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The plumes of Enceladus produce a cloud of neutral H2O molecules and, via dissociation, OH and O. These neutrals are ionized by charge exchange, solar UV, and electron impacts, producing the therma...

Ionospheric outflow is driven by an ambipolar electric field induced due to the separation of electrons and ions in a gravitational field when equilibrium along a magnetic field line is lost. A model of ionospheric outflow at Saturn was developed using transport equations to estimate the number of charged particles that flow from the auroral regions into the magnetosphere. The model evaluates the outflow from 1,400 km in altitude above the 1 bar level, to 3 RS along the field line. The main ion constituents evaluated are R+ and R+3. We consider the centrifugal force exerted on the particles due to a fast rotation rate, along with the effects of field‐aligned currents present in the auroral regions. The total number flux from both auroral regions is found to be 5.5–13.0×1027 s−1, which relates to a total mass source of 5.5–17.7 kg s−1. These values are on average an order of magnitude higher than expected without the additional effects of centrifugal force and field‐aligned currents. We find the ionospheric outflow rate to be comparable to the lower estimates of the mass loading rate from Enceladus and are in agreement with recent Cassini observations. This additional mass flux into the magnetosphere can substantially affect the dynamics and composition of the inner and middle magnetosphere of Saturn.

We conduct a survey of electrostatic electron cyclotron harmonic (ECH) emissions observed at Saturn by the radio and plasma wave science (RPWS) investigation on board the Cassini spacecraft. These emissions are known to be effective at interacting with electrons in the terrestrial inner magnetosphere, producing electron scattering into the loss cone and acceleration [cf. Horne and Thorne, 2000; Thorne et al., 2010]. At Saturn ECH emission occurs with high probability and at strong intensity near the magnetic equator, outside the Enceladus torus in the range ~5 < L < ~10. Inside the inner boundary of the torus, ECH emissions are also observed near the equator and at higher latitude. Intensity levels of ECH emission are comparable to those observed at Earth, higher than Saturn chorus and Z-mode emission, and are likely to scatter electrons into the loss cone as at Earth. ECH waves are particularly intense and extend to higher harmonics within some plasma injection regions. We present results for a survey of over 8 years of Saturn data for fundamental and up to three harmonics of fce, the electron cyclotron frequency.

This thesis presents a study of the inner plasma disk of Saturn. The results are derived from measurements by the instruments on board the Cassini spacecraft, mainly the Cassini Langmuir probe (LP), which has been in orbit around Saturn since 2004. One of the great discoveries of the Cassini spacecraft is that the Saturnian moon Enceladus, located at 3.95 Saturn radii (1 RS = 60,268 km), constantly expels water vapor and condensed water from ridges and troughs located in its south polar region. Impact ionization and photoionization of the water molecules, and subsequent transport, creates a plasma disk around the orbit of Enceladus. The plasma disk ion components are mainly hydrogen ions H+ and water group ions W+ (O+, OH+, H2O+, and H3O+). The Cassini LP is used to measure the properties of the plasma. A new method to derive ion density and ion velocity from Langmuir probe measurements has been developed. The estimated LP statistics are used to derive the extension of the plasma disk, which show plasma densities above ~20 cm-3 in between 2.7 and 8.8 RS. The densities also show a very variable plasma disk, varying with one order of magnitude at the inner part of the disk. We show that the density variation could partly be explained by a dayside/nightside asymmetry in both equatorial ion densities and azimuthal ion velocities. The asymmetry is suggested to be due to the particle orbits being shifted towards the Sun that in turn would cause the whole plasma disk to be shifted. We also investigate the ion loss processes of the inner plasma disk and conclude that loss by transport dominates loss by recombination in the entire region. However, loss by recombination is still important in the region closest to Enceladus (~±0.5 RS) where it differs with only a factor of two from ion transport loss.

Studies of Saturn's magnetosphere with the Cassini mission have established the importance of Enceladus as the dominant mass source for Saturn's magnetosphere. It is well known that the ionosphere is an important mass source at Earth during periods of intense geomagnetic activity, but lesser attention has been dedicated to study the ionospheric mass source at Saturn. In this paper we describe a case study of data from Saturn's magnetotail, when Cassini was located at ≃ 2200 h Saturn local time at 36 R S from Saturn. During several entries into the magnetotail lobe, tailward flowing cold electrons and a cold ion beam were observed directly adjacent to the plasma sheet and extending deeper into the lobe. The electrons and ions appear to be dispersed, dropping to lower energies with time. The composition of both the plasma sheet and lobe ions show very low fluxes (sometimes zero within measurement error) of water group ions. The magnetic field has a swept‐forward configuration which is atypical for this region, and the total magnetic field strength is larger than expected at this distance from the planet. Ultraviolet auroral observations show a dawn brightening, and upstream heliospheric models suggest that the magnetosphere is being compressed by a region of high solar wind ram pressure. We interpret this event as the observation of ionospheric outflow in Saturn's magnetotail. We estimate a number flux between (2.95 ± 0.43) × 109 and (1.43 ± 0.21) × 1010 cm−2 s−1, 1 or about 2 orders of magnitude larger than suggested by steady state MHD models, with a mass source between 1.4 ×102 and 1.1 ×103 kg/s. After considering several configurations for the active atmospheric regions, we consider as most probable the main auroral oval, with associated mass source between 49.7 ±13.4 and 239.8 ±64.8 kg/s for an average auroral oval, and 10 ±4 and 49 ±23 kg/s for the specific auroral oval morphology found during this event. It is not clear how much of this mass is trapped within the magnetosphere and how much is lost to the solar wind., Key Points Tailward observation of ionospheric plasma in the magnetotail lobeEvidence for simultaneous reconnection in the plasma sheetFirst observational estimate at Saturn of mass source rate from the ionosphere to the magnetotail

We develop a new analytical model of the Alfven wing that is generated by the interaction between a planetary moon's ionosphere and its magnetospheric environment. While preceding analytical approaches assumed the obstacle's height-integrated ionospheric conductivities to be spatially constant, the model presented here can take into account a continuous conductance profile that follows a power law. The electric potential in the interaction region, determining the electromagnetic fields of the Alfven wing, can then be calculated from an Euler-type differential equation. In this way, the model allows to include a realistic representation of the “suspension bridge”-like conductance profile expected for the moon's ionosphere. The major drawback of this approach is its restriction to interaction scenarios where the ionospheric Pedersen conductance is large compared to the Hall conductance, and thus, the Alfvenic perturbations are approximately symmetric between the planet-facing and the planet-averted hemispheres of the moon. The model is applied to the hemisphere coupling effect observed at Enceladus, i.e., to the surface currents and the associated magnetic discontinuities that arise from a north-south asymmetry of the obstacle to the plasma flow. We show that the occurrence of this effect is very robust against changes in the conductance profile of Enceladus' plume, and we derive upper limits for the strength of the magnetic field jumps generated by the hemisphere coupling effect. During all 11 reported detections of the hemisphere coupling currents at Enceladus, the observed magnetic field jumps were clearly weaker than the proposed limits. Our findings are also relevant for future in situ studies of putative plumes at the Jovian moon Europa.

