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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.

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

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.

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.

[1] A 2–3 day periodicity observed in Jupiter's magnetosphere (superposed on the giant planet's 9.5 h rotation rate) has been associated with a characteristic mass-loading/unloading period at Jupiter. We follow a method derived by Kronberg et al. (2007) and find, consistent with their results, that this period is most likely to fall between 1.5 and 3.9 days. Assuming the same process operates at Saturn, we argue, based on equivalent scales at the two planets, that its period should be 4 to 6 times faster at Saturn and therefore display a period of 8 to 18 h. Applying the method of Kronberg et al. for the mass-loading source rates estimated by Smith et al. (2010) based on data from the third and fifth Cassini-Enceladus encounters, we estimate that the expected magnetospheric refresh rate varies from 8 to 31 h, a range that includes Saturn's rotation rate of ~10.8 h. The magnetospheric period we describe is proportional to the total mass-loading rate in the system. The period is, therefore, faster (1) for increased outgassing from Enceladus, (2) near Saturn solstice (when the highest proportion of the rings is illuminated), and (3) near solar maximum when ionization by solar photons maximizes. We do not claim to explain the few percent jitter in period derived from Saturn Kilometric Radiation with this model, nor do we address the observed difference in period observed in the north and south hemispheres.

The moon Enceladus, embedded in Saturn’s radiation belts, is the main internal source of neutral and charged particles in the Kronian magnetosphere. A plume of water ice molecules and dust released through geysers on the south polar region provides enough material to feed the E-ring and also the neutral torus of Saturn and the entire magnetosphere. In the time period 2005–2010 the Cassini spacecraft flew close by the moon 14 times, sometimes as low as 25km above the surface and directly through the plume. For the very first time measurements of plasma and energetic particles inside the plume and its immediate vicinity could be obtained. In this work we summarize the results of energetic electron measurements in the energy range 27keV to 21MeV taken by the Low Energy Magnetospheric Measurement System (LEMMS), part of the Magnetospheric Imaging Instrument (MIMI) onboard Cassini in the vicinity of the moon in combination with measurements of the magnetometer instrument MAG and the Electron Spectrometer ELS of the plasma instrument CAPS onboard the spacecraft. Features in the data can be interpreted as that the spacecraft was connected to the plume material along field lines well before entering the high density region of the plume. Sharp absorption signatures as the result of losses of energetic electrons bouncing along those field lines, through the emitted gas and dust clouds, clearly depend on flyby geometry as well as on measured pitch angle/look direction of the instrument. We found that the depletion signatures during some of the flybys show “ramp-like” features where only a partial depletion has been observed further away from the moon followed by nearly full absorption of electrons closer in. We interpret this as partially/fully connected to the flux tube connecting the moon with Cassini. During at least two of the flybys (with some evidence of one additional encounter) MIMI/LEMMS data are consistent with the presence of dust in energetic electron data when Cassini flew directly through the south polar plume. In addition we found gradients in the magnetic field components which are frequently found to be associated with changes in the MIMI/LEMMS particles intensities. This indicates that complex electron drifts in the vicinity of Enceladus could form forbidden regions for electrons which may appear as intensity drop-outs.

Narrowband emission (NB) is observed at Saturn centered near 5 kHz and 20 kHz and harmonics. This emission appears similar in many ways to Jovian kilometric narrowband emission observed at higher frequencies, and therefore may have a similar source mechanism. Source regions of NB near 20 kHz are believed to be located near density gradients in the inner magnetosphere and the emission appears to be correlated with the occurrence of large neutral plasma clouds observed in the Saturn magnetotail. In this work we present the results of a growth rate analysis of NB emission (~20 kHz) near or within a probable source region. This is made possible by the sampling of in-situ wave and particle data. The results indicate waves are likely to be generated by the mode-conversion of directly generated Z-mode emission to O-mode near a density gradient. When the local hybrid frequency is close n fce (n is an integer and fce is the electron cyclotron frequency) with n=4, 5 or 6 in our case, electromagnetic Z-mode and weak ordinary (O-mode) emission can be directly generated by the cyclotron maser instability.

