W. S. Kurth

Publications (217)805.19 Total impact

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  Available from: uiowa.edu

  Article: Energetic electrons in the inner part of the Jovian magnetosphere and their relation to auroral emissions

  A T Tomás, J Woch, N Krupp, A Lagg, K.-H Glassmeier, W S Kurth
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  ABSTRACT: 1] The energetic particle distribution in the magnetosphere of Jupiter changes significantly between the inner and the middle magnetosphere. One of the most prominent changes is a transition of the electron pitch angle distribution (PAD) from a pancake to a bidirectional distribution. The transition is a persistent and localized feature defining a distinct spatial boundary between 10 and 17 R J . We discuss the possible relation between the PAD boundary and some of the observed structures in the Jovian aurora. A comparison between the Hubble Space Telescope observations and the predicted ionospheric footprints of the PAD boundary indicates a good correlation, with a discrete belt of emissions equatorward of the main auroral oval. Furthermore, the precipitation energy flux associated with the energetic electron distribution at the PAD boundary is compatible with the brightness range of these auroral emissions.
  J. Geophys. Res. 01/2407; 109.
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  Available from: M. Volwerk

  Article: ULF waves in Ganymede's upstream magnetosphere

  M Volwerk, X Jia, C Paranicas, W S Kurth, M G Kivelson, K K Khurana
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  ABSTRACT: Ganymede's mini-magnetosphere, embedded in Jupiter's larger one, sustains ULF waves that are analyzed here using data from two Galileo flybys that penetrate deeply into the upstream closed field line region. The magnetometer data are used to identify field line resonances, magne-topause waves and ion cyclotron waves. The plasma densities that are inferred from the interpretation of these waves are compared with the observations made by other plasma and wave experiments on 5 Galileo and with numerical and theoretical models of Ganymede's magnetosphere.
  Annales Geophysicae 01/2013; 31:45. · 1.84 Impact Factor
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  Available from: ucla.edu

  Article: Mapping Magnetospheric Equatorial Regions at Saturn from Cassini Prime Mission Observations

  C. S. Arridge, N. André, H. J. McAndrews, E. J. Bunce, M. H. Burger, K. C. Hansen, H.-W. Hsu, R. E. Johnson, G. H. Jones, S. Kempf, [......], W. S. Kurth, J. S. Leisner, C. Paranicas, E. Roussos, C. T. Russell, P. Schippers, E. C. Sittler, H. T. Smith, M. F. Thomsen, M. K. Dougherty
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  ABSTRACT: Saturn’s rich magnetospheric environment is unique in the solar system, with a large number of active magnetospheric processes and phenomena. Observations of this environment from the Cassini spacecraft has enabled the study of a magnetospheric system which strongly interacts with other components of the saturnian system: the planet, its rings, numerous satellites (icy moons and Titan) and various dust, neutral and plasma populations. Understanding these regions, their dynamics and equilibria, and how they interact with the rest of the system via the exchange of mass, momentum and energy is important in understanding the system as a whole. Such an understanding represents a challenge to theorists, modellers and observers. Studies of Saturn’s magnetosphere based on Cassini data have revealed a system which is highly variable which has made understanding the physics of Saturn’s magnetosphere all the more difficult. Cassini’s combination of a comprehensive suite of magnetospheric fields and particles instruments with excellent orbital coverage of the saturnian system offers a unique opportunity for an in-depth study of the saturnian plasma and fields environment. In this paper knowledge of Saturn’s equatorial magnetosphere will be presented and synthesised into a global picture. Data from the Cassini magnetometer, low-energy plasma spectrometers, energetic particle detectors, radio and plasma wave instrumentation, cosmic dust detectors, and the results of theory and modelling are combined to provide a multi-instrumental identification and characterisation of equatorial magnetospheric regions at Saturn. This work emphasises the physical processes at work in each region and at their boundaries. The result of this study is a map of Saturn’s near equatorial magnetosphere, which represents a synthesis of our current understanding at the end of the Cassini Prime Mission of the global configuration of the equatorial magnetosphere. KeywordsCassini–Saturn–Magnetospheric regions–Plasma processes
  Space Science Reviews 05/2012; 164(1):1-83. · 3.61 Impact Factor
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  Available from: ArXiv

