P51A-1397 0800h
Saturn's Internal Magnetic Field
We present new magnetic field data measured by the Cassini spacecraft during Saturn Orbit Insertion (SOI). The observed data are analyzed using standard inversion techniques, and a preliminary model of the internal field is presented. The zonal coefficients are found to be in good agreement with previous models derived from the Pioneer 11, Voyager 1 and Voyager 2 flybys. One striking feature of past models is the lack of non axisymmetric terms. We have reanalyzed past flyby data, and concluded that no certain conclusion on the axial symmetry of the field can be obtained from flyby observations alone, due to the uncertainty in our knowledge of the planetary rotation rate. We then propose a method for obtaining the first direct measurement of the rotation rate of the magnetic field, which is found to be 10h39m23.0s, in close agreement with the IAU value. A full picture of the internal magnetic field of Saturn, including its rotation state and the degree of its axial symmetry, will be revealed by Cassini, which will map the field at various longitudes and latitudes during the orbital tour.
P51A-1398 0800h
A new Global Model of Saturn's Magnetospheric Field
We present a new model of Saturn's magnetospheric field which is based on observations derived from Pioneer, Voyager and Cassini spacecraft. The model uses techniques and methods used by modelers for the earth's magnetosphere. Our model consists of modules which specify (1) the internal spherical harmonic model, (2) the ring current and the magnetotail current system, (3) the field from the radial current system which reinforces corotation on the outflowing plasma. (4) the shielding fields from an axially symmetric magnetopause (5) and the interconnection magnetic field between solar wind IMF and the magnetosphere. The structural model of the current sheet is specified in the Kronian Solar Magnetospheric (KSM) coordinate system and incorporates the hinging of the current sheet observed at large distances. The model correctly specifies the effect of the 26.7 degrees tilt between the Kronian orbital and equatorial planes. We use Tsyganenko and Peredo [1994] models of disk-shaped current sheets to model the magnetic field of Saturn's current sheet. The tilt, and hinging of the current sheet is introduced by using the general deformation technique [Tsyganenko, 1998]. The shielding field from the magnetopause for the equatorial current sheet and the internal field is specified by Cartesian and cylindrical harmonics, respectively.
P51A-1399 0800h
Plasma Boundaries and Low Frequency Waves at Saturn: Cassini Magnetometer Observations
The Cassini orbit insertion phase yielded a rich dataset from the magnetometer instrument (MAG) which provided magnetic field measurements in the solar wind, magnetosheath and magnetospheric regions of Saturn. During the inbound and outbound pass of the insertion orbit, Cassini MAG detected multiple bowshock and magnetopause crossings. We comment on the structure and variability of these magnetic boundaries implied by the data. We also study the properties of low frequency waves in Saturn's upstream and magnetosheath regions. In particular, observations of non-linear, low-frequency waves called "shocklets", and mirror-mode waves are presented. Their properties, occurrence and origin are also discussed.
P51A-1400 0800h
Possible Signature of Plasma Interchange in the Saturnian Plasmasheet From Magnetometer Observations
The Cassini spacecraft crossed both inbound and outbound the Saturnian plasmasheet during its insertion orbit around Saturn, beginning of July 2004. This particular region of the Saturnian magnetosphere, which contains numerous plasma sources, appeared in the magnetometer data particularly dynamic at that time, suggesting an important redistribution of plasma operating in the region. The mechanism responsible for plasma transport and redistribution in rapidly rotating magnetospheres like the Saturnian magnetosphere is believed to be triggered by the centrifugal instability and to proceed through the interchange of magnetic flux tubes. We present and discuss possible signatures of interchanging flux tubes observed in the magnetometer data in regions between Dione and Rhea. These signatures consist of abrupt and of short duration changes in the magnetic field magnitude and are interpreted in term of mass-loaded or depleted flux tubes moving outward or inward through the plasmasheet.
