P52B-01 

Investigation of Particle and Magnetic Field Periodicities in Saturn's Magnetosphere

* Burch, J L (jburch@swri.edu), Southwest Research Institute, P. O. Drawer 28510, San Antonio, TX 78228-0510, United States Goldstein, J (jgoldstein@swri.edu), Southwest Research Institute, P. O. Drawer 28510, San Antonio, TX 78228-0510, United States Mokashi, P (pmokashi@swri.edu), Southwest Research Institute, P. O. Drawer 28510, San Antonio, TX 78228-0510, United States Young, D T (dyoung@swri.edu), Southwest Research Institute, P. O. Drawer 28510, San Antonio, TX 78228-0510, United States Coates, A J (ajc@mssl.ucl.ac.uk), MSSL-UCL, Holmbury St. Mary, Dorking, SRY RH5 6NT, United Kingdom Dougherty, M (m.dougherty@imperial.ac.uk AF:

Several recent studies have revealed a periodicity in either charged particles or magnetic fields comparable to the rotation rate of Saturn (~10.8 h), with various local time and longitudinal dependences. We analyze Cassini particle and field data together to show that the periodicity is associated with the traversal (by the Cassini spacecraft) of a region of enhanced plasma pressure, decreased magnetic field strength, and a localized region of more dipolar magnetic field orientation, a plasma and field configuration that could result from a locally intensified current sheet or disk. We test several hypotheses concerning possible origins of this configuration, including association with a particular longitude or local time range, and dynamical behavior of the magnetospheric current sheet.

P52B-02 

Why Does Saturn's Auroral Oval Not Respond to the Rotating Cam Perturbations?

* Kivelson, M G (mkivelson@igpp.ucla.edu), Inst. Of Geophys. and Planetary Phys., 6843 Slichter Hall, 405 Hilgard Ave., Los Angeles, CA 90095-1567, United States Southwood, D J (david.southwood@esa.int), European Space Agency, HQ, 8-10 Rue Mario-Nikis, Paris, FRA 75015, Southwood, D J (david.southwood@esa.int), Physics Department, Imperial College, London, SW7 2BZ, United Kingdom

The regular magnetic perturbations in Saturn's magnetosphere have been shown to require source currents flowing in regions external to the planet. The perturbations are also consistent with a rotating source where the current varies with azimuth around the planet as cos φ. From the low latitude data, one deduces that the current seems to cross the equatorial plane and flow from ionosphere to ionosphere on magnetic L-shells in the region 10 to 15. Near the equator, such a current system produces roughly the same perturbations as one with the same azimuthal symmetry flowing on the surface of a sphere of radius 10 to 15 RS, implying that inside the current-carrying shell the perturbation field is uniform and that outside it has the form of an equatorial dipole. Indeed, beyond L=15, but near the equator, the field corresponds approximately to that of an internal dipole tilted by roughly 10 degrees and modified by local plasma contributions. At high latitudes, the spherical model is not applicable and so deductions based on it cannot be made. Evidence for this is seen in the response of the auroral emissions to planetary rotation. Auroral emissions associated with a tilted dipole, as at Earth, move relative to the spin axis as the planetary dipole rotates. At Saturn the auroral emissions move little relative to the spin axis as the planet rotates. The lack of response to the cam currents at high latitudes can be understood by considering current paths in the polar regions, as we shall discuss in this talk.

P52B-03 

The Saturnian Ring Current: The Role of Suprathermal Pressure.

* Sergis, N (nsergis@phys.uoa.gr), Office of Space Research and Technology, Academy of Athens, Soranou Efesiou 4, Papagos, Athens, 115 27, Greece Krimigis, S M (Tom.Krimigis@jhuapl.edu), Office of Space Research and Technology, Academy of Athens, Soranou Efesiou 4, Papagos, Athens, 115 27, Greece Krimigis, S M (Tom.Krimigis@jhuapl.edu), Applied Physics Laboratory, Johns Hopkins University, 11100 Johns Hopkins Road, Laurel, MD 20 723, United States Mitchell, D G (don.mitchell@jhuapl.edu), Applied Physics Laboratory, Johns Hopkins University, 11100 Johns Hopkins Road, Laurel, MD 20 723, United States Roelof, E C (Edmond.Roelof@jhuapl.edu), Applied Physics Laboratory, Johns Hopkins University, 11100 Johns Hopkins Road, Laurel, MD 20 723, United States Hamilton, D C (dch@umd.edu), University of Maryland, Department of Physics, John S. Toll Physics Building, College Park, MD 20 742, United States Krupp, N (krupp@linmpi.mpg.de), Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Str. 2, Katlenburg-Lindau, 37 191, Germany