We present the two-stage method used to calibrate the electron spectrometer (ELS), part of the plasma spectrometer (CAPS) on board the Cassini spacecraft currently in orbit around Saturn. The CAPS-ELS is a top-hat electrostatic analyser designed to measure electron fluxes between 0.5 eV and 26 keV. The on-ground calibration method described here includes the production of photoelectrons, which are energised and passed into the CAPS-ELS in a purpose designed calibration facility. Knowledge of the intensity of these incident electrons and the subsequent instrument output provides an on-ground calibrated geometric factor. Comparative studies of physical quantities such as plasma density and electron differential flux calculated using on-ground calibration factor with the quantities deduced from the wave experiment and high energy electron detector provide in-flight calibration. The results of this are presented together with a comparison of the experimentally calibrated values with simulated calibration values.

During its tour Cassini has observed numerous plasma injection events in Saturn's inner magnetosphere. Here, we present a case study of one “young” plasma bubble observed when Cassini was in the equatorial plane. The bubble was observed in the equatorial plane at ∼7 Saturn radii from Saturn and had a maximum azimuthal extent of ∼0.25 Rs (Rs=Saturn radius ∼60330 km). We show that the electron density inside the event is lower by a factor ∼3 and the electron temperature higher by over an order of magnitude compared to its surroundings. The injection contains slightly increased magnetic field magnitude of 49 nT compared with a background field of 46 nT. Modelling of pitch angle distributions inside the plasma bubble and measurements of plasma drift provide a novel way to estimate that the bubble originated between 9

In this paper we present two methods to derive electron fluid parameters from the CAPS–ELS spectrometer on board the Cassini spacecraft currently in orbit around Saturn. In the first part of the paper we give a basic overview of the instrument and describe the challenges inherent in the derivation of density and temperature values using these techniques. We then describe a method to calculate electron moments by integrating the particle distribution function. We also describe a second technique in which we fit the electron energy spectrum with a Gaussian curve and use the peak energy of this curve to derive density and temperature values. We then compare the two methods with particular emphasis on their application to Cassini SOI observations in the saturnian environment and point out the limitations of the two techniques. We will show that results from the two very different methods are in agreement when the physical properties of the environment and of the observed electron populations have been inferred from inspection of the raw data. Finally we will suggest future developments that will remove these limitations.

We present new and definitive results of Cassini plasma spectrometer (CAPS) data acquired during passage through Saturn's inner plasmasphere by the Cassini spacecraft during the approach phase of the Saturn orbit insertion period. This analysis extends the original analysis of Sittler et al. [2005. Preliminary results on Saturn's inner plasmasphere as observed by Cassini: comparison with Voyager. Geophys. Res. Lett. 32, L14S07, doi:10.1029/2005GL022653 ] to L∼10 along with also providing a more comprehensive study of the interrelationship of the various fluid parameters. Coincidence data are sub-divided into protons and water group ions. Our revised analysis uses an improved convergence algorithm which provides a more definitive and independent estimate of the spacecraft potential ΦSC for which we enforce the protons and water group ions to co-move with each other. This has allowed us to include spacecraft charging corrections to our fluid parameter estimations and allow accurate estimations of fluctuations in the fluid parameters for future correlative studies. In the appendix we describe the ion moments algorithm, and minor corrections introduced by not weighting the moments with sinθ term in Sittler et al. [2005] (Correction offset by revisions to instruments geometric factor). Estimates of the spacecraft potential and revised proton densities are presented. Our total ion densities are in close agreement with the electron densities reported by Moncuquet et al. [2005. Quasi-thermal noise spectroscopy in the inner magnetosphere of Saturn with Cassini/RPWS: electron temperatures and density. Geophys. Res. Lett. 32, L20S02, doi:10.1029/2005GL022508 ] who used upper hybrid resonance (UHR) emission lines observed by the radio and plasma wave science (RPWS) instrument. We show a positive correlation between proton temperature and water group ion temperature. The proton and thermal electron temperatures track each with both having a positive radial gradient. These results are consistent with pickup ion energization via Saturn's rotational electric field. We see evidence for an anti-correlation between radial flow velocity VR and azimuthal velocity Vφ, which is consistent with the magnetosphere tending to conserve angular momentum. Evidence for MHD waves is also present. We show clear evidence for outward transport of the plasma via flux tube interchange motions with the radial velocity of the flow showing positive radial gradient with V R ∼ 0.12 ( L / 4 ) 5.5 km / s functional dependence for 4 D LL ∼ D 0 L 11 for fixed stochastic time step δt). Previous models with centrifugal transport have used D LL ∼ D 0 L 3 dependence. The radial transport seems to begin at Enceladus’ L shell, L∼4, where we also see a minimum in the W+ ion temperature T W ∼ 35 eV . For the first time, we are measuring the actual flux tube interchange motions in the magnetosphere and how it varies with radial distance. These observations can be used as a constraint with regard to future transport models for Saturn's magnetosphere. Finally, we evaluate the thermodynamic properties of the plasma, which are all consistent with the pickup process being the dominant energy source for the plasma.

