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The Cassini spacecraft performed several flybys of Saturn's largest moon, Titan, collecting valuable data. During several passes, altimetric data were acquired. Here, we focus on altimetric measurements collected by Cassini's radar when flying over the same region at different epochs in order to correlate such measurements (crossovers) and investigate differences in altimetry. In our study, we assess altimetric errors associated with three distinct methods for extracting topography from Cassini's radar data: the maximum likelihood estimator (MLE), the threshold method, and the first moment technique. Focusing on crossover events, during which Cassini revisited specific areas of Titan's surface, we conduct a detailed examination of the consistency and accuracy of these three topography extraction methods. The proposed analysis involves closely examining altimetric data collected at different epochs over identical geographical regions, allowing us to investigate potential errors due to the variations in off-nadir angle, relative impact, uncertainties, and systematic errors inherent in the application of these methodologies. Our findings reveal that the correction applied for the off-nadir angle to the threshold and first moment methods significantly reduces the dispersion in the delta difference at the crossover, resulting in a dispersion of the order of 60 m, even lower than what is achieved with the MLE (~70 m). Additionally, an effort is made to assess the potential of Cassini for estimating the tidal signal on Titan. Considering the altimetric errors identified in our study and the relatively low number of crossovers performed by Cassini, our assessment indicates that it is not feasible to accurately measure the tidal signal on Titan using the currently available standard altimetry data from Cassini. Our assessment regarding the accuracy of the Cassini altimeter provides valuable insights for future planetary exploration endeavors. Our study advances the understanding of Titan's complex landscape and contributes to refining topographical models derived from Cassini's altimetry observations. These insights not only enhance our knowledge of Saturn's largest moon but also open prospects for Titan surface and interior exploration using radar systems. [ABSTRACT FROM AUTHOR]

The Cassini spacecraft performed several flybys of Saturn’s largest moon, Titan, collecting valuable data. During several passes, altimetric data were acquired. Here, we focus on altimetric measurements collected by Cassini’s radar when flying over the same region at different epochs in order to correlate such measurements (crossovers) and investigate differences in altimetry. In our study, we assess altimetric errors associated with three distinct methods for extracting topography from Cassini’s radar data: the maximum likelihood estimator (MLE), the threshold method, and the first moment technique. Focusing on crossover events, during which Cassini revisited specific areas of Titan’s surface, we conduct a detailed examination of the consistency and accuracy of these three topography extraction methods. The proposed analysis involves closely examining altimetric data collected at different epochs over identical geographical regions, allowing us to investigate potential errors due to the variations in off-nadir angle, relative impact, uncertainties, and systematic errors inherent in the application of these methodologies. Our findings reveal that the correction applied for the off-nadir angle to the threshold and first moment methods significantly reduces the dispersion in the delta difference at the crossover, resulting in a dispersion of the order of 60 m, even lower than what is achieved with the MLE (~70 m). Additionally, an effort is made to assess the potential of Cassini for estimating the tidal signal on Titan. Considering the altimetric errors identified in our study and the relatively low number of crossovers performed by Cassini, our assessment indicates that it is not feasible to accurately measure the tidal signal on Titan using the currently available standard altimetry data from Cassini. Our assessment regarding the accuracy of the Cassini altimeter provides valuable insights for future planetary exploration endeavors. Our study advances the understanding of Titan’s complex landscape and contributes to refining topographical models derived from Cassini’s altimetry observations. These insights not only enhance our knowledge of Saturn’s largest moon but also open prospects for Titan surface and interior exploration using radar systems.

A mirror mode wave is a fundamental magnetic structure in the planetary space environment that is persistently compressed by solar wind, especially in the magnetosheath. Mirror modes have been widely identified in the magnetosheaths of the Earth and other planets in the solar system, yet the understanding of mirror mode waves on extraterrestrial planets is not as comprehensive as that on the Earth. Using magnetic field data collected by the Cassini spacecraft, we found peak and dip types according to the magnetic morphology (i.e., structures with higher or lower magnetic strengths than the background field). Moreover, mirror mode waves and electromagnetic ion cyclotron waves were found one after the other, implying that the two wave modes may evolve into one another in the Kronian magnetosheath. The results indicate that many fundamental plasma processes associated with the mirror mode structure exist in the Kronian magnetosheath. The energy conversion in Saturn’s magnetosheath may provide key insights that will aid in understanding giant planetary magnetospheric processes.