On July 1, 2007, Cassini completed three full years orbiting Saturn. The Magnetospheric Imaging Instrument (MIMI) onboard the spacecraft, performs comprehensive measurements of the energetic ion population within the magnetosphere of the planet. Observations of energetic ion directional intensities, energy spectra and ion composition are provided by the Charge Energy Mass Spectrometer (CHEMS) over the range 3 to 220 keV/e and by the Low Energy Magnetospheric Measurements System (LEMMS) that measures ions in the range 0.024 to 18 MeV. Using the suprathermal particle pressure (P_part) distribution over the Saturnian magnetosphere, calculated from the (E>3keV) energetic ion fluxes obtained during the three years of Cassini orbits, we address the driving mechanism of the equatorial azimuthal current system in the Saturnian magnetosphere. We report estimates of the equatorial azimuthal ring current intensity (J_phi) resulting from the radial pressure gradient and compare the inertial (centrifugal) stress to suprathermal particle pressures and their relative significance on the ring current formation and dynamics. Preliminary results indicate that the suprathermal pressure (P_part), is systematically larger compared to the ram pressure of the corotational flow (ρ V²), by approximately one order of magnitude (i.e. 5× 10^{- 10}dyne/cm² vs. 6× 10^-11dyne/cm² for r=10R_S), throughout the ring current region. Furthermore, outside r=11R_S the centrifugal stress (ρ Ømega ²r), appears to be substantially smaller compared to the radial gradient of the suprathermal pressure (dP_part/dr), suggesting that the outer part of the azimuthal ring current is driven principally by suprathermal rather than inertial forces.

P52B-04 

Plasma Production and Circulation in Saturn's (and Jupiter's?) Magnetosphere.

* Rymer, A M (abigail.rymer@jhuapl.edu), Johns Hopkins Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, United States Mauk, B H (barry.mauk@jhuapl.edu), Johns Hopkins Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, United States Hill, T W (hill@rice.edu), Department of Physics and Astronomy, Rice University, Houston, TX 77005, United States Paranicas, C (chris.paranicas@jhuapl.edu), Johns Hopkins Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, United States Mitchell, D G (don.mitchell@jhuapl.edu), Johns Hopkins Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, United States Smith, H (h.todd.smith@jhuapl.edu), Johns Hopkins Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, United States Christon, S P (spchriston@aol.com), Johns Hopkins Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, United States Wilson, R J (rjw@lanl.gov), Los Alamos National Laboratory, Los Alamos, Los Alamos, NM 87545, United States Johnson, R E (rej@virginia.edu), University of Virginia, University of Virginia, Charlottesville, VA 22903, United States Andre, N (nandre@rssd.esa.int), Research and Scientific Support Department, Estec, Noordwijk, 2200 AG, Netherlands Sittler, E C (edward.C.Sittler@nasa.gov), NASA, Goddard Space Flight Centre, Greenbelt, MD 21405, United States Thorne, R M (rmt@atmos.ucla.edu), Department of Atmospheric and Oceanic Sciences, Los Angeles, Los Angeles, CA 90095, United States Coates, A J (ajc@mssl.ucl.ac.uk), Mullard Space Science Laboratory, Holmbury St Mary, Dorking, RH5 6NT, United Kingdom Young, D T (dyoung@swri.edu), Southwest Research Institute, SwRI, San Antonio, TX 78228, United States Santos-Costa, D (dsantoscosta@swri.edu), Southwest Research Institute, SwRI, San Antonio, TX 78228, United States Bolton, S J (sbolton@swri.edu), Southwest Research Institute, SwRI, San Antonio, TX 78228, United States Thomsen, M F (mthomsen@lanl.gov), Los Alamos National Laboratory, Los Alamos, Los Alamos, NM 87545, United States Dougherty, M K (m.dougherty@imperial.ac.uk), Department of Space and Atmospheric Physics, Imperial College, London, SW7 2AZ, United Kingdom

Saturn has a distributed source of cold (<100 eV) electrons inside L ~ 12 associated with Saturn's satellites, rings, and extended neutral cloud. Phase space density analyses by Rymer et al. [2007a] have shown that the cold component has a local source, probably due to ionisation of the neutral cloud components. These cold electrons are heated to the observed energies through Coulomb collisions, and other interactions with ions, and transport slowly outward. Like Jupiter, magnetic flux lost through cold plasma outflow is balanced by the injection of hot outer magnetospheric plasma inward. Phase space density contours of the hot (> 100 eV) electron component at Saturn are consistent with a source in the outer magnetosphere which transports inward and heats adiabatically. Several studies have shown that small scale injection events are a ubiquitous feature of Saturn's magnetosphere [Burch et al., 2005, Hill et al., 2005, Leisner et al. 2005, André et al. 2005] and these are thought to be the source of the observed hot electron component. Rymer et al. [2007b] suggest that, along with losses to the neutral cloud, inwardly transported electrons turn around as they drift out of the small inflow channels and flow back to the outer magnetosphere – thus contributing to the hot electron component "butterfly" pitch angle distributions observed by Burch et al. [2007] and attributed to outward flow from an inner magnetospheric source. Here we summarise this electron recirculation picture as it currently stands and discuss how high energy in situ plasma observations and remote energetic neutral observations by MIMI along with proton observations by CAPS add to the current picture. There are significant differences in how Jupiter and Saturn behave in terms of electric and magnetic drift speeds; we will discuss to what extent our picture of Saturn's magnetosphere is true for Jupiter.

