnight to dawn side of the equatorial magnetosphereThe open‐closed field boundary is located at colatitudes of 12.7 ± 0.6° and 14.5 ± 0.6° (north and south) with weak planetary period oscillation modulation in the north [ABSTRACT FROM AUTHOR]

The Cassini spacecraft orbited the planet Saturn from July 2004 to September 2017, and its varied orbital trajectory took it across the magnetopause and bow shock boundaries multiple times, at varying radial distances, local times, latitudes, and phases of the solar cycle. Here we present a comprehensive list of these boundary crossings, derived primarily using data from the Cassini magnetometer instrument, with cross‐validation against the electron spectrometer data where available. There are a multitude of scientific avenues for exploitation of this list. In this work, we examine the variability in boundary location and use the crossing times in concert with models of the bow shock and magnetopause to infer the upstream solar wind dynamic pressure at the times of crossings. This analysis allows us to understand the limitations of the Cassini trajectory for studying boundary physics under a range of solar wind driving conditions. In addition, rapid traversals of the magnetosheath are used to estimate the range of speeds of boundary motion. Key Points: Comprehensive set of magnetopause and bow shock crossings spanning the entire Cassini mission presentedCrossings used in conjunction with boundary models to infer upstream solar wind dynamic pressure and range of standoff distancesRapid traversals of magnetosheath explored in context of rapidly changing external conditions [ABSTRACT FROM AUTHOR]

The Cassini RADAR altimeter has provided broad-scale surface topography data for Saturn’s largest moon Titan. Herein, we adapt the delay/Doppler algorithm to take into account Cassini geometries and antenna mispointing usually occurring during hyperbolic Titan flybys. The proposed algorithm allows up to tenfold improvement in the along-track resolution. Preliminary results are provided that show how the improved topography presented herein can advance our understanding of Titan’s surface characteristics. [ABSTRACT FROM AUTHOR]

Cassini's final orbits in 2016 and 2017 provided unprecedented spatial resolution of Saturn's polar regions from near‐polar spacecraft viewing geometries. Long‐wavelength channels of Cassini's Ultraviolet Imaging Spectrograph instrument detected Saturn's UV‐dark north polar hexagon near 180 nm at planetocentric latitudes near 75°N. The dark polar hexagon is surrounded by a larger, less UV‐dark collar poleward of planetocentric latitude 65°N associated with the dark north polar region seen in ground‐based images. The hexagon is closely surrounded by the main arc of Saturn's UV aurora. The UV‐dark material was locally darkest on one occasion (23 January 2017) at the boundary of the hexagon; in most Ultraviolet Imaging Spectrograph images the dark material more uniformly fills the hexagon. The observed UV‐dark stratospheric material may be a hydrocarbon haze produced by auroral ion‐neutral chemistry at submicrobar pressure levels. Ultraviolet Imaging Spectrograph polar observations are sensitive to UV‐absorbing haze particles at pressures lower than about 10–20 mbar. Key Points: Saturn's polar hexagon is visible in Cassini UVIS dataSaturn's polar hexagon is adjacent to Saturn's auroral emissionsIt is likely that auroral ion chemistry provides a source of dark material for Saturn's polar hexagon [ABSTRACT FROM AUTHOR]

