P43A-1002
Modelling the Neutral Cloud Environment of the Saturnian System
From HST observations and the Cassini measurements, we know that the Saturnian system is immersed in a vast neutral gas cloud of water molecules and their dissociative products like OH, O and H. Most of the gas molecules originate from the water gas jets in the south pole of Enceladus. In addition, the ring system is the source of oxygen molecules and atoms which can be injected into the distant Saturnian magnetosphere via ion- molecule reactions. Titan's exosphere is another major source of neutral gas composed of escaping N2, CH4 and H2. Because of different diffusion and transport processes, the global distribution of the neutral gas is a combination of these separate source regions. In this work, we will examine the structures of these separate gas clouds using the most recent plasma model from Cassini and check how these different pieces can be fit together.
P43A-1003
Enceladus' Plasma Interaction With Saturn's Magnetosphere
We study the local plasma interaction of Enceladus and it's neutral gas plume with the Saturnian magnetosphere by using a 3D MHD model. Thereby, we investigate how the neutral gas environment around Enceladus modifies the magnetospheric plasma flow and magnetic field environment around Enceladus. In our model, we use an asymmetric gas cloud with maximum density around Enceladus south pole. Through charge exchange and ionization in the gas cloud the plasma flow is slowed and diverted. We compare our model results to the Cassini spacecraft measurements and provide estimates of the total mass loading rate in the vicinity of Enceladus.
P43A-1004
3D Multi-fluid Simulations With Ion-Neutral Interactions: Combining Observations and Modeling to Study Enceladus' Plume
The Cassini spacecraft discovered a plume of water ice particles jetting outward from the southern polar region of Saturn's moon Enceladus. Preliminary modeling studies in conjunction with Cassini spacecraft observations have demonstrated the presence of an extended source of ions likely due to charge exchange and ion production in the expanding neutral plume. While the importance of understanding the chemical interactions in this system has been recognized, a self-consistent model examining the neutral and ion dynamics and their interaction has yet to be fully established. In this paper we present results from incorporating neutral fluid components and ion-neutral interactions into an existing multi-fluid modeling infrastructure. The ion and neutral fluids interact through charge exchange, and the production and loss of ions and neutrals are monitored through source and loss terms in each fluid species in each grid cell. The model is used to interpret Cassini spacecraft observations from the July 2005 encounter, the result of which strongly indicates that treating both ions and neutrals consistently throughout a multi-fluid plasma dynamic model is necessary to fully understand the local and extended interaction of Enceladus" plume with Saturn's magnetosphere. Improved understanding of ion-neutral interactions can later be implemented for tracking the numerous sources of neutrals and ions within Saturn's magnetosphere, such as the neutral atmosphere of the rings, icy moons, and Titan.
P43A-1005
Simulation of the Effect of Titan's Induced Magnetosphere on the Kronian Magnetosphere: Titan mass losses and Kronian mass loading
Using a coupled 3-D multi-fluid simulation of the Saturn-Titan plasma interaction we investigate mass loss from Titan and mass loading of the Kronian magnetosphere. The multi-fluid method is ideal for studying Titan's plasma interaction because it incorporates ion cyclotron effects and each fluid species has an independent velocity, energy, and mass. The Saturn-Titan simulation has excellent resolution both in Saturn's inner magnetosphere and around Titan. We will compare mass loss from Titan and the morphology of Titan's ion tail and mass loading region for Titan in Saturn's dayside and nightside magnetosphere. It is shown that Titan is able to produce substantial mass loading to the Kronian system with a plasmatail extending several Saturn radii in length. However, global convection within the Kronian magnetosphere can cause the plasma tail from Titan to move substantially in radial distance so that only a partial plasma torus can develop. The different properties of Titan's plasma response with respect to position within the Kronian magnetosphere are contrasted.
P43A-1006
Numerical Simulation of Coriolis Effects on the Interchange Instability in Saturn¡¯s Magnetosphere
Simulations with the Rice Convection Model (RCM) of the injection-dispersion phenomenon in Saturn's magnetosphere produce interchange convection cells stretching outward from the outer edge of the plasma torus centered near the orbit of Enceladus. We show that the velocity-dependent Coriolis effects can be included by an effective Hall conductance, which is first implemented here using the grid based RCM. The simulation shows the following effects introduced by the Coriolis force: (1) bending of the convection cells in the retrograde direction, and (2) slowing of their growth. Our simulation results support and distinguish the predictions made by Vasyliunas [GRL, 21, 401, 1994] and Pontius [GRL, 24, 2961, 1997].