  Article: Overview of Saturn lightning observations

  G Fischer, U. A. Dyudina, W. S. Kurth, D. A. Gurnett, P. Zarka, T. Barry, M. Delcroix, C. Go, D. Peach, R. Vandebergh, A. Wesley
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  ABSTRACT: The lightning activity in Saturn's atmosphere has been monitored by Cassini for more than six years. The continuous observations of the radio signatures called SEDs (Saturn Electrostatic Discharges) combine favorably with imaging observations of related cloud features as well as direct observations of flash-illuminated cloud tops. The Cassini RPWS (Radio and Plasma Wave Science) instrument and ISS (Imaging Science Subsystem) in orbit around Saturn also received ground-based support: The intense SED radio waves were also detected by the giant UTR-2 radio telescope, and committed amateurs observed SED-related white spots with their backyard optical telescopes. Furthermore, the Cassini VIMS (Visual and Infrared Mapping Spectrometer) and CIRS (Composite Infrared Spectrometer) instruments have provided some information on chemical constituents possibly created by the lightning discharges and transported upward to Saturn's upper atmosphere by vertical convection. In this paper we summarize the main results on Saturn lightning provided by this multi-instrumental approach and compare Saturn lightning to lightning on Jupiter and Earth.
  11/2011;
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  Available from: astrosurf.com

  Article: A giant thunderstorm on Saturn.

  G Fischer, W S Kurth, D A Gurnett, P Zarka, U A Dyudina, A P Ingersoll, S P Ewald, C C Porco, A Wesley, C Go, M Delcroix
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  ABSTRACT: Lightning discharges in Saturn's atmosphere emit radio waves with intensities about 10,000 times stronger than those of their terrestrial counterparts. These radio waves are the characteristic features of lightning from thunderstorms on Saturn, which last for days to months. Convective storms about 2,000 kilometres in size have been observed in recent years at planetocentric latitude 35° south (corresponding to a planetographic latitude of 41° south). Here we report observations of a giant thunderstorm at planetocentric latitude 35° north that reached a latitudinal extension of 10,000 kilometres-comparable in size to a 'Great White Spot'-about three weeks after it started in early December 2010. The visible plume consists of high-altitude clouds that overshoot the outermost ammonia cloud layer owing to strong vertical convection, as is typical for thunderstorms. The flash rates of this storm are about an order of magnitude higher than previous ones, and peak rates larger than ten per second were recorded. This main storm developed an elongated eastward tail with additional but weaker storm cells that wrapped around the whole planet by February 2011. Unlike storms on Earth, the total power of this storm is comparable to Saturn's total emitted power. The appearance of such storms in the northern hemisphere could be related to the change of seasons, given that Saturn experienced vernal equinox in August 2009.
  Nature 07/2011; 475(7354):75-7. · 36.28 Impact Factor
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  Available from: Andrew J. Coates

  Article: Properties of Saturn Kilometric Radiation measured within its source region

  L. Lamy, P. Schippers, P. Zarka, B. Cecconi, C. Arridge, M. K. Dougherty, P. Louarn, N. Andre, W. S. Kurth, R. L. Mutel, D. A. Gurnett, A. J. Coates
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  ABSTRACT: On 17 October 2008, the Cassini spacecraft crossed the southern sources of Saturn kilometric radiation (SKR), while flying along high-latitude nightside magnetic field lines. In situ measurements allowed us to characterize for the first time the source region of an extra-terrestrial auroral radio emission. Using radio, magnetic field and particle observations, we show that SKR sources are surrounded by a hot tenuous plasma, in a region of upward field-aligned currents. Magnetic field lines supporting radio sources map a continuous, high-latitude and spiral-shaped auroral oval observed on the dawnside, consistent with enhanced auroral activity. Investigating the Cyclotron Maser Instability (CMI) as a mechanism responsible for SKR generation, we find that observed cutoff frequencies are consistent with radio waves amplified perpendicular to the magnetic field by hot (6 to 9 keV) resonant electrons, measured locally.
  01/2011;
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  Available from: ArXiv