Scientific findings and new discoveries from the international Cassini‐Huygens mission's exploration of the Saturn system are presented in this special issue of Geophysical Research Letters. In the mission's final year, Cassini dove through the gap between the rings and Saturn for the first time, returning new science in this previously unexplored region. Running low on fuel, Cassini's journey ended on 15 September 2017, as the spacecraft plunged into Saturn's atmosphere and vaporized. Just before the mission concluded, the Grand Finale orbits through the gap provided some of the highest resolution information on Saturn's interior and atmosphere, its rings and inner icy moons, and direct in situ sampling of this intriguing region. Initial results indicate that hydrocarbons appear to be descending into the atmosphere from the rings, that electric currents flow between the inner rings and Saturn, and that a new radiation belt exists in the gap and innermost ring. Plain Language Summary: The international Cassini‐Huygens mission explored Saturn, its magnificent rings, its astonishing array of moons, and magnetic environment for 13 years, returning a wealth of groundbreaking new science. Findings made by the Cassini spacecraft and Huygens probe significantly altered our views of this outer planet system and in some cases challenged long‐held theories. Cassini explored these worlds up close, returning images that are beautiful as well as data that are scientifically priceless. Cassini's last year exploring the Saturn system provided unprecedented new science, discussed in more detail in papers in this Cassini special issue. New discoveries examined in this issue include tiny ring particles with complex hydrocarbons streaming into Saturn's atmosphere, methane from the rings feeding Saturn's upper atmosphere, electric currents flowing between Saturn and its rings, and a new inner radiation belt. Saturn gravity and magnetic field measurements detected deep winds and differential rotation in its upper layers. Results from Cassini's final orbits turned out to be more interesting than we could have imagined. Understanding the interior of Saturn and the interplay between the rings and planet will provide insights into how our solar system formed and evolved and the role of gas giant planets in exoplanet systems. Key Points: Cassini's last year exploring the Saturn system provided unprecedented new scienceNew discoveries included ring particles with complex hydrocarbons, electric currents between Saturn and rings, and inner radiation beltSaturn gravity and magnetic field measurements detected deep winds and differential rotation in its upper layers [ABSTRACT FROM AUTHOR]

How deep do Saturn's zonal winds penetrate below the cloud level has been a decades‐long question, with important implications not only for the atmospheric dynamics but also for the interior density structure, composition, magnetic field, and core mass. The Cassini Grand Finale gravity experiment enables answering this question for the first time, with the premise that the planet's gravity harmonics are affected not only by the rigid body density structure but also by its flow field. Using a wide range of rigid body interior models and an adjoint based optimization for the flow field using thermal wind balance, we calculate the flow structure below the cloud level and its depth. We find that with a wind profile, largely consistent with the observed winds, when extended to a depth of around 8,800 km, all the gravity harmonics measured by Cassini are explained. This solution is in agreement with considerations of angular momentum conservation and is consistent with magnetohydrodynamics constraints. Plain Language Summary: Observations show strong east‐west flows at the cloud level of Saturn. These winds are strongest at the equatorial regions, reaching up to 400 m/s, about 4 times stronger than tornado strength winds on Earth. Yet until now we had no knowledge on how deep these winds penetrate into the interior of the gas giant. The gravity experiment executed during the Grand Finale stage (May–August 2017) of the NASA Cassini mission helps answering this question. It is well established that any large‐scale motion of the fluid would have a signature in the density distribution and therefore in the planet gravity field. If we can estimate the internal structure and shape of the planet, we might be able to decipher the depth of the winds from its signal in the gravity measurements. Moreover, the rigid‐body and flow contribution to gravity field are entangled together, therefore it is necessary to use a wide range of rigid‐body models in order to define the wind‐induced gravity signal. In this work we propose a solution to the problem. We find that the gravity measurements can be explained with a flow pattern, similar to that observed at the cloud level, penetrating to depths of more than 8,000 km into the planet interior. This has important implications not only for the atmospheric dynamics but also for the interior density structure, composition, magnetic field, and core mass. Key Points: Cassini gravity measurements point to deep differential flows within SaturnUsing a wide range of rigid body internal structure models, the required wind‐induced gravity signal is definedWith a conservatively modified cloud‐level wind and an optimized vertical profile, extended to a depth of 8,800km, all gravity measurements are explained [ABSTRACT FROM AUTHOR]