P43A-1007
A Multi-Fluid Study of the Centrifugal Interchange Cycle in Saturn's Inner Magnetosphere
Centrifugal interchange is a process widely observed by the Cassini spacecraft in Saturn's inner magnetosphere. A 3D multi-fluid global model of the Kronian magnetosphere shows that Saturn's gravity, magnetic field, and convection electric field modulate the growth of this interchange process. The model is used to quantify both components: the inward injections and the outward-moving outwelling of the interchange cycle. The model shows finger-like injections of hot tenuous plasma alternating between cooler spiral arm regions of denser plasma. These features are ubiquitous in the equatorial plane between 5-12 RS and persist for several hours, though their generation is dependent on the prevailing conditions in both the inner magnetosphere and the solar wind forcing. Using a distributed Enceladus source of water group ions, and altering its concentration relative to the ionospheric ions, we see that a relative concentration of heavier ions affects the extent and definition of the finger- like cool, dense outwelling plasma. Additionally, the direction of the interplanetary magnetic field (IMF) can modulate the size of the fingers. Results address the radial velocity, ion and electron densities, magnetic and electric fields in these injection regions, as well as the time and spatial scales on which they persist. Model spectrograms are compared to the bulk parameters observed by Cassini.
P43A-1008
Global MHD studies of Titan's interaction
Cassini Spacecraft made a recent pass of Titan on June 13, 2007 when Titan was located at 13.6 SLT. This flyby, referred to as T32 flyby, is the first occasion that Titan was observed to be outside Saturn's magnetopause. In this presentation, we will study Titan's ionospheric responses to such a sudden change in the upstream plasma flow, using a sophisticated multi-species global MHD model. In addition, simulation results of another flyby of Titan, T34 flyby, will also be presented and compared with Cassini observations. During this T34 flyby, the Cassini spacecraft passed the upstream region of Titan in the equatorial plane, provided important constraints for Titan's ionosphere. Moreover, Titan's ionospheric diffusion time is discussed based on both observations and simulation results.
P43A-1009
Numerical Simulations of the Interaction of Enceladus' Interaction With Saturn's Magnetosphere Using a 3D Multi-Species, Hall MHD Model
We have used our new multi-species, Hall MHD model to study the interaction of Saturn's magnetosphere with Enceladus. We used neutral densities, consistent with the values observed during the Cassini's July 14, 2005 flyby of Enceladus. We used a simple ion chemistry scheme and approximated the upstream conditions from CAPS and MAG observations. We compare our calculated plasma and magnetic field values with the observed ones.
P43A-1010
A non Thermal Model for the Titan Extended Exosphere
The recent results of the INMS instrument, onboard Cassini, have shown evidence for a hot Titan corona, with non thermal profiles for N2 and CH4 in the lower exosphere (below 2000 km altitude). We propose here to use these observations for modelling the extended non thermal exosphere of Titan, through the use of non thermal distributions at the exobase. Non thermal extended exospheric profiles are first deduced for the Titan flybys Ta (26th october 2004), Tb (13th december 2004) and T5 (16th april 2005). Then, we propose a non thermal exosphere model averaged over the two first two flybys, whose conditions were very similar, for the five main species at the exobase : H, H2, N, N2 and CH4. This model is then used to simulate the production of energetic neutral atoms (ENAs) in the Titan exosphere, and the results are compared to the observations of the MIMI INCA ENA imager onboard Cassini.
P43A-1011
Energetic Ion Precipitation at Titan
Energetic protons and oxygen ions have been observed in Saturn's outer magnetosphere and can precipitate into Titan's atmosphere where they deposit energy and drive ionospheric chemistry. A simple model is used to estimate ion production rates caused by the magnetospheric ion precipitation. Using an incident proton flux in the 27 keV to 4 MeV energy interval measured by the Cassini MIMI instrument we find that significant ion production rates exist in the 500 km to 1000 km altitude range. The electron density associated with the ion production is estimated to be 200 - 2000 cm-3 between 500 km and 1000 km. For comparison, solar radiation produces a dayside ionosphere with peak electron densities between about 2000 cm-3 and 6000 cm-3 . We find that energetic oxygen ions do not penetrate below about 650 km. We suggest that a few percent of the oxygen flux is converted to negative O ions as a consequence of charge exchange collisions, which might help to explain the negative ions observed near 1000 km by the Cassini CAPS instrument.