We have analyzed the Cassini Radio and Plasma Wave Science Wideband Receiver (WBR) data specifically looking for the presence of bipolar electrostatic solitary waves (ESWs). Typical examples of these ESWs are provided to show that when they are present, several of them may be detected over a few to several millisecond time span. We carried out an event study of an Enceladus encounter which took place on 9 October 2008. Approximately 30 min prior to and during the crossing of the Enceladus dust plume, several ESWs are observed with amplitudes of about 100 μV/m up to about 140 mV/m, and time durations of several tens of microseconds up to 250 µs. The highest amplitudes (over 10 mV/m) were observed only during the closest approach to Enceladus. We also carried out an ESW survey using the WBR for all years from 2004 to 2008 for distances less than 10 Rs. The survey clearly shows that most of the ESWs are found on the nightside, with a high percentage of them in the range of 4–6 Rs. This location is consistent with the densest part of Saturn's E ring and Enceladus' orbit. These are the first extended survey results of ESWs near Saturn and the first reported ESWs in connection with Enceladus. We discuss possibilities for the generation of these nonlinear ESWs, which involve current, beam, and acoustic, including dust, instabilities.

Equatorial electron density measurements from the Cassini Radio and Plasma Wave Science experiment are derived from the upper hybrid resonance frequency from Saturn Orbit Insertion (SOI) on 1 July 2004 through 21 May 2013. These densities are used to determine the characteristics of the plasma in the inner magnetosphere of Saturn between the outer edge of the A Ring and the orbit of Enceladus. Electron densities obtained when Cassini first arrived at Saturn on 1 July 2004 showed a plasma distribution decreasing radially outward from Saturn in the direction of Enceladus, the expected distribution of a centrifugally driven plasma expanding radially outward from a source in the main rings. We examine equatorial electron densities in the region between 2.4 and 4.0 Rs and show that the density measurements in this region exhibit a strong seasonal dependence resulting from photon-induced decomposition of icy particles on the ring surfaces, a decomposition process which is controlled by the solar incidence angle. This seasonal dependence will have plasma density implications for Cassini when the spacecraft returns to the region just beyond the A Ring in 2016.

We examine the variation of energetic electron pitch angle distribution at the magnetic equator and loss rate of precipitated electrons into Saturn's atmosphere through pitch angle scattering due to elastic collisions with neutral H2O along Saturn's magnetic field line around Enceladus. To examine the variation of those, we perform one-dimensional test-particle simulation when the co-rotating electron flux tube passes through the dense H2O region in the vicinity of Enceladus (~6.4 min). We focus on 1 keV as a typical kinetic energy of the electrons in this study. The initial pitch angle distribution is assumed to be isotropic. Results show that the equatorial electron pitch angle distribution near the loss cone ( 160°) decreases with time through pitch angle scattering due to elastic collisions. It is found that the electrons of ~11.4% to the total number of equatorial electrons at the initial condition are lost in ~380 s. The calculated loss time is twice faster than the loss time under the strong diffusion.

Prior to Cassini's arrival at Saturn, the nitrogen-rich dense atmosphere of Titan was considered as a significant, if not dominant, source of heavy ions in Saturn's magnetosphere. While nitrogen was detected in Saturn's magnetosphere based on Cassini observations, Enceladus instead of Titan appears to be the primary source. However, it is difficult to imagine that Titan's dense atmosphere is not a source of nitrogen. In this paper, we apply the Rymer et al.'s (2009) Titan plasma environment categorization model to the plasma environment along Titan's orbit when Titan is not present. We next categorize the Titan encounters that occurred since Rymer et al. (2009). We also produce an empirical model for applying the probabilistic occurrence of each plasma environment as a function of Saturn local time (SLT). Finally, we summarized the electron energy spectra in order to allow one to calculate more accurate electron-impact interaction rates for each plasma environment category. The combination of this full categorization versus SLT and empirical model for the electron spectrum is critical for understanding the magnetospheric plasma and will allow for more accurate modeling of the Titan plasma torus.

Enceladus' southern plume is one of the major discoveries of the Cassini mission. The water neutrals and water ice particles (dust) ejected by the cryovolcanic activity populate Saturn's E ring and the neutral torus, and they interact with the plasma environment of Saturn's magnetosphere. The plasma neutrality inside Enceladus' plume has been shown by the Langmuir probe measurement to be modified by the presence of the dust particles. We present an independent method of determining the electron density inside the plume. Sometimes, after dust impacts, plasma oscillations (ringing) were detected by the Cassini Radio and Plasma Wave Science instrument. The frequencies of these oscillations have been shown to be consistent with the local plasma frequency, thus providing a measurement of the local electron density.

Encedalus' plume of charged dust has been shown to strongly modify the plasma structures. Therefore, we analyze the magnetic signatures of dust to constrain the dust plume. For the first time, the mutual feedback between the nanograins and their plasma environment is investigated. Our model combines plasma simulations by means of the A.I.K.E.F. (Adaptive Ion-Kinetic Electron-Fluid) code with Monte Carlo simulations of the 3-D profiles of the gas and dust plumes. Data from several instruments of Cassini are considered: the neutral plume model is in good agreement with INMS data, whereas theoretical predictions of the peak ion density are compared against CAPS data, and properties of the dust are obtained by comparing our results with MAG data from the recent E14–E19 flybys. Our main results are (1) due to the ion-neutral chemistry, H3O+ is the predominant ion species within the plume; (2) the high nanograin densities observed by CAPS require an increased effective ionization frequency to fulfill quasi-neutrality; (3) the nanograin pickup current makes only a minor contribution to the current systems; i.e., the major contribution of the dust arises from electron absorption; (4) the pickup of nanograins is clearly visible in the magnetic field signatures, even including the distant encounter E15; (5) MAG data indicate a southward extension of the charged dust plume of at least 4 Enceladus radii; (6) the modification of the currents by the nanograins is responsible for the surprising fact that Cassini did not detect a region with a reduced magnetic field strength.