We overview the properties of the azimuthal magnetic fields observed during the periapsis passes of the final 23 orbits of the Cassini spacecraft, including the partial orbit at end of mission, on near equatorial field lines passing inside of Saturn's D ring. The signatures are variable in form and amplitude, though generally approximately symmetric about the point where the spacecraft trajectory lies tangent to a flux shell, corresponding to where the ionospheric field line feet map closest to the equator, consistent with the effect of interhemispheric field‐aligned currents. The perturbations usually begin and end near symmetrically at some point on field lines threading the D ring and extend into the interior region, but in no case do they clearly extend outward onto field lines passing through the C ring. About 35% of cases display a ~20‐40 nT single positive central field peak indicative of southward field‐aligned current flow, while a further ~30% display two or three weaker ~10‐20 nT positive peaks indicative of multiple sheets of northward and southward current. Significant smaller‐scale >5 nT peak‐to‐peak field fluctuations are commonly superposed. A further ~20% of cases exhibit unique profiles within the data set, including two with ~20‐30 nT negative fields and two with only <10 nT fluctuating fields. The variable nature of the signatures is not connected with the pass altitude, local time, planetary period oscillation phase, or D68 ringlet phase but may relate to variable structured thermospheric winds and/or ionospheric conductivities that suggest a significant dynamical role for D ring‐atmosphere interactions. Key Points: Azimuthal magnetic fields observed on Saturn's intra‐D ring (IDR) field lines exhibit a range of profiles on the 23 Cassini proximal passesOne, two, or three near‐symmetrical positive peaks ~10‐40 nT are typical, but two passes have ~20‐30 nT negative peaks and two <10 nT fieldsVariable IDR field signatures are not organized by periapsis radius, local time, planetary period oscillation phase, or D68 ringlet phase [ABSTRACT FROM AUTHOR]

The megaelectron volt proton radiation belts of Saturn are isolated from the middle and outer magnetosphere, and the source of these high‐energy protons is thought to be linked to the access of galactic cosmic rays (GCRs) in the system. To validate this hypothesis, it is first of all necessary to determine the realistic spectrum of GCRs at Saturn. Previously, only analytical attempts were performed in order to calculate the GCR spectra. In this letter we provide for the first time the numerical solution for the determination of the GCR access to the upper atmosphere and rings of Saturn. The proposed method is based on the charged particle tracing technique. As a result the GCRs access energies to the atmosphere and to the equatorial plane of the magnetosphere as a function of radial distance to the planet were obtained and the GCRs spectra were reconstructed. Dependencies of the spectral parameters such as the time or the incidence direction were also obtained offering all necessary information for simulating the interaction of GCRs with the Saturnian system during different phases of the Cassini mission. Plain Language Summary: In this research letter we propose the numerical solution for the calculation of the galactic cosmic rays (GCRs) access energies to Saturn and its magnetosphere. Previously, only analytical approach was used to address this problem in the Saturnian system providing uncertain results. The new numerical approach on the basis of particle tracing brings much more accurate access energies, GCR spectra, and integrated GCR flux to the planet and across the magnetosphere and allows to estimate the Cosmic Ray Albedo Neutron Decay process on Saturn, calculate its contribution to the Saturnian radiation belts, and evaluate the impact of GCRs on the atmosphere, rings and moons of Saturn. Key Points: We provide GCR access energies to the Saturnian atmosphere and across the magnetosphereGCR spectra and integrated flux to Saturn and to the equatorial plane of the magnetosphere are calculatedAdvantages of a particle tracing approach over the analytical solution are demonstrated [ABSTRACT FROM AUTHOR]

Planetary magnetospheres receive plasma and energy from the Sun or moons of planets and consequently stretch magnetic field lines. The process may last for varied timescales at different planets. From time to time, energy is rapidly released in the magnetosphere and subsequently precipitated into the ionosphere and upper atmosphere. Usually, this energy dissipation is associated with magnetic dipolarization in the magnetosphere.This process is accompanied by plasma acceleration and field‐aligned current formation, and subsequently auroral emissions are often significantly enhanced. Using measurements from multiple instruments on board the Cassini spacecraft, we reveal that magnetic dipolarization events at Saturn could reoccur after one planetary rotation and name them as recurrent dipolarizations. Three events are presented, including one from the dayside magnetosphere, which has no known precedent with terrestrial magnetospheric observations. During these events, recurrent energizations of plasma (electrons or ions) were also detected, which clearly demonstrate that these processes shall not be simply attributed to modulation of planetary periodic oscillation, although we do not exclude the possibility that the planetary periodic oscillation may modulate other processes (e.g., magnetic reconnection) which energizes particles. We discuss the potential physical mechanisms for generating the recurrent dipolarization process in a comprehensive view, including aurora and energetic neutral atom emissions. Plain Language Summary: Using measurements from the Cassini spacecraft, we reveal a new feature of magnetic dipolarization at Saturn, that is, the magnetic signature repeat after one planetary rotation, which is named recurrent dipolarization. Up to hundreds of kiloelectron volt electrons and ions are identified for the recurrent dipolarization events, suggesting that these particles have experienced efficient acceleration and cannot be purely due to planetary modulation. It remains a mystery why the magnetic dipolarization process associated with energetic ions and electrons could reoccur after one planetary rotation. Moreover, dipolarization process in Saturn's dayside magnetosphere is reported for the first time at Saturn, which has no known precedent with terrestrial or other planetary magnetospheric observations. The results demonstrate that magnetosphere‐ionosphere coupling dynamics at Saturn and Earth have fundamental similarities and differences. Key Points: We report a recurrent type of magnetic dipolarization at SaturnThe associated processes could efficiently accelerate electrons and ions to tens to hundreds of kiloelectron voltsCorotation of magnetosphere and planetary periodic oscillation are suggested as possible mechanisms [ABSTRACT FROM AUTHOR]