P43A-1012
Saturn's Magnetosphere During the Recent HST Observations
Using our 3D global MHD model we intend to study the periods during which the Hubble Space Telescope recently made images of the Saturn's ionosphere (Jan/Feb 2007). Using upstream solar wind conditions propagated from the Earth to Saturn as input, we have made dynamic simulations of the magnetosphere- ionosphere system and its response to the solar wind. In this paper we will present results of the simulations as well as comparisons to observations made by the HST.
P43A-1013
Global Shape Modelling of Saturn's Bow Shock
We present a new model of Saturn's bow shock surface. Shock crossings are identified in Cassini magnetic field and plasma data and these are added to the crossings made by Pioneer 11, Voyager 1 and Voyager 2. Using electron densities determined from Langmuir wave observations by the Radio and Plasma Wave System, the solar wind dynamic pressure for the Cassini crossings is estimated. Corrections for the planet's orbital motion and solar wind dynamic pressure variation are made and a conic section is fitted to the crossings using a least squares technique. We examine and discuss the role played by different parameters in determining the size and shape of the Kronian bow shock.
P43A-1014
Plasma Convection and Injections in Kronian Magnetosphere
Steady-state access of plasma from the magnetotail to the inner magnetosphere of Saturn is hampered by the large size of corotation-dominated region which extends all the way to the dayside magnetopause (~20RS). Production of plasma by Saturn's moons (e.g. oxygen ions from Enceladus) has been identified as a dominant driver of plasma convection in the inner magnetosphere. Mass loading of flux tubes results in their outward convection and consequent stretching of magnetic field lines consistent with observed disc-like magnetic field configuration. Since the new plasma material does not produce any additional magnetic flux, magnetic field stretching is limited. The convection crisis can be resolved via bursts of magnetic reconnection and partial tailward release of plasma in plasmoids. Flux tubes after reconnection are more tenuous and hotter due to non- adiabatic heating processes. Their volume per unit magnetic flux also decreases, which provides the buoyancy force moving flux tubes back inwards towards the planet. We show that while the outward plasma convection is smooth, the inward injections of the hot plasma observed by the Cassini CAPS instrument are inherently bursty. The injections events can be attributed to reconnection and consequent inward motion of entropy-depleted flux tubes. We present stability analysis which includes corotation effects and possible generation of field-aligned currents.
P43A-1015
Analysis of Electron Cyclotron Waves Observed in the Saturn Magnetosphere
Electron cyclotron waves, sometimes with harmonics, are observed for many hours on a number of orbits of the Cassini spacecraft, and appear to be enhanced within plasma depletion regions. These waves are similar to those observed in the terrestrial magnetosphere by the Polar spacecraft. Possible free energy sources include loss cones and electron beams. When electron phase space distributions are available we are conducting a linear dispersion analysis of possible wave modes in an effort to better understand the physical origin of these waves.
P43A-1016
Correlations Between Narrowband Radio Emissions and Transient Rotating Plasma Clouds in Saturn's Magnetosphere
The RPWS (Radio and Plasma Wave Science) instrument onboard the Cassini spacecraft has frequently detected a series of narrowband electromagnetic emissions from the inner magnetosphere of Saturn from May 2005 to July 2007. Frequency-time spectrograms show that the strongest narrowband emissions tend to occur at frequencies near 5 kHz with bandwidths of about 2 to 3 kHz. Other apparently associated bands also occur at higher frequencies, sometimes as high as 30 kHz. Analysis of all the events detected over a two-year period shows that the emissions tend to be observed more frequently on the night side of the planet, as well as at latitudes away from the equator. The longitude of the Sun using a longitude system recently developed by Kurth et al. organizes the narrowband radio emissions better than the longitude of the spacecraft, indicating that the modulation of the radio emissions is acting more like a flashing light than a rotating beacon. The narrowband radio emissions are believed to be produced by mode conversion from electrostatic waves near the upper hybrid frequency at about L ~ 7 to 8 and latitudes of ±30 degrees. Our studies show that the transient hot plasma clouds rotating around the planet inside the orbit of Titan, may be the source for narrowband radio emissions for three reasons. First, the hot plasma clouds are produced by ENA (energetic neutral atom) emissions, which are detected by MIMI (INCA), through charge exchange. ENA images of Saturn's magnetosphere show that the hot plasma clouds rotate around the planet in the region from 7 to 8 RS, and this agrees with the source location required by the above mode conversion model. Second, ENA emissions tend to show stronger intensities on the night side of Saturn, consistent with the narrowband radio emissions that are also observed more frequently on the night side. Third, the narrowband radio emissions and ENA emissions are well correlated in time. In most cases (20 out of 30), narrowband radio emissions are accompanied by clear ENA emissions. Of the remaining cases, 8 narrowband radio emissions fall in the gaps of ENA data, and the correlations between them can not be verified.