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]

Wavelike perturbations in the ionosphere of Titan, the largest satellite of Saturn, are explored based on the Cassini Ion Neutral Mass Spectrometer (INMS) measurements. Strong wavelike perturbations are identified for more than twenty ion species, from simple ones such as N+ and CH4+ to complex ones such as C2H3CNH+ and C4H7+. Simultaneous wavelike perturbations in background N2, indicative of atmospheric gravity waves, are also observed, motivating us to speculate that the INMS-derived ion perturbations are wave-driven. The amplitudes of the ion perturbations are found to be larger than that of the N2 perturbations. Clear compositional variation is revealed by the data: heavier ion species exhibit greater amplitudes. Such observations might be understood based on considerations either of force balance or chemical loss in Titan’s ionosphere.

The observations of Southwood et al. (2021, https://doi.org/10.1029/2020JA028473), using data from the Cassini magnetometer from the final (proximal) orbits of the mission at Saturn, show large scale azimuthally polarized magnetic signals are always present near periapsis. The signals were attributed to standing Alfvén waves excited on the magnetic shells planetward of the Saturn D‐ring. The apparent absence of any systematic variation in frequency as the spacecraft crossed magnetic shells, implied that the signals were not simply locally excited standing Alfvén modes, but were pumped by coupling to global compressional eigenmodes excited in a cavity formed in the dayside magnetosphere. In this study, we use a numerical magnetohydrodynamic (MHD) model to test such theoretical explanations for the observations, by examining in detail the MHD wave coupling and large scale spatial structure of the signals. The modeling not only shows good agreement with the data but further provides new insight into features previously overlooked in the data. In particular, we show how the apparent frequency of a single spacecraft observation is affected by the phase variation present in a local field line resonance. Plain Language Summary: On the final orbits of the Cassini Saturn Orbiter, one surprise was the discovery of large magnetic oscillations perpendicular to the background planetary field. The disturbances were present each time the spacecraft passed planetward of the innermost rings. This study models what a spacecraft would see if the signals were large scale magnetohydrodynamic waves excited within a high‐density plasma. The strong alignment of the magnetic oscillations transverse to the background led to the original proposal that the signals are highly localized Alfvén waves in what is known as field line resonance. The simulations, however, reveal subtle aspects of the overall picture and how the system is being excited. In particular, it is shown that on the quasi‐polar orbits such as Cassini followed one expects some unexpected features, such as the signal frequency detected in the largest field component differs between hemispheres. Moreover, the frequency detected on the spacecraft might substantially differ in field components in different directions. The conclusion that the signals are resonantly excited Alfvén waves hold up but the simulations show that such excitation must have a complicated spatial amplitude and phase structure. A re‐examination of the Cassini data has revealed some of the effects discovered in the computer simulations. Key Points: Simulation results are used to test and validate the interpretation by Southwood et al. (2021, https://doi.org/10.1029/2020JA028473) wave observations at SaturnThe spatial phase structure in field line resonances can shift the observed frequency on a moving spacecraft between in/outbound passesWe highlight how such frequency changes can be accounted for in both the spacecraft and simulation data [ABSTRACT FROM AUTHOR]

