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The Ion Neutral Camera (INCA) on the Cassini spacecraft made images of energetic H atoms (25-55 keV) over a 3-day span in 2017. The images were projected onto the equatorial plane of Saturn, and a keogram was made by interpolating the projections in local time at 9 RS (1 RS = 60268 km). The keogram intensities show strong periodicities near the 10.79h period of Saturn's energetic particles and exhibit a slope commensurate with corotation at that period. These periodic fluxes intensify near midnight but are weaker near noon. A “midnight flash” model can explain this behavior in terms of a searchlight rotating at 10.79h that intensifies in the midnight sector. The model can also describe similar activity in Saturn's kilometric radiation and magnetic fields, although the “flash” must be shifted to the dawn-to-noon sector.

Lomb periodogram analyses have been applied to magnetic field observations made by the Cassini spacecraft during its high-latitude orbits from 2006 to 2009. Only data from open-field regions (OFRs), identified by absence of thermal electrons, were used to separate pure north and south periodic signals. In agreement with previous investigations, the periodograms reveal signals at 10.6 h in the northern OFR and at 10.8 h in the southern OFR but only for the Bθ and Bϕ components. The Br component exhibited essentially no periodicity in either the north or south, or at least its periodicity amplitude was too small for detection. In addition, the Bθ and Bϕ components displayed signals at ~10.0 h and ~11.2 h in the north and ~10.2 h and ~11.5 h and possibly ~9.6 h in the south. These periods can be reproduced by a simulation by using a rotating “searchlight” model with different north and south periods and an r−3 dependence. This investigation employs a Lomb-Scargle method to analyze magnetic field periodicities and confirms that the magnetic fields have “pure” north and south periods in the respective hemispheres. The results also imply that a radial dependence in these fields exists, which is expected if the fields are produced by field-aligned currents. Using this model, the effects of spacecraft motion and position can be readily detected in the Lomb analyses.

Lomb periodogram analyses were applied to all thermal electron data from the Cassini Plasma Science (CAPS) instrument from mid-2004 to mid-2011. Very strong periods at ~26 days were observed in the fluxes of electrons with energies less than 1000 eV. At higher energies, such a strong solar wind period was not apparent, and numerous signals appeared between ~5 d and ~22 d for E > 100 eV. The amplitudes of all these signals greatly exceeded those recognized in the electrons at the ~10.7 h period related to planetary rotation. A simulation using Cassini orbits and a 2D model of electron fluxes indicate that the 5-22 d periods were caused by the spacecraft orbits in to and out of electron radiation belts. Definitive signals do not exist at the periods of Saturn's moons.

Short-period amplitude modulations (~60 min period) have been detected in the ~100 Hz plasma wave emissions observed by the Radio Plasma Wave Science instrument on the Cassini spacecraft in orbit around Saturn. These periodicities were detected throughout the mission from mid-2004 to the present, and this is the first statistical study of them. The modulations are observed throughout the magnetosphere and can last from ~2 h to ~20 h, although the duration may be biased by spacecraft observing conditions. The periodicities are statistically much more likely to be seen at high latitudes, both north and south, and at local times between dusk and midnight. When corrected for latitude and local time, the occurrence frequency has declined in time since 2005. Considering all observations, the mean period of these events is 65.3 ± 20.7 min, with a peak (modal value) at 63.3 min. The period has no dependence on local time or latitude. Alfven waves have interhemispheric transit times commensurate with the mean periods and should thus be considered principal candidates for their production.

Rather than a clock-like strobe, a rotating spiral may underlie the ~10.7 h periodicities observed in many phenomena in Saturn's magnetosphere. This spiral is a density or flux wave propagating outward from the planet, and the periodicity is generated when a spacecraft encounters the wave. The wave moves outward with the Alfven speed, which can be computed from the magnetic field strength and plasma mass density. Using data from the first 200 days of 2010, the observed field strength and plasma density are used to compute this speed and construct the spiral. When the Cassini spacecraft “flies through” this model on a real trajectory, the model produces a strong main period at 10.7 h with weaker secondary periods at 10.4 h and 11.0 h resulting from Doppler effects. Periodograms of observed phenomena from the same interval show a main peak at 10.7 h but with spurious secondary peaks due to noise.

Over the course of about 6 h on Day 129, 2008, the UV imaging spectrograph (UVIS) on the Cassini spacecraft observed a repeated intensification and broadening of the high latitude auroral oval into the polar cap. This feature repeated at least 5 times with about a 1 h period, as it rotated in the direction of corotation, somewhat below the planetary rotation rate, such that it moved from noon to post-dusk, and from roughly 77° to 82° northern latitudes during the observing interval. The recurring UV observation was accompanied by pronounced ∼1 h pulsations in auroral hiss power, magnetic perturbations consistent with small-scale field aligned currents, and energetic ion conics and electrons beaming upward parallel to the local magnetic field at the spacecraft location. The magnetic field and particle events are in phase with the auroral hiss pulsation. This event, taken in the context of the more thoroughly documented auroral hiss and particle signatures (seen on many high latitude Cassini orbits), sheds light on the possible driving mechanisms, the most likely of which are magnetopause reconnection and/or Kelvin Helmholtz waves.