P43A-1017
Source Location of Narrowband Radio Emission Detected at Saturn
Since Cassini's arrival at Saturn in 2004, the Radio and Plasma Wave Science (RPWS) instrument has detected a series of narrowband radio emissions in Saturn's magnetosphere. It is believed that these narrowband emissions are first excited as electrostatic waves by electrons when the matching condition fuh = (n+1/2)fce is met, where fuh is the upper hybrid frequency and fce is the electron cyclotron frequency. These upper hybrid waves can mode convert to electromagnetic L-O waves in the presence of density gradients. We determined the polarization of the narrowband emissions and found it to be consistent with the prediction of linear mode conversion theory. To model the source region of narrowband emissions, we numerically calculated the locations where the matching condition is met in a meridian plane, using a scale height electron density model [Persoon et al., 2006] and a dipole magnetic field model. For the 5 kHz narrowband emission, the matching condition fuh = 3/2 fce is met at a radial distance of 5.6 RS and latitude of 32.6 degrees. For the 20 kHz emission, fuh = 3/2fce is met at 3.3 RS and 15.7 degrees latitude. In several selected events, Cassini went through the source region of narrowband emissions, as indicated by a strong electrostatic band preceding the radio emission. The coordinates of Cassini when the source of narrowband emissions are encountered closely match that predicted by the model. For these encounters with the narrowband emission source, we investigate the electron distribution functions obtained from the electron spectrometer (ELS) onboard Cassini, and perform an initial wave growth analysis for the electrostatic waves. For the electromagnetic radio component of narrowband emission, we employ a polarization reversal technique to determine the source locations. While the Cassini spacecraft is rotating, the apparent polarization of the radio emission measured by RPWS switches its rotation sense when the source goes through the antenna plane. By finding the intersection of the plane which contains the source with a ring of possible source locations predicted by our model, we can determine the longitudinal coordinate of the source.
P43A-1018
Goniopolarimetry of the SKR: Study of a Perikrone.
We present goniopolarimetric (aka direction-finding) results of the Saturn Kilometric Radiation (SKR), using the Cassini/RPWS/HFR data. Tools to retrieve the characteristics of the source of the emissions have been developed that allow to measure the localization and beaming angle of the SKR sources as well as the localization of the foot prints of the active magnetic field lines. We present results from these analysis on a SKR burst observed during a perikrone (09/25/2006). These results are providing for the first time the beaming angle, the invariant latitude and the local time of a SKR burst. These parameters are essential to constraint the models for electron acceleration that lead to auroral precipitations in one hand, and the radio emission processes on the other hand. We show that the foot print latitudes of the active magnetic field lines are compatible with the position of the UV aurorae.
P43A-1019
Energy Flow in Saturn's Ion Cyclotron Wave Belt
When molecules in Saturn's water-group neutral cloud are ionized, they are accelerated by the electric field associated with the motion of the magnetospheric plasma. This acceleration brings the pick-up ions to the bulk plasma speed and gives them free energy that leads to the growth of ion cyclotron waves. Ion cyclotron waves propagating along the magnetic field have been observed near the equatorial plane on almost all of Cassini's orbits through the inner magnetosphere. On near-equatorial orbits, these waves are observed to peak below the gyrofrequencies of water-group (O, OH, H2O) and molecular oxygen (O2) ions. On inclined orbits, the velocity of the spacecraft along the waves' direction of propagation produced Doppler shifts in the wave frequency. Using this shift, we calculate the phase velocity of the waves through the wave belt. We find that near the magnetic equator, the waves are propagating both parallel and anti-parallel to the magnetic field and have phase velocities of 40-50 km/s, one-half to one-third of the local Alfven speed. Beyond +/- 0.04 Rs of the magnetic equator, the water-group waves are propagating away from that plane and increasing in power until about +/- 0.25 Rs. Then they quickly damp. The wave power profile is similar for molecular oxygen waves, except these waves are about one-tenth as strong. Beyond +/- 0.3 Rs of the magnetic equator the water-group waves are absent, but the molecular oxygen waves persist until higher latitudes, although with low power. We discuss the energy carried by these waves, considering both ion species and where the background plasma gains energy from their absorption.