The Cassini spacecraft made in‐situ measurements of Titan's plasma environment during 126 close encounters between 2004 and 2017. Here we report on observations from the Radio and Plasma Waves System/Langmuir probe instrument (RPWS/LP) from which we have observed, primarily on the outbound leg, a localized increase of the electron density by up to 150 cm−3 with respect to the background. This feature, appearing as an electron density spike in the data, is found during 28 of the 126 flybys. The data from RPWS/LP, the electron spectrometer from the Cassini Plasma Spectrometer package , and the magnetometer is used to calculate electron densities and magnetic field characteristics. The location of these structures around Titan with respect to the nominal corotation direction and the sun direction is investigated. We find that the electron density spikes are primarily observed on the dayside and ramside of Titan. We also observe magnetic field signatures that could suggest the presence of current sheets in most cases. The density spikes are extended along the trajectory of the spacecraft with the horizontal scale of ∼537 ± 160 km and vertical scale ∼399 ± 163 km. We suggest that the density spikes are formed as a result of the current sheet formation. Key Points: Electron density spikes of up to ∼150 cm−3 from RPWS/LP data are observed during the outbound leg of the Cassini's Titan encountersThe correlation between the density spikes and a decrease of the magnetic field strength is consistent with the presence of a current sheetThe analysis of 126 encounters reveals the presence of these enhancements in 28 cases [ABSTRACT FROM AUTHOR]

The recently published Interstellar Probe (ISP) study report describes a pragmatic mission concept with a launch window that starts in 2036 and is expected to reach several hundreds of astronomical units past the heliopause within a time frame of ≥50 years (https://interstellarprobe.jhuapl.edu/Interstellar-Probe-MCR.pdf). Following the ISP report, this paper, that will also be accessible from the Bulletin of the AAS (BAAS) in the framework of the Decadal Survey for Solar and Space Physics (Heliophysics) 2024–2033 (Dialynas et al., A future Interstellar Probe on the dynamic heliosphere and its interaction with the very local interstellar medium: In-situ particle and fields measurements and remotely sensed ENAs, 2022a), aims to highlight the importance of studying the physics of the interactions pertaining to the expanding solar wind that meets the plasma, gas and dust flows of the very local interstellar medium, forming the complex and vast region of our astrosphere. We focus on three fundamental open science questions that reveal the dynamical nature of the heliosphere A) Where are the heliosphere boundaries and how thick is the heliosheath B) Is there a “missing” pressure component towards exploring the dynamics of the global heliosheath and its interaction with the very local interstellar medium C) Why does the shape and size of the global heliosphere appear different in different Energetic Neutral Atom energies? We argue that these questions can only be addressed by exploiting a combination of in-situ charged particle, plasma waves and fields measurements with remotely sensed Energetic Neutral Atoms that can be measured simultaneously from the instruments of a future Interstellar Probe mission, along its trajectory from interplanetary space through the heliosheath and out to the very local interstellar medium.

Many quaternions with the coefficients selected from special integer sequences such as Fibonacci and Lucas sequences have been investigated by a great number of researchers. This article presents new classes of quaternions whose components are composed of symmetrical hyperbolic Fibonacci functions. In addition, the Binet's formulas, certain generating matrices, generating functions, Cassini's and d'Ocagne's identities for these quaternions are given.

The exploration of interplanetary space and our solar bubble, the heliosphere, has made a big leap over the past two decades, due to the path-breaking observations of the two Voyager spacecraft, launched more than 44 years ago. Their in-situ particle and fields measurements were complemented by remote observations of 5.2 to 55 keV Energetic Neutral Atoms (ENA) from the Cassini mission (Ion and Neutral Camera-INCA), revealing a number of previously unanticipated heliospheric structures such as the "Belt", a region of enhanced particle pressure inside the heliosheath. The Suprathermal Time Of Flight (HSTOF) instrument on the Solar and Heliospheric Observatory (SOHO) also provided information of 58–88 keV ENAs from the heliosphere. In this chapter we provide a brief discussion for the contribution of the Voyager 1 and 2 Low Energy Charged Particle (LECP) observations that provided "ground truth" to the ENA images from Cassini/INCA towards addressing fundamental questions for the heliosphere's interaction with the Very Local Interstellar Medium. [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]

