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Banded structures observed in energetic particle spectrograms in the Earth's inner radiation belt and slot region, that is, "zebra stripes," have been resolved in the Saturnian magnetosphere with Cassini. This study implements a large‐scale statistical analysis of Saturnian zebra stripe properties in association with the noon‐to‐midnight electric field of the inner magnetosphere to which the stripes' origin was recently established. Cassini has detected zebra stripes extending between L‐shells (L) of 5–9 for more than half of the orbits that crossed inward of L = 9. The amplitude of the stripes is 15−20% on average above the background differential energy flux, and their age is estimated to be 20–60 hr. The regular observation of zebra stripes suggests that their regeneration and the corresponding electric field enhancements develop over timescales comparable to their estimated lifetime (days), revealing that internal processes contribute to the electric field dynamics, in addition to a solar wind‐induced variability indicated by previous investigations. The flux‐enhanced stripes are traced back to the dayside, preferentially from postnoon, indicating an electric field orientation from postnoon to postmidnight. Our results further suggest that the electric field's offset from the noon‐midnight line is subject to both L‐shell and temporal dependencies, confirming the previous inferred variability. Key Points: Energy‐discretized electron spectra (zebra stripes) were seen in the majority of Cassini's orbitsZebra stripes develop within tens of hours and spread to the whole inner magnetosphereThe electric field responsible for the stripes has an L‐shell and time‐dependent deviation from the noon‐midnight orientation [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]

Based on the measurements from Radio and Plasma Wave Science instrument onboard the Cassini spacecraft over 13 years, we perform a statistical analysis of electron cyclotron harmonic (ECH) wave spatial distribution in Saturn's magnetosphere. The wave occurrence rates, amplitudes, and peak frequencies as functions of magnetic local time, L‐shell, and magnetic latitude are investigated in detail. Our results confirm that the fundamental band ECH waves mainly occur near the magnetic equator (|λ| < 5°) and also at high latitudes (20° < |λ| ≤ 40°), with averaged amplitudes of about 0.01–0.1 mV/m. By comparison, the amplitudes and occurrence rates of higher harmonics n decrease significantly, and the higher harmonic bands tend to occur at lower L‐shells along with the enhanced day‐night asymmetry. Overall, Saturnian ECH emissions are preferentially a nightside phenomenon and exhibit a gap in the midlatitude regions (5° < |λ| ≤ 20°) for the occurrence rates. The occurrence rates of ECH waves are higher at high latitudes than those near the magnetic equator, but the statistically averaged amplitudes of high latitude emissions are relatively weaker. Furthermore, the averaged peak frequencies of each ECH wave harmonic band are generally below (n + 1/2) fce and have the smallest deviation from (n + 1/2) fce near the equator. These results can be readily adopted as representative inputs to quantify the electron scattering effect of multi‐harmonic Saturnian ECH waves and investigate the underlying contribution of ECH emissions to the evolution of the Saturnian radiation belt. Plain Language Summary: Similar to Earth, Saturn possesses a strong internal magnetic field, which reacts to the solar wind flows to form the magnetosphere. There are a variety of plasma waves in Saturn's magnetosphere, which can either accelerate or decelerate energetic electrons via wave‐particle interaction processes. As an electrostatic wave mode frequently observed in Saturn's magnetosphere, electron cyclotron harmonic (ECH) waves near the half‐integral harmonics of electron cyclotron frequency, that is, (n + 0.5) fce (n = 1, 2, 3), remain rather limitedly understood in the morphological characteristics, including their occurrence pattern, wave amplitudes and frequency spectra. Based on long‐term wave data over 13 years from the Cassini spacecraft, we investigate comprehensively the global distribution properties of Saturian ECH waves within ∼ 5–10 Rs, which are essential to quantify the particular role of ECH waves in the energetic electron dynamics in the Saturnian magnetosphere. Our results clearly indicate that Saturnian ECH waves are more likely to be a nightside phenomenon and occur more frequently at high latitudes than the equatorial regions, while the high‐latitude wave amplitudes are statistically weaker. It is also revealed that in statistics the peak wave frequencies of Saturian ECH emissions are generally below (n + 1/2) fce, with the smallest deviation from (n + 1/2) fce in the near‐equatorial regions. Key Points: The averaged amplitudes of Saturnian electron cyclotron harmonic (ECH) waves have a pronounced magnetic local time asymmetry, and they are larger on the nightside than the daysideThe ECH wave occurrence rates are higher at high latitudes than those near the equator, but high‐latitude averaged amplitudes are weakerThe ECH wave averaged peak frequencies are generally below (n + 1/2) fce, with the smallest deviation from (n + 1/2) fce near the equator [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. [ABSTRACT FROM AUTHOR]

One of the grand challenge problems of the giant planet magnetospheres is the issue of nonadiabatic plasma heating. Simple turbulent heating models consider the energy cascade rate from one scale to another where the energy density is based on perpendicular magnetic fluctuations of counterpropagating Alfvén waves. Analytical expressions from turbulence theory for the heating rate density have yielded promising results for the observed ion heating at Jupiter and Saturn. Here, we compare ion heating using hybrid simulations of the Kelvin--Helmholtz instability and analytical estimates in an effort to validate turbulence theory and further understand the nature of the ion heating. Heating rate densities ~10-15 W/m³ are produced in our three-dimensional Kelvin--Helmholtz simulations during the nonlinear growth phase and compare favorably with analytical estimates. Results targeting Saturn will be discussed in the broader context of radial plasma transport in the rapidly rotating magnetospheres. [ABSTRACT FROM AUTHOR]

Low-frequency waves are closely related to magnetospheric energy dissipation processes. The Cassini spacecraft explored Saturn's magnetosphere for over 13 years, until September 2017, covering a period of more than a complete solar cycle. Using this rich heritage data set, we systematically investigated key physical parameters of low-frequency waves in Saturn's magnetosphere, including their local time distribution and the dependence on solar activity. We found that the wave activity peaked in the near noon sector. For the nightside, the wave intensity also appeared to peak pre and postmidnight. Due to the limited local time coverage for each solar phase, we were not able to draw a firm conclusion on the wave's dependence on solar activity. In general, the wave power showed a monotonically decreasing trend toward larger distances in nightside sectors especially during the declining phase, which implied that low-frequency waves mainly originate from the relatively inner regions of the magnetosphere. On the dayside, stronger waves were mostly located at/within ~25 Rs, near the magnetopause. The study shows a global picture of low-frequency waves in Saturn's magnetosphere, providing important implications for how magnetospheric energy dissipates into Saturn's polar ionosphere and atmosphere. [ABSTRACT FROM AUTHOR]

Energetic particles (>~25 keV) have been observed routinely in the terrestrial magnetosheath, but have not been well studied at the magnetosheaths of the outer planets. Here we analyze energetic electrons and ions (mostly protons) in the vicinity (±1 RS) of Saturn's magnetopause, using particle data acquired with the low-energy magnetosphere measurements system, one of the three sensors of the magnetosphere imaging instrument on board the Cassini spacecraft, during a period of ~14 years (2004--2017). It is found that energetic particles, especially ions, are also common in Saturn's magnetosheath. A clear inward (toward Saturn) gradient in the electron differential flux is identified, suggestive of magnetospheric sources. Such an inward gradient does not appear in some of the ion channels. We conclude that Saturn's magnetopause acts as a porous barrier for energetic electrons and, to a lesser extent, for energetic ions. A dawn-dusk asymmetry in the gradient of particle flux across the magnetopause is also identified, with a gradual decrease at the dawn and a sharp decrease at the dusk magnetopause. It is also found that magnetic reconnection enhanced flux levels just outside of the magnetopause, with evidence suggesting that these particles are from magnetospheric sources. These findings strongly suggest that Saturn's magnetosphere is most likely the main source of energetic particles in Saturn's magnetosheath and magnetosphere leakage is an important process responsible for the presence of the energetic particles in Saturn's magnetosheath. [ABSTRACT FROM AUTHOR]