P43A-1020
Thermal Plasma Flow in Saturn's Inner Magnetosphere.
Ion counting data from the Cassini plasma spectrometer (CAPS) in Saturn's inner magnetosphere are utilized to calculate bulk plasma moments including densities, temperatures, and flow velocities. The study covers radial distances from about 5.5 to 10.0 RS, outside of the Encleadus orbit and the region where significant fresh pick up ions are observed1. In order to generate the ion moments from the non-spinning craft and a restricted view of phase space, a forward modeling approach is utilized where two Maxwellian populations (a light ion (H+) and a water group ion (W+)) are fit to the available data. Data are processed from the equatorial plane during periods of actuator arm activity yielding good CAPS viewing, i.e. through the predominant plasma flow direction on each actuator sweep. The magnetospheric plasma is found to be sub co-rotating in this region at about 80%. This is in good agreement with velocity results from MIMI data despite their results not being confined to the equatorial plane, while the densities calculated also agree well with density values from RPWS. The W+ temperature anisotropy appears to be >1 at low values of RS and becomes more isotropic at larger distances. Using the generated ion moments, estimates of the mass loading throughout this region are given. 1.) Tokar, R.L. et al., this meeting.
P43A-1021
Titan at Saturn's magnetopause: CAPS results from T32
Titan spends most of its time inside Saturn's magnetosphere, thus most of Cassini's encounters to date have been with Titan surrounded by magnetospheric plasma. During the encounter on 13 June 2007, however, the magnetosheath was close enough that the encounter itself happened very close to the magnetopause. In this talk we present the upstream plasma conditions, and show the measured electron and ion spectra during this so far unique event. Magnetosphere and magnetosheath plasma are both clearly seen, and at times there are mixed populations. Titan's ionosphere is clearly distinguished by cold plasma, and very heavy negative ions are seen by CAPS ELS near closest approach. We discuss the plasma results and their implications.
P43A-1022
Titan in Saturn's Magnetosheath: Cassini MAG Observations During the T32 Encounter
With a magnetopause average stand-off distance of at least 21 Kronian radii, Titan spends most of its time in Saturn's rotating magnetosphere. However, for Saturn local times near noon and during periods of high solar wind pressure, Titan is expected to be found within the magnetosheath. An analysis of Cassini magnetometer data obtained during the T32 flyby suggests that Titan was found in Saturn magnetosheath at the time of the encounter, in the middle of a series of strong magnetopause compressions and expansions. However, the topology of the heavily massloaded, draped field lines observed in Titan's magnetic pileup region suggest that a few minutes before the encounter, the satellite was within the magnetosphere. This interpretation is supported by Cassini plasma data. This is the first time that Saturn's major satellite is found in the shocked solar wind.
P43A-1023
Joint Control of Substorm Onset by Titan and Saturn
Terrestrial substorms occur with a sudden release of energy into the night magnetosphere and aurora zone. This energy is ultimately derived from the bulk flow of the solar wind coupled to the magnetosphere via dayside reconnection. Substorms at Jupiter and Saturn are in many respects similar to their terrestrial counterparts except that the energy is supplied from the rotational energy of the planet that is transferred from the ionosphere by field- aligned currents to the ions added to the magnetosphere by ionization of the E-ring and the Titan atmospheres. When the magnetospheric field is no longer able to contain the build up of rotational energy, tail reconnection occurs releasing the "excess" plasma down the tail and returning the magnetosphere to a lower stress state. Menietti and coworkers have reported that SKR is modulated by the phase of Titan in its orbit about Saturn. We examine the phase of Titan during substorms observed by their north-south field changes in the tail and find a strong association with Titan being very close to local midnight. In fact, Titan is usually within one Saturn rotation of midnight when the substorm occurs. This suggests that the periodic stretching of the magnetic field in synchronism with Saturn's rotation is the final factor leading to instability and release of the plasmoid. Thus both Titan and Saturn play a role in controlling the onset of saturnian substorms.