Moments of the charged particle distribution function provide a compact way of studying the transport, acceleration, and interactions of plasma and energetic particles in the magnetosphere. We employ κ‐distributions to describe the energy spectra of H+ and O+, based on >20 keV measurements by the three detectors of Cassini's Magnetospheric Imaging Instrument, covering the time period from DOY 183/2004 to 016/2016, 5 < L < 20. From the analytical spectra we calculate the equatorial distributions of energetic ion moments inside Saturn's magnetosphere and then focus on the distributions of the characteristic energy (Ec=IE/In), temperature, and κ‐index of these ions. A semiempirical model is utilized to simulate the equatorial ion moments in both local time and L‐shell, allowing the derivation of the polytropic index (Γ) for both H+ and O+. Primary results are as follows: (a) The ∼9 < L < 20 region corresponds to a local equatorial acceleration region, where subadiabatic transport of H+ (Γ∼1.25) and quasi‐isothermal behavior of O+ (Γ∼0.95) dominate the ion energetics; (b) energetic ions are heavily depleted in the inner magnetospheric regions, and their behavior appears to be quasi‐isothermal (Γ<1); (c) the (quasi‐) periodic energetic ion injections in the outer parts of Saturn's magnetosphere (especially beyond 17–18 RS) produce durable signatures in the energetic ion moments; (d) the plasma sheet does not seem to have a ground thermodynamic state, but the extended neutral gas distribution at Saturn provides an effective cooling mechanism that does not allow the plasma sheet to behave adiabatically. Key Points: Derivation of energetic ion moments, κ‐index, characteristic energy, temperature, and polytropic index in Saturn's magnetospherePresentation of a semiempirical analytical model for the 20 keV energetic ion Pressure, density, and temperatureThe neutral gas at Saturn provides an effective cooling mechanism and does not allow the plasma sheet to behave adiabatically [ABSTRACT FROM AUTHOR]

We present new models of Saturn's equatorial ionosphere based on the first in situ measurements of its upper atmosphere. The neutral spectrum measured by Cassini's Ion and Neutral Mass Spectrometer, which includes substantial methane, ammonia, and organics in addition to the anticipated molecular hydrogen, helium, and water, serves as input for unexpectedly complex ionospheric chemistry. Heavy molecular ions are found to dominate Saturn's equatorial low‐altitude ionosphere, with a mean ion mass of 11 Da. Key molecular ions include H3O+ and HCO+; other abundant heavy ions depend upon the makeup of the mass 28 neutral species, which cannot be uniquely determined. Ion and Neutral Mass Spectrometer neutral species lead to generally good agreement between modeled and observed plasma densities, though poor reproduction of measured H+ and H3+ variability and an overabundance of modeled H3+ potentially hint at missing physical processes in the model, including a loss process that affects H3+ but not H+. Plain Language Summary: Cassini's Grand Finale enabled the first‐ever direct measurements of Saturn's upper atmosphere. Here we use Cassini's unique measurements to construct new models of the plasma in this important boundary region that separates the dense lower atmosphere from space. Based on the complex array of observed gases, we find that heavy molecular ions are dominant near Saturn's equator. This surprising result demonstrates that the chemistry in Saturn's equatorial upper atmosphere is substantially more complex than anticipated. The presence of these unexpected ions potentially represents a new method of monitoring Saturn's ionosphere remotely. Furthermore, as other Cassini measurements indicate that the complex chemistry is likely driven by an influx of ring‐derived material, such observations may even help to track the evolution of Saturn's rings as they lose mass to its atmosphere. Key Points: Unexpectedly complex influx of ring material is found in Saturn's equatorial upper atmosphere, including organics, water, and nanograinsRing influx leads to reduction in major ions (H+ and H3+); heavier molecular ions dominate Saturn's low‐altitude equatorial ionosphereMajor molecular ions at low‐altitude are still uncertain but are likely to include H3O+ and HCO+, and the mean modeled ion mass is 11 Da [ABSTRACT FROM AUTHOR]

