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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

The location of the auroral oval and the intensity of the auroral precipitation within it are basic elements in any adequate characterization of the state of the magnetosphere. Yet despite the many ground-based and spacecraft-borne instruments monitoring various aspects of auroral behavior, there are no clear and consistent answers available to those wishing to locate the auroral oval or to quantify its intensity. The purpose of OVATION is to create a tool which does so. OVATION is useful both for archival purposes and for space weather nowcasting. The long-running DMSP particle data set, which covers both hemispheres, and has operated since the early 1980s, and which will continue to operate well into the next decade, is chosen as a calibration standard. Other data sets, including global images from Polar UVI, SuperDARN boundaries, and meridian scanning photometer images, are cross-calibrated to the DMSP standard. Each incorporated instrument has its average offset from the DMSP standard determined as a function of MLT, along with the standard deviations. The various data can, therefore, be combined in a meaningful manner, with the weight attached to a given boundary measurement varying inversely with the variance (square of the standard deviation). OVATION currently spans from December 1983 through the present, including real-time data. Participation of additional experimenters is highly welcomed. The only prerequisites are a willingness to conduct the prescribed cross-calibration procedure, and to make the data available online. The real-time auroral oval location can be found here: http://sd-www.jhuapl.edu/Aurora/ovation live/northdisplay.html.Key words. Magnetospheric physics (auroral phenomena; energetic particles, precipitating; magnetosphere – ionosphere interactions)

Pitch angle distributions (PADs) of very energetic electrons (110–365 keV) are examined during the ring‐grazing and proximal orbits of the Cassini spacecraft, from day 320 2016 (15 November) to day 257 2017 (14 September). These repeating orbits allowed a statistical evaluation of the PADs within the magnetopause on the nightside of Saturn. Along L‐shells (i.e., equatorial crossing distances of magnetic field lines) near and outside that of Titan and north of the equator, the electron fluxes were unidirectionally field‐aligned going away from Saturn. Along L‐shells inside Titan's and south of the equator, the electrons had bidirectional or pancake (trapping) PADs. This behavior suggests that the field lines within Titan's L‐shell are generally closed, while those outside of that L‐shell are generally open. This result strictly applies only to the nightside local times sampled during the final Cassini orbits, but one may infer a similar behavior at other times.

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.

Contrary to the permanent MeV ion belts which are relatively stable in intensity over both short and long time scales and are modulated by a single Galactic Cosmic Ray driven source, the electron belts of Saturn appear to be much more complex in both structure and temporal evolution. In order to understand the responses of this system to the different factors that may control it (internal or external/solar sources) we study its long-term, temporal evolution. We achieve that by tracking the equatorial distance of the belts׳ outer boundary, using MIMI/LEMMS energetic charged particle observations over a period of more than 7 years. This boundary is defined at the distance that a selected count rate level is measured in a LEMMS channel that has the properties of an omnidirectional, integral energy detector. Simulated solar wind moments, energetic neutral atom (ENA) observations and solar irradiance data are used to support the analysis. In many cases, correlations of the different datasets are weak, suggesting that the electron belts are modulated in time scales that are much shorter than the sampling of the electron belt boundary (typically every 10–30 days). Still, we find several cases of persistent, long term and strong perturbations in the system that appear to have corresponding disturbances in the extension of the electron belts, even on such long time scales. From the analysis of those intervals we believe that we have established a solid link with the planetary ring current as the primary source of the electron belts of Saturn. This is concluded mostly on the basis of an extended ring current decay in 2011 (inferred through ENA observations), coinciding with a similar, unusual drop in the electron belt extension (and intensity). This means that the electron belts should reflect also the modulation of the ring current. We suggest that possible sources of long term modulation are both the solar UV irradiance of the Saturnian thermosphere and the solar wind. The former appears to acquire large values and exhibit significant changes during the period of the electron belt/ring current decay, while an approximate 2-week periodicity (characteristic for the case of two solar wind streams per solar rotation) appears in the electron belts in the form of recurrent expansions or intensifications. The latter finding indicates that external forcing by the solar wind can have a considerable impact on the planet׳s electron belts, contrary to the expectations that Saturn׳s magnetospheric dynamics are rotationally dominated and internally driven.