Based on magnetic field fluctuations, Saturn's magnetosphere can be divided into quiet (little or not much fluctuation) and disturbed periods (large fluctuation of the magnetic field). Kaminker et al. (2017, https://doi.org/10.1002/2016JA023834) showed that the average heating rate density of entire magnetosphere of Saturn is ~10-17 W m-3 based on magnetic field fluctuations. Here, we categorize the magnetosphere of Saturn on the basis of magnetic field fluctuation. Using the eigenvalues of the variance analysis of the magnetic field, it has been found that the magnetodisc is in a disturbed state almost 6%-7% of the time. Dwell time normalization of the disturbed events suggests that most of the events happen at all latitudes between -5° and +30° and mostly on the dayside. Kinetic turbulent heating due to magnetic field fluctuation (dB) could potentially provide a significant amount of the required power. [ABSTRACT FROM AUTHOR]

The moon Dione orbits Saturn at 6.2 Saturn radii RS deep in the Kronian magnetosphere. In situ studies of the moon‐magnetosphere interaction processes near Dione were possible with the Cassini/Huygens mission which flew by close to Dione five times at distances between 99 and 516 km. In addition, Cassini crossed Dione's L‐shell more than 400 times between 2004 and 2017 and documented the variability of Saturn's magnetosphere. Different flyby geometries allowed to study the interaction processes upstream, in the low‐energy wake, and above the north pole of Dione. We describe here the energetic particle measurements from the Low Energy Magnetospheric Measurement System (LEMMS), part of the Magnetosphere Imaging Instrument (MIMI) onboard Cassini. We also use hybrid simulation results from "A.I.K.E.F." to interpret the signatures in the particle fluxes. This paper is a continuation of Krupp et al. (2013, https://doi.org/10.1016/j.icarus.2013.06.007) and Kotova et al. (2015, https://doi.org/10.1016/j.icarus.2015.06.031). The key results are as follows: (1) Saturn's magnetosphere at Dione's orbit is highly variable with changes in energetic charged particle fluxes by 1–2 orders of magnitude. (2) The dropout signatures near Dione are basically consistent with a fully absorbing obstacle, but some features point to more complex interaction processes than plasma and energetic particle absorption. (3) Absorption signatures are found to be asymmetric with respect to the orientation of the moon, indicative of the presence of radial drift components for electrons. (4) The deepest absorption signatures were observed at the edge of the low‐energy wake pointing to gradient‐B drifts strongest in that part of the interaction region. Key Points: Dropout signatures in energetic particle fluxes near Dione are basically consistent with a fully absorbing obstacleAbsorption signatures are found to be asymmetric with respect to the orientation of the moon, pointing to radial drift of electronsThe deepest absorption signatures were observed at the edge of the low‐energy wake pointing to gradient‐B drifts [ABSTRACT FROM AUTHOR]

Enceladus' plume is the dominant source of neutrals and plasma in Saturn's magnetosphere. The plasma results from the ionization of icy particles and water vapor, which are vented into Saturn's inner magnetosphere through fissures in Enceladus' southern polar region. These fissures are subjected to tidal stresses that can vary as Enceladus moves in a slightly eccentric orbit around Saturn. Plume activity and brightness have also been shown to vary with the moon's orbital position, reaching a maximum when Enceladus is farthest away from Saturn in its orbit (the Enceladus orbital apoapsis). In this paper we will show that temporal variations in the thermal electron density distribution correlate with the position of Enceladus in its orbit around Saturn, with the strongest density enhancements in the vicinity of Enceladus when the moon is in the post‐apoapsis sector of its orbit. Key Point: There is evidence of enhanced electron densities in the near‐moon environment when Enceladus is in the post‐apoapsis sector of its orbit [ABSTRACT FROM AUTHOR]

The thickness and variability of the Saturnian magnetodisc current sheet is investigated using the Cassini magnetometer data set. Cassini performed 66 fast, steep crossings of the equatorial current sheet where a clear signature in the magnetic field data allowed for a direct determination of its thickness and the offset of its center. The average, or nominal, current sheet half‐thickness is 1.3 RS, where RS is the equatorial radius of Saturn, equal to 60,268 km. This is thinner than previously calculated, but both spatial and temporal dependencies are identified. The current sheet is thicker and more variable by a factor ∼2 on the nightside compared to the dayside, ranging from 0.5–3 RS. The current sheet is on average 50% thicker in the nightside quasi‐dipolar region (≤15 RS) compared to the dayside. These results are consistent with the presence of a noon‐midnight electric field at Saturn that produces a hotter plasma population on the nightside compared to the dayside. It is also shown that the current sheet becomes significantly thinner in the outer region of the nightside, while staying approximately constant with radial distance on the dayside, reflecting the dayside compression of the magnetosphere by the solar wind. Some of the variability is well characterized by the planetary period oscillations (PPOs). However, we also find evidence for non‐PPO drivers of variability. Plain Language Summary: A key discovery of the Cassini orbital mission at Saturn was that the small moon Enceladus was geologically active. Water from a subsurface ocean escapes as vapor through geysers in the moon's southern hemisphere. A significant portion becomes ionized and interacts with the magnetic environment that surrounds Saturn, called its magnetosphere. Due to the rapid rotation rate of the planet, this layer of charged particles sourced from Enceladus is stretched into a thin disc around Saturn, known as the magnetodisc current sheet. In this study, we explore the thickness of the current sheet, identifying its average structure and potential sources of variability. We find that the current sheet thickness depends on both the distance from Saturn and position in local time with respect to the Sun. We also find that some of the variability about its average state is somewhat described by the presence of a magnetic perturbation system that occurs close to the rotation period of Saturn. These results are essential for accurately modeling the magnetosphere of Saturn and understanding how the system responds to different drivers of variability. Key Points: The median current sheet half‐thickness is 1.3 Saturn radii, but spatial and temporal variability is identifiedThe current sheet is 50% thicker on the nightside close to the planet but reduces more significantly with radial distance than the daysideThe planetary period modulations organize some of the variability, but there is evidence of other temporal drivers [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]

Electron density measurements have been obtained by the Cassini Radio and Plasma Wave Science (RPWS) instrument covering the period from 30 June 2004 to 19 April 2017, spanning latitudes up to ~30° and L values from 2.4 to 10. Near the F ring, electron densities are derived from RPWS measurements of electron plasma oscillations at high latitudes and from the Langmuir Probe (RPWS/LP) sweep data at low latitudes. The electron density measurements from the ring‐grazing orbits, beginning in December 2016, have made it possible to extend the work of a previous diffusive equilibrium model to include the distribution of the ring plasma. Beyond the ring‐grazing orbits, the densities are derived from RPWS measurements of the upper hybrid resonance frequency. These density measurements are used to anchor the fit of an expanded diffusive equilibrium density model for a two‐species plasma consisting of water group and hydrogen ions in Saturn's inner magnetosphere. Density contour plots for the two ion species and the electrons are presented. The distribution of the derived plasma densities is consistent with two primary sources, the Enceladus plume and the extended ring atmosphere. There is also an indication of a weaker plasma source at Dione. In the region just outside the A ring and in the region including the Enceladus orbit, the diffusive equilibrium model also shows the expansion of lighter ions and electrons to higher latitudes along the magnetic field lines with evidence of a weaker plasma expansion between the orbits of Tethys and Dione. Key Point: The diffusive equilibrium density model provides a global map of a two‐species plasma distribution in Saturn's inner magnetosphere [ABSTRACT FROM AUTHOR]