A new composite analysis of ∼13 years of Cassini's Charge Energy Mass Spectrometer measurements, presented by Allen et al. (2018, https://doi.org/10.1029/2018JA025262), suggests that the asymmetrically distributed suprathermal He++ in Saturn's magnetosphere is due to Dungey cycle related reconnection and Kelvin‐Helmholtz instabilities that provide paths to the solar wind plasma to reach distances even deep in the inner magnetosphere and accumulate in the ring current region, together with the internally sourced plasma. The results provide a significant addition to the ongoing discussions concerning the contribution of the solar wind input to the dynamics of an otherwise rotationally driven system, such as Saturn's magnetosphere. Key Points: Detection of He++ in Saturn's magnetosphere points to solar wind entry due to Dungey cycle reconnection and Kelvin‐Helmholtz instability [ABSTRACT FROM AUTHOR]

Abstract: We report on a set of clear and abrupt decreases in the high‐frequency boundary of whistler mode emissions detected by Cassini at high latitudes (about ±40°) during the low‐altitude proximal flybys of Saturn. These abrupt decreases or dropouts have start and stop locations that correspond to L shells at the edges of the A and B rings. Langmuir probe measurements can confirm, in some cases, that the abrupt decrease in the high‐frequency whistler mode boundary is associated with a corresponding abrupt electron density dropout over evacuated field lines connected to the A and B rings. Wideband data also reveal electron plasma oscillations and whistler mode cutoffs consistent with a low‐density plasma in the region. The observation of the electron density dropout along ring‐connecting field lines suggests that strong ambipolar forces are operating, drawing cold ionospheric ions outward to fill the flux tubes. There is an analog with the refilling of flux tubes in the terrestrial plasmasphere. We suggest that the ring‐connected electron density dropouts observed between 1.1 and 1.3 Rs are connected to the low‐density ring plasma cavity observed overtop the A and B rings during the 2004 Saturn orbital insertion pass. [ABSTRACT FROM AUTHOR]

Abstract: We present images of Saturn from the final phases of the Cassini mission, including images with 0.5 km per pixel resolution, as high as any Saturn images ever taken. Notable features are puffy clouds resembling terrestrial cumulus, shadows indicating cloud height, dome and bowl shaped cloud structures indicating upwelling and downwelling in anticyclones and cyclones respectively, and filaments, which are thread‐like clouds that remain coherent over distances of 20,000 km. From the coherence of the filaments, we give upper bounds on the diffusivity and kinetic energy dissipation. A radiative transfer analysis by Sanz‐Requena et al. (2018) indicates that methane‐band imagery is most useful in determining cloud and haze properties in the 60–250 mbar pressure range. Our methane‐band imagery finds haze in this pressure range covering 64°‐74°planetocentric latitude. Filaments lie within the haze, and cumulus clouds lie below it, but pressure levels are uncertain below the 250 mbar level. [ABSTRACT FROM AUTHOR]

Abstract: Using images from the Cassini spacecraft, we analyzed three ribbon waves in Saturn's 42°N eastward jet at 45°N, 42°N, and 39°N planetocentric latitudes. In this report, we demonstrate that the morphology, wavelength, and propagation of the ribbon waves are consistent with barotropic Rossby waves with a smaller baroclinic component. We report on the appearance and disappearance of these waves during Cassini's mission. We suggest that the temporal evolution of these waves are related to the great Saturn storm of 2010–2011. [ABSTRACT FROM AUTHOR]