  Article: Emission and propagation of Saturn kilometric radiation: magneto-ionic modes, beaming pattern and polarization state

  L. Lamy, B. Cecconi, P. Zarka, P. Canu, P. Schippers, W. S. Kurth, R. L. Mutel, D. A. Gurnett, J. D. Menietti, P. Louarn
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  ABSTRACT: The Cassini mission crossed the source region of the Saturn kilometric radiation (SKR) on 17 October 2008. On this occasion, the Radio and Plasma Wave Science (RPWS) experiment detected both local and distant radio sources, while plasma parameters were measured in situ by the magnetometer (MAG) and the Cassini Plasma Spectrometer (CAPS). A goniopolarimetric inversion was applied to RPWS 3-antenna electric measurements to determine the wave vector k and the complete state of polarization of detected waves. We identify broadband extraordinary (X) as well as narrowband ordinary (O) mode SKR at low frequencies. Within the source region, SKR is emitted just above the X mode cutoff frequency in a hot plasma, with a typical electron-to-wave energy conversion efficiency of 1% (2% peak). The knowledge of the k-vector is then used to derive the locus of SKR sources in the kronian magnetosphere, that shows X and O components emanating from the same regions. We also compute the associated beaming angle at the source theta'=(k,-B) either from (i) in situ measurements or a model of the magnetic field vector or from (ii) polarization measurements. Obtained results, similar for both modes, suggest quasi-perpendicular emission for local sources, whereas the beaming pattern of distant sources appears as a hollow cone with a frequency-dependent constant aperture angle: theta'=75{\deg}+/-15{\deg} below 300kHz, decreasing at higher frequencies to reach theta'(1000kHz)=50{\deg}+/-25{\deg}. Finally, we investigate quantitatively the SKR polarization state, observed to be strongly elliptical at the source, and quasi-purely circular for sources located beyond approximately 2 kronian radii. We show that conditions of weak mode coupling are achieved along the ray path, under which the magneto-ionic theory satisfactorily describes the evolution of the observed polarization.
  01/2011;
• Article: Hybrid Simulations of Plasma‐Neutral‐Dust Interactions at Enceladus

  N. Omidi, C. T. Russell, R. L. Tokar, W. M. Farrell, W. S. Kurth, D. A. Gurnett, Y. D. Jia, J. S. Leisner
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  ABSTRACT: Through ejection from its southern hemisphere, Enceladus is a dominant source of neutral gas and dust in Saturn’s inner magnetosphere. The interaction of the corotating plasma with the gas and dust modifies the plasma environment around Enceladus. We use 3‐D hybrid (kinetic ions, fluid electrons) simulations to examine the effects of gas and dust on the nature of the interaction region and use Cassini observations to constrain their properties.
  AIP Conference Proceedings. 12/2010; 1302(1):237-242.
• Article: Saturn's equinoctial auroras

  J ~D Nichols, S ~V Badman, E ~J Bunce, J ~T Clarke, S ~W ~H Cowley, F ~J Crary, M ~K Dougherty, J -C Gérard, D Grodent, K ~C Hansen, W ~S Kurth, D ~G Mitchell, W ~R Pryor, T ~S Stallard, D ~L Talboys, S Wannawichian
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  ABSTRACT: We present the first images of Saturn's conjugate equinoctial auroras, obtained in early 2009 using the Hubble Space Telescope. We show that the radius of the northern auroral oval is tilde1.5deg smaller than the southern, indicating that Saturn's polar ionospheric magnetic field, measured for the first time in the ionosphere, is tilde17% larger in the north than the south. Despite this, the total emitted UV power is on average tilde17% larger in the north than the south, suggesting that field-aligned currents (FACs) are responsible for the emission. Finally, we show that individual auroral features can exhibit distinct hemispheric asymmetries. These observations will provide important context for Cassini observations as Saturn moves from southern to northern summer.
  grl. 12/2009; 36:24102.
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  Available from: ulg.ac.be