An extensive analysis of Cassini Ion and Neutral Camera (INCA) and Charge Energy Mass Spectrometer (CHEMS) measurements of ~6–231 keV ion anisotropies acquired during selected spin and stare periods for nearly all mission orbits has been completed. Based on this analysis, we find that the computed azimuthal speed of Saturn's magnetodisk plasma increases within the inner and middle magnetosphere. Beyond the orbit of Titan, in the magnetotail, the calculated rotation speed remains roughly constant with increasing distance from Saturn. The component of convection parallel to Saturn's spin axis is smaller and on average nearly zero. The radial speed of plasma shows distinct local time dependence and increases outward with increasing distance down the magnetotail. The magnetodisk flow remains primarily azimuthal to large distances, indicating the plasma is still under the influence of Saturn as it transits across the nightside. Tailward flows have been observed in the region near the dawn and dusk magnetotail flanks. The plasma flow in the predawn quadrant is much more disorganized than that in the premidnight quadrant. The hydrogen and oxygen hot ion temperatures increase with decreasing distance to Saturn. Some plasma evidently escapes into a dusk boundary layer at the dusk magnetotail flank, while the remaining plasma primarily moves across the magnetotail and is entrained into the flow of a boundary layer at the dawn flank of the magnetotail. When scaled to the magnetopause standoff distance and corotation fraction, the convection speed of plasma in the magnetospheres of Jupiter and Saturn is similarly organized. Key Points: Saturn's magnetospheric convection pattern has been extracted from Cassini dataThe escape of plasma from the system occurs primarily at the magnetotail flanksThe magnetotail convection pattern contains a strong dawn‐dusk asymmetry [ABSTRACT FROM AUTHOR]

The energetic electron spectra of Saturn's radiation belts are studied using Cassini's 13 years of measurements by the Magnetosphere Imaging Instrument/Low Energy Magnetospheric Measurement System detector. We find that between L shells (L) of 10 and 4.5 the differential flux spectrum of 0.3‐ to 1.6‐MeV electrons evolves from a single power law to two power law functions separated at an energy cutoff (Ec). We show that inside L∼8, Ec has an L shell dependence that tracks consistently the energy of corotation drift cancelation (or resonance) ECDR, that is, the energy at which magnetic electron drifts and azimuthal corotation cancel, and is not associated with the absorbing effects of Saturn's moons. Ec also deviates from what conservation of the first adiabatic invariant would dictate. Our results verify that electrons around ECDR can be transported radially very efficiently by variable convective flows, such as those related to the noon‐midnight electric field in Saturn's magnetosphere. Plain Language Summary: The electron radiation belt of Saturn is a region that reaches out several planetary radii outward of the main rings and contains significant intensities of relativistic electrons. It can be rather dynamic in terms of particle intensity and spatial extent. How these electrons are supplied is still debated, as several viable physical processes have been proposed. Using Cassini's 13 years of observations, we verified that a special class of energetic electrons, whose westward magnetic drift cancels out with the eastward corotation imposed by the planet, could be transported and accelerated very effectively. At the energy where this drift cancelation occurs, the intensities of energetic electrons show a sharp change in intensity, as would be expected if this transport process dominates over others mechanisms which invoke interactions of electrons with high‐frequency waves. The same acceleration process may be readily applicable to Jupiter, as its electron radiation belts are exposed to a qualitatively similar environment as of Saturn's. This implication would be crucial for establishing realistic, physical electron radiation belt models for the outer planets. Key Points: Electron spectra around 1 MeV are described with two power law functions separated at a cutoff energyThe L shell dependence of the cutoff energy tracks consistently that of corotation drift cancelation (or resonance)Transport and acceleration of ∼1‐MeV electrons is affected by variable convective flows [ABSTRACT FROM AUTHOR]

The large‐scale structure of Saturn's magnetosphere is determined by internal and external factors, including the rapid planetary rotation rate, significant internal hot and cold plasma sources, and varying solar wind pressure. Under certain conditions the dayside magnetospheric magnetic field changes from a dipolar to more disk‐like structure, due to global force balance being approximately maintained during the reconfiguration. However, it is still not fully understood which factors dominantly influence this behavior, and in particular how it varies with local time. We explore this in detail using a 2‐D force‐balance model of Saturn's magnetodisk to describe the magnetosphere at different local time sectors. For model inputs, we use recent observational results that suggest a significant local time asymmetry in the pressure of the hot (>3 keV) plasma population, and magnetopause location. We make calculations under different solar wind conditions, in order to investigate how these local time asymmetries influence magnetospheric structure for different system sizes. We find significant day/night asymmetries in the model magnetic field, consistent with recent empirical studies based on Cassini magnetometer observations. We also find dawn‐dusk asymmetries in equatorial current sheet thickness, with the varying hot plasma content and magnetodisk radius having comparable influence on overall structure, depending on external conditions. We also find significant variations in magnetic mapping between the ionosphere and equatorial disk, and ring current intensity, with substantial enhancements in the night and dusk sectors. These results have consequences for interpreting many magnetospheric phenomena that vary with local time, such as reconnection events and auroral observations. Key Points: We use hot plasma pressure observations to update the UCL/AGA model of Saturn's magnetodiskWe investigate how local time variations in hot plasma and effective disk radius influence the magnetic field structureWe find variability in current sheet thickness, density, and ionospheric field line mappings [ABSTRACT FROM AUTHOR]

We develop a simple physical model of the planetary period oscillations (PPOs) observed in Saturn's magnetosphere. The model couples together flows and currents in five two‐dimensional systems: an upper and lower atmospheric layer in each hemisphere, and the equatorial magnetosphere. We neglect the curved geometry of the planet and the magnetosphere but use a simple scaling argument to account for the very different sizes of the two systems. We show that it is possible to drive a PPO current system with many of the observed properties by invoking a twin vortex system in the Hall conducting layer of either hemisphere. The twin vortex system is able to generate divergent currents because our model includes a representation of the latitudinal variation of conductance. The large inertia and low Pedersen conductance of this layer of the atmosphere means that the twin vortex system and the PPO currents have a very stable rotation velocity and a very long dissipation timescale, explaining the long term persistence and stability of the PPO current systems. For a range of realistic parameters, part of the PPO current system closes through the equatorial magnetosphere, and part through the opposite hemisphere, as observed. We make predictions about the phase relationships between these closure currents that allow our model to be tested. Although the wind speeds required to produce the observed currents are implausibly large, interaction with enhanced electron density in the auroral regions reduces the necessary wind speeds to plausible values. Key Points: We develop a coupled five‐layer linear model of PPO disturbances in Saturn's magnetosphereIt includes a latitude gradient in the Hall conductance, allowing it to drive current divergencesThe model predicts highly stable PPO wave modes with long dissipation timescales [ABSTRACT FROM AUTHOR]

The ring current of Saturn is obtained by taking the curl of the observed magnetic field. An analytical model of this ring current can then be obtained by fitting it to a lognormal in ρ and a Gaussian in z. Local time dependence is included as a harmonic expansion in ϕ for coefficients of the lognormal; radial dependence is included as a quadratic expansion in ρ of the width of the Gaussian. In this model, the ring current peaks at ~0.25 MA/RS2 at ~10 RS on the nightside near ~1 hr. The ring current strength minimizes at ~0.17 MA/RS2 on the dayside near ~16 hr. The cross‐sectional shape of the ring current is a teardrop extending from ~5 to ~20 RS or more. Key Points: Using the curl of the magnetic field obtained from Cassini data, the ring current of Saturn can be determined in three dimensionsAn analytical model of the ring current is constructed by fitting to a lognormal in range and a Gaussian in z, with the fit coefficients dependent on local timeRather than a block shape, the ring current has a tear drop cross‐sectional shape at all local times [ABSTRACT FROM AUTHOR]