We use an analytical model of magnetosphere-ionosphere coupling to show that an asymmetric ring current (RC) pressure with an m=1 longitudinal dependence can initiate a rotating two-cell interchange potential. The model extends prior similar work by considering both cold plasma interchange and warm plasma pressure. This model predicts that within 7 h the magnitude of the interchange potential equals the RC seed potential. Within 13 to 26 h, the model reproduces the degree of cold density nonaxisymmetry at the outer density gradient of the Enceladus plume, as observed by the Cassini Plasma Spectrometer. The interchange growth time bears a strong dependence on the particular value of height-integrated ionospheric conductivity and a weaker dependence on the magnitude of the initial RC perturbation. The model also extends prior work by including an outer boundary. We discuss the qualitative effect of a realistically shaped magnetopause that is anchored to the nonrotating frame. For high-m interchange, the magnetopause presence has no significant effect. In contrast, low-m interchange modes experience a rotating, asymmetric boundary condition that alternately enhances and inhibits interchange growth each rotation period. Several published studies have proposed interchange-driven, rotating two-cell convection; our results suggest that an asymmetric RC pressure distribution, coupled to Saturn's ionosphere, is one possible means of generating it. Our model predicts that the two-cell interchange potential should be long-lived and relatively insensitive to subsequent ring current injections, because after two Saturnian rotations the interchange potential is an order of magnitude larger than the seed potential that initiated it.

Swept forward field is the term given to configurations of magnetic field wherein the field lines deviate from the meridional planes of a planet in the direction of its rotation. Evidence is presented for swept-forward field configurations on Cassini orbits around Saturn from the first half of 2008. These orbits were selected on the basis of high inclination, spatial proximity, and temporal proximity, allowing for the observation of swept-forward field and resolution of dynamic effects using data from the Cassini magnetometer. Nine orbits are surveyed; all show evidence of swept-forward field, with typical sweep angle found to be 23°. Evidence is found for transient events that lead to temporary dramatic increases in sweep-forward angle. The Michigan Solar Wind Model is employed to investigate temporal correlation between the arrivals of solar wind shocks at Saturn with these transient events, with two shown to include instances corresponding with solar wind shock arrivals. Measurements of equatorial electron number density from anode 5 of the Cassini Plasma Spectrometer instrument are investigated for evidence of magnetospheric compression, corresponding with predicted shock arrivals. Potential mechanisms for the transfer of momentum from the solar wind to the magnetosphere are discussed.

[1] We analyze the large-scale structures of electrons in Saturn's inner magnetosphere equatorial plane, from 2.8 to about 10 Saturnian radii (RS). The electron total density and core temperature are obtained using the quasi-thermal noise spectroscopy method, based on the HF power spectra measurements acquired with the Cassini/Radio and Plasma Wave Science instrument around the local plasma frequency from July 2004 to May 2012. The results reveal the existence of two regions. An inner region around Enceladus orbit (3–5 RS) is characterized by a high variability of the electron density, an increasing core temperature profile (∝ R2.7), and a strong correlation of the density and temperature. An outer region, beyond 5 RS, is characterized by a decrease of the density (∝ R−4.19) and a slight decrease of the core temperature with distance from the planet (∝ R−0.3). The electron temperature profile is consistent with heating by thermalization of the electrons with the ions in the inner region and with thermalization balanced by cooling due to outward radial transport in the outer region. In the outer region, we identify a local time asymmetry of both the density and the temperature: higher core temperatures are observed in the nightside while higher densities are observed in the dayside. We finally estimate the plasma scale height from a few orbits at quasi-constant altitude. We find that it varies as R1.53 inside Dione's orbit and displays a bell-shaped profile between 7 and 9 RS, consistent with the maximum in the corotation lag previously observed.

[1] The origin of the periodicities in the radio, plasma, and magnetic fields of Saturn has long been debated. Given the high degree of alignment of Saturn's dipole with its rotation axis, no strong rotational periodicities are expected. However, Cassini data demonstrated the existence of such periodicities not only in Saturn's kilometric radio emissions (SKR) but in the plasma and magnetic field signatures. It is shown that the development of the centrifugal interchange instability that originates from mass loading from the Enceladus torus contains information about the planetary rotation period. However, the planetary period is masked by high-frequency components of the instability. The presence of Titan is shown to damp the high-frequency components and enables the fundamental frequency near the planetary rotation frequency to grow at the expense of the high-frequency components. As a result, the interchange instability is seen to change from one where five to seven large interchange fingers dominate to one where there are about three which cause the modulation of magnetospheric parameters near the planetary period. This modulation includes the movement of the magnetopause, the injection of energetic particles into the inner magnetosphere and the plasma density at high latitudes, both of which control SKR. Controlling factors on the frequency include Titan's drag on the plasma sheet which produces asymmetries between the Northern and Southern Hemispheres, solar wind conditions, and the density of the Enceladus plasma torus. The resultant magnetic perturbations are shown to have similar size and frequency as that seen in Cassini data.

[1] Suprathermal singly charged molecular ions, O2+ (at ~32 Da/e) and the Mass-28 ion group 28M+ (ions at ~28 Da/e, with possible contributions from C2H5+, HCNH+, N2+, and/or CO+), are present throughout Saturn's ~4–20 Rs (1 Saturn radius, Rs = 60,268 km) near-equatorial magnetosphere from mid-2004 until mid-2012. These ~83–167 keV/e heavy ions measured by Cassini's CHarge-Energy-Mass Spectrometer have long-term temporal profiles that differ from each other and differ relative to the dominant water group ions, W+ (O+, OH+, H2O+, and H3O+). O2+/W+, initially ~0.05, declined steadily until equinox in mid-2009 by a factor of ~6, and 28M+/W+, initially ~0.007, declined similarly until early-2007 by a factor of ~2. The O2+/W+ decline is consistent with Cassini's in situ ring-ionosphere thermal ion measurements, and with proposed and modeled seasonal photolysis of Saturn's rings for thermal O2 and O2+. The water ice-dominated main rings and Enceladus plume depositions thereon are the two most likely O2+ sources. Enceladus' dynamic plumes, though, have no known long-term dependence. After declining, O2+/W+ and 28M+/W+ levels remained low until late-2011 when O2+/W+ increased, but 28M+/W+ did not. The O2+/W+ increase was steady and became statistically significant by mid-2012, indicating a clear increase after a decline, that is, a possibly delayed O2+ “seasonal” recovery. Ring insolation is driven by solar UV flux which itself varies with the sun's 11 year activity cycle. The O2+/W+ and 28M+/W+ declines are consistent with seasonal ring insolation. No O2+/W+ response to the late-2008 solar-cycle UV minimum and recovery is evident. However, the O2+/W+ recovery from the postequinox baseline levels in late-2011 coincided with a strong solar UV enhancement. We suggest a scenario/framework in which the O2+ observations can be understood.