P51A-1401 0800h
Titan's Interaction With the Saturnian Magnetospheric Plasma - Results From an Advanced 3D MHD Model and Comparison With Cassini MAG Data.
We have developed an advanced 3D model for Titan's interaction with the Saturnian magnetospheric plasma. The ideal magnetohydrodynamic (MHD) equations are extended in order to account for the effect of Titan's dense neutral atmosphere has on the plasma interaction. The ion-neutral friction plays a key role in the deceleration of the magnetospheric plasma. From the neutral atmosphere the ionosphere is created within the model by applying a 3D radiation model for the solar EUV radiation. Impact ionization rates by photoelectrons are calculated effectively. In order to account for the process of impact ionization by magnetospheric electrons we have developed a time dependent flux tube model which accounts for flux tube motion through the atmosphere and electron heat conduction along the magnetic field lines. On Titan's nightside magnetospheric electron impact ionization is the only source for ion production. Thus, we obtain a 3D ionosphere that depends on the Saturnian local time (SLT) and the subsolar latitude (SSL). As a reference for the incident magnetospheric plasma parameters we use the parameters measured by Voyager 1. On the 26th of October the first close Cassini flyby at Titan occurs. We have applied our model to the nominal situation at the time of TA, i.e. SLT=10.6 and SSL=-23.2. It is most likely that Titan is inside of the Saturnian magnetosphere during the flyby. In that case we will compare the results from our model with the Cassini MAG data along the trajectory and discuss the results.
P51A-1402 0800h
Initial results from the Cassini CAPS Instruments at the Magnetopause of Saturn
Cassini entered orbit about Saturn on 1st July 2004 providing the first opportunity since Voyager to sample the Saturnian magnetospheric plasma regions. In this presentation we concentrate on the series of magnetopause encounters made by the spacecraft during its inbound trajectory, in particular we present data from the Cassini CAPS Electron Spectrometer (ELS) instrument which measures electrons from 0.58eV-26keV. The ELS instrument collected comprehensive electron data with an unprecedented temporal and energy response throughout the duration of these magnetopause crossings with the timing of the crossings being identified from the change of characteristics of the low energy electron populations. All crossings were made at local times of 07:43 to 07:48 MLT at magnetic latitudes of $\sim$ -15 degrees. The electron density and temperature within the magnetosheath regions were ~ 1.3$\times10^5$m$^{-3}$ and $\sim$ 40eV respectively. Within the magnetosphere the plasma was more rarefied (~ 1.2$\times10^4$m$^{-3}$) with an increased electron temperature of up to several 100eV. The crossings appeared to exhibit varied features associated with different magnetopause configurations. Some are associated with very sharp discontinuities in the electron data, possibly indicating a closed boundary, while others appear to be associated with an interior boundary layer. Some brief magnetosphere entries reveal unusual plasma regions separate from the magnetopause and immersed in the ambient plasma possibly indicating FTE's or boundary layers. We present results from an initial analysis of each of these crossings using the low energy electrons and incorporating results from the CAPS ion instruments where possible. Magnetic field data has also been used to put the particle results in context.
P51A-1403 0800h
Plasma Transition Across the Bow Shock at Saturn: Initial Results From the Cassini Plasma Spectrometer
The Cassini spacecraft entered the Saturn system in June 2004. The first observation of Saturn's bow shock occurred at 09:48 UT on 27 June 2004 at an absolute distance of 49.2 Rs from the planet. At this time Cassini was approaching Saturn towards dawn, ~30 degrees below the orbital plane with a velocity of ~7.3 kms-1. During this, and six subsequent crossings on the inbound leg, the Cassini plasma spectrometer (CAPS) measured the flux of 0.5 eV to 28.0 keV electrons as they were slowed and heated at the shock front and made some measurements of ions in the downstream region. After the successful insertion burn Cassini encountered the bow shock a further ten times from 82.5 to 87.0 Rs. At this time the orbiter was exiting the Saturn system with a velocity of ~2.6 kms-1 within ~15 degrees of Saturn's equator. During these crossings CAPS was oriented such that the instrument was able to measure the fluxes of 1 eV to 50 keV ions in addition to the electron measurements (which are always available irrespective of spacecraft pointing). The CAPS observations show that the solar wind plasma is strongly modified at the bow shock with electron density enhancements up to a factor of 4 and electron temperature increasing by as much as a factor of ~20. The ion bulk flow in the sheath was typically ~250 kms-1 with a thermal speed of ~100 kms-1. In this presentation we will characterise the plasma transitions observed and compare them with previous studies made by the same instrument at Jupiter. Supporting data from the Cassini magnetometer and the radio and plasma wave instruments will also be presented where appropriate.