Radial transport is an important dynamical process in Saturn's internally driven magnetosphere. Radial transport is presumed to occur by a centrifugally driven interchange instability, determined by the gradient of flux tube content and flux tube entropy. Plasma produced in the inner magnetosphere must be transported radially outward. The outward motion of the plasma stretches the magnetic field lines, leading to magnetic reconnection. Reconnection allows the mass to be transported radially while allowing the flux to circulate back to the inner magnetosphere. Both radial transport of mass and magnetic flux in Saturn's magnetosphere have been estimated based on Cassani Plasma Spectrometer data provided by Wilson et al. (2017, https://doi.org/10.1002/2017JA024117) and suggesting the radial transport rate of plasma of around 55 kg/s. The net magnetic flux transport should be 0, but the data suggest a net outward magnetic flux transport indicating the existence of different possible transport mechanisms in Saturn's magnetodisc. Key Points: We have calculated the radial transport of plasma mass and magnetic flux in Saturn's magnetosphere using Cassini CAPS and magnetometer dataWe found radial transport rate of plasma around 55 kg/s and found net deficit in magnetic flux calculationWe see dawn‐dusk asymmetry in radial flows [ABSTRACT FROM AUTHOR]

Saturn's magnetosphere has been extensively studied over the past 13 years with the now retired Cassini mission. Periodic modulations in a variety of magnetospheric phenomena have been observed at periods close to those associated with the emission intensity of Saturn kilometric radiation (SKR). Resulting from Rayleigh‐Taylor like plasma instabilities, interchange is believed to be the main plasma transport process in Saturn's inner to middle magnetosphere. Here we examine the organization of equatorially observed interchange events identified based on high‐energy (3–22 keV) H+ intensifications by several longitude systems that have been derived from different types of measurements. The main question of interest here is as follows: Do interchange injections undergo periodicities similar to the Saturn kilometric radiation or other magnetospheric phenomena? We find that interchange shows enhanced occurrence rates in the northern longitude systems between 30° and 120°, particularly between 7 and 9 Saturn Radii. However, this modulation is small compared to the organization by local time. Additionally, this organization is weak and inconsistent with previous findings based on data with a limited time span. Plain Language Summary: When estimating the rotation rate of Jupiter and Saturn, scientists often use a periodic signal of radio emission from the planet's auroral region. At Saturn this emission is called the Saturn kilometric radiation (SKR), and unlike Jupiter, the period of SKR is observed to vary over time. Similar periodic variations have also been observed in particle energy and magnetic fields, suggesting that this periodicity is a fundamental property of the Saturn space environment. In this work, we ask if these same repetitions can be seen in a process called interchange injection. To do this, we analyze interchange's occurrence rate, as observed in particle data from the Cassini spacecraft, with respect to two longitude systems previously derived from the observed periods of the SKR emission. We find that interchange occurrence shows only weak organization in these longitude systems as compared to organization by local time. Key Points: Interchange injections, identified from an automated detection method, shows strongest organization in local time compared to longitudeLongitude system dependence of equatorial interchange injections exists, but it is weak and inconsistent to previous worksInterchange occurrence rates weakly peak at ~90° in northern SLS‐5 and PPO between 7 and 9 Saturn Radii but occur at all longitudes and seasons [ABSTRACT FROM AUTHOR]

The magnetosphere of Saturn is known to be warped into a bowl shape, and the severity of the warping varies with the seasonal tilt of Saturn's spin axis in its orbital plane. This peculiar geometry complicates the analysis of many magnetospheric phenomena, because they are symmetric about the warped magnetodisk equator rather than the spin equator. To reorganize the Cassini data, a new coordinate system is proposed wherein a zA coordinate represents the perpendicular distance above or below the warped surface, and a ρA coordinate represents the arc length along the surface from the origin to where the perpendicular intersects the surface. Using the Cassini spacecraft trajectory, examples show how these disk coordinates (ρA, zA) significantly deviate from the standard spin‐equatorial coordinates (ρ, z), especially outside the orbit of Titan. Another example indicates that fluxes of energetic electrons are well organized in this new system. Supporting information tabulates the disk coordinates as a function of time during the Cassini mission. Plain Language Summary: The sundry phenomena within Saturn's magnetosphere are typically organized in a "flat" coordinate system with Cartesian (x,y,z) locations. However, Saturn's magnetosphere is known to have a "bowl" shape that reflects a curved magnetodisk. The curvature of the bowl depends on season, so that the bowl is more curved near the solstices and has no curvature at the equinoxes. A new curvilinear coordinate system is proposed that is based on the bowl shape and which varies with season. An example shows how energetic electron fluxes are well‐organized in the new coordinate system. Key Points: Normal spin‐equator coordinates are inadequate to organize phenomena that are symmetric about a warped, bowl‐shaped magnetodiskA new curvilinear system is proposed that is aligned with the curved disk and changes with seasonal warping, as at SaturnThe new system differs from the spin‐equator system, especially at the solstices, but consistently reorganizes magnetospheric phenomena [ABSTRACT FROM AUTHOR]

The equatorial current sheet at Saturn is the result of a rapidly rotating magnetosphere. The sheet itself exhibits periodic seasonal and diurnal movements as well as aperiodic movements of a currently unknown origin, along with periodic thickening and thinning of the magnetodisc, and azimuthal changes in the thickness due to local effects in the magnetosphere. In this paper aperiodic movements of the magnetodisc are utilized to calculate the height‐integrated current density of the current sheet using a Harris current sheet model deformed by a Gaussian wave function. We find a local time asymmetry in both the radial and azimuthal height‐integrated current density. We note that the local time relationship with height‐integrated current density is similar to the relationship seen at Jupiter, where a peak of ∼0.04 A/m at ∼3 SLT (Saturn local time) is seen inside 20 RS. The divergence of the radial and azimuthal current densities are used to infer the parallel currents, which are seen to diverge from the equator in the prenoon sector and enter the equator in the premidnight sector. Key Points: Current density is estimated using a deformed Harris current sheet modelCurrent density generally decreases with radial distance with some local time asymmetries mainly in the azimuthal currentDivergence of the perpendicular current density used to infer parallel field‐aligned currents that can enhance auroral emission premidnight [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]

We conduct a statistical analysis of 2,094 reconnection events in Saturn's near‐equatorial magnetotail previously identified in Cassini magnetometer data from intervals during 2006 and 2009/2010. These consist of tailward propagating plasmoids and planetward propagating dipolarizations, with approximately twice as many plasmoids as dipolarizations. We organize these by three related planetary period oscillation (PPO) phase systems, the northern and southern PPO phases relative to noon, the same phases retarded by a radial propagation delay, and the local retarded phases that take account of the azimuth (local time) of the observation. Clear PPO modulation is found for both plasmoid and dipolarization events, with local retarded phases best organizing the event data with the modulation in event frequency propagating across the tail as the PPO systems rotate. This indicates that the events are localized in azimuth, rather than simultaneously affecting much of the tail width. Overall, events occur preferentially by factors of ~3 at northern and southern phases where the tail current sheet is expected locally to be thinnest in the PPO cycle, with field lines contracting back from their maximum radial displacement, compared with the antiphase conditions. Separating the events into those representing the start of independent reconnection episodes, occurring at least 3 hr after the last, and events in subsequent clusters, shows that the above phases are predominantly characteristic of the majority cluster events. The phases at the start of independent reconnection episodes are typically ~60° earlier. Key Points: We analyze a catalogue of 2,094 reconnection events in Saturn's magnetotail and organize them according to northern and southern PPO phasesEvents are best organized by local PPO phases rather than global phases, implying that reconnection is more locally than globally triggeredEvents are best organized by phases where the tail current sheet should be locally thin and the field lines near maximum radial displacement [ABSTRACT FROM AUTHOR]