[1] Electron density measurements derived from the upper hybrid resonance frequency have been obtained from the Cassini Radio and Plasma Wave Science experiment over a 7 year period from 28 October 2004 through 7 November 2011. Additional density measurements inside the orbit of Enceladus and outside the orbit of Rhea have made it possible to expand a previously published density model to radial distances from 2.6 to 10.0 Saturnian radii (RS). The distribution of density data is compared to a simple scale height density model for a single-species plasma within 8° of the magnetic equatorial plane. There is a broad peak in the equatorial density distribution between 4 RS and 5 RS with the plasma falling off both inward and outward from the peak region. The radial dependence of the equatorial density profile varies as R4.0 from 2.6 RS to 4 RS and as R–4.8 from 5 RS to 10 RS. The plasma distribution outside 5 RS remains consistent with earlier density models but additional density measurements inside 4 RS provide information on the plasma distribution in this inner region that will be the focus of the F-Ring and proximal orbits near the end of the Cassini mission.

Saturn's magnetosphere is populated by plasma created from neutrals ejected by the moon Enceladus. These neutrals are ionized and picked up by the planetary magnetic field requiring large amounts of angular momentum to be transferred from Saturn's upper atmosphere to the magnetospheric plasma. The resulting upward currents that supply this angular momentum are associated with electrons, which travel toward the planetary atmosphere. At high magnetic latitudes along the flux tube, parallel electric fields may develop to enhance the field-aligned current density flowing between the two regions. We show that, similar to the Jovian system, the current-voltage relation in the Saturnian system must be evaluated at the top of the acceleration region, which occurs at ~1.5 RS along the magnetic field line as measured from the center of the planet. Owing to the large abundance of protons in the Saturnian system, cold electrons carry the majority of the field-aligned current for net potential drops less than 500 V. For the flux tube intersecting the equatorial plane at 4 RS, field-aligned potentials of 50-130 V are consistent with the energy fluxes inferred from the Enceladus emission. In the middle magnetosphere, field-aligned potentials of ∼1.5 kV produce ionospheric electron energy fluxes of 0.3 mW/m2 when hot electrons comprise 0.3% of the magnetospheric electron population. Key Points Current-voltage relation must be evaluated at high magnetic latitudes. Cold electrons contribute strongly to field-aligned current density. Full Knight (1973) current-voltage relation must be applied to Saturnian system. ©2013. American Geophysical Union. All Rights Reserved.

[1] We present an analytical model of the Poynting flux that is generated by the interaction between the plume of Enceladus and Saturn's magnetospheric plasma. Our purpose is to analyze the influence of two key elements of Enceladus' magnetospheric interaction on the electromagnetic energy radiated away in the moon's Alfven wings. First, the north-south asymmetry of the obstacle generates a system of hemisphere coupling currents which allows a transport of electromagnetic energy into Saturn's northern hemisphere, even if the field-aligned currents connecting to the plume are completely blocked at the non-conducting icy crust of Enceladus. Second, the presence of electron-absorbing dust grains within the plume was recently found to drastically modify the electromagnetic field configuration within Enceladus' Alfven wings (anti-Hall effect), thereby also altering the energy flux radiated away from the interaction region. By systematically studying the impact of varying strengths of the hemisphere coupling currents and varying electron absorption fractions on the energy flux, we come to the following conclusions: (1) The integrated Poynting flux into Saturn's southern hemisphere always exceeds the integrated flux into the northern hemisphere. In particular, the power transmitted towards the south may become several orders of magnitude larger than the power transmitted towards the north. (2) The search for Enceladus' auroral footprint has so far mainly focused on Saturn's northern hemisphere. However, based on the Poynting fluxes radiated away by the interaction, detections of the footprint should occur more likely in the giant planet's southern hemisphere, if no other far-field effects play a role. (3) Electron absorption by the dust grains within the plume makes a measurable contribution to the energy flux.

[1] The Cassini spacecraft has made 20 close flybys of the icy moon Enceladus between its arrival at Saturn in 2004 and 2012. Of those 20, strong whistler mode emissions (often called auroral hiss) were clearly observed on seven encounters. The propagation paths of these emissions are determined by the background magnetic field, which allows their source regions to be studied using simple ray-tracing codes. In this paper, we trace the auroral hiss observations from Cassini's trajectories to possible source locations near Enceladus. We find that all of the detected emissions could be generated by field-aligned electron beams in one of two regions around the moon: upstream of the Saturnward terminator and downstream of the anti-Saturnward terminator. These results suggest that electron beam acceleration near the solid body is a quasi-time-stationary feature of the plasma interaction and that the auroral hiss generated by these beams may be used to remotely study plasma processes in regions separated from the spacecraft.

[1] We examine the contribution of photo-dissociation under quiet solar conditions to the global OH and O distributions in Saturn's inner magnetosphere by performing a Monte Carlo simulation. We first calculate the H2O distribution generated by H2O sources, namely Enceladus' cryo-volcanic plumes, satellite sputtering, and E ring sputtering. We calculate the OH distribution through photo-dissociation reactions using the calculated H2O distribution and then calculate the O distribution from the obtained H2O and OH distributions. We quantitatively evaluate the role of the energy increment of produced OH and O particles due to photo-dissociation by comparing the resultant distribution of OH and O particles with and without the energy increment. To quantitatively examine the effect of photo-dissociation on the spreading of OH and O clouds, we use the H2O model including charge exchange and neutral/neutral collisions based on Cassidy and Johnson (2010), as the initial distribution. For the OH (O) density in the region outside 5 Rs (6 Rs), the density with energy increment is greater than that without energy increment. The contribution of calculated OH density with energy increment to the observation is more than ∼10%. The OH ratio outside 6 Rs decreases with radial distance from Saturn. On the other hand, the contribution of calculated O density with energy increment to the observation is less than 10% except for around Enceladus.

[1] Radiation belts cause irreversible damages to on-board instrument materials. Studies about radiation belts can provide precious information for future interplanetary missions. Pioneer 11, Voyager 2 and nowadays Cassini missions give the characteristics of Saturn's inner magnetosphere: Saturn's famous rings, various moons, neutral particles ejected from Enceladus and waves. Thanks to its experience with the Earth's and Jupiter's radiation belt studies, ONERA is now able to develop an electron radiation belt model for Saturn's environment, i.e., a new version of Salammbo. The study of Saturn's inner magnetosphere emphasizes the most important physical processes governing radiation belt dynamics: electrons losses due to dense rings are the dominant physical process near the planet (L ∼50 keV), radial diffusion and local losses due to the moons are predominant from L = 2.3 to the boundary condition (L = 6). The analysis of spacecraft in situ data (Pioneer 11, Voyager 2, and Cassini) allows a boundary condition to be built for the model and the Salammbo results to be validated. The Salammbo Kronian model has also been compared to the empirical radiation belt model SATRAD (for SATurn RADiation model) based on Pioneer 11 and Voyager 2 data. Comparisons of Salammbo results with in situ data and SATRAD confirm that Salammbo is a good mean model for Saturn electron radiation belts, for energies from about a hundred keV to a few MeV.