  Article: Recurrent energization of plasma in the midnight-to-dawn quadrant of Saturn's magnetosphere, and its relationship to auroral UV and radio emissions

  D G Mitchell, S M Krimigis, C Paranicas, P C Brandt, J F Carbary, E C Roelof, W S Kurth, D A Gurnett, J T Clarke, J D Nichols, J.-C Gé Rard, D C Grodent, M K Dougherty, W R Pryor
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  ABSTRACT: a b s t r a c t We demonstrate that under some magnetospheric conditions protons and oxygen ions are accelerated once per Saturn magnetosphere rotation, at a preferred local time between midnight and dawn. Although enhancements in energetic neutral atom (ENA) emission may in general occur at any local time and at any time in a Saturn rotation, those enhancements that exhibit a recurrence at a period very close to Saturn's rotation period usually recur in the same magnetospheric location. We suggest that these events result from current sheet acceleration in the 15–20 Rs range, probably associated with reconnection and plasmoid formation in Saturn's magnetotail. Simultaneous auroral observations by the Hubble Space Telescope (HST) and the Cassini Ultraviolet Imaging Spectrometer (UVIS) suggest a close correlation between these dynamical magnetospheric events and dawn-side transient auroral brightenings. Likewise, many of the recurrent ENA enhancements coincide closely with bursts of Saturn kilometric radiation, again pointing to possible linkage with high latitude auroral processes. We argue that the rotating azimuthal asymmetry of the ring current pressure revealed in the ENA images creates an associated rotating field aligned current system linking to the ionosphere and driving the correlated auroral processes.
  05/2009;
• Article: Low frequency equatorial waves at Earth, Polar and Cassini observations

  G. B. Hospodarsky, W. S. Kurth, D. A. Gurnett, J. D. Menietti, O. Santolik, M. K. Dougherty
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  ABSTRACT: Low-frequency electromagnetic emissions are often detected by spacecraft near the Earth's geomagnetic equator. These fast magnetosonic waves can accelerate electrons and are believed to play an important role in the Van Allen radiation belt dynamics. The Polar Plasma Wave Instrument (PWI) often detected these waves near the geomagnetic equator. These emissions were usually detected below a few hundred Hertz and showed a wide range of frequency structure, from macroscopic structure (funnel-shaped spectrum) to finer frequency structure (narrow frequency bands with a spacing of only a few Hertz). The Cassini spacecraft also detected low frequency emissions near the Earth's magnetic equator during its Earth flyby. A survey of properties and of these emissions will be presented and the similarities and differences between the two spacecraft will be discussed.
  03/2009; 11:5568.
• Article: Titan s plasma wake geometry from RPWS and MAG observations

  R. Modolo, P. Canu, C. Bertucci, L. Rosenqvist, W. S. Kurth, D. Gurnett, M. K. Dougherty
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  ABSTRACT: Up to now, several tens of Titan flybys have been successfully completed by Cassini and have revealed a highly dynamic structure of the near space environment of Titan. The upstream condition of the plasma flow is expected to affect Titan's induced magnetosphere. The Titan's plasma wake has been investigated using observations from the Radio and Plasma Wave Science (RPWS) instruments (Gurnett et al, 2004) and the dual Magnetometer Technique MAG instruments (Dougherty et al, 2004). Electric field emissions were detected by the RPWS antennas during Cassini passes through Titan's wake. These narrow band emissions are identified as upper hybrid resonance emissions and therefore can provide a density estimate of the Titan's cold plasma. Some of Titan's wake flybys show a very strong asymmetry between the inbound and the outbound pass. Good examples are Ta and Tb flybys (Wahlund et al, 2005). Both flybys have a similar trajectory in Titan Interaction coordinate System (TIIS) and have the same illumination condition but the density profiles present major differences. Some of the Cassini flybys have been set back in the DRAP coordinate system (Neubauer et al, 2006) such that the upstream direction of the magnetic field is fixed, in order to determine the geometry of the plasma wake and study asymmetries. Maps of cold plasma in Titan's environment are presented. Information concerning the geometry of the wake is crucial to estimate accurately the plasma escape. Dougherty, M. K., et al. (2004), The Cassini Magnetic Field Investigation, Space Sci. Rev, 114, 331- 383, doi:10.1007/s11214-004-1432-2 Gurnett, D. A., et al. (2004), The Cassini radio and plasma wave investigation, Space Sci. Rev., 114, 395- 463, doi:10.1007/s11214-004-1434-0 Neubauer, F. M., et al. (2006), Titan's near magnetotail from magnetic field and electron plasma observations and modeling: Cassini flybys TA, TB, and T3, J. Geophys. Res., 111, A10220, doi:10.1029/2006JA011676 Wahlund, J.-E., et al. (2005), Cassini measurements of cold plasma in the ionosphere of Titan, Science, 308, 986 - 989, doi:10.1126/ science.1109807
  03/2009; 11:5912.
• Article: Elliptical polarization of Saturn Kilometric Radiation (SKR) observed by Cassini/RPWS