P52B-05 INVITED 

How important is Enceladus in Saturn's global magnetosphere?

* Delamere, P (delamere@lasp.colorado.edu), Laboratory for Atmospheric and Space Physics, University of Colorado, CB 392 Duane Physics D133, Boulder, CO 80309, United States

Our knowledge of Saturn's neutral gas-dominated inner magnetosphere is based on measurements made by Pioneer 11, Voyager 1, Voyager 2, Cassini, and on observations using the Hubble Space Telescope. As was recently revealed by Cassini, the source for Saturn's neutral torus is Enceladus, venting 10²⁷ to 10²⁸ H₂O s^-1 from its south pole. Ionization of the neutral torus produces a tenuous plasma torus with electron densities < 100 cm^-3. Outside 1.9 {R_s}, the plasma torus is subject to a flux tube interchange instability provided that the total number of ions per unit of magnetic flux decreases radially outward. Consequently the radial transport of plasma from Enceladus can influence magnetospheric dynamics just as Io's plasma source drives Jupiter's magnetosphere. A key parameter that determines radial transport rates is the plasma mass per second transported outward, coupled with Saturn's ionospheric Pedersen conductivity. To assess the influence of Enceladus on Saturn's global magnetosphere we have adapted our Io plasma torus chemistry and magnetosphere-ionosphere coupling models to Saturn following Richardson et al. [1986] and Jurac and Richardson [2005] for implementing the water group chemistry. Compared to Jupiter, our results suggest that ionization at Saturn is fundamentally limited by the slower corotational flow velocity at Enceladus (26 km/s), resulting in a lower ion pickup temperature. The net result of cooler ions at Enceladus is a cooler thermal electron population (~ 2 eV) that is insufficient to generate significant ionization. Instead, the Enceladus plasma torus is maintained by the hot electron population (i.e. 1000 eV, measured by the Cassini Plasma Spectrograph [ Young et al., 2005]), and we suggest that the hot electrons are derived from global plasma dynamics. In addition, our initial results suggest that longitudinal variations in the hot electron abundance can explain the factor-to-two variation in electron density reported by Gurnett et al. [2006]. Preliminary estimates of the plasma mass outflow rate will be presented to address the question, "How important is Enceladus in Saturn's global magnetosphere?"

P52B-06 

Detecting the Enceladus Neutral Torus via Water Group Pick Up Ions

* Tokar, R L (rlt@lanl.gov), Space Science and Applications, Los Alamos National Laboratory, Los Alamos, NM 87545, United States Wilson, R J (rjw@lanl.gov), Space Science and Applications, Los Alamos National Laboratory, Los Alamos, NM 87545, United States Henderson, M G (mhenderson@lanl.gov), Space Science and Applications, Los Alamos National Laboratory, Los Alamos, NM 87545, United States Thomsen, M F (mthomsen@lanl.gov), Space Science and Applications, Los Alamos National Laboratory, Los Alamos, NM 87545, United States Sittler, E C (Edward.C.Sittler@nasa.gov), NASA, Goddard Space Flight Center, Greenbelt, MD 20771, United States Johnson, R E (rej@virginia.edu), Engineering Physics and Astronomy, University of Virginia, Charlottesville, VA 22904, United States