We analyze electron plasma, energetic ion, and magnetic field data from four almost vertical Cassini passes through the nightside plasma sheet of Saturn (segments of the high-latitude orbits of the spacecraft) separated in two subsets: two passes of identical geometry from January 2007 with Cassini crossing the equatorial plane in the postmidnight sector at a distance of similar to 21 Saturn radii (R-S) and two passes from April 2009, also of identical geometry, with Cassini crossing the equatorial plane in the premidnight sector again at a distance of similar to 21 R-S. The vertical structure and variability of the plasma sheet is described for each individual pass, and its basic properties (scale height, vertical displacement, tilt angle, hinging distance) are computed. The plasma sheet presents an energy-dependent vertical structure, being thicker by a factor of similar to 2 in the energetic particle range than in the electron plasma. It further exhibits intense dynamical behavior, evident in the energetic neutral atom emission. In two of the four passes, we observe a clear north-south asymmetry, presumably a combined result of vertical plasma sheet motion and short time scale dynamics. Comparison between the 2007 and 2009 passes reveals a clear change in the tilt and vertical offset of the planetary nightside plasma sheet, which progressively becomes aligned to the solar wind direction as we approach Saturnian equinox (August 2009). Temperature, pressure, and number density in the center of the sheet remain relatively stable and essentially unaffected by the seasonal change.

[1] In the inner part of a rapidly rotating magnetosphere such as that of Saturn, the major observable signature of radial plasma convection is a series of longitudinally localized injections and simultaneous drift dispersions of hot tenuous plasma from the outer magnetosphere. The Cassini Plasma Spectrometer (CAPS) and the Cassini Magnetospheric Imaging Instrument (MIMI) have observed signatures of these processes frequently, thus providing direct evidence for Saturn's magnetospheric convective motions, in which the radial transport of plasma comprises hot, tenuous plasma moving inward and cooler, denser plasma moving outward. On the basis of an extended statistical sample of these injection/dispersion events, we find that the inflow channels occupy only a small fraction (∼7%) of the total available longitudinal space, indicating that the inflow speed is much larger than the outflow speed. We assume that the plasma is largely confined to a thin equatorial sheet and calculate its thickness by deriving the centrifugal scale height profile based on the CAPS observations. We also present the radial and longitudinal dependences of flux tube mass content as well as the total ion mass between 5 and 10 Saturn radii. Combining these results, we estimate a global plasma mass outflow rate ∼280 kg/s.

[1] We present energetic electron data obtained by Cassini’s Magnetospheric Imaging Instrument in the inner magnetosphere of Saturn. We find here that inward transport and energization processes are consistent with conservation of the first two adiabatic invariants of motion. We model several injections near local midnight, one injection has a maximum energy of hundreds of keV, that are consistent with data. We also present mission‐ averaged data that shows an injection boundary in radial distance. Inward of this boundary, fluxes fall off toward the planet. Around this inner boundary, strong local time asymmetries are present in the averaged data with peak fluxes near midnight.

[1] We study the azimuthal plasma velocity in Saturn's magnetosphere between 3 and 13 Saturn radii (Rs) by analyzing energetic particle injection events using data of the Magnetospheric Imaging Instrument (MIMI) onboard the Cassini spacecraft in orbit around Saturn. Due to the magnetic drifts, the injected particles at various energies begin to disperse and leave an imprint in the electron as well as in the ion energy spectrograms of the MIMI instrument. The shape of these profiles strongly depends on the azimuthal velocity distribution of the magnetospheric plasma and the age of the injection event. Comparison of theoretically computed dispersion profiles with observed ones enables us to characterize the azimuthal flow of the plasma. The measured flow profile clearly shows that the plasma subcorotates with velocities as low as 80% of full corotation at radial distances between 8 Rs to 13 Rs. With knowledge of the flow profile and the ages of each injection event we can calculate the location where the energetic particles were injected into the inner magnetosphere. The night and morning sector of the Kronian magnetosphere are preferred regions for the generation of hot plasma injections.