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.

The kappa distribution function has become an essential form for analyzing trapped distributions of charged particles in space. It is loosely based on the Maxwellian distribution and can be thought of as a generalization of it, specifically with the capacity to describe higher fluxes at high energy, i.e., exhibiting a power law tail at the highest energies that roll over at lower energies to become Maxwellian at the lowest energies. Given that the charged particle populations themselves can have many different kinds of sources (solar wind, atmosphere, volcanic moons, etc.) and sinks (collisions with ambient neutrals or moon or ring surfaces, reconnection, etc.) and move under the influence of strong and dynamic planetary magnetic fields, it is advantageous to have a function flexible enough to capture the essential properties of entire particle distributions, e.g., plasma ( 5 keV). However, some distributions are well modeled by combinations (or modifications) of kappa distributions, since the most accurate quantitative characterizations often need even more flexibility than afforded by the standard kappa function. In this chapter we will review the use of kappa distributions to describe charged particles in the magnetospheres of the gas giant planets (Jupiter, Saturn, Uranus, and Neptune) and the inner planets (Earth and Mercury) that have allowed the determination of physical quantities of the plasma and energetic particle spectra such as temperatures, densities, pressures, spectral indices, and convection bulk velocities for one or more species, as available. These are critical quantities for understanding the structure and dynamics of magnetospheres. The dependence of these quantities on spatial and magnetic field parameters has rendered the kappa distribution a powerful diagnostic tool for understanding processes in space plasma physics. We also consider the degree to which the kappa distribution succeeds or fails to characterize planetary particle distributions under different conditions.

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.

We demonstrate that under some magnetospheric conditions protons and oxygen ions are accelerated once per Saturn magnetosphere rotation, at a preferred local time between midnight and dawn. Although enhancements in energetic neutral atom (ENA) emission may in general occur at any local time and at any time in a Saturn rotation, those enhancements that exhibit a recurrence at a period very close to Saturn's rotation period usually recur in the same magnetospheric location. We suggest that these events result from current sheet acceleration in the 15–20 Rs range, probably associated with reconnection and plasmoid formation in Saturn's magnetotail. Simultaneous auroral observations by the Hubble Space Telescope (HST) and the Cassini Ultraviolet Imaging Spectrometer (UVIS) suggest a close correlation between these dynamical magnetospheric events and dawn-side transient auroral brightenings. Likewise, many of the recurrent ENA enhancements coincide closely with bursts of Saturn kilometric radiation, again pointing to possible linkage with high latitude auroral processes. We argue that the rotating azimuthal asymmetry of the ring current pressure revealed in the ENA images creates an associated rotating field aligned current system linking to the ionosphere and driving the correlated auroral processes.

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.

During the Leonid meteor shower on 18 November 1999, the five spectrographic imagers onboard the Midcourse Space Experiment (MSX) satellite recorded the first complete meteor spectra from 110 to 860 nm. The observation occurred at 00:23:36.2 UT, at which time satellite was pointed at a tangent altitude of 100 km over 37.2°N and 78.2°E. The spectrograph slits were oriented approximately parallel to the horizon, and the meteor passed approximately perpendicular through the slits’ fields of view. All five spectrographic images serendipitously observed the passage of the object and each recorded 1–2 frames of data. Common meteor emissions from iron, sodium, and oxygen were observed in the visible, but the ultraviolet spectrum displayed a wealth of new features. The most intense features appeared in the middle ultraviolet (250–300 nm) from neutral and singly ionized iron and singly ionized magnesium. Far ultraviolet (110–130 nm) features included oxygen and nitrogen, a bright hydrogen Lyman α emission, and ionized iron and magnesium emissions. Also seen in the far ultraviolet were weak but recognizable emissions from neutral carbon. The ultraviolet emissions were much stronger than those in the visible and near-infrared. The meteor had a visual magnitude of −2.8 at 100 km, but the energy of emissions in the ultraviolet (110–337 nm) exceeded the energy of the visible (337–560 nm) by a factor of at least 5. The new Lyman α and carbon emissions in the far UV as well as the overall intensity of the ultraviolet spectrum suggest important applications for ultraviolet observations of meteors.