[1] Observations of Saturn's ultraviolet and infrared aurora show structures that, when traced along the planetary magnetic field, map to the inner, middle, and outer magnetosphere. From low to high latitudes the structures seen in the UV are the Enceladus footprint, which maps to an equatorial radius of 4 RS (Saturn radii); a diffuse emission that maps to a broad equatorial region from 4–11 RS on the nightside; and a bright ring of emission that maps to ∼15 RS. With the exception of the Enceladus spot, the magnetospheric drivers for these auroral emissions are not yet fully understood. We apply a 1D spatial, 2D velocity space Vlasov solver to flux tubes mapping from equatorial radii of 4, 6, 9, and 13 RS to Saturn's southern atmosphere. The aim is to globally characterize the field-aligned potential structure and plasma density profiles. The ionospheric properties - the field-aligned current densities at the ionospheric boundary, energy intensity profiles and fluxes of the electrons precipitating into the ionosphere - are also determined. We then couple our results to an ionospheric model to calculate the Pedersen conductances at the foot of the relevant flux tubes. We find that for a zero net potential drop between the ionosphere and magnetosphere, there exists a sharp potential drop at ∼1.5 RS along the magnetic field line as measured from the planetary center. The strength of this potential drop is approximately equal to that of the ambipolar potential resulting from the centrifugal confinement of the heavy, cold magnetospheric ion population. We also find that the ionospheric properties respond to changes in the magnetospheric plasma population, which are reflected in the nature of the precipitating electron population. For the flux tube mapping to 9 RS (−70°), the incident electron energy flux into the ionosphere resulting from a magnetospheric plasma population with a small fraction of hot electrons is an order of magnitude less than that inferred from observations, implying that significant high-latitude field-aligned potentials (up to 1.5 keV) may exist in the saturnian magnetosphere. Calculated ionospheric Pedersen conductances range from 3.0–18.9 mho, and are thus not expected to limit the currents flowing between the ionosphere and magnetosphere.

Observations by instruments onboard the Cassini spacecraft of Saturn's icy satellite Enceladus have shown that a plume containing water vapor and ice grains is present in the southern hemisphere. Energy distributions of electrons in this plume were measured by the electron sensor part of the Cassini plasma spectrometer (CAPS – ELS). A significant suprathermal electron population was detected. The nature of the electron population is important for understanding the composition and chemistry of the plume plasma because the electron-ion recombination rate depends on the energy distribution and because ionizing collisions by energetic electrons creates new plasma. We present the results of a two-stream electron transport model for plume electrons that includes neutral densities that agree with Cassini Ion and Neutral Mass Spectrometer (INMS) data. Electron production within the plume due to photoionization by solar radiation and by electron impact ionization was included, as were energy losses due to electron-neutral collisions. Model cases were considered that both included and did not include electron inputs from outside the plume. Comparisons are made of model fluxes with measured fluxes by CAPS – ELS on October 9, 2008. The model-data comparisons indicate that photoelectrons (10 eV–70 eV energies) locally produced within the plume can explain the data. The possible role of electron-grain collisions was also explored and it was determined that nanograin densities in excess of 106 cm−3 would be needed to affect the electron distribution.

[1] Enceladus is one of Saturn's most active moons. It ejects neutral gas and dust particles from its southern plumes with velocities of hundreds of meters per second. The interaction between the ejected material and the corotating plasma in Saturn's magnetosphere leads to flow deceleration in ways that remain to be understood. The most effective mechanism for the interaction between the corotating plasma and the neutral gas is charge exchange which replaces the hotter corotating ions with nearly stationary cold ions that are subsequently accelerated by the motional electric field. Dust particles in the plume can become electrically charged through electron absorption and couple to the plasma through the motional electric field. The objective of this study is to determine the level of flow deceleration associated with each of these processes using Cassini RPWS dust impact rates, Cassini Plasma Spectrometer (CAPS) plasma data, and 3-D electromagnetic hybrid (kinetic ions, fluid electrons) simulations. Hybrid simulations show that the degree of flow deceleration by charged dust varies considerably with the spatial distribution of dust particles. Based on the RPWS observations of dust impacts during the E7 Cassini flyby of Enceladus, we have constructed a dust model consisting of multiple plumes. Using this model in the hybrid simulation shows that when the dust density is high enough for complete absorption of electrons at the point of maximum dust density, the corotating flow is decelerated by only a few km/s. This is not sufficient to account for the CAPS observation of flow stagnation in the interaction region. On the other hand, charge exchange with neutral gas plumes similar to the modeled dust plumes but with base (plume opening) densities of ∼109 cm−3 result in flow deceleration similar to that observed by CAPS. The results indicate that charge exchange with neutral gas is the dominant mechanism for flow deceleration at Enceladus.

[1] There have been three Cassini encounters with the south-pole eruptive plume of Enceladus for which the Cassini Plasma Spectrometer (CAPS) had viewing in the spacecraft ram direction. In each case, CAPS detected a cold dense population of heavy charged particles having mass-to-charge (m/q) ratios up to the maximum detectable by CAPS (∼104 amu/e). These particles are interpreted as singly charged nanometer-sized water-ice grains. Although they are detected with both negative and positive net charges, the former greatly outnumber the latter, at least in the m/q range accessible to CAPS. On the most distant available encounter (E3, March 2008) we derive a net (negative) charge density of up to ∼2600 e/cm3 for nanograins, far exceeding the ambient plasma number density, but less than the net (positive) charge density inferred from the RPWS Langmuir probe data during the same plume encounter. Comparison of the CAPS data from the three available encounters is consistent with the idea that the nanograins leave the surface vents largely uncharged, but become increasingly negatively charged by plasma electron impact as they move farther from the satellite. These nanograins provide a potentially potent source of magnetospheric plasma and E-ring material.