P43A-1024
Magnetospheric Storms at Saturn and Earth
The terrestrial magnetospheric storms are a well-known phenomenon in which plasma from the solar wind and the ionosphere is convected into the inner magnetosphere ("ring current") and energized by betatron acceleration and rapid changes in the magnetic field (substorms). Here we compare terrestrial storm characteristics with similar, newly found characteristics of Saturn's magnetosphere. We characterize Saturn's magnetospheric response to solar wind variability by using remote energetic neutral atom (ENA) measurements with simultaneous in-situ solar wind measurements when Cassini was outside the Saturnian magnetosphere. The Ion and Neutral Camera on board the Cassini spacecraft have obtained global energetic neutral atom (ENA) images of the hot plasma of Saturn's magnetosphere since February 2004. INCA obtains ENA images in the ~3-200 keV/nuc of protons and O+. The typical observations show hot plasma distributed roughly between 6 to 30 RS orbiting the planet with a period around the 10h45min rotation period depending on energy and species. However, some observations show how ENA intensity builds up on the nightside during intervals longer than the rotation period which indicates a gradual source of plasma. The intervals are often ended by a dramatic ENA intensification followed by a rotation of the newly injected plasma around the planet. We have selected a few of such intervals when Cassini was in the solar wind and could obtain solar wind parameters and simulataneous ENA image sequences. We use the Magnetic Field Experiment (MAG), the Cassini Charge Energy Mass Spectrometer (CHEMS), and the Cassini Plasma Spectrometer Subsystem (CAPS) to study the IMF, solar wind speed and density during these events and find that Saturn's magnetospheric activity most likely depends more on solar wind pressure than magnetic field orientation.
P43A-1025
The solar wind upstream of Saturn a comparison of pre- and post-SOI interplanetary magnetic field structure.
The interplanetary medium upstream of Saturn can have a very strong impact on magnetospheric dynamics. During the declining phase of the solar cycle, the heliosphere is highly structured by Corotating Interaction Regions (CIRs), while this pattern breaks down somewhat closer to solar minimum. We compare Cassini magnetometer data from the cruise to Saturn, and from a period after Saturn Orbit Insertion (SOI) where Cassini had a prolonged excursion into the solar wind. We present the predicted and observed values for the Parker Spiral angle over this long time interval upstream of Saturn and discuss the implications of this. The pre-SOI data encompass an interval where the solar wind was highly disturbed by the "Halloween Storms", a period of extreme solar activity during October/November 2003. The effects of these storms have been widely studied in the vicinity of the Earth, where some of the highest solar wind speeds ever were recorded. We examine how this unusual solar wind evolved by the time it reached Cassini, and discuss the expected magnetospheric response. This is then in turn contrasted with the more typical solar wind conditions, and solar cycle effects taken into account.
P43A-1026
Phase Space Density of Energetic Electron in Saturn's Inner Magnetosphere From Cassini Observation
Energetic particles in magnetized planet have been studied to understand particle acceleration and loss processes. Phase space density (PSD) gives a clue to determine internal or external source of energetic particles. We investigate source of energetic electrons (20 keV - a few MeV) in Saturnfs inner magnetosphere (2<L<10 Rs) by performing phase space density analyses for the Saturn orbit insertion (SOI) of Cassini. The analyses show that there is a difference of characteristics of electron PSD between inbound and outbound in 6 - 8 Rs as follows. 1. Electron PSD of outbound is greater than that of inbound in 6 - 8 Rs. 2. Electron PSD of inbound decreases with decreasing of radial distance. 3.Electron PSD of outbound has a local peak in 6 - 8 Rs. The results of 1 and 2 are consistent with PSD analyses from the Voyager data of Armstrong et al., [1983] although they did not calculate PSD for the region more than 8 Rs. The local peak in 6 - 8 Rs is a newly confirmed result and shows an evidence of the internal source region such as a local acceleration due to wave-particle interactions. Pancake or flat pitch angle distribution is observed in the internal source region. One of possible mechanisms explaining these distributions would be pitch angle scattering due to wave-particle interactions. On the other hand, butterfly distribution appears in inbound. Possible mechanism explaining the butterfly distribution would be a ring absorption. In this presentation, we will discuss causes of the difference of electron PSD between inbound and outbound by comparing observed pitch angle distribution and performing numerical calculations about the ring absorption, possibility of wave-particle interactions, and diffusion equation. Acknowledgments We grateful thank for the Magnetospheric Imaging Instrument (MIMI) group and the Radio and Plasma Wave Science (RPWS) group.