International audience; Cassini discovered Saturn's innermost radiation belt during the end of its mission. The belt is populated with relativistic protons, probably up to the trapping limit of ≈20 GeV. It extends from Saturn's dense atmosphere into and throughout the D-ring. The A-C rings separate this belt entirely from the previously known radiation belts, suggesting that the innermost radiation belt is populated entirely via cosmic ray albedo neutron decay. We find that the proton pitch angle distributions are consistent with being shaped by losses to the D-ring and the upper atmosphere rather than, for example, wave-particle interactions. This supports that the main loss process of this new radiation belt is energy loss in neutral material, different from Saturn's other radiation belts. This property constrains the overall scale height of Saturn's exosphere to

Cassini's final phase of exploration The Cassini spacecraft spent 13 years orbiting Saturn; as it ran low on fuel, the trajectory was changed to sample regions it had not yet visited. A series of orbits close to the rings was followed by a Grand Finale orbit, which took the spacecraft through the gap between Saturn and its rings before the spacecraft was destroyed when it entered the planet's upper atmosphere. Six papers in this issue report results from these final phases of the Cassini mission. Dougherty et al. measured the magnetic field close to Saturn, which implies a complex multilayer dynamo process inside the planet. Roussos et al. detected an additional radiation belt trapped within the rings, sustained by the radioactive decay of free neutrons. Lamy et al. present plasma measurements taken as Cassini flew through regions emitting kilometric radiation, connected to the planet's aurorae. Hsu et al. determined the composition of large, solid dust particles falling from the rings into the planet, whereas Mitchell et al. investigated the smaller dust nanograins and show how they interact with the planet's upper atmosphere. Finally, Waite et al. identified molecules in the infalling material and directly measured the composition of Saturn's atmosphere. Science , this issue p. eaat5434 , p. eaat1962 , p. eaat2027 , p. eaat3185 , p. eaat2236 , p. eaat2382

Cassini's final phase of exploration The Cassini spacecraft spent 13 years orbiting Saturn; as it ran low on fuel, the trajectory was changed to sample regions it had not yet visited. A series of orbits close to the rings was followed by a Grand Finale orbit, which took the spacecraft through the gap between Saturn and its rings before the spacecraft was destroyed when it entered the planet's upper atmosphere. Six papers in this issue report results from these final phases of the Cassini mission. Dougherty et al. measured the magnetic field close to Saturn, which implies a complex multilayer dynamo process inside the planet. Roussos et al. detected an additional radiation belt trapped within the rings, sustained by the radioactive decay of free neutrons. Lamy et al. present plasma measurements taken as Cassini flew through regions emitting kilometric radiation, connected to the planet's aurorae. Hsu et al. determined the composition of large, solid dust particles falling from the rings into the planet, whereas Mitchell et al. investigated the smaller dust nanograins and show how they interact with the planet's upper atmosphere. Finally, Waite et al. identified molecules in the infalling material and directly measured the composition of Saturn's atmosphere. Science , this issue p. eaat5434 , p. eaat1962 , p. eaat2027 , p. eaat3185 , p. eaat2236 , p. eaat2382

Saturn's magnetospheric periodicities are commonly thought to have a dual nature, one period originating from the southern hemisphere and a slightly different period from the northern. Both periods vary a few percent over time intervals of years and apparently merged a few months after Saturn equinox. These periodicities have not been explained. The dual-period waveform is generally represented as the superposition of two sinusoids with nearly equal periods. However, dual-period waves can also result from an amplitude modulation of a carrier periodicity, from frequency modulation of the carrier, from random phase jumps in the carrier, and from small, random changes in the period itself. While such simple phenomena are well known in the radio community, they can serve as possible explanations for how a single planetary period can appear as the dual (or multiple) periods observed in Saturn's magnetosphere. Candidates for modulation and randomization include the solar wind pressure and speed, the orbital periods of moons of Saturn, or even the trajectory of the Cassini orbiter itself.

The Cassini orbiter has observed the magnetosphere of Saturn in situ from July 2004 to the present. The spacecraft has visited nearly all local times and a large range of latitudes, including both northern and southern hemispheres, for a large fraction of a Saturn year (=29 Earth years). Local time asymmetries have been observed in the thermal plasma, the energetic particles, energetic neutral atoms, magnetic fields and aurora. Some of these are dawn-to-dusk asymmetries and have Earth-like analogies. Unlike Earth’s magnetosphere, however, Saturn’s magnetosphere is rotationally dominated, has no observable tilt relative to the spin axis, and has a major internal plasma and neutrals source in the icy moon Enceladus. These factors contribute to a number of local time asymmetries that are not dawn-to-dusk. This paper reviews Saturn’s local time asymmetries in charged particles, magnetic fields, and energetic neutral atoms, showing how some are Earth-like and some are not.

Planetary magnetospheres contain two general classes of charged particles: low energy “thermal” particles (generally E 10 keV), variously called superthermal or “energetic” particles, which are described by a power law distribution. The kappa or κ function combines aspects of both Maxwellian and power law forms to provide a reasonably complete description of the particle distribution from low to high energies. Fits of the data to the kappa distribution can reveal particle density, temperature, pressure, and convection velocity, all of which are key parameters of magnetospheric physics. This paper summarizes the use of the kappa distribution and its variants to investigate the plasma properties of the magnetospheres of the giant planets Jupiter, Saturn, Uranus, and Neptune. While the κ function was used to derive convective motions in previous investigations, temperatures, and pressures at Jupiter and Saturn, it was not an optimum fit for hot plasmas at Uranus and Neptune, and in many instances may not be the optimum representation of magnetospheric plasmas.