  G. Fischer, B. Cecconi, L. Lamy, S.-Y. Ye, U. Taubenschuss, W. Macher, P. Zarka, A. Lecacheux, W. S. Kurth, D. A. Gurnett
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  ABSTRACT: The high-inclination orbits of the Cassini spacecraft from autumn 2006 until spring 2007 allowed the Cassini/RPWS (Radio and Plasma Wave Science) instrument to observe Saturn Kilometric Radiation (SKR) from latitudes up to 60° for the first time. This has revealed a surprising new property of SKR: Above 30° in observational latitude a significant amount of SKR is strongly elliptically polarized, in marked contrast to previous observations from low latitudes which showed only circular polarization. There are transitional latitudes where the elliptical polarization occurs in "patches" in the time-frequency spectrograms next to regions of still completely circularly polarized SKR. From 45°-60° in latitude it is found that almost all of SKR (especially from the northern hemisphere and less from the southern hemisphere) is elliptically polarized throughout its entire frequency range with an average degree of 0.7 in linear polarization. We demonstrate the ellipticity of SKR by using the concept of "apparent polarization" in case of 2-antenna measurements, but also show 3-antenna measurements from which the polarization can be unambiguously determined.
  03/2009; 11:7008.
• Article: The electron density of Saturn's magnetosphere

  M. W. Morooka, Modolo R, Wahlund J.-E, André M, A. I. Eriksson, A. M. Persoon, D. A. Gurnett, W. S. Kurth, A. J. Coates, G. R. Lewis, K. K. Khurana, Dougherty M
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  ABSTRACT: We have investigated statistically the electron density below 5 cm−3 in the magnetosphere of Saturn (7–80 RS, Saturn radii) using 44 orbits of the floating potential data from the RPWS Langmuir probe (LP) onboard Cassini. The density distribution shows a clear dependence on the distance from the Saturnian rotation axis (√X2+Y2) as well as on the distance from the equatorial plane (|Z|), indicating a disc-like structure. From the characteristics of the density distribution, we have identified three regions: the extension of the plasma disc, the magnetodisc region, and the lobe regions. The plasma disc region is at LL is the radial distance to the equatorial crossing of the dipole magnetic field line, and confined to |Z|RS. The magnetodisc is located beyond L=15, and its density has a large variability. The variability has quasi-periodic characteristics with a periodicity corresponding to the planetary rotation. For Z>15 RS, the magnetospheric density distribution becomes constant in Z. However, the density still varies quasi-periodically with the planetary rotation also in this region. In fact, the quasi-periodic variation has been observed all over the magnetosphere beyond L=15. The region above Z=15 RS is identified as the lobe region. We also found that the magnetosphere can occasionally move latitudinally under the control of the density in the magnetosphere and the solar wind. From the empirical distributions of the electron densities obtained in this study, we have constructed an electron density model of the Saturnian nightside magnetosphere beyond 7 RS. The obtained model can well reproduce the observed density distribution, and can thus be useful for magnetospheric modelling studies.
  Annales Geophysicae. 01/2009;
• Article: Titan's ionosphere in the magnetosheath: Cassini RPWS results during the T32 flyby