We use hyperspectral-imaging observations from the Visual and Infrared Mapping Spectrometer (VIMS) to identify and explain processes of Titan's equator through the qualitative spatial relationships between geographic features (i.e. geospatial-topology). Our geographic features are defined by their spectra and geomorphology. We use tens of millions of VIMS pixels between 30°S and 30°N with incidence and emission angles <75°, and a pixel spatial scale of 200 km or less. Our dataset is several orders of magnitude larger than previous studies. This is possible through our use of novel techniques to reduce scattering and improve inter-flyby comparisons. We validate the dataset produced by these techniques by reproducing the results of previous studies. We use vector quantization, dimension reduction, and the Monte Carlo method to identify 14–16 spectrally and spatially distinct units within our dataset, a priori maps or images. These spectral units occur in distinct sequences, indicating that there is a discrete number of spectral pathways to describe the transitions across the surface. Using the same methodology used to identify the spectral units, we determine the spectral transition between units can be explained by five spectroclines. We define a spectrocline as the geographic expression of change in spectra between two spectrally distinct units; it is the spectral equivalent of an ecocline. We compare the spectra of the five spectroclines to the USGS spectral library to identify candidates for the change in compositions across the equator. We find evidence among the spectra of the spectroclines for changes in abundance of water-ice, acetylene, benzene, and alkane species. With the help of the geomorphological units identified in Cassini RADAR images, we discuss the significance of the spectral units and the spectroclines based on their geospatial-topology including their distribution, frequency, size, patterns, and sequences. From the correlations we identify between the spectra and geomorphology, we propose mechanisms for the formation and evolution of Titan's equatorial surface features. •.At Titan's equator, variance in the spectra can be explained by 14–16 units and in the spectral gradient by 5 spectroclines. • Compositional candidatesfor changing spectra are identified including water-ice, acetylene, benzene, and alkanes. • Hummock rich plains, like Xanadu, show evidence of being more organic rich than the undifferentiated plains on the surface. • Windward vs. leeward spectral topology shows aeolian transport effects on equatorial evolution. [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.

Titan's ionosphere hosts a globally distributed non‐trivial dusty ion‐ion plasma, providing an environment for studies of dusty ionospheres that is in many aspects unique in our solar system. Thanks to the Cassini mission, Titan's ionosphere also features one of the largest dusty plasma data sets from 126 flybys of the moon over 13 years, from 2004 to 2017. Recent studies have shown that negatively charged dust dramatically alters the electric properties of plasmas, in particular planetary ionospheres. Utilizing the full plasma content of the moon's ionosphere (electrons, positive ions, and negative ions/dust grains), we derive the electric conductivities and define the conductive dynamo region. Our results show that using the full plasma content increases the Pedersen conductivities at ∼1,100–1,200 km altitude by up to 35% compared to the estimates using only electron densities. The Hall conductivities are in general not affected but several cases indicate a reverse Hall effect at ∼900 km altitude (closest approach) and below. The dayside conductivities are shown to be factor ∼7–9 larger than on the nightside, owing to higher dayside plasma densities. Plain Language Summary: Titan (largest moon of Saturn) is famous for its signature orange haze, formed in the top layer of its atmosphere–ionosphere. The complex organic chemistry initiated mainly by sunlight forms grains of dust that at ∼1,000 km altitude reach a few nanometers in size (comparable to finely ground flour). In the ionosphere, these grains of dust absorb the free electrons (depleting them) and become charged. Below ∼1,000 km altitude there is very little electrons and the plasma consists primarily of ions–called "ion‐ion" or "dusty" plasma. In the absence of light electrons (negative charge), the positively charged ions instead become the dominant mobile charge carrier, as the negatively charged dust is much heavier. Such a reversal of charge mobility has a large impact on the electric properties of an ionosphere, increasing its electric conductivity and changing the direction of its electric currents. We use a Cassini mission data set spanning an entire solar cycle, nearly half a Titan year (≈15 Earth years), to calculate the electric conductivities of Titan's ionosphere and show that dusty plasma typically contributes up to 35%. We also find indications of the charge mobility reversal below 1,000 km although it is not a persistent feature. Key Points: Titan's ionospheric conductivities from in‐situ measurements with full plasma content (electrons, pos. ions, neg. ions/dust grains)Indications of reverse Hall effect near ∼900 km altitudeMost impact of dusty plasma at ∼1,100–1,200 km altitude—up to 35% increase for Pedersen conductivities [ABSTRACT FROM AUTHOR]