The radiation belts and magnetospheres of Jupiter and Saturn show significant intensities of relativistic electrons with energies up to tens of megaelectronvolts (MeV). To date, the question on how the electrons reach such high energies is not fully answered. This is largely due to the lack of high‐quality electron spectra in the MeV energy range that models could be fit to. We reprocess data throughout the Galileo orbiter mission in order to derive Jupiter's electron spectra up to tens of MeV. In the case of Saturn, the spectra from the Cassini orbiter are readily available and we provide a systematic analysis aiming to study their acceleration mechanisms. Our analysis focuses on the magnetospheres of these planets, at distances of L > 20 and L > 4 for Jupiter and Saturn, respectively, where electron intensities are not yet at radiation belt levels. We find no support that MeV electrons are dominantly accelerated by wave‐particle interactions in the magnetospheres of both planets at these distances. Instead, electron acceleration is consistent with adiabatic transport. While this is a common assumption, confirmation of this fact is important since many studies on sources, losses, and transport of energetic particles rely on it. Adiabatic heating can be driven through various radial transport mechanisms, for example, injections driven by the interchange instability or radial diffusion. We cannot distinguish these processes at Saturn with our technique. For Jupiter, we suggest that the dominating acceleration process is radial diffusion because injections are never observed at MeV energies. Plain Language Summary: Space is filled with a radiation of protons and electrons moving with almost light speed. While in free space this is cosmic radiation of extreme energies, magnetized planets trap radiation particles of lower energies, typically in the megaelectronvolt range. These are the so called radiation belts that are found, for example, around Jupiter, Saturn, and Earth. All radiation particles initially start out from being almost at rest and are over time accelerated to almost light speed. The physical mechanisms responsible for this are the subject of ongoing research. Here we focus on the processes accelerating electrons around Jupiter and Saturn based on data from the Galileo and Cassini orbiters. During their life, electrons change their orbits around a planet, get closer to the planetary surface, and are exposed to stronger magnetic fields that these planets are producing. It is known that such changes in magnetic field exposure are in principle able to accelerate particles. Here we find that this exposure is indeed the main reason for acceleration of electrons at relatively large distances from Jupiter and Saturn, not other candidate processes that could in principle also have been responsible. Key Point: Electrons in Saturn's and Jupiter's magnetosphere, outside the most intense radiation belts, are accelerated through adiabatic transport [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]

The Cassini spacecraft orbited Saturn from 2004 to 2017, and in 2006 it started exploring the deep magnetotail, reaching distances of about 68 RS (where RS is the equatorial radius of Saturn). Since Cassini covered a broad area of Saturn's magnetosphere, this raises the question of what is the typical and atypical plasma composition in different regions of Saturn's environment. In this paper, we present a survey of the bulk plasma composition using time‐of‐flight data from the Plasma Spectrometer/Ion Mass Spectrometer instrument on Cassini, from 2004 through 2012. This is the most comprehensive study ever made of relative abundances of thermal plasma at Saturn, maximizing the use of Cassini's orbital coverage in Saturn's magnetosphere during those years, and, therefore, the sensitivity to seasonal or natural variability of the system. We studied the ratio of counts between ions with E/q≃1.19–21,300 eV/q and mass per charge equal to 2 (either H 2+ or He++) and ionized hydrogen ([(m/q = 2)]/[H+]), and a mixture of ions (H2O+, H3O+, OH+, and O+), known as the water group (W+) and ionized hydrogen ([W+]/[H+]). We present the data as a function of position in the magnetosphere, radial distance and local time, and distance from the planet and longitude with respect to the moons Enceladus, Dione, Rhea, and Titan. We found that the plasma composition in Saturn's magnetosphere presents significant local time asymmetries and variability. Key Points: Data show an enhancement in the ratio between ions with mass per charge equal to 2 and protons in the dusk sectorEnhancement is independent of the location of the moons with respect to the spacecraft and of seasonPlasma stagnation point occurs at earlier local time than suggested by previous modeling [ABSTRACT FROM AUTHOR]

The magnetometer data for the Cassini mission (2004–2016) are used to derive a model of Saturn's meridional magnetic field lines. Using a bin map of the Bρ and Bz field components, the field lines can be traced from the equator to high latitudes in the inner magnetosphere. The traces reveal a magnetic field greatly compressed on the dayside and highly elongated on the nightside, which are presumably the effects of solar wind compression and viscous flow around the magnetosphere. A model of the traced field lines can accommodate this day‐night asymmetry and offer a way to connect various places in the magnetosphere. This model can be adjusted for magnetodisk warping by using solar latitude. The field line traces can be mapped down to the ionosphere using an offset dipole. The dayside magnetopause (L = 21) is mapped to colatitudes of ~13° in the north and ~16° in the south, while for L > 25 on the nightside, the mapped field lines approach asymptotic limits of ~16° in the north and ~18° in the south. Key Points: The meridional magnetic field lines of Saturn were statistically determined using measurements from 2004 to 2016The magnetic field is compressed on the dayside and elongated on the nightside, as expected from viscous interaction with the solar windThe field lines can be represented by using an L extremum parameter divided by a power law expansion in sin (λ) [ABSTRACT FROM AUTHOR]

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

Parallel propagating electromagnetic electron cyclotron (EMEC) waves in the extended plasma sheet (~12RS) and in the outer magnetosphere (~18RS) of Saturn have been studied. A dispersion relation for parallel propagating relativistic EMEC waves has been applied to the magnetosphere of Saturn, and comparisons have been made with the data of Voyager 1 at these radial distances. The detailed investigations for EMEC waves have been done in the presence of the perpendicular AC electric field, using the kappa distribution function. The relativistic temporal growth rate is calculated by the method of characteristic solution with the data provided by Voyager 1. The effect of the suprathermal electron density, temperature anisotropy, frequency of AC electric field, thermal energy of ions, and relativistic factor on the temporal growth rate of EMEC wave emission has been studied. The simulation results show that the growth of parallel propagating EMEC waves is significantly affected by variations in the temperature anisotropy, electron density, ion thermal energy, and relativistic factor in both the extended plasma sheet and the outer magnetosphere of Saturn. The temperature anisotropy (T⊥/T║), ion thermal energy (KBT║i), and electron density (n0) have been found to be a major source of free energy for parallel propagating EMEC waves in both regions. [ABSTRACT FROM AUTHOR]

In this paper parallel flow velocity shear Kelvin-Helmholtz instability has been studied in two different extended regions of the inner magnetosphere of Saturn. The method of the characteristic solution and kinetic approach has been used in the mathematical calculation of dispersion relation and growth rate of K-H waves. Effect of magnetic field (B), inhomogeneity (P/a), velocity shear scale length ( A i ), temperature anisotropy ( T ⊥ / T | | ), electric field (E), ratio of electron to ion temperature ( T e / T i ), density gradient ( ε n ρ i ) and angle of propagation ( θ ) on the dimensionless growth rate of K-H waves in the inner magnetosphere of Saturn has been observed with respect to k ⊥ ρ i . Calculations of this theoretical analysis have been done taking the data from the Cassini in the inner magnetosphere of Saturn in the two extended regions of Rs ∼4.60–4.01 and Rs ∼4.82–5.0. In our study velocity shear, temperature anisotropy and magnitude of the electric field are observed to be the major sources of free energy for the K-H instability in both the regions considered. The inhomogeneity of electric field, electron-ion temperature ratio, and density gradient have been observed playing stabilizing effect on K-H instability. This study also indicates the effect of the vicinity of icy moon Enceladus on the growth of K-H instability. [ABSTRACT FROM AUTHOR]