Cassini has recently completed its 13-year mission at Saturn leaving a vast data set. A large interest among the scientific community is to investigate plasma waves and instabilities at Saturn. It is no longer feasible to manually search through Cassini’s vast data set to identify all such waves of interest. Thus, the feasibility of using artificial neural networks (ANNs) to identify plasma waves at Saturn is demonstrated using Cassini data. A convolutional neural network (CNN) was trained to identify low-frequency plasma waves that occur in the upstream region of Saturn using images constructed from the Cassini magnetometer time series data. By systematically varying the network architecture during training and validation, a CNN was obtained that can identify upstream waves with an accuracy of 94% ± 2%. The CNN’s high accuracy for wave identification demonstrates that it is, in fact, feasible to use ANNs to identify plasma waves at Saturn and by extension in other planetary and lunar plasma environments using spacecraft data. [ABSTRACT FROM AUTHOR]

Abstract: 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. [ABSTRACT FROM AUTHOR]

Abstract: Reconnection is a fundamentally important process in planetary magnetospheres, with both local and global effects. At Saturn, observations of the magnetotail reconnection site (or x‐line) are rare, with only one in situ encounter reported to date. In this work, an extensive database of plasmoids and dipolarizations (Smith et al., 2016, https://doi.org/10.1002/2015JA022005) was investigated from a multi‐instrument perspective in order to probe the location and variability of the magnetotail x‐line. Several clear intervals were identified in which the x‐line location could be indirectly inferred to move on relatively short timescales. Two case studies are presented, the first of which concerns short‐lived flows, suggesting the reconnection sites can be either short‐lived (∼10 minutes) or extremely azimuthally limited (∼3RS/0.4 hr of local time). The second interval concerns the tailward motion of the reconnection site (or sites), inferred from the increasing electron temperature (and diminishing electron density) associated with the flows. This tailward motion occurs over ∼2.5 hr (approximately a quarter of a planetary rotation). The composition of the suprathermal plasma suggests that this could be an example of the gradual depletion of mass‐loaded flux tubes (that must occur prior to lobe reconnection). These case studies are consistent with previous statistical work that suggested that the site of reconnection in the Kronian magnetotail can be highly dynamic. [ABSTRACT FROM AUTHOR]

Abstract: We have used data from the Cassini Composite Infrared Spectrometer to map the temperatures in Saturn's polar cyclones at the highest spatial resolution obtained during the Cassini mission. We find temperature contrasts of 7 K in the upper troposphere within 1.4° of both poles, roughly 50 percent larger than earlier measurements at lower spatial resolution. The polar hot spots weaken with depth, disappearing near 500 mbar. In the stratosphere, the polar hot spot becomes broader, extending 4° from the poles, and weakens with altitude disappearing near 1 mbar. A thermal relaxation model shows that the tropospheric hot spot is consistent with adiabatic heating from subsidence with a vertical velocity of about −0.05 mm/s above 500 mbar. The observed temperature gradients imply that the winds in the polar cyclone decay with increasing altitude over roughly three pressure scale heights above the 200‐mbar level. [ABSTRACT FROM AUTHOR]

Plasma composition observations provide a useful mechanism for investigating plasma sources and subsequent evolution within planetary magnetospheres. While He++ is the second most abundant ion species in the solar wind, there are no known sources of He++ ions within the magnetosphere of Saturn, allowing He++ to serve as a tracer of solar wind ions within the Kronian magnetosphere. Meanwhile, water group ions (W+, consisting of O+, OH+, H2O+, and H3O+) and H2+, known to originate within the magnetosphere of Saturn, serve as a tracer of internally ionized plasma. In this paper, we investigate the relative abundances and properties of energetic (32–220 keV) ion species originating from sources within the magnetosphere and from the solar wind. Solar wind‐originating ions are observed to have significant relative abundance (up to ~0.05) in the midnight, dawn, and noon local time quadrants at high radial distances (~40, ~45, and ~20 RS, respectively). Several possible entry processes, such as reconnection and Kelvin‐Helmholtz instabilities, are outlined in this paper as well as a discussion of subsequent transport. Plain Language Summary: While the sources of plasma within the magnetosphere of Saturn have long focused on internal generation, this study seeks to estimate the amount of plasma entering into the magnetosphere from the solar wind. Using a mission averaged vantage point of the global relative abundances of 32 to 220 keV ions, the internal to external plasma is investigated in order to estimate the relative amount of solar wind‐originating ions within the magnetosphere of Saturn, as well as the regions of their importance. Solar wind‐originating ions are predominantly seen within the midnight and dawn quadrants of Saturn, while the dayside and inner magnetosphere is dominated by internally generated plasma. The energetic plasma is also seen to peak in density near a radial distance of 10 RS as a consequence of loss processes closer to the planet and radial diffusion further from Saturn. Key Points: Energetic (32‐220 keV) H+, H2+, He+, He++, and W+ densities are used to investigate internal versus external plasma sourcesEnergetic solar wind He++ observed in the magnetotail suggest access through Kelvin‐Helmholtz Instabilities and reconnection processesBoth internally and externally generated plasma species observe a peak in density at a radial distance of ~10 RS [ABSTRACT FROM AUTHOR]