The periodicities of energetic neutral atoms (90–170 keV oxygens) at Saturn are determined by applying Lomb-Scargle periodogram analyses to energetic neutral atom (ENA) fluxes observed in eight local time sectors of the equatorial plane between 5 and 15 RS (1 RS = 60,268 km). The analyses come from four long intervals (>180 days each) of high-latitude viewing from 2007 to 2013 and represent an essentially global view of Saturn's periodicities. The periodograms display rich and complex structures in local time. Sectors near midnight generally exhibit the strongest periodicities (in terms of highest signal-to-noise ratios) and often show the dual or single periods of the Saturn kilometric radiation (SKR). Sectors near noon display single or multiple periodicities or none. Furthermore, dayside periods may be much shorter (~10.3 h) than SKR periods. Sectors near dawn or dusk display periodicities intermediate between midnight and noon or may show no periodicities whatsoever. These patterns of local time dependence do not remain constant from interval to interval.

All available observations (July 2004 to June 2011) made by the electron spectrometer (ELS) of the Cassini Plasma Science instrument were used to generate meridional maps of thermal electron fluxes (10–20,000 eV) separated by dayside and nightside. The maps had a spatial resolution of 1 × 1 RS (1 RS = 60,238 km), 10° resolution in pitch angle, and full ELS energy resolution. These maps indicate that electron fluxes tend to accumulate along the field lines between the L shells of ~6 and ~13 in apparent association with the flux tube of Rhea. In the vicinity of Rhea's flux tube, the electrons tend to have field-aligned pitch angle distributions near the equator, especially between ~10 eV and ~500 eV, but can be isotropic or butterfly at different energies north or south of the equator, and there was no strong evidence of field-aligned electron pitch angle distributions at higher latitudes. The electron fluxes display strong day-night asymmetries in flux intensity and pitch angle distribution. However, the day-night asymmetry observed in the ions, seen as a thicker plasma sheet on the dayside, is not observed in the electrons. Finally, the flux distributions approximately resemble those expected from propagation along field lines using conservation of the first adiabatic invariant.

Keograms are constructed from azimuthal profiles of energetic hydrogen atoms (25–55 keV) from Saturn's magnetosphere. The keograms exhibit linear structures or “tracks” that reveal prograde rotational motion of features or “blobs” in the energetic neutral atom (ENA) images. From polynomial fits, the first derivatives of these tracks are used to estimate the rotational speeds of the blobs. The total blob speed consists of plasma convective drift plus gradient drift, so the convective speed can be approximated by subtracting the gradient drift of the protons from which the ENA derive. This subtraction gives plasma convection speeds that are ~28°/h at ~5 RS and decrease to a constant ~21°/h between 10 RS and 20 RS, which are consistently below corotation (~33.3°/h) and in substantial agreement with estimates of plasma convection made in situ. The speeds also show a local time dependence, decreasing as much as 4–6°/h as the blobs move from midnight through noon to midnight.

The Cassini Ion and Neutral Camera measures intensities of hydrogen and oxygen ions and neutral atoms in the Saturnian magnetosphere and beyond. We use the measured intensity spectrum and anisotropy of energetic hydrogen and oxygen ions to detect, qualify, and quantify plasma convection. We find that the plasma azimuthal convection speed relative to the local rigid corotation speed decreases with radial distance, lagging the planetary rotation rate, and has no significant local time dependences. Plasma in the dusk-midnight quadrant sub-corotates at a large fraction of the rigid corotation speed, with the primary velocity being azimuthal but with a distinct radially outward component. The duskside velocities are similar to those obtained from earlier orbits in the midnight-dawn sector, in contrast to the depressed velocities measured at Jupiter using Energetic Particles Detector measurements on the Galileo spacecraft in the dusk-midnight quadrant. We find significant radial outflow in most of the nightside region. The radial component of the flow decreases with increasing local time in the midnight-dawn sector and reverses as dawn is approached. This and previous results are consistent with a plasma disk undergoing a centrifugally induced expansion as it emerges into the nightside, while maintaining partial rotation with the planet. The magnetodisk expansion continues as plasma rotates across the tail to the dawnside. We do not see evidence in the convection pattern for steady state reconnection in Saturn's magnetotail. The outermost region of the magnetodisk, having undergone expansion upon emerging from the dayside magnetopause confinement, is unlikely to recirculate back into the dayside. We conclude that plasma in the outer magnetodisk [at either planet] rotates from the dayside, expands at the dusk flank, but remains magnetically connected to the respective planet while moving across the tail until it interacts with and is entrained into the dawnside magnetosheath flow. This interaction causes plasma in the outer magnetospheric regions of Jupiter and Saturn to decouple from the planet and exhaust tailward down a dawnside low latitude boundary layer. Magnetospheric plasma will also interact with the dayside magnetosheath plasma, moving across the boundary [enhanced by shear instability] and into the magnetosheath, where it is lost to the magnetosphere with the magnetosheath flow.

[1] A simple wavy magnetodisk model can explain periodicities in energetic charged particles observed in Saturn's outer magnetosphere (>20 RS). The model's free parameters are the tilt of magnetodisk in inner magnetosphere (~1.8°), speed of outgoing spiral wave (~8 RS/h), critical radius (~10 RS), and period of rotation (10.64 h). The fidelity of the model is not judged by a least squares fit to the actual data, but rather by the model's accuracy in reproducing the Lomb periodogram of the periodicities. The model accurately simulates the main spectral feature near ~10.7 h plus a secondary (“dual”) period near ~10.95 h. The ability of the wavy magnetodisk with one period to produce the observed dual periodicity in the observations suggests that models having “dual” periods need not be invoked to explain some of the periodicities in Saturn's outer magnetosphere.