P43A-1027
Examining the Dynamics of Llow-Energy Electrons in Saturn's Magnetosphere by combining Cassini CAPS-ELS Data with Charged Particle Transport Model.
Our first analysis of electron populations using a diffusion theory model allowed the discussion of plasma production and its distribution in Saturn's inner magnetosphere. Our results in modeling the interactions between low-energy electrons and different components of the Kronian system (satellites, dust, and neutral clouds) showed that 1) part of the cold plasma observed by the Cassini Plasma Spectrometer is produced during the impact-ionization of neutrals, and 2) during such a process, the hot component of the electron populations is redistributed along the field lines. Our primary result was then the theoretical demonstration that the impact- ionization process contributes to the `bimodal' energy distributions and `butterfly' pitch-angle distributions. We now present our recent investigation of the sources, sinks and transports of electron plasma populations obtained by combining CAPS-ELS data with our physical particle transport model. We will focus on presenting our results for the period where Cassini was orbiting near the equatorial plane (from late 2005 to early 2006). Plasma data will be used for constraining our modeling, and assisting with the validation of our new simulations. Interaction with neutrals will be studied in the purpose of analyzing the various Saturnian plasma domains. We will also reexamine the diffusive radial transport by discussing magnetospheric processes susceptible to drive inward transport and outward plasma flow.
P43A-1028
Analysis of Intercalibrated Plasma Observations in Saturn's Magnetosphere
The magnetosphere of Saturn, an environment rich in plasma sources, has been systematically investigated by the Cassini spacecraft since 2004. Using the plasma particle measurements, the aim of the present study is to produce an overview of the plasma dynamics inferred from observations and to establish a three dimension kronian plasma distribution mapping. It consists in studying the properties of the plasma energy spectra in the different regions crossed by Cassini and determining the characteristic fluid parameters (temperature, density, pressure, spectral index) by a forward modeling method. For this purpose, we use data from the two plasma instruments onboard Cassini spacecraft: the CAPS electron and ion spectrometer (ELS & IMS) measuring the low energy plasma (eV to keV) and the Low Energy Magnetospheric Measurement System (LEMMS) of the MIMI instrument measuring the energetic plasma (keV to MeV).The calculated radial, latitudinal, and local time profiles allow to deduce some first results on the plasma configuration and dynamics and also constraint the possible inward and outward transport mechanisms. The fluid parameters are finally used as input for the equilibrium and transport equations parallel and perpendicular to the magnetic field.
P43A-1029
On the Variability of Observations in the Inner Saturnian Magnetosphere: Exploring the Parameter Space
The inner Saturnian magnetosphere is a complicated multiphase environment. Its structure depends both on several independent spatial coordinates (radial distance, latitude, longitude, local time, and the orbital phase of Enceladus, the principal plasma source) and on the time variability of the Enceladus dust, neutral and plasma torus. We will explore the hypothesis that a centrifugally driven instability of the torus results in a two-cell rotating convection pattern, as proposed by Gurnett et al. [Science, 2007] and Goldreich and Farmer [JGR, 2007], by detailing low-energy electron plasma, magnetometer and plasma wave observations inside the inner Saturnian magnetosphere. We will take advantage of particular Cassini orbits with similar radial, latitudinal, local time coverage in order to examine the time repeatability of observations and the possible existence of an azimuthally- restricted sector of plasma outflow.
P43A-1030
Statistical Morphology of ENA Emissions at Saturn
The Magnetospheric Imaging Instrument (MIMI) on the Cassini spacecraft is providing the first energetic neutral particle (ENA) measurements in the magnetosphere of Saturn. Advantageous spacecraft orbits during the first 120 days of 2007 allowed ENA observations to be mapped to the equatorial plane of the planet and surveyed as a statistical ensemble in a Sun-synchronous coordinate system. When projected onto the equatorial plane, emissions from both energetic hydrogen atoms (20-50 keV) and energetic oxygen atoms (64-144 keV) form toroidal rings nearly concentric with the planet. The H ring has a mean radius of 10.2 +/- 1.3 RS, while the O ring has a mean radius of 7.3 +/- 0.8 RS (1 RS = 60268 km). The H emissions display a distinct azimuthal peak at a local time of ~0.4 hours (just past midnight), while the O emissions essentially no peak in local time. The H peak seems to be a regular feature of these emissions over the 120-day interval surveyed, suggesting the hot spot may be caused by injection of particles from Saturn's magnetotail. Using a multipole model of the internal field and a ring current external field, magnetic field mapping of the ENA emissions to Saturn's ionosphere reveals that both H and O emissions appear equatorward of the aurora and are NOT connected to it.