P51A-1404 0800h
Preliminary Results on Saturn's Inner Plasma Sheet as Observed by Cassini: Comparison with Voyager
We will present preliminary results of our analysis of Saturn's inner plasma sheet as observed by the Cassini Plasma Spectrometer (CAPS) experiment during Cassini's initial entry into Saturn's magnetosphere and when the spacecraft was put into orbit around Saturn. For this initial analysis ion fluxes are divided into two sub-groups: protons and water group ions. Depending on the status of our preliminary analysis we will discuss the ion composition and details of the fluid parameters. These results will eventually allow us to solve the force balance equation along the magnetic field (ions and electrons) and predict the vertical distribution of the plasma along the magnetic field. Once this is done we will be in a position to make detailed comparisons with the Voyager results.
P51A-1405 0800h
Transverse Waves in the Inner Saturn Magnetosphere
The Cassini magnetic field investigation detected two very different types of transverse fluctuations during the Saturn Orbit Insertion pass, in the frequency range that is resonant with heavy ions. Neither wave has an analogue in the Galileo measurements at Jupiter. The first wave phenomenon was seen inbound about 6 Rs planetocentric, surrounding a significant diamagnetic depression of unknown origin. The waves were almost purely transverse with little compressional component but they were also linearly polarized and not circularly polarized as expected for ion resonance. A second period of transverse waves appeared outbound from about 4.26 Rs to 5.27 Rs 5 hours after the orbit insertion burn. These waves were close to the local gyro frequency for ions with mass to charge ratios from that of singly ionized water to singly ionized carbon dioxide. These waves were left-hand elliptically polarized, propagating at a finite angle to the magnetic field with a smaller but significant compressional component. The mass range of these ions coincide with the expected major components of the exhaust of Cassini's injection engine. Thus these observations appear to be the first detection of the artificial production of waves in a magnetosphere other than that of the Earth. Observations during Cassini's second orbit will also be discussed.
P51A-1406 0800h
The Ring-dust Plasma Torus as Observed by Cassini RPWS
We present new results indicating that ring-dust is a major plasma source for the magnetosphere of Saturn. This study of the cold plasma near the ring plane of Saturn is based on observations by the Radio and Plasma Wave Science (RPWS) instruments on board the Cassini spacecraft. A dense dusty ring plasma was detected both during the inbound and outbound crossings of the ring plane. The Langmuir probe and plasma emission measurements indicated an increase of the plasma density up to 100 cm(-3), and the dense plasma was centred around the maximum count rate of impacting micrometer sized dust particles on the spacecraft. Furthermore, the Langmuir probe observations pointed toward a cold plasma population with electron temperatures around 0.5-1 eV and ion temperatures well below 100 eV. The ram current to the spherical probe indicated an average ion mass between 20-40 amu, which was confirmed by the INMS instrument that the dominant ion was molecular oxygen (O2+). The plasma density decreased to very low values when Cassini passed over (northward) the visible rings of Saturn, which suggest that the ring-plasma is most dense just outside the F ring. The visible rings presumably absorb magnetically mirrored charged particles on conjugate magnetic field lines, and hence the plasma density is low inside the F-ring. Except during the Saturn Orbit Injection (SOI) spacecraft burn, the spacecraft potential was determined under the rings. From the information of the thermal plasma and the spacecraft potential over the rings we make an attempt to infer the electric charge of dust particles in the rings themselves. The Langmuir probe measurements therefore may provide observational constraints on the problem how the co-rotating magnetosphere of the planet Saturn affect the dynamics and structure of ring-dust.