[1] With the discovery by the Cassini spacecraft of an oxygen atmosphere over Saturn's main rings, and a strong source of water products from the plumes of Saturn's moon Enceladus, our picture of the physics of Saturn's magnetosphere from the main rings to inside the orbit of Enceladus has changed dramatically. This region contains oxygen ions from the ring atmosphere and water-group ions from the Enceladus torus. The purpose of this study is to examine ion densities, temperatures, and composition from several equatorial periapsis passes from 2004 to 2010 for the region from 2.4 to 3.5 Saturn radii (∼60,300 km) in addition to Voyager 2 in order to separate contributions from Saturn's ring atmosphere from the water products in the Enceladus torus and to describe the temporal variations in the plasma. Because of the high background due to so-called penetrating radiation in this region, only six orbits are used in this study. Our analysis indicates that large variations in ion density, temperature, and composition occurred between the Voyager 2 flyby, 2004, and 2010. Although the Enceladus plumes may be variable, we propose that the large change in the ion density from 2004 to equinox near 2010 is due to the seasonal variation in the ring atmosphere. Our interpretation of the plasma data is supported by a simple photochemical model, combining the water products from Enceladus and the seasonal variations in the ring atmosphere.

We present in situ Cassini Radio Plasma Wave Science observations in the vicinity of Enceladus and in the E ring of Saturn that indicate the presence of dusty plasma. The four flybys of Enceladus i ...

[1] We apply the hybrid simulation code AIKEF (adaptive ion kinetic electron fluid) to the interaction between Enceladus' plume and Saturn's magnetospheric plasma. For the first time, the influence of the electron-absorbing dust grains in the plume on the plasma structures and magnetic field perturbation, the Alfven wing, is taken into account within the framework of a global simulation. Our work continues the analytical calculations by Simon et al. (2011), who showed that electron absorption within the plume leads to a negative sign of the Hall conductivity. The resulting twist of the magnetic field, referred to as the Anti-Hall effect, has been observed during all targeted Enceladus flybys between 2005 and 2010. We show that (1) applying a plume model that considers both, the neutral gas and the dust allow us to quantitatively explain Cassini Magnetometer (MAG) data, (2) dust enhances the anti-Saturnward deflection of the ions, causing asymmetries which are evident in the MAG data, and (3) the ions in the plume are slowed down below 1 km s−1; and we compare our results to MAG data in order to systematically analyze variations in the plume activity and orientation for selected pairs of similar flybys: (E5, E6), (E7, E9) and (E8, E11).

[1] Ion cyclotron waves have been observed at Saturn by all spacecraft that passed through the inner magnetosphere near the equatorial plane, typically from slightly inside Enceladus' orbit to outside of Dione's. In 2005 and 2006, the Cassini spacecraft made high-inclination crossings of the equatorial plane in this region. The magnetometer observed that the waves were characteristically not uniform with distance from the equatorial plane. Instead, waves with weak and constant amplitude were observed in a small region around the magnetic equator where they propagated bidirectionally. Above and below that plane, the wave amplitude varied strongly, and the wave propagated away from the equator. We draw comparisons between these waves and those at the Earth and ion cyclotron waves associated with neutral sources in the Jovian magnetosphere. These behaviors may be common and should be considered when using the wave amplitude to infer the neutral ionization rates at Saturn, in other planetary magnetospheres, and at bodies in the solar wind.

Radial profiles of the azimuthal current density between similar to 3 and 20 R-S in Saturn's magnetosphere have been derived using plasma and magnetic field data from 11 near-equatorial Cassini orbits spanning a 10 month interval. The current density generally shows only modest variations with local time and from pass to pass within this region, rising rapidly near similar to 5 R-S to peak at similar to 90 pAm(-2) at similar to 9 R-S and falling more gradually to below similar to 20 pA m(-2) at 20 R-S. The pressure gradient current is overall the most important component, the dominant inertia current in the inner region being significantly canceled by the oppositely directed pressure anisotropy current. These characteristics principally reflect the properties of the warm water plasma originating from the Enceladus torus to distances of similar to 10 R-S encompassing the usual current peak, inside of which distance the plasma properties are generally unvarying within factors of less than similar to 2. Increased variability is present at larger distances where the pressure of the hot magnetospheric plasma plays the more important role. In this region the dominant pressure gradient current is found to be strongest in the dusk to midnight sector and declines modestly, by factors of similar to 2 or less, in the midnight to dawn and dawn to noon sectors. Pass-to-pass temporal variability by factors of similar to 2-3 is also present in the outer region, particularly in the dawn to noon sector, probably reflecting both hot plasma injection events as well as solar wind-induced variations.

[1] We present an analytical model of the Alfven wing system that is generated by the interaction between the plume of Enceladus and the corotating plasma in Saturn's inner magnetosphere. Our primary purpose is to explain the orientation of the magnetic field perturbations detected in Enceladus' Alfven wings by the Cassini magnetometer (MAG) instrument. Observational data from numerous close Enceladus flybys show both the Bx and By components (in Enceladus interaction coordinates: Bx, along corotation direction; By, toward or away from Saturn) in the center of the northern wing tube to possess a negative sign, whereas the opposite case of Bx and By being positive was observed within the southern wing. So far, none of the available models of Enceladus' magnetospheric interaction is able to reproduce this correlation between the directions of Bx and By. On the basis of the analytical calculations of Neubauer (1980, 1998) and Saur et al. (1999, 2007), we demonstrate that the observed orientation of the magnetic field may arise from the presence of negatively charged dust grains in the plume of Enceladus, serving as a sink for “free” magnetospheric electrons. Although the current carried by these particles does not make a noteworthy contribution to the magnetic field distortions in the interaction region, the negative charge accumulated by them needs to be accounted for in the quasi-neutrality condition of the plasma. The depletion of magnetospheric electrons within the plume is therefore far from causing only some localized perturbations of the magnetic field, but it drastically alters the nature of the interaction: we show that this process yields a reversal in the sign of the Hall conductivity, thereby giving rise to the observed field signatures. By applying a modified version of the Alfven wing model developed by Saur et al. (2007), we demonstrate that the magnetic field observations from Cassini's targeted Enceladus flybys can be understood by taking into account the influence of electron-absorbing dust grains. In contrast to what is claimed in recent literature, we therefore propose that magnetic field observations near Enceladus can be completely understood in terms of a local interaction model, i.e., that it is not necessary to consider the large-scale dynamics of the flux tubes in Saturn's magnetosphere. In addition, we provide first in situ evidence that the hemisphere coupling current system predicted by Saur et al. (2007) and the associated magnetic field discontinuities are indeed present at Enceladus. The field perturbations caused by these hemisphere coupling currents arise from the partial blockage of the Alfven wing at the nonconducting icy crust of Enceladus. This effect needs to be taken into account when interpreting Cassini MAG data from flybys that intersected the Enceladus flux tube and can only be reproduced by models that apply adequate boundary conditions to the surface of the icy moon.