The water-group ion cyclotron waves (ICWs) in Saturn's magnetosphere were studied using the magnetic field data provided by the MAG magnetometer on board the Cassini satellite. The period from January 2005 to December 2009, when the Cassini radial distance is smaller than 8 R , was used. ICWs were identified by their left-hand circularly polarized magnetic perturbations and wave frequencies near the water-group ion gyrofrequencies. We obtained the spatial distribution of ICW amplitude and found that the source region of ICWs is mostly located in the low-latitude region, near the equator and inside the 6 R radial distance. However, it can extend beyond 7 R in the midnight region. In general, the wave amplitude is peaked slightly away from the equator, for all local time sectors in both the Northern and Southern Hemispheres. By assuming that the water-group ions are composed of pickup ions and background thermal ions, we obtained the local instability condition of the ICWs and estimated their growth rate along the field lines. If the wave amplitude is correlated with the growth rate, the observed latitudinal dependence of the wave amplitude can be well explained by the local stability analysis. Also, latitudinal location of the peak amplitude is found to depend on the local time. This implies a local time dependence for the water-group ion parallel temperature T|, as determined from the theoretical calculations. [ABSTRACT FROM AUTHOR]

In present paper whistler mode waves in magnetosphere of Saturn have been investigated. Linear properties of ring velocity distribution function are used to derive dispersion relation for whistler mode waves in ambient magnetic field of Saturn in extended plasma sheet of planet’s magnetosphere. The derivation for perturbed distribution function, dispersion relation and growth rate have been determined by using the method of characteristic and kinetic approach. Analytical expressions for growth rate and real frequency of whistlers propagating parallel to magnetic field direction are attained. The analysis shows that temperature anisotropy, increases in number density and energy density increases the growth rate of whistler waves along with significant shift in wave number. It has been shown that whistler mode waves have grown due to loss of perpendicular kinetic energy of ring electrons. Calculations have been performed at two radial distances in Saturn’s magnetosphere. The results are of importance in analyzing observed VLF emissions over wide spectrum of frequency range in Saturnian magnetosphere. The analytical model developed can also be used to study various types of instabilities in planetary magnetospheres. [ABSTRACT FROM AUTHOR]

Based on magnetic field data from Cassini's high-inclination orbits of Saturn (radius R S = 60 , 268 km ), we analyze the latitudinal distribution of ion cyclotron waves in the giant planet's magnetosphere. Our survey takes into account magnetic field data from all high-inclination orbits between 2004 and 2015. We analyze the dependency of the occurrence rate and amplitude of the ion cyclotron waves on radial distance ρ to Saturn's rotation axis, vertical distance z to Saturn's equatorial plane, and magnetic latitude λ . The occurrence rate of ion cyclotron waves is approximately 100% in Saturn's equatorial plane between the orbits of Enceladus and Dione and decreases to 50% at altitudes of | z | ≈ 0.6 R S . Ion cyclotron waves were detected up to | z | = 2.0 R S . The occurrence rate displays strong, non-monotonic variations with respect to ρ , z , and λ . The vertical amplitude profile of the waves exhibits an M-like pattern with two distinct peaks near z = ± 0.3 R S and the central minimum at z =0. Compared to earlier observations, we find this M-like structure to be inflated in± z direction by a factor of three. The available magnetic field data provides only weak evidence for a local impact of Enceladus and Dione on the ion cyclotron wave field. Using the observed Doppler shift of the ion cyclotron wave frequency during Cassini's high-inclination orbits, we demonstrate the existence of a narrow band of bidirectional wave propagation. This band is centered around Saturn's equatorial plane and possesses a half-width of | z | = 0.15 R S , which agrees well with the vertical scale height of Saturn's neutral cloud. To the north of this band, all ion cyclotron waves propagate towards the north ( z > 0 ); and to the south, all waves propagate towards the south ( z < 0 ). In companion with our previous study (Meeks et al., 2016), this survey provides the complete three-dimensional picture of the ion cyclotron wave field between the orbits of Enceladus and Rhea during the Cassini era. [ABSTRACT FROM AUTHOR]

Images obtained by the Cassini spacecraft between 2012 and 2015 reveal a periodic brightness variation in a region of Saturn’s D ring that previously appeared to be rather featureless. Furthermore, the intensity and radial wavenumber of this pattern have decreased steadily with time since it was first observed. Based on analogies with similar structures elsewhere in the D ring, we propose that this structure was created by some event that disturbed the orbital motions of the ring particles, giving them finite orbital eccentricities and initially aligned pericenters. Differential orbital precession then transformed this structure into a spiral pattern in the ring’s optical depth that became increasingly tightly wound over time. The observed trends in the pattern’s radial wavenumber are roughly consistent with this basic model, and also indicate that the ring-disturbing event occurred in early December 2011. Similar events in 1979 may have generated the periodic patterns seen in this same region by the Voyager spacecraft. The 2011 event could have been caused by debris striking the rings, or by a disturbance in the planet’s electromagnetic environment. The rapid reduction in the intensity of the brightness variations over the course of just a few years indicates that some process is either damping orbital eccentricities in this region or causing the orbital pericenters of particles with the same semi-major axis to become misaligned. [ABSTRACT FROM AUTHOR]

The Hera Saturn entry probe mission is proposed as an M-class mission led by ESA with a contribution from NASA. It consists of one atmospheric probe to be sent into the atmosphere of Saturn, and a Carrier-Relay spacecraft. In this concept, the Hera probe is composed of ESA and NASA elements, and the Carrier-Relay Spacecraft is delivered by ESA. The probe is powered by batteries, and the Carrier-Relay Spacecraft is powered by solar panels and batteries. We anticipate two major subsystems to be supplied by the United States, either by direct procurement by ESA or by contribution from NASA: the solar electric power system (including solar arrays and the power management and distribution system), and the probe entry system (including the thermal protection shield and aeroshell). Hera is designed to perform in situ measurements of the chemical and isotopic compositions as well as the dynamics of Saturn's atmosphere using a single probe, with the goal of improving our understanding of the origin, formation, and evolution of Saturn, the giant planets and their satellite systems, with extrapolation to extrasolar planets. Hera 's aim is to probe well into the cloud-forming region of the troposphere, below the region accessible to remote sensing, to the locations where certain cosmogenically abundant species are expected to be well mixed. By leading to an improved understanding of the processes by which giant planets formed, including the composition and properties of the local solar nebula at the time and location of giant planet formation, Hera will extend the legacy of the Galileo and Cassini missions by further addressing the creation, formation, and chemical, dynamical, and thermal evolution of the giant planets, the entire solar system including Earth and the other terrestrial planets, and formation of other planetary systems. [ABSTRACT FROM AUTHOR]

Most of the phenomena that describe the magnetized plasma filling the huge magnetosphere of Saturn exhibit periodic behavior. The fundamental period reflected in many magnetospheric phenomena is the rotational period of the planet, but the relationship is not at all trivial. In most cases clear periodic behavior can be found only for relatively short time intervals, and often even in these intervals abrupt phase-shifts occur and non-rotational frequencies appear. Several sophisticated methods have been developed to filter out interfering fluctuations and find the basic periodicity and phase of the variations. Although these methods proved to be very useful, some information is inevitably lost in the process. To recover this otherwise lost information we follow a different strategy to analyse the quasi-periodic variations of the plasma properties. We assume that the motion of the magnetodisk is periodic and that the observed quasi-periodic variations are due to the interplay of this periodic motion and the effects governing the spatial dependence of the plasma parameters ( F ), especially their dependence on the distance ( d ) from the central sheet of the magnetodisk. We found that relatively simple F ( d ) functions are able to reproduce the observed complex temporal dependence of the plasma properties. [ABSTRACT FROM AUTHOR]