P51A-1407 0800h
Narrowband Emissions within Saturn's Density Torus
The Cassini spacecraft made a close approach to Saturn on 1 July 2004, flying through the rings to within ~1.4 RS of the planet. The Cassini Radio and Plasma Wave Science (RPWS) investigation detected a clear emission at the upper hybrid resonance frequency that enabled the identification of a high density plasma torus surrounding Saturn, with peak plasma densities located at about 2.3 RS and a very abruptly decreasing plasma density at the torus inner edge. Emitted from this inner torus edge is a set of narrowband emissions similar in nature to those emitted from the terrestrial plasmapause. We will discuss the morphology of the narrowband emission observations, including their drifts in frequency as a function of time. We will also place these observations in context of ongoing theoretical analysis of narrowband emission from density gradients.
P51A-1408 0800h
Observation of Flux Modulations in SKR and Low Frequency Waves, Relations with Other Measurements and Possible Implications on the Magnetospheric Activity.
Pronounced modulations of the flux of low frequency waves (whistler mode around 50 Hz) have been observed (RPWS experiment) on day 181 and 182 as Cassini was above the plasmasheet at a distance of 25-15 Rs from Saturn. They present a typical periodicity of 50- 90 minutes. During the same time interval, the fluxes of the auroral radio emission (SKR) and of the energetic particles (INCA experiment) present similar modulations. The magnetic field (MAG instrument) also fluctuates, suggesting that global scale magnetospheric oscillations are observed (analogue to pi2/pc5 at Earth ?). Considering other time intervals, we show that similar modulations are often detected in the flux of SKR with periodicity of several tens of minutes. The phenomena would thus be frequent, perhaps systematic, suggesting that these oscillations are an important element of the activity of Saturn's magnetosphere.
P51A-1409 0800h
Auroral Hiss-Like Emissions Observed by Cassini Near the Rings of Saturn
During the Cassini spacecraft pass over the rings of Saturn on July 1, 2004, a well-defined funnel-shaped electric field emission was detected by the plasma wave instrument onboard the spacecraft. The funnel-shaped emission has spectral characteristics that are very similar to a type of whistler-mode emission called `auroral hiss' that is commonly observed in Earth's auroral region. This is the first detection of such emissions at Saturn. Using a dipole magnetic field and a simple electron density model based on the electron density profile measured by Cassini, ray tracing computations have been performed and it is found that the low altitude boundary of the source is located very close to the B ring at a distance near the synchronous rotation point in the rings. At Earth, such auroral hiss emissions are caused by field-aligned low-energy (100 eV to 1 keV) electron beams associated with the auroral current system. The existence of similar emissions originating from near Saturn's rings suggest that an electrodynamic interaction between the rings and Saturn's co-rotating magnetospheric plasma may be driving field-aligned beams and currents similar to those occurring in Earth's auroral regions.