Auroral Kilometric Radiation (AKR) is terrestrial radio emission that originates in particle acceleration regions along magnetic field lines, coinciding with discrete auroral arcs. AKR viewing geometry is complex due to the confinement of the source regions to nightside local times (LTs) and the anisotropy of the beaming pattern, so observations are highly dependent on spacecraft viewing position. We present a novel, empirical technique that selects AKR emission from observations made with the spin‐axis aligned antenna of the Wind/WAVES instrument, based on the rapidly varying amplitude of AKR across spacecraft spin timescales. We apply the technique to Wind/WAVES data during 1999 day of year 227–257, when the Cassini spacecraft flew past Earth and provided an opportunity to observe AKR from two remote locations. We examine the AKR flux and power, with observations made from LTs of 1700–0300 hr having an average power up to 104 Wsr‐1 larger than those on the dayside and an increasing AKR power observed at higher magnetic latitudes. We perform a linear cross‐correlation between the Wind AKR power and the spacecraft magnetic latitude, showing positive then negative correlation as Wind travels from the Northern to Southern magnetic hemisphere. Statistically significant diurnal modulations are found in the whole 30‐day period and in subsets of the data covering different local time sectors, indicative of a predominantly geometrical effect for remote AKR viewing. The reproduction of well‐known features of the AKR verifies the empirical selection and shows the promise of its application to Wind/WAVES observations. Plain Language Summary: Auroral Kilometric Radiation (AKR) is naturally occurring radio emission from the Earth's Northern and Southern polar regions, which becomes more intense as the aurora brightens. In this work, we examine data from the Wind spacecraft WAVES instrument from a 30‐ day interval in 1999 when a second spacecraft, Cassini, was also flying near Earth and measuring the AKR from a different viewpoint. In this work, we select the AKR using an empirical measure of the variability observed by the WAVES instrument, and compare the distribution and time profile of AKR intensity. Comparing measurements of this radio emission from different spacecraft positions help us to understand how the AKR is best viewed and illustrate the constrained beaming of the emission. This information is important for anyone wanting to attempt to interpret measurements of the AKR. Key Points: Novel, empirically based method to extract Auroral Kilometric Radiation (AKR) from Wind/WAVES is presented and applied to observations made during the Cassini flybySelected data show a distribution of AKR power with expected longitudinal and latitudinal visibility constraintsDiurnal temporal modulation observed, suggesting the dominance of a geometric viewing effect and agreeing with previous AKR observations [ABSTRACT FROM AUTHOR]

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 have used Spacecraft Plasma Interaction Software (SPIS) simulations to study the characteristics (i.e., dimensions, ion depletion, and evolution with the changing spacecraft attitude) of the Cassini ion wake. We focus on two regions, the plasma disk at 4.5–4.7 RS, where the most prominent wake structure will be formed, and at 7.6 RS, close to the maximum distance at which a wake structure can be detected in the Cassini Langmuir probe (LP) data. This study also reveals how the ion wake and the spacecraft plasma interaction have impacted the Cassini LP measurements in the studied environments, for example, with a strong decrease in the measured ion density but with minor interference from the photoelectrons and secondary electrons originating from the spacecraft. The simulated ion densities and spacecraft potentials are in very good agreement with the LP measurements. This shows that SPIS is an excellent tool to use for analyses of LP data, when spacecraft material properties and environmental parameters are known and used correctly. The simulation results are also used to put constraints on the ion temperature estimates in the inner magnetosphere of Saturn. The best agreement between the simulated and measured ion density is obtained using an ion temperature of 8 eV at ∼4.6 RS. This study also shows that SPIS simulations can be used in order to better constrain plasma parameters in regions where accurate measurements are not available. Key Points: The Cassini ion wake is characterized and it is demonstrated that the ion wake can have a substantial impact on the Cassini LP measurementsOur study indicates that earlier estimates of the ion temperature in the inner magnetosphere of Saturn are overestimatedThe software SPIS provides accurate simulation results for typical magnetospheric conditions [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]