Abstract: We investigate the azimuthal magnetic field signatures associated with high‐latitude field‐aligned currents observed during Cassini's F‐ring orbits (October 2016–April 2017). The overall ionospheric meridional current profiles in the northern and southern hemispheres, that is, the regions poleward and equatorward of the field‐aligned currents, differ most from the 2008 observations. We discuss these differences in terms of the seasonal change between data sets and local time (LT) differences, as the 2008 data cover the nightside while the F‐ring data cover the post‐dawn and dusk sectors in the northern and southern hemispheres, respectively. The F‐ring field‐aligned currents typically have a similar four current sheet structure to those in 2008. We investigate the properties of the current sheets and show that the field‐aligned currents in a hemisphere are modulated by that hemisphere's “planetary period oscillation” (PPO) systems. We separate the PPO‐independent and PPO‐related currents in both hemispheres using their opposite symmetry. The average PPO‐independent currents peak at ~1.5 MA/rad just equatorward of the open closed field line boundary, similar to the 2008 observations. However, the PPO‐related currents in both hemispheres are reduced by ~50% to ~0.4 MA/rad. This may be evidence of reduced PPO amplitudes, similar to the previously observed weaker equatorial oscillations at similar dayside LTs. We do not detect the PPO current systems' interhemispheric component, likely a result of the weaker PPO‐related currents and their closure within the magnetosphere. We also do not detect previously proposed lower latitude discrete field‐aligned currents that act to “turn off” the PPOs. [ABSTRACT FROM AUTHOR]

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Abstract: We present a statistical study of dipolarization fronts within Saturn's magnetotail. Automated methods were used to identify 28 significant southward rotations of the field coupled with enhancements in the electron energy. The observed dipolarizations cover the majority of the magnetotail, though possess a strong dawn‐dusk asymmetry (79% occur postmidnight). Almost half (43%) of dipolarizations occur within 3 hr of another event, though these chains are solely observed postmidnight. Most pitch angle distributions of the heated electron populations show increased relative fluxes parallel or perpendicular to the field, likely due to nonlocal heating effects. The electron temperature and density following the passage of a front are anticorrelated; the temperature increases are accompanied by a decrease in their density. The temperature increases by factors of 4–12, while the density drops by factors of 3–10. Premidnight events consistently show the smallest relative heating and density depletion, suggesting they are observed closer to their generation. In contrast, the location of the postmidnight x‐line is inferred to be more variable, with a large variety of heating factors observed. Forty percent of the events show a strong reduction in water (W+) group fraction, likely related to either the preferential loss of equatorial heavy ions in departing plasmoids or the closure of open field. Two of these events show significant compositional changes suggesting the addition of plasma of external origin; we suggest that these events involved the closure of open field. [ABSTRACT FROM AUTHOR]

Advances in sustainable, resilient building can have little impact unless they are both affordable and easily repeatable. The SU+RE House was conceived with both of these imperatives firmly in mind. Guest-Editor Clarke Snell, who was in the project's leadership group, highlights its emphasis on familiar rather than ultrahigh- tech materials, both minimising costs and facilitating the construction process. Describing its mechanisms in detail, he reports on how its flood-proofing system was inspired by existing marine-industry technology, with elements designed as standalone products that could be used in other settings. [ABSTRACT FROM AUTHOR]