[1] Based on periodicities in the kilometric radio emissions, the Saturn Longitude System 4 (SLS4) was used to organize the far ultraviolet (120–150 nm) aurora observed by the Ultraviolet Imaging Spectrograph on the Cassini spacecraft. Individual Ultraviolet Imaging Spectrograph pixels were projected onto the ionosphere of Saturn, transformed into the SLS4 north and SLS4 south longitude systems, accumulated over all over auroral observations from 2007 to early 2009, and binned into 1°×1° bins of colatitude. The intensity of the northern aurora showed little variation in its SLS4 north system, but the intensity of the southern aurora exhibited an enhancement of over ~10 kR between ~140–280° SLS4 south longitude. This enhancement may represent the auroral signature of a southern ionospheric vortex proposed in MHD models of Saturn's magnetosphere to explain its periodicities. The loci of the northern intensity peaks and the 3 kR boundaries varied little over 360° of longitude, while the equatorward boundary of the southern aurora varied by ~5° in SLS4 south longitude, reaching its most equatorward location of ~23° colatitude between 100° and 180° longitude. The polygonal centroids of the aurora in both north and south were consistent with offsets of no more than ~1° in both hemispheres.

[1] Energetic protons (2.8–78 keV) and water group ions (8.8–78 keV) observed in Saturn's magnetosphere using the Magnetospheric Imaging Instrument/Charge-Energy-Mass Spectrometer instrument from 2005 through 2012 were subjected to a Lomb periodogram analysis with the period window extending from 0.5 to 50 days, and the data constrained to the spatial region between 10 RS (1 RS = 60,268 km) and the magnetopause. Both the protons and water group ions exhibited solar periodicity at ~26 days and harmonics thereof. For all ions, the 26 day periodicities were strong on the dayside and duskside, weak at dusk, and virtually nonexistent on the nightside. The solar periodicity was evident during both the first and second halves of the 7 year time span, but strongest in the first half from January 2004 to July 2008. Some of the ion spectral peaks, especially for the water group ions, can be associated with orbital motion of the spacecraft. The 26 day periodicities are likely caused by corotating interaction regions in the solar wind that periodically sweep past Saturn, regularly compressing its dayside magnetosphere, but not its nightside.

We characterize the relative importance of energetic electrons and protons to the weathering of five of the inner satellites of Saturn. To do this, we present data from the Magnetospheric Imaging Instrument on the Cassini spacecraft, some of which is averaged over the whole mission to date. We also compute averaged proton and electron energy spectra relevant to the distances of these inner satellites. Where data are available, we estimate the power per unit area into a satellite's surface. For electron energy deposition into satellite leading hemispheres, we find the power per unit area is greatest at Mimas and falls off with distance from Saturn. Using fluxes of 1–50 MeV protons detected within the sweeping corridors of Mimas and Enceladus, we find the corresponding deposition would be about 2×108 and 3.7×107 eV/cm2 s.

Composite images of hydrogen and oxygen energetic neutral atoms (ENA) obtained from 2005 through 2015 from the Ion Neutral Camera (INCA) on Cassini reveal the structure of Saturn's plasma sheet out to ~40 RS (1 RS = 60268 km). Seen from either the dawnside or duskside at low latitude, these composites reveal the plasma sheet is concave upwards (northwards) near Saturn's southern solstice, has no concavity near equinox, and is concave downwards (southwards) near Saturn's northern solstice. This seasonal variation confirms the Arridge “bowl” model developed early in the Cassini mission based on limited magnetometer data, with the concavity depending on the tangent of the Sun's latitude at Saturn and a “hinge” parameter rH. The best fits to the ENA data indicate rH ≈ 25-30 RS, which is close to the 29 RS originally suggested by the magnetometer results. The bowl structure suggests other magnetodisks in the solar system and beyond may also undergo similar warping dynamics and may not have a “flat” geometry.

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

[1] When subject to Lomb periodogram analyses, fluxes of energetic electrons (27–496 keV) observed during the first 245 days of 2012 exhibit both mono and dual periods depending on energy. For E < 100 keV electrons, dual periods at 9.95 hours and 10.64 hours are evident, with the strongest signal at the shortest period. For higher energy electrons, only the period at 10.64 hours is evident. The 9.95 hour period is the shortest period ever measured for Saturn's magnetosphere, and is even shorter than the ∼10.5 hour period suggested by studies of Saturn's gravity field and cloud vorticity. If Saturn's magnetospheric periodicities are driven by ionospheric vortices, as suggested by some models, then their speeds would need to be super-rotational to sustain the 9.95 hour period reported here.

[1] Since Saturn's vernal equinox in August 2009 (day 223), energetic electrons (110–365 keV) have exhibited a variety of periodic and aperiodic behavior within a spectral window of 5–15 hours. From late 2009 through the end of 2010, when the observed at dusk, a single period near 10.7 hours dominated the Lomb spectra of these particles. Near the end of 2010, however, the energetic electrons displayed multiple periods, with the strongest at 10.65 hours. The periodicity observed after equinox has a mean value of 10.69 ± 0.06 hours and agreed closely with that of Saturn kilometric radio (south) emissions. By early 2011, when the observer had moved to the dayside, the periodicities abruptly disappeared and the Lomb spectra show no periodicity. This behavior may suggest changes in Saturn's ionosphere as a result of seasonal change, or may alternately imply a local time dependence of periodicity caused by magnetodisk thickness asymmetry.