Background: The objective of this study was to compare the central corneal curvature and astigmatism measurements obtained with three different systems in healthy eyes and to assess the level of interchangeability between them.Methods: This was a comparative study examining 30 healthy eyes of 30 patients (age 15-53 years). A complete eye examination was performed in all cases including analysis of anterior corneal curvature and astigmatism with three devices: the colour-LED topography system Cassini (i-Optics) (CAS), the Scheimpflug-based system Pentacam (Oculus Optikgeräte) (PTC) and the optical biometer IOL-Master 500 (Carl Zeiss Meditec) (IOLM). Differences between devices in terms of curvature in the flattest (flat K) and steepest meridians (steep K) as well as in the magnitude (AST) and power vector components of astigmatism (J0 and J45 ) were evaluated. The interchangeability between devices was evaluated with the Bland-Altman method.Results: Statistically significant differences between devices were found in steep K and flat K (p < 0.001). No statistically significant differences between devices were found in AST (p = 0.057) and J0 power vector (p = 0.185). However, differences between devices in J45 did reach statistical significance (p = 0.039). Ranges of agreement for curvature measures ranged from 0.123 (flat K, CAS-PTC) to 0.165 mm (steep K, CAS-PTC). Ranges for the magnitude of astigmatism were 0.868, 1.059 and 0.739 D in CAS-IOLM, PTC-IOLM and CAS-PTC comparisons, respectively. For J0 and J45 , ranges of agreement were below 0.522 D.Conclusions: Measurements of central corneal curvature and astigmatism obtained with the three devices evaluated cannot be used interchangeably. [ABSTRACT FROM AUTHOR]

The Saturn‐orbiting Cassini spacecraft completed 126 close Titan flybys from 2004 until 2017. During almost all of them the Cassini Radio and Plasma Wave Science (RPWS) instrument was turned on to search for radio emissions attributed to Titan lightning. Here we report about their nondetection after close inspection of all Titan flybys throughout the Cassini mission. We also infer new and strong constraints on the permissible flash energy and flash rate of potential Titan lightning. The nondetection of lightning flashes by Cassini observations implies that any lightning on Titan must be either very weak, very rare, or does not exist at all, and the latter could be due to cloud electric fields being too low to initiate a discharge. This finding holds important implications for the prebiotic chemistry of Titan and also implies that lightning will not be a significant hazard to the upcoming Dragonfly mission. Plain Language Summary: During its Saturn tour the Cassini spacecraft performed 126 close Titan flybys. The RPWS (Radio and Plasma Wave Science) instrument would have been able to detect radio emissions from potential lightning in Titan's atmosphere, similar to the easy detection of Earth lightning during the Cassini Earth flyby in August 1999. A careful inspection of RPWS data has revealed no radio signals that could be attributed to Titan lightning. The long observation times make it very likely that Titan lightning does not exist, is very weak, or is very rare. Key Points: All Cassini Titan flybys were examined for radio emissions from Titan lightningNo radio emissions from Titan lightning were found in Cassini RPWS dataNew upper limits for potential but unlikely Titan lightning activity were set [ABSTRACT FROM AUTHOR]