The Ultraviolet and Visible Imaging and Spectrographic Imaging (UVISI) instrument on the Midcourse Space Experiment (MSX) satellite obtained unique ultraviolet (200–350 nm) images and spectrographic images of polar mesospheric clouds over both the northern and southern poles during the summers of 1997–1999. Imager observations revealed that PMCs have essentially the same altitudes in the north and south (82.7 ± 1.3 km) and that cloud altitudes remain essentially constant with latitude. Imagery from below the horizon showed PMC structures with scales ranging from 100 to over 500 km. Integrated from 235 nm to 265 mn, PMC limb intensities were essentially the same in both hemispheres, averaging 12 mega-Rayleighs at peak altitude. Intensities increased with latitude toward the poles. Spectrographic observations reveal the clouds have a solar-like spectrum from 200 nm to 600 nm, but that short of 315 nm the spectrum is contaminated with ground albedo effects. Between 200 nm and 315 nm, the scattering ratio (PMC spectral intensity to solar irradiance) is less steep than that expected from pure Rayleigh scattering. When the scattering ratio is least-squares fit to a log-normal distribution of Mie scatterers, the distribution has a mode radius of 65 nm and a dispersion of 1.2 with no dependence on latitude or time from solstice. The slope of the scattering ratio changes as a function of altitude relative to the altitude peak of the PMC intensity, suggesting that particles 3 km above this peak are smaller than particles at the peak and 3 km below.

During the Leonid meteor shower on 18 November 1999, the five spectrographic imagers onboard the Midcourse Space Experiment (MSX) satellite recorded the first complete meteor spectra from 110 to 860 nm. The observation occurred at 00:23:36.2 UT, at which time the satellite was pointed at a tangent altitude of 100 km over 37.2°N and 78.2°E. The spectrograph slits were oriented approximately parallel to the horizon at a tangent altitude of 100 km, and the meteor passed approximately perpendicular through the slits’ fields of view. All five spectrographic imagers observed the passage of a bright object (mv

A middle ultraviolet imager over the northern and southern polar regions has measured the scattered intensities of polar mesospheric clouds (PMC) during the austral summer of 1997–98 and the arctic summer of 1999. The intensities, corrected for solar zenith angle, are those at the peak in the PMC radiance profiles for a waveband of 235–263 nm. The peak radiance averaged 12.0±11.0 mega-Rayleighs (MR) for 13 passes over the south polar region, where the scattering angle was ∼111°. The peak radiance averaged 11.4±7.6 MR for 12 passes over the north polar region, where the scattering angle was ∼48°. The PMC intensities increased with increasing latitude in both the north and the south, but the rate of increase was twice as fast in the south (0.60 MR/° latitude) as in the north (0.32 MR/° latitude).

Simultaneous measurements of auroral limb H Lyman α, H Balmer α, H Balmer β, N2+ 1 NG 391.4-nm, and N2 2 PG 337.1-nm emissions excited by combined proton/hydrogen/electron precipitation are reported. The data were recorded by the Ultraviolet and Visible Imaging and Spectrographic Imaging (UVISI) spectrographic imagers on the Midcourse Space Experiment (MSX) satellite on November 10, 1996, while viewing a diffuse emission region during a limb-scanning data collection event. Energy fluxes associated with the electron and proton/hydrogen precipitation components are estimated with one-dimensional fits to selected limb profiles using a transport-theoretic model. Spectral radiances (110–900 nm) are also presented. The data presented here are but one example of the large number of MSX/UVISI data sets that have been collected offering opportunities for scientific investigations of auroral emission phenomena and for retrieval of composition and particle precipitation parameters from optical remote sensing data.