  Garnier P, Wahlund J.-E, Rosenqvist L, Modolo R, Ågren K, Sergis N, Canu P, Andre M, D. A. Gurnett, W. S. Kurth, S. M. Krimigis, Coates A, Dougherty M, J. H. Waite
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  ABSTRACT: The Cassini mission has provided much information about the Titan environment, with numerous low altitude encounters with the moon being always inside the magnetosphere. The only encounter taking place outside the magnetopause, in the magnetosheath, occurred the 13 June 2007 (T32 flyby). This paper is dedicated to the analysis of the Radio and Plasma Wave investigation data during this specific encounter, in particular with the Langmuir probe, providing a detailed picture of the cold plasma environment and of Titan's ionosphere with these unique plasma conditions. The various pressure terms were also calculated during the flyby. The comparison with the T30 flyby, whose geometry was very similar to the T32 encounter but where Titan was immersed in the kronian magnetosphere, reveals that the evolution of the incident plasma has a significant influence on the structure of the ionosphere, with in particular a change of the exo-ionospheric shape. The electrical conductivities are given along the trajectory of the spacecraft and the discovery of a polar plasma cavity is reported.
  Annales Geophysicae. 01/2009;
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  Available from: uiowa.edu

  Article: Elliptical polarization of Saturn Kilometric Radiation observed from high latitudes

  G Fischer, B Cecconi, L Lamy, S.-Y Ye, U Taubenschuss, W Macher, P Zarka, W S Kurth, D A Gurnett
  [show abstract] [hide abstract]
  ABSTRACT: 1] The high-inclination orbits of the Cassini spacecraft from autumn 2006 until spring 2007 allowed the Cassini/RPWS (Radio and Plasma Wave Science) instrument to observe Saturn Kilometric Radiation (SKR) from latitudes up to 60° for the first time. This has revealed a surprising new property of SKR: above $30° in observational latitude, a significant amount of SKR is strongly elliptically polarized, in marked contrast to previous observations from low latitudes, which showed only circular polarization. There are transitional latitudes where the elliptical polarization occurs in ''patches'' in the time-frequency spectrograms next to regions of still completely circularly polarized SKR. From $45° to 60° in northern latitude, it is found that most of the SKR is elliptically polarized throughout its entire frequency range with an average degree of $0.7 in linear polarization. We demonstrate the ellipticity of SKR by using the concept of ''apparent polarization'' in case of two-antenna measurements, but also show three-antenna measurements from which the polarization can be unambiguously determined. Possible reasons for the variation of SKR polarization with the observer's latitude will be discussed.
  Journal of Geophysical Research 01/2009; 114. · 3.02 Impact Factor
• Chapter: Auroral Processes

  W ~S Kurth, E ~J Bunce, J ~T Clarke, F ~J Crary, D ~C Grodent, A ~P Ingersoll, U ~A Dyudina, L Lamy, D ~G Mitchell, A ~M Persoon, W ~R Pryor, J Saur, T Stallard
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  ABSTRACT: Cassini has afforded a number of unique opportunities to understand auroral processes at Saturn and to highlight both differences and similarities with auroral physics at both Earth and Jupiter. A number of campaigns were coordinated with the Hubble Space Telescope such that Cassini could provide either ground truth on the impinging solar wind or in situ measurements of magnetospheric conditions leading to qualitative and sometimes quantitative relationships between the solar wind influence on the intensity, the morphology and evolution of the auroras, and magneto-spheric dynamics. The Hubble UV images are enhanced by Cassini's own remote sensing of the auroras. Cassini's in situ studies of the structure and dynamics of the magnetosphere discussed in other chapters of this book provide the context for understanding the primary drivers of Saturn's auroras and the role of magnetospheric dynamics in their variations. Finally, Cassini's three dimensional prime mission survey of the magnetosphere culminates in high inclination orbits placing it at relatively small radial distances while on auroral field lines, providing the first such in situ observations of auroral particles and fields at a planet other than Earth. The new observations have spawned a number of efforts to model the interaction of the solar wind with the magnetosphere and understand how such dynamics influence the auroras.
  01/2009: pages 333;
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  Available from: George Hospodarsky