Doppler tracking of Solar System probes is used for spacecraft navigation, planetary geodesy, and tests of the theory of General Relativity. The spacecraft radial velocity is measured by observing the Doppler shift of a radio signal transmitted from an Earth station to the spacecraft and then re-transmitted back, while preserving phase coherence, to the same station (two-way link) or to a different station (three-way link). Specialized orbit determination software is then used to reconstruct the spacecraft trajectory and estimate planetary gravity field coefficients or relativistic parameters. The measurement noise is a crucial element for the accuracy of the final estimates, thus considerable effort has been devoted to improve the range rate accuracy by adopting higher frequency links to reduce the dispersive noise from interplanetary and ionospheric plasmas, and by calibrating the tropospheric path delays with microwave radiometers. While Ka-band radio links (32–34 GHz) allowed a successful suppression of plasma noise, reducing tropospheric noise and ground antenna mechanical noise has been more challenging. The Time-Delay Mechanical noise Cancellation (TDMC) technique is a promising method to reduce mechanical and tropospheric noises and to improve further the accuracy of Doppler measurements. The TDMC is a linear combination of simultaneous Doppler data from a main antenna providing the two-way link and a three-way antenna (generally smaller and stiffer). If the listen-only, three-way antenna is also located in a particularly dry site, the TDMC can considerably reduce both tropospheric and antenna mechanical noises, which are the leading disturbances in two-way Ka-band radio links. For an operational test of this method, we applied the TDMC to Doppler data at X-band (7.2–8.4 GHz) from the Cassini spacecraft acquired during the Saturn tour phase of the mission. Although X-band links are generally dominated by the highly-variable interplanetary plasma noise and are not suitable for the TDMC, we found that, when local noises are particularly large at the two-way antenna, this technique may still lead to up to a factor-of-three noise reduction (at 60-s integration time) with respect to the two-way link. The TDMC can maximize the data quality during unique events, mainly planetary or satellite flybys (such as those considered in the Europa Clipper and JUICE missions), where the scientific results could be severely hampered by adverse conditions at the tracking station. • Reducing noise in Doppler links improves the accuracy of the estimated parameters. • With favorable conditions, TDMC technique reduces two-way antenna's local noises. • Simultaneous two-way and three-way links are required to apply the TDMC. • We present examples of successful two-way noise reduction with Cassini Doppler data. [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]

Cassini Radar observations of Titan have revealed diverse landforms resulting from a variety of geologic processes. Many landforms can be unambiguously interpreted as resulting from atmospheric processes (dunes, rivers, and lakes) or impact cratering. Here we argue from morphological evidence such as nested collapses, elevated ramparts, halos, and islands or floor mountains that some of the abundant small depressions in the north polar region of Titan are volcanic collapse craters. A few similar depressions occur near the south pole; the restriction of this volcanism to polar regions is possibly related to predicted warmer and thinner‐than‐normal ice crust at the low‐elevation poles. The close association of the proposed volcanic craters with polar lakes is consistent with a volcanic origin through explosive eruptions, as either maars or calderas. The apparent freshness of some craters may mean that volcanism has been relatively recently active on Titan or even continues today. Plain Language Summary: The Cassini mission revealed many landforms on Saturn's moon Titan that are like those found on Earth. Sand dunes, river valleys, and lakes are all a result of actions by the atmosphere on the surface, driven by solar heating. We demonstrate there is also evidence for internal heat, manifest at the surface as cryovolcanoes, made from melting the water ice crust into liquid water and erupting it onto the surface. These features are roughly round, with raised rims, and they sometimes overlap each other. They are consistent with the shapes of other volcanic landforms on Earth formed by explosion, excavation, and collapse. That these features are at the polar regions, near the lakes of methane, may indicate methane or some other volatile can power them. The features appear relatively fresh, meaning they could still be forming today. Key Points: Landforms with characteristics consistent with a volcanic origin are found at Titan's north polar regionElevated rims, subround shapes, and large sizes indicate an origin by explosion and collapseThe relative youth of the landforms indicates they may be forming today [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]

Ion velocities have been measured during the Enceladus E3 and E5 flybys using the Cassini Plasma Spectrometer (CAPS) instrument on the Cassini spacecraft. Data from three sensors in the CAPS instrument have been examined from two flybys that occurred during 2008. Positive ion measurements from the CAPS Ion Beam Spectrometer and Ion Mass Spectrometer have been used to measure positive ion velocities. The CAPS Electron Spectrometer has been used to complement the positive ion findings with measurements of negative ion velocities. Two velocities for the positive ions are found, with the fast ions (2.3–5.8 km/s) originating from the high‐speed neutral gas emission and slow ions (0.2–2.2 km/s) associated with the low‐speed thermal gas emission from Enceladus. Negative ions were found to be near stationary or northerly traveling, implying a deceleration mechanism within the plume. A tentative detection of fast negative ions was also recorded for one of the flybys. These findings will aid in future modeling of plume dynamics. Key Points: Velocities of positive and negative ions have been measured in the Enceladus plume using CAPS sensorsTwo distinct ion velocities are found, in agreement with fast gas and slow thermal emissions of neutralsThermal negative ions are near stationary relative to Enceladus, implying a deceleration mechanism such as an ambipolar electric field [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]