[1] The Saturnian system is immersed in an extended cloud of neutrals. Although water vapor ejected from Enceladus' south pole is the dominant neutral source, photolysis and radiolysis of ices can release H2O, O2, and H2 from the icy ring particles and the icy satellites, and Titan's atmosphere is a source of H2. Once ionized, these neutrals are the source of the observed magnetospheric plasma. To understand the H2+ ion densities observed by the Cassini plasma spectrometer (CAPS), we developed a Monte Carlo test particle model to simulate the spatial morphology of the neutral H2 cloud and the resulting H2+ ion source rates. The H2 lifetime is constrained by its local chemistry, which is computed from the latest plasma measurements by Cassini CAPS data. The main rings, Enceladus' water torus, Rhea, and Titan are considered as the primary sources of H2 in our model. It is seen that H2 accumulates over Saturn's main rings because of thermal accommodation with the ring particles, and Titan is the dominant source of H2 in the outer magnetosphere (>∼6 RS). From ∼6 to ∼2.5 RS, photodissociation of water from Enceladus and H2 scattered from the ring atmosphere are comparable sources. The newly formed H2+ ions are lost by collisions with the ring particles inside ∼2.5 RS, by interchange processes in the middle magnetosphere, and by flow down the tail in the outer magnetosphere. The density distribution of H2+ estimated from our ion source rates roughly agrees with CAPS observations, and we show that the H2+ density near the equator over the main rings is at least 1 order of magnitude smaller than O2+, possibly consistent with the nondetection of H2+ by CAPS at Saturn orbit insertion.

Io's interaction with the Jovian magnetosphere generates a power of about 1012 W which propagates as Alfvn waves along the magnetic field lines and is partly transferred to electrons, resulting in intense auroral emissions. A recent study of the power transmission along the Io flux tube and of the electron acceleration at high latitudes showed that the power of the observed emissions is well explained by assuming filamentation of the Alfvn waves in the torus and the acceleration of the electrons at high latitude. At Jupiter, UV footprints related to Europa and Ganymede have also been observed. At Saturn recent observations revealed a weak UV footprint of Enceladus. We apply the Io interaction model to the Europa and Enceladus interactions. We show that the Alfvn wave filamentation leads to a precipitating electron power consistent with the power of the observed UV footprints.

[1] The major momentum‐loading source in Saturn’s magnetosphere, Enceladus, has been studied with seven Cassini flybys between 2005 and 2008. In this paper, we first use parameter tests with our 3‐D magnetohydrodynamic simulation to demonstrate and determine the sensitivity of the interaction to both electron impact rates and charge‐ exchange rates. We also investigate the reasons behind our previous discovery that in the plume, within about two Enceladus radii of the plume’s source, the momentum‐loading rates per unit ion and neutral density are orders of magnitude lower than at greater distances. We find that depletion of hot electrons and variations in charge‐exchange rates are two possible explanations for such a reduction of the momentum‐loading rates. Assisted by the Cassini observations, we use our understanding of the plasma interaction to determine the temporal variation of Enceladus’ neutral plume, which is important in understanding its origin, as well as the geological evolution of this icy moon. We base our study on magnetometer observations during all seven flybys to present the first comparative analysis to all flybys in 2005 and 2008. It is found that the maximum variation in gas production rates is one third the largest rate. The plasma momentum‐ loading rate ranges from 0.8 to 1.9 kg/s, which is consistent with previous studies.

[1] In 2008, the Cassini ion neutral mass spectrometer (INMS) investigation made in situ measurements of neutral species near Saturn's equatorial plane within 0.5 Saturn radii (RS) of the orbit of Enceladus. After removing the large background and modeling to interpret instrumental effects, the data provide rough constraints on the neutral distribution and composition. These data show an azimuthal asymmetry in the neutral densities and provide measurements used to compare to simulations of neutral H2O emitted from Enceladus. Far from Enceladus, the neutral water densities, at a few times 103 molecules/cm3, are near the detection limit of INMS. Near Enceladus, but outside of the plumes and north of the equatorial plane, the INMS detects particles within 5000 km of Enceladus, with the density increasing to approximately 105 molecules/cm3 at the equatorial plane. The observations also show CO2 in the form of its dissociated product, CO. On the basis of the spatial distribution of CO2 counts, the scale height of the neutral cloud above and below the equatorial plane is less than 7000 km. Far from Enceladus, the concentration of CO2 with respect to H2O increases, a consequence of the predicted decline in H2O density. Relatively high counts at 2 amu are infrequently observed. These measurements indicate H2 released during ice-grain impacts and provide a constraint on the frequency and distribution of small ice grains.

[1] Neutral particle dominance over charged particles in Saturn's magnetosphere was evident prior to Cassini's arrival at Saturn in 2004. The observation of active plumes emanating from the south pole of Enceladus suggests that this small moon is likely to be the principal source of neutrals in Saturn's magnetosphere. Cassini has flown through the plumes on several occasions, and the resulting data imply the source rate is variable (∼1027 to 1028 water molecules/s). Here we use Cassini plasma spectrometer and Cassini magnetospheric imaging instrument observations to update neutral particle lifetimes and then use the most recent processed versions of Cassini ion neutral mass spectrometer observations made during encounters E2, E3, and E5 to constrain a 3-D multispecies neutral particle model. This procedure improves constraints on the plume source rate, ejection velocity, and plume divergence. We find that the plume source rate varies by at least a factor of 4 over the 7 month period considered. Additionally, we find that previous estimates of the plume source rates based on E2 observations are most likely overestimated because the background neutral torus has not been adequately account for. On the basis of these results, we discuss the implications of this variability on global neutral particle distributions.

[1] In addition to being the major source of neutral gas and dust particles for the Saturnian E‐ring and, ultimately, heavy ions for the Saturnian inner magnetosphere, Enceladus exhibits geological activity that has made it an object of recent intensive study. The interest has significantly increased after Cassini flybys in 2005 provided a detailed map of its surface, showing that most of its activity occurs in a region around the south pole of the satellite. Dust jets that were discovered during the flybys can be related to a set of localized gas sources that dominate the supply of material into the rarefied atmosphere of Enceladus. A comprehensive data analysis involves developing physical models that include all major processes occurring in the atmosphere. Such models can be used not only for calibration and understanding of data already available, but also could have a practical application for planning upcoming flybys. This work presents the results of the development and application of a kinetic model of the Enceladus’ atmosphere consisting of a gas described in terms of its distribution function. The paper describes the basic principles of the modelandgivesacomparisonwiththeobservationaldataobtainedwithCassiniinstruments.

[1] We report on the measurements of the Cassini Ion Neutral Mass Spectrometer (INMS) of the density and structure of Enceladus' south polar plume during the E3 and E5 flybys on 12 March and 9 October 2008. Using a Monte Carlo simulation, we analyze the dependence of the INMS gas inlet transmittance on spacecraft pointing and the effect on the measurements at E3. We apply a finite element analysis to correct for water physisorption in the inlet and obtain a maximum plume density almost twice that suggested by the raw INMS data. The results indicate uniform spreading of the plume vapor from the source with a source rate of at least 100 kg/s. We also analyze the detection of ice grains by the INMS and find that, in contrast to the plume's vapor component, the grains are concentrated within the plume jets seen in Cassini imaging, supporting the suggestion that the jets are composed of fine-grained ice.