We present a comprehensive analysis of ion cyclotron waves in the equatorial magnetosphere of Saturn, considering all magnetic field data collected during the Cassini era (totaling to over 4 years of data from the equatorial plane). This dataset includes eight targeted flybys of Enceladus, three targeted flybys of Dione, and three targeted flybys of Rhea. Because all remaining orbits of Cassini are high-inclination, our study provides the complete map of ion cyclotron waves in Saturn's equatorial magnetosphere during the Cassini era. We provide catalogs of the radial and longitudinal dependencies of the occurrence rate and amplitude of the ion cyclotron fundamental and first harmonic wave modes. The fundamental wave mode is omnipresent between the orbits of Enceladus and Dione and evenly distributed across all Local Times. The occurrence rate of the fundamental mode displays a Fermi–Dirac-like profile with respect to radial distance from Saturn. Detection of the first harmonic mode is a rare event occurring in only 0.49% of measurements taken and always in conjunction with the fundamental mode. We also search for a dependency of the ion cyclotron wave field on the orbital positions of the icy moons Enceladus, Dione, and Rhea. On magnetospheric length scales, the wave field is independent of the moons' orbital positions. For Enceladus, we analyze wave amplitude profiles of seven close flybys (E9, E12, E13, E14, E17, E18, and E19), which occurred during the studied trajectory segments, to look for any local effects of Enceladan plume variability on the wave field. We find that even in the close vicinity of Enceladus, the wave amplitudes display no discernible dependency on Enceladus' angular distance to its orbital apocenter. Thus, the correlation between plume activity and angular distance to apocenter proposed by Hedman et al. (2013) does not leave a clearly distinguishable imprint in the ion cyclotron wave field. [ABSTRACT FROM AUTHOR]

A multifluid model is used to investigate how Saturn's magnetosphere affects ionosphere. The model includes a magnetospheric plasma temperature of 2 eV as a boundary condition. The main results are: (1) H + ions are accelerated along magnetic field lines by ambipolar electric fields and centrifugal force, and have an upward velocity of about 10 km/s at 8000 km; (2) the ionospheric plasma temperature is 10,000 K at 5000 km, and is significantly affected by magnetospheric heat flow at high altitudes; (3) modeled electron densities agree with densities from occultation observations if the maximum neutral temperature at a latitude of 54˚ is about 900 K or if electrons are heated near an altitude of 2500 km; (4) electron heating rates from photoelectrons (≈100 K/s) can also give agreement with observed electron densities when the maximum neutral temperature is lower than 700 K (note that Cassini observations give 520 K); and (5) the ion temperature is high at altitudes above 4000 km and is almost the same as the electron temperature. The ionospheric height-integrated Pedersen conductivity, which affects the magnetospheric plasma velocity, varies with local time with values between 0.4 and 10 S. We suggest that the sub-corotating ion velocity in the inner magnetosphere depends on the local time, because the conductivity generated by dust–plasma interactions in the inner magnetosphere is almost comparable to the ionospheric conductivity. This indicates that magnetosphere–ionosphere coupling is highly important in the Saturn system. [ABSTRACT FROM AUTHOR]

A survey of Cassini MIMI/LEMMS data acquired between 2004 and 2015 has led to the identification of 13 energetic electron microsignatures that can be attributed to particle losses on one of the several faint rings of the planet. Most of the signatures were detected near L-shells that map between the orbits of Mimas and Enceladus or near the G-ring. Our analysis indicates that it is very unlikely for these signatures to have originated from absorption on Mimas, Enceladus or unidentified Moons and rings, even though most were not found exactly at the L-shells of the known rings of the saturnian system (G-ring, Methone, Anthe, Pallene). The lack of additional absorbers is apparent in the L-shell distribution of MeV ions which are very sensitive for tracing the location of weakly absorbing material permanently present in Saturn’s radiation belts. This sensitivity is demonstrated by the identification, for the first time, of the proton absorption signatures from the asteroid-sized Moons Pallene, Anthe and/or their rings. For this reason, we investigate the possibility that the 13 energetic electron events formed at known saturnian rings and the resulting depletions were later displaced radially by one or more magnetospheric processes. Our calculations indicate that the displacement magnitude for several of those signatures is much larger than the one that can be attributed to radial flows imposed by the recently discovered noon-to-midnight electric field in Saturn’s inner magnetosphere. This observation is consistent with a mechanism where radial plasma velocities are enhanced near dusty obstacles. Several possibilities are discussed that may explain this observation, including a dust-driven magnetospheric interchange instability, mass loading by the pick-up of nanometer charged dust grains and global magnetospheric electric fields induced by perturbed orbits of charged dust due to the act of solar radiation pressure. Indirect evidence for a global scale interaction between the magnetosphere and Saturn’s faint rings that may drive such radial transport processes may also exist in previously reported measurements of plasma density by Cassini. Alternative explanations that do not involve enhanced plasma transport near the rings require the presence of a transient absorbing medium, such as E-ring clumps. Such clumps may form at the L-shell range where microsignatures have been observed due to resonances between charged dust and corotating magnetospheric structures, but remote imaging observations of the E-ring are not consistent with such a scenario. [ABSTRACT FROM AUTHOR]

The paraboloid model of Saturn's magnetosphere describes the magnetic field as being due to the sum of contributions from the internal field of the planet, the ring current, and the tail current, all contained by surface currents inside a magnetopause boundary which is taken to be a paraboloid of revolution about the planet-Sun line. The parameters of the model have previously been determined by comparison with data from a few passes through Saturn's magnetosphere in compressed and expanded states, depending on the prevailing dynamic pressure of the solar wind. Here we significantly expand such comparisons through examination of Cassini magnetic field data from 18 near-equatorial passes that span wide ranges of local time, focusing on modelling the co-latitudinal field component that defines the magnetic flux passing through the equatorial plane. For 12 of these passes, spanning pre-dawn, via noon, to post-midnight, the spacecraft crossed the magnetopause during the pass, thus allowing an estimate of the concurrent subsolar radial distance of the magnetopause R1 to be made, considered to be the primary parameter defining the scale size of the system. The best-fit model parameters from these passes are then employed to determine how the parameters vary with R1, using least-squares linear fits, thus providing predictive model parameters for any value of R1 within the range. We show that the fits obtained using the linear approximation parameters are of the same order as those for the individually selected parameters. We also show that the magnetic flux mapping to the tail lobes in these models is generally in good accord with observations of the location of the open-closed field line boundary in Saturn's ionosphere, and the related position of the auroral oval. We then investigate the field data on six passes through the nightside magnetosphere, for which the spacecraft did not cross the magnetopause, such that in this case we compare the observations with three linear approximation models representative of compressed, intermediate, and expanded states. Reasonable agreement is found in these cases for models representing intermediate or expanded states. [ABSTRACT FROM AUTHOR]

We discuss the planetary period oscillations (PPOs) observed by the Cassini spacecraft in Saturn's magnetosphere, in particular the relationship between the properties of the PPOs in the post-equinox interval as observed in magnetic field data by Andrews et al. (2012) and Provan et al. (2013, 2014) and in Saturn kilometric radiation (SKR) emissions by Fischer et al. (2014, 2015), whose results are somewhat discrepant. We show that differences in the reported PPO periods, a fundamental property which should be essentially identical in the two data sets, can largely be accounted for by the phenomenon of dual modulation of the SKR emissions in polarization-separated data, in which the modulation associated with one hemisphere is also present in the other. Misidentification of the modulations results in a reported reversal in the SKR periods in the initial post-equinox interval, south for north and vice versa, relative to the magnetic oscillations whose hemispheric origin is more securely identified through the field component phase relations. Dual modulation also results in the apparent occurrence of phase-locked common periods in the northern and southern SKR data during later intervals during which two separate periods are clearly discerned in the magnetic data through beat modulations in both phase and amplitude. We further show that the argument of Fischer et al. (2015) concerning the phase relation between the magnetic field oscillations and the SKR modulations is erroneous, the phase difference between them revealing the local time (LT) of the upward field-aligned current of the PPO current system at times of SKR modulation maxima. Furthermore, this LT is found to vary significantly over the Cassini mission from dawn, to dusk, and to noon, depending on the LT of apoapsis where the spacecraft spends most time. These variations are consistent with the view that the SKR modulation is fundamentally a rotating system like the magnetic perturbations, though complicated by the strong LT asymmetry in the strength of the sources, and rule out a mainly clock-like (strobe) modulation as argued by Fischer et al. (2015), for which no physical mechanism is suggested. We also elucidate the nature of the magnetic periods, criticized by Fischer et al. (2015), which have previously been derived in ∼100–200 day post-equinox intervals between abrupt changes in PPO properties, and further show that their argument that the magnetic phase data provide evidence for the occurrence of common phase-locked magnetic oscillations in some intervals is fallacious. The most important consequence of our results, however, is that they demonstrate the essential compatibility of the post-equinox magnetic field and SKR data, despite the contrary results published to date. They also show that due to the dual modulation effect in polarization-separated SKR data, analysis and interpretation may contain more subtleties than previously realized. Joint examination of the combined magnetic and SKR data clearly provides greater insight and enhanced confidence compared with analyses of these data sets individually. [ABSTRACT FROM AUTHOR]