[1] Cassini orbits during days 200–366 in 2004 afforded the opportunity to continuously observe energetic neutral atom (ENA) emissions from long range (>50 RS, 1 RS = 60268 km) on Saturn's dawn side. Images of energetic neutral hydrogen (25–55 keV) and oxygen (90–160 keV) were projected onto the noon-midnight plane, corrected for travel time from Saturn, averaged into half hour time bins and finally averaged into a 60 × 40 RS spatial bin. The time profiles of these bin averages were then subjected to a Lomb periodogram analysis. The H periodogram exhibits a weak periodicity (SNR = 9.1) with a major peak at 10.78 hours and several minor peaks. The O periodogram displays strong periodicities (SNR = 36.2) with a major peak at 10.78 hours and a various secondary peaks. A cross correlation of the SKR signal with the ENA signals reveals that the H signal leads the SKR by 1.46 ± 0.08 hours, while the O signal leads the SKR by 2.21 ± 0.14 hours.

[1] Pitch angle distributions of energetic electrons (110-365 keV) at Saturn are statistically analyzed for 2005-2009. Using a nondipolar model magnetic field, pitch angle distributions are mapped to the magnetospheric equator and sorted by equatorial crossing distance. The results are quantified using a standard function for the pitch angle distribution, f(α) = Asin K α (where α is the pitch angle and K is the power). Inside of ~10 R s , the distributions are mostly peaked at 90° (K 0) with maxima near 0° and 180°. The 10 R s boundary maps to Saturn's ionosphere at latitudes equatorward of the aurora. Very few "flat" distributions are observed (K ≈ 0). The pitch angle distributions are not as well organized in local time as they are in radial distance, but over the 5 year survey between 10 and 20 R s field-aligned distributions appear most often near midnight, while trapping distributions are found elsewhere.

[1] The charge-energy-mass spectrometer instrument in the Cassini spacecraft measured differential fluxes of protons (2.8–236 keV) and oxygen ions (8.8–236 keV) from July 2004 to August 2007. The fluxes were bin-averaged in Saturn longitude system (SLS) longitude within ±5 RS of the equator and between 8 and 12 RS in radial distance (1 RS = 60,238 km) to determine their global morphology. The 3-year time period is the range of validity of the SLS, which is based on a variable period of Saturn kilometric radiation. Fluxes at all energies of H+ and O+ display an essentially sinusoidal variation in longitude, often with peak-to-trough ratios of 2:1. For E 77 keV; the maxima shift to ∼250°. The ion distributions closely resemble those of energetic neutral hydrogen and neutral oxygen atoms observed by the ion neutral camera onboard Cassini.

[1] The intensities of energetic neutral atoms (ENAs) from Saturn's ring current were cross-correlated with Saturn kilometric radiation (SKR, 100–400 kHz) to determine their phase relations. The cross correlations were obtained for three intervals in early 2007 when Cassini was at high northern latitudes and the Ion and Neutral Camera (INCA) could optimally view the entire ring current. During these intervals, the hydrogen, oxygen, and SKR signals all exhibited ∼10.8 h periodicities. A consistent phase relation was not observed between the ENA and the SKR. Cross-correlation amplitudes in these intervals varied from 0.25 to 0.79, and the phase offsets varied from ∼1 h to several hours and differed according to ENA species. Not only did the SKR and ENA signal show phase offsets, but the H and O signals also displayed offsets. That is, all periodicities at ∼10.8 h had no consistent synchronization.

[1] Fluxes of energetic electrons (110–365 keV) have been subjected to a long-term Lomb periodogram analysis from the middle of 2004 to the middle of 2009. Fluxes within 20 RS and outside the magnetopause were excluded, so only fluxes within the magnetosphere were included in the analysis. The periodicity “box” was expanded from 5 hours to 50 days. In addition to the familiar rotational period near 10.8 hours, the electron fluxes exhibited a strong periodicity near 26 days (the solar wind period) and also a weaker periodicity near 13 days (half the solar wind period). A simulated periodogram using a “rotating anomaly” as it would be seen from the Cassini orbit does not display 26-day and 13-day periods, so the solar wind periodicity cannot result from a possible orbital resonance. The long-term electron periodogram does not show any periods associated with the periods of Saturn's moons.

[1] Energetic electron fluxes (110-485 keV) observed by the MIMI/LEMMS instrument on the Cassini mission to Saturn were averaged into azimuthal bins and L shell bins for the period from day 150 2004 to day 366 2008. In local time, the electrons fluxes maximize on the night side between the Mimas and Rhea L shells and have a minimum near noon. This local time behavior may be a result of nightside injection combined with subcorotational drift in a nondipolar field. In SLS longitude, the electrons are smoothly distributed, showing no signs of spiral patterns on this multiyear timescale. The lower energy electrons (110-365 keV) form an inner belt near the Mimas L shell and an outer belt between the Dione and Rhea L shells.

[1] Ion conics accompanied by electron beams are observed regularly in Saturn's magnetosphere. The beams and conics are seen throughout the outer magnetosphere, on field lines that nominally map from well into the polar cap (Ldipole > 50) to well into the closed field region (Ldipole < 10). The electron beams and ion conics are often observed together but also sometimes separately. Typically, the ion conics are prominent at energies between about 30 keV and 200 keV. The electron beams extend from below the ∼20 keV lower energy threshold for the instrument to sometimes as high as 1 MeV. The electrons may be either unidirectional (upward) or bidirectional; the ions are exclusively unidirectional upward. The ion conics are usually seen in conjunction with enhanced broadband electromagnetic noise in the 10 Hz to few kHz frequency range. Most of the wave energy appears below the local electron cyclotron frequency, hence, is propagating in the whistler mode, although some extension to higher frequencies is sometimes observed, suggesting an electrostatic mode. Sometimes the particle phenomena and the broadband noise occur in pulses of roughly 5 min duration, separated by tens of minutes. At other times they are relatively steady over an hour or more. Magnetic signatures associated with some of the pulsed events are consistent with field aligned current structures. The ions are almost exclusively light ions (H, H2, H3, and/or He) with only occasional hints of oxygen or other heavier species, suggesting an ionospheric source. Taken together, the observations are strikingly similar to those made at Earth in association with auroral zone downward sheet currents, except that in the case of Saturn the particle energies are 20 to 100 times higher.