P43A-1031
Determination of Ionization Rates in the E ring Based on Magnetic Fluctuation Amplitudes
Recent Cassini observations have established that the icy moon Enceladus is actively venting and ejecting water molecules, indicating strongly that it is the origin of the torus atmosphere. The interaction between the corotating plasma and neutrals is a critical process with significant implications for the evolution of both. Ionization of the neutral gasses through charge exchange creates a population of slower moving ions which interact with the corotating plasma and gain energy. When neutralized, these ions form faster moving neutrals that travel to larger radial distances from Saturn or escape from the system. Ionization of neutrals leads to a velocity phase space distribution function known as a ring distribution which results in generation of ULF waves in the torus and can be used for determining the ionization rates throughout the region. To do so, it is necessary to establish the saturation amplitude of the waves as a function of ionization rate. Given the nonlinear nature of this process, we use 2.5-Dimensional electromagnetic hybrid (kinetic ions, fluid electrons) simulations to investigate the generation and nonlinear evolution of ion cyclotron and mirror waves by ring distribution functions. Specifically, we utilize Cassini plasma measurements at a number of different radial distances in the E ring to initialize the simulations and determine the ionization rates needed to account for the observed wave amplitudes at these distances. Assuming charge exchange to be the dominant ionization process, we also estimate neutral densities at these distances and compare to model predictions.
P43A-1032
Molecular Oxygen Ions in Saturn's Inner Magnetosphere for the First 24 Cassini Orbits
We present an analysis of molecular oxygen ions in Saturn\'s inner magnetosphere for the first 24 orbits of Cassini about Saturn. Apoapses of these orbits have local times between 3 and 8 MLT. Data from the Ion Mass Spectrometer (IMS) are summed to achieve better statistics and to bring out features that are otherwise difficult to resolve from an individual orbital pass. In particular, O2+ is well-resolved by this method. Through a combination of peak fitting and baseline subtraction, we isolate O2+ counts from the tail of the W+[O+, OH+, H2O+, H3O+] mass distribution, and from background radiation. We detect O2+ from L = 4.5 out to L = 11 with high confidence, and tentatively detect O2+ inward to L = 3.5 and outward to L = 12. Molecular oxygen ions were initially reported to account for approximately 1-2% of the total ion population in the inner magnetosphere based on IMS data from the inbound Saturn orbital insertion (SOI) pass [Young et al., 2005]. Through refined analysis techniques, we have found O2+ to account for approximately 0.5% of the W+ peak at L = 4.5 with only minor fluctuations in relative density out to L = 12. The mean O2+ energy lies above co-rotation, but nevertheless tracks theoretical co-rotation out to L = 9.5, beyond which its energy falls.
P43A-1033
Abundances and Energetics for Water Group Ions in Saturn's Magnetosphere
We present magnetospheric ion composition results obtained from the Cassini Plasma Spectrometer (CAPS) time-of-flight instrument for orbits over a two year period in which Cassini was almost entirely in the dawn magnetosphere. Water group ions (O+,\ OH+,\ H2O+,\ H3O+; or collectively,\ W+) dominate over H+ in the inner magnetosphere to approximately L = 20. W+ ions are seen sporadically out to L = 36. Within the water group,\ O+ dominates at all distances, H3O+ is second to O+ for L < 5, H2O+ is second from L = 5 to L = 8, and OH+ is second for L > 10. The density of H3O+ peaks at L = 3 and drops off rapidly, becoming undetectable beyond L = 11 . The W+ group plasma has both a core and a high energy tail; characteristics of each component have a separate dynamical history and vary as a function of radial distance. The ratio of number densities for the tail to the core is seen to have a broad peak between 10 < L < 20,\ indicating a source mechanism in this location,\ possibly the ring current. Finally,\ we show how the time variation of W+ number densities at specific locations is indicative of bursty events in the magnetosphere.