P51A-1410 0800h
Nonlinear electrostatic structures associated with Saturn's bow shock
We report on nonlinear electrostatic structures including pulse chains and isolated pulses observed to be abundant at the bow shock transition layer and the downstream magnetosheath of Saturn. The characteristic time scales involved in these nonlinear structures are 5-10 ms. Pulse chains, isolated pulses, and large amplitude (up to $\sim$ 10 mV/m) fluctuations of very similar time scales are also observed to be the dominant waveforms in the downstream bow shock current layer at Earth. Considering the widely different densities and magnetic field strengths in the two magnetosheaths, we rule out those wave modes whose characteristic frequencies scale with the plasma frequency or the cyclotron frequency. The only parameters that are comparable in the two magnetosheaths are the plasma streaming velocity and the electron temperature. We therefore conclude that the responsible wave mode is the low frequency (below the ion plasma frequency) ion acoustic mode. The solitary unipolar, bipolar, and tripolar electric-field structures of these time scales are thus most likely to be nonlinear descendants of low frequency ion acoustic waves. These nonlinear electrostatic structures may hold important information on how a collisionless shock boundary is formed and sustained.
P51A-1411 0800h
SATURN'S GLOBAL ION DISTRIBUTION FROM CASSINI/INCA ENA AND IN-SITU MEASUREMENTS
The Saturn Orbit Insertion (SOI) of Cassini took place on 1 July 2004. The Ion and Neutral Camera (INCA) obtained global energetic neutral atom (ENA) images of the magnetosphere during 1-2 July, while the LEMMS detector obtained {\it in-situ} ion and electron intensities and pitch-angle distributions. Since Cassini made a cut through the morning (inbound) and post-midnight (outbound) sector, we have enough information to use the INCA images and LEMMS in situ intensities to constrain the global ion distribution using simulations with a parametrized model adapted from Roelof and Skinner [2000]. The model for the neutral gas distribution was adapted from Richardson [1998]. From the ENA images, the ion distribution appeared to peak on the nightside, while the LEMMS ion and electron intensities displayed two strong peaks between 3 to 8 RS on both the inbound and outbound passes, with the outbound pass displaying stronger intensities. The LEMMS pitch-angle distributions exhibited clear depletions around 90 deg within these regions, consistent with ion/gas interactions contributing to the minima between the intensity peaks. Additional resolved ENA images and in situ intensities will be obtained after the first close Titan flyby (Ta) on 26 October 2004.
http://sd-www.jhuapl.edu/CASSINI/
P51A-1412 0800h
Near-Saturn Solar Wind Speeds Determined from MIMI/ CHEMS Measurements of Pickup Ion Spectra at the Cassini Spacecraft
During the Cassini spacecraft's approach to Saturn and now while in orbit (beginning July 1, 2004), the varying energy spectrum of singly charged, $\sim$3--220 keV/e He ions is measured by the MIMI/CHEMS instrument. At these energies, He pickup ions are a significant population, well observed by CHEMS. The cutoff in pickup ion phase space densities at twice the solar wind speed provides a means to measure the speed without directly sampling the solar wind plasma; i.e., without requiring a particular orientation of the spacecraft. Since the state of the solar wind plays a fundamental role in the dynamics of the Saturnian magnetosphere, the existence of a solar wind speed monitor should contribute to a range of research efforts. We develop a method to determine the solar wind speed by fitting the pickup He spectra with a mathematical model when Cassini is in the solar wind, which is the great majority of the time. Except during periods of very low flux or passing interplanetary shocks, an automated procedure will allow routine analysis of the solar wind speed down to 1-hour resolution. The time dependent solar wind speeds that we compute will be used in conjunction with observations from other MIMI sensors to investigate energetic particle phenomena in the Saturn system.