International audience; For many years it has been known that Saturn emits intense radio emissions at kilometer wavelengths and that this radiation is modulated by the rotation of the planet at a rate that varies by up to one percent on a time scale of years. Recent radio observations from the Cassini spacecraft have revealed the appearance of a second component, with a rotation period of about 10.6 hours, significantly less than the period of the previously known component, which is currently about 10.8 hours. In this paper we show that the first component originates from the southern auroral region, and that the second component originates from the northern auroral region. This north-south asymmetry in the rotation period has potentially important implications on how angular momentum is transferred from the interior to the magnetosphere.

[1] The ENA images from the Ion Neutral CAmera (INCA) on the Cassini spacecraft are projected onto the noon-midnight plane of Sun-Saturn orbital coordinates, and a composite “image” of Saturn's plasma sheet is constructed from dawn-side observations of 20–50 keV hydrogens obtained from days 352 to 361 in 2004. The maxima in the intensity contours define the center of the plasma sheet in the noon-midnight plane. This plasma sheet surface displays a distinct bending or “warping” above Saturn's equatorial plane at radial distances of beyond ∼15 RS on the nightside. On the dayside, the plasma sheet lies close to the equator all the way to the magnetopause. The observed warping agrees with the “bowl” model derived from measurements of Saturn's magnetic field, but fits more closely a simple third-order polynomial.

[1] From the vantage of the dawn sector, the INCA instrument on Cassini imaged neutral hydrogen atoms (20–50 keV) emitted from the center of the Saturn's plasma sheet for five days during late 2004. Points along the center of the plasma sheet were found from contoured images projected onto the noon-midnight plane; points within 20 RS of Saturn were fitted to straight lines, and the slopes of these lines were examined as a function of time at one hour resolution. The slopes vary between ∼17° and ∼24° with a period of 10.80 hours, the same as that of Saturn kilometric radiation (SKR). This periodic tilting of the plasma sheet is in phase with SKR radiation in the sense that the maximum tilt angle occurs when the maximum in the SKR power occurs, and the tilt angle periodicity has a phase angle of ∼47° in SLS-3 longitude.

[1] An explanation for the morphology and evolution of the ring current in Saturn's magnetosphere is provided. We use global Energetic Neutral Atom (ENA) images of the 20–140 keV proton distribution obtained by the Ion and Neutral Camera (INCA) on board the Cassini spacecraft. A case where the ring current displays an exceptionally clear spiral shape is analyzed and by using a simple model of rotational velocities and curvature-gradient drifts we reproduce the observed spiral patterns and dispersions. The spiral evolve from an initial large-scale injection around midnight consistent with INCA observations. The drift patterns of the inner magnetosphere have to be relatively undisturbed in order for a clear spiral to evolve. Clear spiral patterns like this do not always occur, which reminds us that the inner magnetosphere of Saturn is indeed very dynamic.

[1] The Magnetospheric Imaging Instrument (MIMI) on the Cassini spacecraft is providing the first energetic neutral particle (ENA) measurements in the magnetosphere of Saturn. Advantageous spacecraft orbits during the first 120 days of 2007 allowed ENA observations to be mapped to the equatorial plane of the planet and surveyed as a statistical ensemble in a Sun-synchronous coordinate system. When projected onto the equatorial plane, emissions from both energetic hydrogen atoms (20–50 keV) and energetic oxygen atoms (64–144 keV) form toroidal distributions nearly concentric with the planet. The hydrogen torus is continuous in local time, but the oxygen torus has a gap from dawn to noon. When fitted to circles, the H torus has a mean radius of 11.0 ± 0.5 RS, while the O torus has a mean radius of 7.9 ± 0.8 RS (1 RS = 60268 km). Both tori display peaks at just before midnight at local times of 23.6 h (H) and 21.8 h (O). These maxima seem to be regular features over the 120-day interval surveyed, suggesting the hot spot may be caused by injection of particles from Saturn's magnetotail, as would be the case during substorms. However, the persistence of the ENA emissions suggests they are continuously driven by processes internal to Saturn's magnetosphere. When mapped along magnetic field lines to Saturn's ionosphere, both H and O emissions appear equatorward of the aurora and are distinct from it.

[1] The Magnetospheric Imaging Instrument (MIMI) on the Cassini spacecraft has observed energetic neutral atoms (ENA) at Saturn for three years from 2004 to 2007. The 20–50 keV hydrogen and 64–144 keV oxygen neutrals were examined for periodic behavior by projecting their images onto a plane perpendicular to Saturn's equatorial plane and summing the emissions along the vertical. The resulting “strip” images were then subjected to a Lomb periodogram analyses for the period between mid-2004 to mid-2007. The hydrogens display erratic periods in the 8–13 hour range, while the oxygens exhibit a very strong, repeatable period of 10.8 ± 0.2 hours, which is close to the nominal period of Saturn kilometric radiation as well as the period of energetic particles in Saturn's outer magnetosphere.