One of the major discoveries1 of Cassini to date is the south polar icy plume at Enceladus (R ~ 4 RS). Models2 predict that this plume may be a source of both the extended (2-8 RS) OH neutral cloud observed by the Hubble space telescope3, and a new feature, a narrow (~0.5 to 1.0 RS) neutral water group torus centered on the Enceladus orbit. As the corotating and magnetically confined thermal plasma (mostly water group ions) streams through the gravitationally bound water group neutrals, charge exchange between the ions and neutrals is expected4 to occur yielding slower ions subsequently "picked up" by Saturn's magnetic field. The phase space density of these ions should show characteristics of a ring velocity distribution within the source, combined with subsequent scattering into a shell and possible adiabatic cooling at larger radial distances. Therefore, via analysis of Cassini in situ ion counting data, it may be possible to indirectly detect the neutral Enceladus torus, confirming the predictions in (2). In this study, Cassini plasma spectrometer (CAPS) data for equatorial orbits with favorable viewing5 are analyzed. The radial distance range of about 3.5 to 6.5 RS is considered covering data across the Enceladus orbit. Assuming flow speeds near co-rotation as reported in (6) yields a modeled water group ion core that is subtracted from the measured data. The resulting residual ion counting data has velocity space signatures resembling pick up ions. The strongest source region is identified about the Enceladus orbit with radial extent at least 1 RS, in qualitative agreement with predictions. Peak phase space density of these ions is perpendicular to the magnetic field, resembling a ring, as expected within the source region. At larger radial distances (e.g. R = 6 RS), the ring signature has evolved to a shell and the expected adiabatic cooling due to transport from the source outward is observed. Similarly strong pick up ion sources are not observed near the orbits of either Tethys or Dione. 1.) Science, "Cassini at Enceladus", special section, 10 March 2006. 2.) Johnson, R.E. et al., The Astrophysical Journal, pg L137, 20 June 2006. 3.) Shemansky et al., Nature, 27 May 1993. 4.) Johnson, R.E., M. Liu, and E.C. Sittler, Jr., Geophys. Res. Letts., 32, 17 Dec 2005. 5.) Wilson, R.J. et al., this meeting. 6.) Sittler, E.C. et al., Geophys. Res. Letts., 32, 15 June 2005.

P52B-07 

The character of the Enceladus water plume during the three Cassini flybys based on a 3-D MHD model comparison

* Jia, Y D (yingdong@ucla.edu), IGPP, UCLA, 595 Charles Young Dr. East, Los Angeles, CA 90095, Khurana, K K (kkhurana@igpp.ucla.edu), IGPP, UCLA, 595 Charles Young Dr. East, Los Angeles, CA 90095, Russell, C T (ctrussel@igpp.ucla.edu), IGPP, UCLA, 595 Charles Young Dr. East, Los Angeles, CA 90095, Gombosi, T I (tamas@umich.edu), UMich, SRB, 2455 Hayward, Ann Arbor, MI 48109,

The Cassini spacecraft made three close flybys of Enceladus in 2005. The magnetic field data collected from these flybys have helped to clarify the importance of Enceladus particle production for the maintenance of the E ring surrounding Saturn. However, some new questions arise based on these flybys, including how is the neutral cloud distributed and how does it vary temporally, as well as how does the neutral cloud affect the plasma distribution in the strong ambient Saturnian magnetic field. In this work we apply our 3-D magnetohydrodynamic model to the simulation of the interaction between the Saturnian corotational plasma flow and the Enceladus obstacle.The effect of different neutral distributions, ionization rates, and charge exchange rates are examined and compared with the Cassini magnetometer data. Based on this MHD result, the field perturbation by dust particles is also investigated.

P52B-08 

Prediction of Solar Wind Properties at Saturn: Models and Validation

* Zieger, B (bzieger@umich.edu), University of Michigan, 2455 Hayward St., Ann Arbor, MI 48109, United States Hansen, K C (kenhan@umich.edu), University of Michigan, 2455 Hayward St., Ann Arbor, MI 48109, United States Cohen, O (oferc@umich.edu), University of Michigan, 2455 Hayward St., Ann Arbor, MI 48109, United States Gombosi, T I (tamas@umich.edu), University of Michigan, 2455 Hayward St., Ann Arbor, MI 48109, United States

To study the role of solar wind driving in the dynamics of the Saturnian magnetosphere, one needs information about the solar wind plasma in the foreshock region. Due to the lack of continuous in-situ measurements, we have to predict the solar wind properties at Saturn by means of solar wind propagation models. So far, mainly 1-D MHD models have been used to propagate the solar wind observed near the Earth to the outer planets. Recently we have developed a 2-D version of our solar wind propagation code, which is expected to further improve solar wind predictions in the ecliptic plane, and can make use of multipoint solar wind input data, e.g. from the Stereo spacecraft. In addition, we are working on a completely different approach of solar wind propagation: a 3-D semi- empirical solar wind model driven by synoptic magnetic charts observed on the solar surface. In this paper, the advantages and limitations of these three models are discussed. Furthermore, we present an extensive validation study of our 1-D model based on the solar wind plasma and interplanetary magnetic field data of major heliospheric missions like the Pioneer, Voyager, and Ulysses spacecraft. We investigate the efficiency of solar wind predictions as a function of azimuthal distance from opposition (when the Earth and the given spacecraft are located at the same helioecliptic longitude). A possible effect of solar cycle phase on the prediction efficiency is also examined. Finally, model results are compared with Cassini measurements of the solar wind.

Author(s) (2007), Title, Eos Trans. AGU, 88(52), Fall Meet. Suppl., Abstract #####-##.