P51A-1413 0800h
Energetic particle injections in Saturns magnetosphere
The Low Energy Magnetosphere Measurement System (LEMMS) and the CHarge Energy Mass Spectromenter (CHEMS) on the Cassini Spacecraft, both part of the Magnetospheric Imaging Instrument (MIMI), have discovered energy-time dispersed energetic ion and electron signatures (greater than 20 keV) within Saturn's inner and middle magnetosphere (L from 4 to 7.5 Rs). They were discovered during the Saturn Orbit Insertion (SOI) orbit of the Cassini mission on 1-2 July 2004. These dispersive events are interpreted here as being caused by sudden planetward injections of energetic particles within regions that are narrowly confined in the longitude direction, followed by the slow and dispersive azimuthal drift of the injected particle to the location of the Cassini spacecraft. This interpretation is suggested by the analysis of similar features discovered by the Galileo mission at Jupiter. The injections at both Jupiter and Saturn may be analogous to the so-called substorm injection phenomenon at Earth. The dispersive properties of five injections at Saturn have been analyzed quantitatively to reveal the longitude and local time of the sudden injections. All five injections occurred on the nightside of Saturn's magnetosphere with best estimate angles with respect to the midnight line ranging between -70 and +60 degrees (the possible range grows to -105 to +135 degrees when random errors are included). If this local time ordering continues with the future observations of injections during future orbits of Cassini, then one would conclude that the solar wind interaction with Saturn participates in the generation of the injections as is observed at Earth and contrary to what appears to occur at Jupiter.
P51A-1414 0800h
Energetic Neutral Atom Emissions Associated With Titan: Observations During Cassini's First Orbits of Saturn
Titan's nitrogen-rich atmosphere is directly bombarded by energetic ions, due to its lack of a significant intrinsic magnetic field. Singly-charged energetic ions from Saturn's magnetosphere undergo charge exchange collisions with neutral atoms in Titan's exosphere, being transformed into energetic neutral atoms (ENAs). The Ion and Neutral Camera (INCA), one of the three sensors that comprise the Magnetospheric Imaging Instrument (MIMI) on the Cassini/Huygens mission to Saturn and Titan, images the ENA emissions from various ion/gas interaction regions in the Saturnian magnetosphere. MIMI also directly measures the parent energetic ion population in situ along the Cassini trajectory, using the CHEMS and LEMMS ion sensors. During Cassini's first orbit around Saturn the INCA data revealed, during the T0 Titan flyby (July 2, 2004, altitude: 339,000 km) the emission of ENAs from a globular region clearly centered on Titan. There is an extended emission region around the Titan cloud, but it is much dimmer than the globular cloud itself. During Cassini's second orbit around Saturn the spacecraft will perform the TA Titan flyby (October 26, 2004), at an altitude of only 1200 km. INCA data acquired during this targeted close flyby will be analyzed and compared to model predictions.
P51A-1415 0800h
Composition and Sources of Suprathermal Ions in Saturn's Magnetosphere
The Charge-Energy-Mass Spectrometer (CHEMS), one of three sensors comprising the MIMI investigation on Cassini, measures the mass and charge state of ions in the energy per charge range 3--220 keV/e. CHEMS has determined the composition of the suprathermal ions during Cassini's first pass through Saturn's magnetosphere. The most abundant species were H$^{+}$, H$_{2}^{+}$, and O$^{+}$. Also present were the molecular ions OH$^{+}$, H$_{2}$O$^{+}$, and O$_{2}^{+}$ as well as doubly charged oxygen O$^{++}$. We have observed a near absence of N$^{+}$ ions. This composition, which features water products, indicates that Saturn's rings and icy moons are strong plasma sources while Titan, with its largely nitrogen atmosphere, is not. We will discuss composition variations with position in Saturn's magnetosphere as well as any temporal changes observed between Cassini's first and second orbits.