[1] A statistical analysis of observations of the Magnetospheric Imaging Instrument (MIMI) on the Cassini spacecraft at Saturn reveals the morphology and dynamics of energetic neutral atoms (ENA). The ENA (20–50 keV hydrogens) images are smoothed and projected onto Saturn's equatorial plane, and the intensities are slant-corrected using a thin-disk approximation. The resulting images exhibit intensity “blobs” that move in a prograde (corotational) sense about Saturn. Blob centroids are determined and tracked for as long as they persist as recognizable objects. The radial distribution of the centroids maximizes near the orbit of Rhea (∼9 RS, 1 RS = 60,268 km). The angular speeds of all blobs lasting longer than 5 h vary from near zero to nearly twice the rigid corotation speed. However, the longer-lived blobs (those lasting longer than 10 h) exhibit a sharp cutoff at the corotation speed. Inside the orbit of Rhea, the angular speeds decrease linearly with radial distance with a slope of −11.4°/h/RS, while outside the orbit of Rhea, the speeds are essentially constant at 13.1 ± 1.6°/h. Generally, the angular speeds differ from corotation (33.4°/h). The angular speeds tend to be somewhat slower near midnight than at other local times. The radial speed distribution peaks at the slightly inward value of −0.26 RS/h. Radial speeds exhibit a slight outflow from 6 to 10 h local time and a slight inflow from 16 to 20 h.

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

[1] The periodicities in electrons observed in Saturn's magnetosphere are examined using the new longitude system based on a drifting signal of Saturn kilometric radiation (SKR). When averaged into longitude and range bins over 50-day time periods, 28–48 keV electron intensities clearly evidence patterns that peak at successively increasing longitudes with increasing radial distance from Saturn. That is, the electrons form a spiral pattern in the quasi-corotational frame of SKR longitude. The spiral has only one “arm” that extends from ∼10 RS to as far as ∼60 RS from the planet (where 1 RS = 60268 km); the “arm” migrates an average of ∼3.4° in longitude for every RS of radial distance. The spiral does not remain fixed in SKR longitude, but changes its relative position on time scales of ∼50 days. The “base” of one spiral appears connected with a postulated convection outflow at ∼330° longitude.

[1] A Lomb periodogram analysis is applied to charged particle data from the LEMMS/CHEMS instruments on the Cassini spacecraft. The data represent count rates, averaged within 30 min bins, from electrons (28-330 keV) and protons and oxygen ions (2.8-236 keV) during 350 days in 2005 and all 365 days in 2006. Sun effects, spacecraft maneuvers, and measurements within 20 R S of Saturn were removed from the data prior to analysis. The main peaks in the frequency periodograms (or power spectra) were found within a frequency window from 9.5 hours to 12.5 hours. For signal-to-noise ratios exceeding 8, the periodograms within this window reveal a consistent peak near 10.80 hours (10 hours 48 min 36 sec) for all the charged particles regardless of energy or species. Even for lower signal-to-noise ratios, a peak near this period is generally present. The Lomb analyses are consistent with an azimuthal anomaly that rotates with a period of 10.80 hours.

[1] The Magnetospheric Imaging Instrument/Low-Energy Magnetospheric Measurements Sensor (MIMI/LEMMS) instrument on the Cassini spacecraft has measured energetic electrons in the energy range 20–300 keV within Saturn's magnetosphere. In the outer magnetosphere, beyond ∼20 RS, these electrons and their spectral index display strong variations with periods comparable to the 10.76 hour (10 hours, 45 min, 36 s) period measured by radio observations of Cassini. Inside ∼20 RS, such electron variations may be present but are masked by satellite and ring effects. Electron periodicities are most easily recognized on the “nightside” segments of the Cassini orbits, although they are also observed to some extent on the dayside. For both dayside and nightside a wavelet analysis of detrended count rates in the 20–40 RS region reveals a mean period of 10.52 ± 0.74 hours (10 hours, 31 min ± 44 min) for the six electron channels investigated. If constrained to the nightside only, a wavelet analysis gives a mean period of 10.88 ± 0.52 hours (10 hours, 53 min ± 31 min). Covering five orbits of the Cassini spacecraft during the 2-year period from Saturn Orbit Insertion (July 2004 to July 2006), this analysis indicates that a day-night asymmetry exists in the electron periodicity in Saturn's outer magnetosphere and suggests that dayside periodicities are much less prominent than nightside or dawnside periodicities.

The low-energy charged particle instrument on Voyager 1 measured low-energy electrons and ions (energies/= 26 and/= 40 kiloelectron volts, respectively) in Saturn's magnetosphere. The first-order ion anisotropies on the dayside are generally in the corotation direction with the amplitude decreasing with decreasing distance to the planet. The ion pitch-angle distributions generally peak at 90 degrees , whereas the electron distributions tend to have field-aligned bidirectional maxima outside the L shell of Rhea. A large decrease in particle fluxes is seen near the L shell of Titan, while selective particle absorption (least affecting the lowest energy ions) is observed at the L shells of Rhea, Dione, and Tethys. The phase space density of ions with values of the first invariant in the range approximately 300 to 1000 million electron volts per gauss is consistent with a source in the outer magnetosphere. The ion population at higher energies (/= 200 kiloelectron volts per nucleon) consists primarily of protons, molecular hydrogen, and helium. Spectra of all ion species exhibit an energy cutoff at energies/= 2 million electron volts. The proton-to-helium ratio at equal energy per nucleon is larger (up to approximately 5 x 10(3)) than seen in other magnetospheres and is consistent with a local (nonsolar wind) proton source. In contrast to the magnetospheres of Jupiter and Earth, there are no lobe regions essentially devoid of particles in Saturn's nighttime magnetosphere. Electron pitch-angle distributions are generally bidirectional andfield-aligned, indicating closed field lines at high latitudes. Ions in this region are generally moving toward Saturn, while in the magnetosheath they exhibit strong antisunward streaming which is inconsistent with purely convective flows. Fluxes of magnetospheric ions downstream from the bow shock are present over distances/= 200 Saturn radii from the planet. Novel features identified in the Saturnian magnetosphere include a mantle of low-energy particles extending inward from the dayside magnetopause to approximately 17 Saturn radii, at least two intensity dropouts occurring approximately 11 hours apart in the nighttime magnetosphere, and a pervasive population of energetic molecular hydrogen.