Abstract: Thermal infrared spectra acquired by Cassini/Composite InfraRed Spectrometer (CIRS) in limb‐viewing geometry in 2015 are used to derive 2‐D latitude‐pressure temperature and thermal wind maps. These maps are used to study the vertical structure and evolution of Saturn's equatorial oscillation (SEO), a dynamical phenomenon presenting similarities with the Earth's quasi‐biennal oscillation (QBO) and semi‐annual oscillation (SAO). We report that a new local wind maximum has appeared in 2015 in the upper stratosphere and derive the descent rates of other wind extrema through time. The phase of the oscillation observed in 2015, as compared to 2005 and 2010, remains consistent with a ∼15 year period. The SEO does not propagate downward at a regular rate but exhibits faster descent rate in the upper stratosphere, combined with a greater vertical wind shear, compared to the lower stratosphere. Within the framework of a QBO‐type oscillation, we estimate the absorbed wave momentum flux in the stratosphere to be on the order of ∼7 × 10−6 N m−2. On Earth, interactions between vertically propagating waves (both planetary and mesoscale) and the mean zonal flow drive the QBO and SAO. To broaden our knowledge on waves potentially driving Saturn's equatorial oscillation, we searched for thermal signatures of planetary waves in the tropical stratosphere using CIRS nadir spectra. Temperature anomalies of amplitude 1–4 K and zonal wave numbers 1 to 9 are frequently observed, and an equatorial Rossby (n = 1) wave of zonal wave number 3 is tentatively identified in November 2009. [ABSTRACT FROM AUTHOR]

Abstract: We present statistical results from the Cassini Radio and Plasma Wave Science (RPWS) Langmuir probe measurements recorded during the time interval from orbit 3 (1 February 2005) to 237 (29 June 2016). A new and improved data analysis method to obtain ion density from the Cassini LP measurements is used to study the asymmetries and modulations found in the inner plasma disk of Saturn, between 2.5 and 12 Saturn radii (1 RS=60,268 km). The structure of Saturn's plasma disk is mapped, and the plasma density peak, nmax, is shown to be located at ∼4.6 RS and not at the main neutral source region at 3.95 RS. The shift in the location of nmax is due to that the hot electron impact ionization rate peaks at ∼4.6 RS. Cassini RPWS plasma disk measurements show a solar cycle modulation. However, estimates of the change in ion density due to varying EUV flux is not large enough to describe the detected dependency, which implies that an additional mechanism, still unknown, is also affecting the plasma density in the studied region. We also present a dayside/nightside ion density asymmetry, with nightside densities up to a factor of 2 larger than on the dayside. The largest density difference is found in the radial region 4 to 5 RS. The dynamic variation in ion density increases toward Saturn, indicating an internal origin of the large density variability in the plasma disk rather than being caused by an external source origin in the outer magnetosphere. [ABSTRACT FROM AUTHOR]

Although auroral emissions at giant planets have been observed for decades, the physical mechanisms of aurorae at giant planets remain unclear. One key reason is the lack of simultaneous measurements in the magnetosphere while remote sensing of the aurora. We report a dynamic auroral event identified with the Cassini Ultraviolet Imaging Spectrograph (UVIS) at Saturn on 13 July 2008 with coordinated measurements of the magnetic field and plasma in the magnetosphere. The auroral intensification was transient, only lasting for ∼30 min. The magnetic field and plasma are perturbed during the auroral intensification period. We suggest that this intensification was caused by wave mode conversion generated field-aligned currents, and we propose two potential mechanisms for the generation of this plasma wave and the transient auroral intensification. A survey of the Cassini UVIS database reveals that this type of transient auroral intensification is very common (10/11 time sequences, and ∼10% of the total images). [ABSTRACT FROM AUTHOR]

The small (approximately 500 km in diameter) satellite Enceladus is moving near the equatorial plane and deep in the gravity field of its parent planet Saturn. Owing to tidal interaction with its parent, Enceladus has adopted a pronounced 3-axial ellipsoidal shape and is tidally locked, with rotational and orbital periods of about 1.37 days. As the equator of Saturn is inclined to the planet's orbital plane, Enceladus, like most of the other satellites of Saturn, undergoes pronounced seasons. This paper gives a summary of the current status as well as shortcomings of our current knowledge regarding Enceladus' geodetic and dynamic parameters. [ABSTRACT FROM AUTHOR]