[1] The plasma environment around Enceladus is governed by the interaction between the corotating plasma with the body of the moon and the neutral gas associated with Saturn's extended cloud and plumes ejected from its southern polar region. To understand the nature of this interaction, we use 3-D electromagnetic hybrid simulations that treat ions kinetically through particle-in-cell methods and treat the electrons as a charge-neutralizing fluid. In these simulations, plasma interaction with the neutrals takes place through charge exchange. The results show that plasma absorption by Enceladus forms a tail-like density cavity behind the moon and a depletion wake which is confined in the direction perpendicular to the magnetic field but extends many Enceladus radii along the magnetic field. Except in the cavity tail, the flow is not slowed by the interaction but is diverted toward the cavity in a symmetric fashion. Interaction of the corotating plasma with the extended neutrals results in the generation of ion cyclotron waves and plasma deceleration to velocities below the corotation speed. The results also show that the extent to which the presence of a plume in the southern pole affects the nature of the interaction depends on its density. For plumes with base densities of ∼106/cm3 (6.5 × 1029 molecules in the plume) or lower we find no significant impacts, while plumes with base densities of ∼107/cm3 and larger are found to greatly impact the interaction region. Specifically, the interaction is no longer symmetric with a density cavity tail in the Northern Hemisphere and a density enhancement tail in the Southern Hemisphere. At base densities of ∼108/cm3, a strong Alfven wing is also generated in the interaction. Preliminary comparisons of the simulations with the Cassini Plasma Spectrometer data during the 12 March 2008 encounter with Enceladus suggest plume base densities of ∼5 × 108/cm3 corresponding to production of ∼4 kg/s of new ions.

[1] The interaction of Saturn's inner magnetosphere with its moons ranges from the addition of significant quantities of gas, dust, and plasma, causing significant consequences for the dynamics and energetics of the entire Saturnian magnetosphere, to the simple absorption of plasma and energetic particles by the icy moons with non-electrically conducting interiors. The interaction with these moons is complex with the contribution of many physical processes, depending on the geometry of any plume, the structure of the atmosphere, and its interaction with the surface and interior of the moon, the latter by induced fields. Our ultimate goal is to understand the complexities of this interaction and its temporal variations, especially at Enceladus. In this paper we use magnetohydrodynamics (MHD) code for addressing the flow around obstacles that are simpler than the Enceladus interaction. These simulations both help us understand the interaction with other icy moons and prepare us for the simulation of the flow around Enceladus. The processes involved include ordinary collisions, impact ionization, photoionization, and charge exchange. We examine a series of simple canonical interactions before we later apply our simulation where the multiple processes are occurring simultaneously with asymmetric geometries. We apply our 3-D MHD model to simulate the interaction between the Saturnian corotational plasma flow for the following cases: an absorbing body having an insulating surface; ion pickup via photo and impact ionization from a spherically symmetric neutral cloud; charge exchange with such a neutral cloud; and ion pickup at an insulating, absorbing body with an atmosphere acted upon by the sum of the three ionization processes. In addition to validating the model and obtaining a deeper understanding of the consequences of each interaction, we can immediately make some conclusions about the Enceladus interaction. We find that the magnetometer data are most consistent with the surface of Enceladus being absorbing and insulating, rather than the surface being reflecting and electrically conducting. For the conditions in the corotating flow at Enceladus, the perturbation to the plasma flow produced by photo/impact ionization is an order of magnitude smaller than that produced by charge exchange. Moreover, the perturbation to the magnetic field Bz component by a spherically symmetric mass loading source alone is an order of magnitude smaller than that observed in the neighborhood of the plume. Thus, the perturbation observed in the magnetometer data is primarily due to the mass loading in the plume, which is primarily ion-neutral charge exchange. The geometry and source strength of the plume are investigated in a following paper.

[1] We present an analysis of the ∼11 h oscillations in Saturn's near-equatorial magnetic field, using Cassini data acquired during 2004–2007. We assume the oscillation period is given by the magnetic phase model derived by Provan et al. (2009) over the same interval, and use this to combine the data to determine the variation of the oscillation amplitude and phase of all three spherical polar field components with radial distance (∼3–30 RS) and local time (RS is Saturn's radius, 60,268 km). The oscillatory field behavior can be divided into two regions at a radial distance of ∼15 RS. In the inner region the radial and azimuthal components form a rotating field that to a first approximation is quasi-uniform, but shows major suppression and deflection effects around the near-planet region. Associated rotating field-aligned currents in the Enceladus torus are estimated to carry ∼±1 MA. In the outer region these field components form a rotating partial twin-vortex centered in the nightside, with associated North–South directed currents carrying ∼±6 MA. Individual current regions of a given sign emerge first at dusk, propagate via midnight, and dissipate near dawn, avoiding the dayside sector of weaker more uniform oscillatory fields. Oscillations in the colatitudinal field are also present throughout, that are generally in phase with the radial component. The oscillation phases of all components are found to increase with radial distance at all local times, indicating outward radial propagation with phase speeds of ∼200 km s−1 on the nightside and ∼500 km s−1 on the dayside.

[1] The Saturnian moons in the inner magnetosphere are immersed in a plasma disk that rotates much faster than the moon's Keplerian speed. The interaction of the rotating plasma with the moons results in a disturbance in the Saturnian magnetospheric plasma that depends on the nature of obstacle that the moon represents. In particular at Enceladus, such perturbations in the magnetic field and flowing plasma enable us to infer the 3-D shape of the Enceladus plume and its outgassing rate. In this paper, we apply our 3-D magnetohydrodynamic model to extensively study the effects of different plume and disk plasma conditions on the interaction. By finding the best agreement with the observations of two diagnostic flybys, one with its point of closest approach on the upstream side and the other on the downstream side, we determine the plume intensity and configuration. We find that mass loading in the plume is less efficient close to the surface of the moon, where the neutral density is the highest. For E2 and E5, the opening angle of the plume is about 20°, and the plume is tilted toward the corotating direction. The upstream density has a significant effect on the mass loading rate, while its effect on the magnitude of the magnetic perturbation is less significant. An upstream velocity component in the Saturn direction helps to explain the observed magnetic perturbation in the By component and signals the need to consider Enceladus's effect on the global plasma circulation in addition to the local effect. Quantitative comparisons of the simulated and observed interaction are provided.