We report initial results on the particle pressure distribution and its contribution to ring current density in the equatorial magnetosphere of Saturn, as measured by the Magnetospheric Imaging Instrument (MIMI) and the Cassini Plasma Spectrometer (CAPS) onboard the Cassini spacecraft. Data were obtained from September 2005 to May 2006, within +/- 0.5 R-S from the nominal magnetic equator in the range 6 to 15 RS. The analysis of particle and magnetic field measurements, the latter provided by the Cassini magnetometer (MAG), allows the calculation of average radial profiles for various pressure components in Saturn's magnetosphere. The radial gradient of the total particle pressure is compared to the inertial body force to determine their relative contribution to the Saturnian ring current, and an average radial profile of the azimuthal current intensity is deduced. The results show that: ( 1) Thermal pressure dominates from 6 to 9 RS, while thermal and suprathermal pressures are comparable outside 9 RS with the latter becoming larger outside 12 RS. ( 2) The plasma beta (particle/magnetic pressure) remains >= 1 outside 8 RS, maximizing (similar to 3 to similar to 10) between 11 and 14 RS. ( 3) The inertial body force and the pressure gradient are similar at 9-10 R-S, but the gradient becomes larger >= 11 R-S. ( 4) The azimuthal ring current intensity develops a maximum between approximately 8 and 12 RS, reaching values of 100-150 pA/m(2). Outside this region, it drops with radial distance faster than the 1/r rate assumed by typical disk current models even though the total current is not much different to the model results.

[1] Narrowband emission is observed at Saturn centered near 5 kHz and 20 kHz and harmonics of 20 kHz. This emission appears to be in many ways similar to Jovian narrowband emission observed at higher frequencies. We analyze one example of this emission near a possible source region. In situ electron distributions suggest narrowband emission has a source region associated with electrostatic cyclotron harmonic and upper hybrid emission. Linear growth rate calculations indicate that the observed plasma distributions are unstable to the growth of electrostatic harmonic emissions. In addition, it is found that when the local upper hybrid frequency is close to 2 fce or 3 fce (fce is the electron cyclotron frequency), electromagnetic Z mode and weak ordinary (O mode) emission can be directly generated by the cyclotron maser instability. In the presence of density gradients, Z mode emission can mode-convert into O mode emission, and this might explain the narrowband emission observed by the Cassini spacecraft.

We present a model wherein electrons produced in Saturn's inner magnetosphere circulate through a combination of outward and inward motions driven by the centrifugal interchange instability and azimuthal motion through gradient and curvature drifts. Cool (< 100 eV) electrons produced inside L similar to 12 move slowly outward. To balance outflowing flux, inward transport occurs in small scale injection events. Electrons in these inwardly moving flux tubes are heated adiabatically to energies greater than 100 eV and their pitch angle distributions evolve from isotropic to "pancake'' ( peaked at 90 degrees). We show that this evolution is observed, and that the pitch angle distributions observed inside a plasma injection are consistent with loss free inward adiabatic transport from L similar to 11. As the flux tube moves inward the warm electrons undergo energy dependent gradient and curvature drifts out of the inwardly moving flux tube and find themselves superposed on cold, locally produced, plasma. At this point they turn around and are transported along with the cold plasma back toward the outer magnetosphere. With reasonable assumptions about scattering and loss this motion can naturally lead to the "butterfly'' electron pitch angle distributions ( with local minima at both 90 degrees and 0/180 degrees) that are observed in the warm electron plasma component. We note that we cannot reproduce the butterfly distributions using loss free outward adiabatic transport alone. This is to be expected because there exist pitch angle dependent losses in the form of Saturn's neutral gas cloud and E-ring.