P51A-1416 0800h
Energetic Proton and Electron Distributions in Saturn Magnetosphere as Revealed by Cassini / Voyager Observations and Proposed by Models
Before the arrival of Cassini at Saturn in July 2004, our knowledge of energetic protons and electrons (energy > 10 KeV) in Saturn's magnetosphere, radial distances r < 12 Rs was mainly based on the particle observations made during the flybys of Pioneer 11 (September 1979), Voyager 1 (November 1980), and Voyager 2 (August 1981). Models of radiation belts have then been developed from direct processes of particles data (LECP) (empirical model) or from physical models drawn on the adiabatic invariant theory and the resolution of the governing Fokker-Planck transport equation. These models have been used to increase our understanding of Saturn radiation belt dynamics, and to predict Cassini observations of particle fluxes (MIMI/LEMMS instrument) and energetic neutral atom emissions (MIMI/INCA instrument). The measurements obtained during the insertion of Cassini at Saturn displayed important differences between predictions and observations. Furthermore, an unexpected new radiation belt in the innermost part of the magnetosphere has been discovered. These new observations have demonstrated that Cassini will provide important new knowledge of Saturn's magnetosphere and revisions of models will need to be considered. We present a preliminary update to our model based on recent LEMMS data. We will discuss both the differences between LEMMS observations and predictions, and the changes made in our model (plasma injection, physical processes, magnetic field model, etc.) in order to reproduce the recent Cassini observations.
P51A-1417 0800h
New Simulations of Saturn's Polar Wind
We present preliminary results of a new simulation of Saturn's polar wind. The results include new vertical ion and electron density profiles, fluxes, and net source rates. Our model solves the 13 moment gyrotropic transport equations. The gyration dominated assumption allows us to assume that the flow is field aligned, and justifies the use of a one dimensional model. Furthermore, the effect of solar zenith angle and optical depth on the solar flux is taken into account. While capable of modeling transient behavior, only steady state results are presented. Similarly, the ability to include field aligned currents is built into the model, but their effect is not yet studied. By combining polar wind model output with a global MHD simulation of the magnetosphere-ionosphere system, we are able to calculate the net ionospheric source due to the polar wind. Like Earth, we expect that Saturn's ionosphere is an important source of magnetospheric plasma. The recent arrival of Cassini is giving unprecedented amounts of data. The time is therefore ripe for an attempt to characterize the ionospheric source at Saturn, and this simulation will help further this objective.
P51A-1418 0800h
Solar Wind Modeling of Cassini-Approach Observations
The CISM (Center for Integrated Space Weather Modeling) inner heliospheric model CORHEL is extended to 10 AU to investigate how well this solar magnetogram-based 3D MHD model describes the solar wind influence on Saturn's magnetosphere. The CORHEL model describes the steady solar wind stream structure and its origins in the corona. It has been tuned to provide a simulation of the solar wind parameters at 1 AU including plasma moments and interplanetary magnetic field magnitude and polarity in the absence of disturbances from coronal transients. Comparisons with the Cassini CAPS plasma and magnetometer field observations during Cassini's approach to Saturn show good agreement with the model, in part because of a low solar activity and a particularly simple and steady interplanetary sector pattern during the months before the Saturn orbit insertion (SOI). It is known that Saturn's magnetosphere responds to solar wind dynamic pressure enhancements, which are evident in this period with striking regularity. The model agreement suggests that a viable option is available both to predict such enhancements from co-rotating stream interaction regions, as well as to provide an approximation to solar wind conditions when Cassini is inside Saturn's magnetosphere. Moreover, as radial alignment with other close-in spacecraft is not required, this study illustrates the potential use of state-of-the-art numerical models to study space weather on a solar-system-wide scale.
P51A-1419 0800h
Planetary Auroral Storms Trace a CME-driven Interplanetary Shock Throughout the Solar System, from the Sun to Saturn at 9 AU
Hubble Space Telescope FUV images taken in December 2000 revealed for the first time au auroral storm on Saturn. The Sun, the Earth, Jupiter and Saturn were practically aligned at that time, allowing the solar wind plasma to flow by all three planets successively within ~1 month. Observations of Jupiter coordinated with Cassini measurements in the nearby solar wind were also executed during this period. Using a recently developped MHD code and solar wind measurements in the Earth vicinity, we establish that (1) the strong auroral event on Saturn was related to the interaction of an interplanetary shock with its magnetosphere, (2) this shock was initiated by a series of CMEs on the Sun observed by SOHO. We follow the propagation of the shock throughout the solar system, from the Earth where auroral storms are recorded, to Jupiter where the auroral activity is strongly enhanced, and to Saturn where it ultimately activates the observed unusual polar source. This is the first report of consecutive auroral responses to a propagating interplanetary shock. It indicates that shocks retain their properties and their ability to trigger planetary auroral activity thoughout the solar system, thereby unifying our understanding of solar-planetary interactions. We discuss also the similarities and differences observed between the planetary auroral responses.