The in-situ exploration of the magnetospheres of Jupiter and Saturn has revealed different periodic processes. In particular, in the Saturnian magnetosphere, several studies have reported pulsations in the outer magnetosphere with a periodicity of about 1 h in the measurements of charged particle fluxes, plasma wave, magnetic field strength and auroral emissions brightness. The Low-Energy Magnetospheric Measurement System detector of the Magnetospheric Imaging Instrument (MIMI/LEMMS) on board Cassini regularly detects 1-hour quasi-periodic enhancements in the intensities of electrons with an energy range from a hundred keV to several MeV. We extend an earlier survey of these relativistic electron injections using 10 years of LEMMS observations in addition to context measurements by several other Cassini magnetospheric experiments. The one-year extension of the data and a different method of detection of the injections do not lead to a discrepancy with the results of the previous survey, indicating an absence of a long-term temporal evolution of this phenomenon. We identified 720 pulsed events in the outer magnetosphere over a wide range of latitudes and local times, revealing that this phenomenon is common and frequent in Saturn’s magnetosphere. However, the distribution of the injection events presents a strong local time asymmetry with ten times more events in the duskside than in the dawnside. In addition to the study of their topology, we present a first statistical analysis of the pulsed events properties. The morphology of the pulsations shows a weak local time dependence which could imply a high-latitude acceleration source. We provide some clues that the electron population associated with this pulsed phenomenon is distinct from the field-aligned electron beams previously observed in Saturn’s magnetosphere, but both populations can be mixed. We have also investigated the signatures of each electron injection event in the observations acquired by the Radio and Plasma Wave Science (RPWS) instrument and the magnetometer (MAG). Correlated pulsed signatures are observed in the plasma wave emissions, especially in the auroral hiss, for 12% of the electron injections identified in the LEMMS data. Additionally, in about 20% of the events, such coincident pulsed signatures have been also observed in the magnetic field measurements, some of them being indicative of field-aligned currents. This analysis combined with the multi-instrument approach sets constraints on the origin and significance of the pulsed events. Hence, our results suggest that the acceleration process providing the quasi-periodic relativistic electrons takes place at high-latitudes. [ABSTRACT FROM AUTHOR]

A proposed resolution of the unexplained 10.7-h periodicities in Saturn’s magnetosphere is a system of atmospheric vortices in the polar regions of the planet. We investigate a description of such vortices in terms of planetary-scale waves. Approximating the polar regions as flat, we use theory developed originally by Haurwitz (Haurwitz, B. [1975]. Geophys. Bioklimatol. 24, 1–18) to find circumpolar Rossby wave solutions for Saturn’s upper stratosphere and lower thermosphere. We find vertically propagating twin vortex solutions that drift slowly westwards at <1% of the deep planetary angular velocity and are thus ideal candidates for explaining the observed periodicities. To produce integrated field-aligned currents of the order of 1 MA we require wind velocities of ∼ 70 ms - 1 . A particular class of vertically propagating solutions are potentially consistent with wave energy being ‘trapped’ between the deep atmosphere and lower thermosphere, at altitudes suited to the production of the necessary field-aligned current systems. [ABSTRACT FROM AUTHOR]

Interactions between the solar wind and planetary magnetospheres provide important diagnostic information about the magnetospheric dynamics. The lack of monitoring of upstream solar wind conditions at the outer planets, however, restrains the overall scientific output. Here we apply a new method, using Cassini nanodust stream measurements, to derive the interplanetary magnetic field structure during the 2013 Saturn aurora campaign. Due to the complex dynamical interactions with the interplanetary magnetic field, a fraction of fast nanodust particles emerging from the Saturnian system is sent back into the magnetosphere and can be detected by a spacecraft located within. The time-dependent directionality caused by the variable interplanetary magnetic field enable these particles to probe the solar wind structure remotely. Information about the arrival time of solar wind compression regions (coupled with the heliospheric current sheet crossings) as well as the field direction associated with the solar wind sector structure can be inferred. Here we present a tentative identification of the interplanetary magnetic field sector structure based on Cassini nanodust and radio emission measurements during the 2013 Saturn aurora campaign. Our results show that, the interplanetary magnetic field near Saturn during 2013-080 to 176 was consistent with a two-sector structure. The intensifications of aurora and the radio emission on 2013-095, 112 and 140 coincide with the IMF sector boundaries, indicating that the encounter of the compressed solar wind is the main cause of the observed activities. [ABSTRACT FROM AUTHOR]

Quasi-periodic, short-period injections of relativistic electrons have been observed in both Jupiter’s and Saturn’s magnetospheres, but understanding their origin or significance has been challenging, primarily due to the limited number of in-situ observations of such events by past flyby missions. Here we present the first survey of such injections in an outer planetary magnetosphere using almost nine years of energetic charged particle and magnetic field measurements at Saturn. We focus on events with a characteristic period of about 60–70 min (QP60, where QP stands for quasi-periodic). We find that the majority of QP60, which are very common in the outer magnetosphere, map outside Titan’s orbit. QP60 are also observed over a very wide range of local times and latitudes. A local time asymmetry in their distribution is the most striking feature, with QP60 at dusk being between 5 and 25 times more frequent than at dawn. Field-line tracing and pitch angle distributions suggest that most events at dusk reside on closed field lines. They are distributed either near the magnetopause, or, in the case of the post-dusk (or pre-midnight) sector, up to about 30 R S inside it, along an area extending parallel to the dawn–dusk direction. QP60 at dawn map either on open field lines and/or near the magnetopause. Both the asymmetries and varying mapping characteristics as a function of local time indicate that generation of QP60 cannot be assigned to a single process. The locations of QP60 seem to trace sites that reconnection is expected to take place. In that respect, the subset of events observed post-dusk and deep inside the magnetopause may be directly or indirectly linked to the Vasyliunas reconnection cycle, while magnetopause reconnection/Kelvin–Helmholtz (KH) instability could be invoked to explain all other events at the duskside. Using similar arguments, injections at the dawnside magnetosphere may result from solar-wind induced storms and/or magnetopause reconnection/KH-instability. Still, we cannot exclude that the apparent collocation of QP60 with expected reconnection sites is coincidental. given also the large uncertainties in field line tracing with the available magnetic field models. The intensity of the QP60 spectrum is strong enough such that if transport processes allow, these injections can be a very important source of energetic electrons for the inner saturnian magnetosphere or the heliosphere. We also observe that electrons in a QP60 can be accelerated at least up to 6 MeV and that the distribution of QP60 appears to trace well the aurora’s local time structure, an observation that may have implications about high-latitude electron acceleration and the connection of these events to auroral dynamics. Despite these new findings, it is still unclear what determines the rather well-defined 60 to 70-min period of the electron bursts and how electrons can rapidly reach several MeV. [ABSTRACT FROM AUTHOR]