Saturn's ring current was observed by Pioneer 11 and the two Voyager spacecraft to extend 8 - 16 R-S in the equatorial plane and appeared to be driven by stress balance with the centrifugal force. We present Cassini observations that show thin current sheets on the dawn flank of Saturn's magnetosphere, symptomatic of the formation of a magnetodisc. We show that the centrifugal force is the dominant mechanical stress in these current sheets, which reinforces a magnetodisc interpretation - the formation of the current sheet is fundamentally rotational in origin. The stress balance calculation is also used to estimate the mass density in the disc, which show good agreement with independent in-situ measurements of the density. We estimate the total mass in the magnetodisc to be similar to 10(6) kg.

[1] We investigate the sources of two different electron components in Saturn's inner magnetosphere (5 100 eV) is Saturn's middle or outer magnetosphere, perhaps transported to the inner magnetosphere by radial diffusion regulated by interchange-like injections. Hot electrons undergo heavy losses inside L ∼ 6 and the distance to which the hot electron component penetrates into the neutral cloud is energy-dependent, with the coolest fraction of the hot plasma penetrating to the lowest L-shells. This can arise through energy-dependent radial transport during the interchange process and/or loss through the planetary loss cone.

[1] Radial convective transport of plasma in a rotation-dominated magnetosphere implies alternating longitudinal sectors of cooler, denser plasma moving outward and hotter, more tenuous plasma moving inward. The Cassini Plasma Spectrometer (CAPS) has provided dramatic new evidence of this process operating in the magnetosphere of Saturn. The inward transport of hot plasma is accompanied by adiabatic gradient and curvature drift, producing a V-shaped dispersion signature on a linear energy-time plot. Of the many (∼100) such signatures evident during the first two Cassini orbits, we analyze a subset (48) that are sufficiently isolated to allow determination of their ages, widths, and injection locations. Ages are typically

We present observations of thermal ( similar to 0.6 - 100eV) electrons observed near Saturn's main rings during Cassini's Saturn Orbit Insertion (SOI) on 1 July 2004. We find that the intensity of electrons is broadly anticorrelated with the ring optical depth at the magnetic footprint of the field line joining the spacecraft to the rings. We see enhancements corresponding to the Cassini division and Encke gap. We suggest that some of the electrons are generated by photoemission from ring particle surfaces on the illuminated side of the rings, the far side from the spacecraft. Structure in the energy spectrum over the Cassini division and A-ring may be related to photoelectron emission followed by acceleration, or, more likely, due to photoelectron production in the ring atmosphere or ionosphere.

[1] We present an analysis of Saturn's inner plasmasphere as observed by the Cassini Plasma Spectrometer (CAPS) experiment during Cassini's initial entry into Saturn's magnetosphere when the spacecraft was inserted into orbit around Saturn. The ion fluxes are divided into two sub-groups: protons and water group ions. We present the relative amounts of these two groups and the first estimates of their fluid parameters: ion density, flow velocity and temperature. We also compare this data with electron plasma measurements. Within the plasmasphere and inside of Enceladus' orbit, water group ions are about a factor of ∼10 greater than protons in number with number densities exceeding 40 cm−3. Within this inner region the spacecraft acquires a negative potential so that the electron density is underestimated. The electron and proton temperatures, which could not be measured in this region by Voyager, are T ∼ 2 eV at L ∼ 3. Also, within this inner region the protons, because of a negative spacecraft potential, appear to be super-corotating. By enforcing the condition that protons and water group ions are co-moving we may be able to acquire an independent estimate of the spacecraft potential relative to that estimated when comparing ion-electron measurements. Using our estimates of plasma properties, we estimate the importance of the rotating plasma on the stress balance equation for the inner magnetosphere and corresponding portion of the ring current.

Plasma waves observed in the Saturn magnetosphere provide an indication of the plasma population present in the rotationally dominated inner magnetosphere. Electrostatic cyclotron emissions often with harmonics and whistler mode emission are a common feature of Saturn's inner magnetosphere. The electron observations for a region near 5 RS outside and near a plasma injection region indicate a cooler low-energy (E>1000 eV) more pancake or butterfly distribution. We model the electron plasma distributions to conduct a linear dispersion analysis of the wave modes. The results suggest that the electrostatic electron cyclotron emissions can be generated by phase space density gradients associated with a loss cone that may be up to 20° wide. This loss cone is sometimes, but not always, observed because the field of view of the electron detectors does not include the magnetic field line at the time of the observations. The whistler mode emission can be generated by the pancake-like distribution and temperature anisotropy (T⊥/T||>1) of the warmer plasma population.