P51A-1420 0800h
Interaction of Saturnian Dust Stream Particles Detected by the Cassini Cosmic Dust Analyser (CDA) With the Interplanetary Medium on Approach to Saturn
On approach to Saturn, during the first half of 2004, the Cosmic Dust Analyzer (CDA) onboard Cassini detected streams of small, fast dust particles thought to emanate from the Saturnian system. In this study, using magnetic field data obtained by the Cassini magnetometer (MAG) and solar wind velocity data obtained by Cassini Plasma Spectometer (CAPS), we simulate the trajectories of dust particles ejected from the Saturnian system as they propagate through the interplanetary medium where they are strongly influenced by the interplanetary magnetic field. By comparing the simulation results with data obtained by CDA we attempt to determine whether Saturn is indeed the point of origin and further characterize the nature of the dust particles detected onboard Cassini.
P51A-1421 0800h
Latest Cassini HFR-RPWS Direction Finding Results at Saturn
The Cassini spacecraft arrived at Saturn on july 1st 2004, starting a 4 year exploration of the kronian environment. Its high frequency receiver HFR, part of the Radio and Plasma Wave Science experiment (RPWS), has direction finding (DF) capabilities, i.e. allows us to retrieve the flux intensity of an incoming electromagnetic wave, its direction of arrival, and its polarization state. We have applied the DF inversion techniques to the data stored since last summer. Two radio components have been mainly studied : the SKR (saturn kilometric radiation coming the auroral regions) and the SEDs (saturn electrical discharges caused by atmospheric lightnings). While the results confirm the Voyager 1 & 2 data, they also unveil puzzling features that remain to be explained.
P51A-1422 0800h
Nitrogen In Saturn's Magnetosphere
We are analyzing CAPS instrument data on Cassini to look for nitrogen ions in Saturn's magnetosphere. Because Voyager could not separate oxygen and nitrogen, there has been considerable controversy on nitrogen's presence and relative importance. Two principal sources have been suggested: Titan's atmosphere and nitrogen species trapped in Saturn's icy satellite surfaces (Sittler et al 2004). The latter may be primordial nitrogen, likely as NH3 in ice (Stevenson 1982; Squyers et al. 1983) or nitrogen ions that have been implanted in the surface (Delitsky and Lane 2002). We will present the results of Saturnian nitrogen cloud modeling and relevant CAPS observations. We recently described the Titan source (Michael, et al. 2004; Shematovich et al. 2003; Smith et al. 2004; Sittler et al. 2004) in preparation for Cassini's Saturnian plasma measurements. Two components were identified: energetic nitrogen ions formed near Titan and energized as they diffused inward (Sittler et al. 2004) and neutrals in orbits with small perigee that became ionized in the inner magnetosphere (Smith et al 2004). The latter component would be a source of lower energy, co-rotating nitrogen ions in the inner magnetosphere. Such a component would have an energy spectrum similar to nitrogen species sputtered from the icy satellite surfaces (Johnson and Sittler 1990). However, the mass spectrum would differ, likely containing NHx and NOx species also, and, hence, may be separated from the Titan source. Our preliminary analysis for nitrogen species in the CAPS data will be compared to our models. Of interest will be the energy spectra, which can indicate whether any nitrogen present is formed locally or near Titan's orbit and diffused inward. This work is supported by the NASA Planetary Atmospheres, NASA Graduate Student Research, Virginia Space Grant Consortium Graduate Research Fellowship and CAPS Cassini instrument team programs.