The low energy charged particle (LECP) experiment on the Voyager 1 and 2 spacecraft made measurements of the intensity, energy spectra, and spatial distributions of ions (30 keV ≲ E ≲ 150 MeV) and electrons (22 keV ≲ E ≲ 20 MeV) during encounters with the Saturnian magnetosphere in November 1980 and August 1981, respectively. Detailed analysis of the data has revealed the following: (1) Energetic ions are present in the interplanetary medium both upstream (to ∼200 Rs) and off the dawn bow shock (to ∼400 Rs) of the magnetosphere, with maximum energies ∼100 keV. (2) Low-energy (≳22 keV) electrons are generally depleted inward of L ∼ 10 Rs, while low-energy (≳30 keV) ions are greatly enhanced in the same region. (3) The composition of low-energy ions is most likely dominated by protons in the outer magnetosphere but is consistent with oxygen in the inner (L ≲ 9) magnetosphere. (4) The ion spectrum is described well by the κ distribution with characteristic temperatures kTH ranging from ∼15 to ∼55 keV; the hot plasma region is generally confined between the L shells of Tethys and Rhea but exhibits substantial variability. (5) The electron energy spectrum at L ≲ 10 develops a secondary peak at E ≳ 200 keV that shifts to higher (∼1 MeV) energies inside the orbits of Enceladus and Mimas, indicative of electron resonance interactions with the planetary satellites. (6) There is a noon-dawn asymmetry in ion and electron intensities with peak fluxes near the Rhea-Dione L shells at local morning; this is the region in local time where Saturn kilometric radiation is modulated by the presence of Dione. (7) The ion energy density (≳30 keV) represents a significant fraction of the field energy density in the outer magnetosphere of the planet (L ≳ 13 Rs), with values of β ranging from 0.1 up to ∼4, when projected to the equator. (8) Comparison of electron and ion intensities measured by Voyagers 1 and 2 in the inner (L ≲ 6) magnetosphere at common points in B, L space shows that the radiation belts are substantially stable over periods of ∼9 months; both ion and electron intensities compared well with Pioneer 11 observations in 1979. It is evident from the results that the inner satellites of Saturn play a dominant role in the determination of intensity and spectral features of energetic particles at L ≲ 10. These aspects of the data are discussed in the context of proposed physical mechanisms expected to be operating within the magnetosphere of Saturn.

The low-energy charged particle instrument on Voyager 2 measured low-energy electrons and ions (energies greater, similar 22 and greater, similar 28 kiloelectron volts, respectively) in Saturn's magnetosphere. The magnetosphere structure and particle population were modified from those observed during the Voyager 1 encounter in November 1980 but in a manner consistent with the same global morphology. Major results include the following. (i) A region containing an extremely hot ( approximately 30 to 50 kiloelectron volts) plasma was identified and extends from the orbit of Tethys outward past the orbit of Rhea. (ii) The low-energy ion mantle found by Voyager 1 to extend approximately 7 Saturn radii inside the dayside magnetosphere was again observed on Voyager 2, but it was considerably hotter ( approximately 30 kiloelectron volts), and there was an indication of a cooler (20 kiloelectron volts) ion mantle on the nightside. (iii) At energies greater, similar 200 kiloelectron volts per nucleon, H(1), H(2), and H(3) (molecular hydrogen), helium, carbon, and oxygen are important constituents in the Saturnian magnetosphere. The presence of both H(2) and H(3) suggests that the Saturnian ionosphere feeds plasma into the magnetosphere, but relative abundances of the energetic helium, carbon, and oxygen ions are consistent with a solar wind origin. (iv) Low-energy ( approximately 22 to approximately 60 kiloelectron volts) electron flux enhancements observed between the L shells of Rhea and Tethys by Voyager 2 on the dayside were absent during the Voyager 1 encounter. (v) Persistent asymmetric pitch-angle distributions of electrons of 60 to 200 kiloelectron volts occur in the outer magnetosphere in conjunction with the hot ion plasma torus. (vi) The spacecraft passed within approximately 1.1 degrees in longitude of the Tethys flux tube outbound and observed it to be empty of energetic ions and electrons; the microsignature of Enceladus inbound was also observed. (vii) There are large fluxes of electrons of approximately 1.5 million electron volts and smaller fluxes of electrons of approximately 10 million electron volts and of protons greater, similar 54 million electron volts inside the orbits of Enceladus and Mimas; all were sharply peaked perpendicular to the local magnetic field. (viii) In general, observed satellite absorption signatures were not located at positions predicted on the basis of dipole magnetic field models.