  Article: Source locations of narrowband radio emissions detected at Saturn

  Sheng-Yi Ye, D A Gurnett, G Fischer, B Cecconi, J D Menietti, W S Kurth, Z Wang, G B Hospodarsky, P Zarka, A Lecacheux
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  ABSTRACT: 1] Since Cassini's arrival at Saturn in 2004, the Radio and Plasma Wave Science instrument has detected numerous narrowband (NB) radio emission events. These emissions, mostly detected around 5 and 20 kHz, usually occur periodically for several days after intensifications of Saturn kilometric radiation. We present calculations based on an electron density profile of Saturn's plasma torus and a dipole magnetic field model showing that the NB emissions originate from the northern and southern edges of Saturn's plasma torus at L shells $ 8 to 10 for 5-kHz NB and L $ 4 to 7 for 20-kHz NB. In many cases, Cassini passes through the source region of the 20-kHz NB, as indicated by intense electrostatic upper hybrid (ESUH) waves in close proximity to electromagnetic emissions on spectrograms. The positions of the spacecraft when intense ESUH waves are observed agree with the model predictions of the NB source locations. Source locations determined by goniopolarimetric (also known as direction-finding) analysis of the NB emissions also support the above results, although sometimes the directions of arrival point toward the region interior to Saturn's plasma torus. A polarization reversal technique is applied to localize the NB emissions observed during spacecraft rotation, on the basis of the fact that the source is within the antenna plane when the apparent circular polarization degree switches sign. The NB emissions are found to be L-O mode polarized, which is consistent with the prediction of linear/nonlinear mode conversion theory. It is also found that sometimes right-hand polarized NB emissions are generated at second harmonic frequencies of the 20-kHz NB; in which case, wave-wave interactions between oppositely propagating ESUH waves may play an important role in the mode conversion process.
  J. Geophys. Res. 01/2009; 114.
• Article: Coordinated measurements of auroral processes at Saturn from the Cassini spacecraft and HST

  D. G. Mitchell, W. S. Kurth, G. B. Hospodarsky, D. A. Gurnett, N. Krupp, J. Saur, B. A. Mauk, J. F. Carbary, S. M. Krimigis, P. C. Brandt, M. K. Dougherty, J. T. Clarke, J. D. Nichols, J. Gerard, D. Grodent, W. R. Pryor, E. J. Bunce, F. J. Crary
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  ABSTRACT: One of the primary Cassini mission objectives at Saturn is to characterize Saturn's aurora-its spatial morphology, associated particle energization, radio wave generation, and magnetospheric currents, relationship with solar wind pressure and magnetic field, and its large scale mapping to the magnetosphere. By design, the Cassini orbital tour included high inclination and low periapsis orbits late in the prime mission specifically to address many of these topics. In this presentation, we will provide a snapshot of the current state of our investigation into the relationship between magnetospheric measurements of particles and fields, and the aurora. For in situ data, we will show measurements of upward traveling light ion conics (~30 keV to 200 keV), often accompanied by electron beams (<20 keV to ~1 MeV) and enhanced broadband noise (10 Hz to a few kHz), throughout the outer magnetosphere on field lines that nominally map from well into the polar cap (dipole L > 50) to well into the closed field region (dipole L < 10). Sometimes the particle phenomena and the broadband noise occur in pulses of roughly five-minute duration, separated by tens of minutes. At other times they are relatively steady over an hour or more. Magnetic signatures associated with some of the pulsed events are consistent with field aligned current structures. Correlative observations of solar wind (Cassini) and aurora (HST) have established a strong relationship between solar wind pressure and auroral activity (brightness) (Crary et al., Nature, 2005; Clarke et al., JGR, 2008). A similar correspondence between bright auroral arcs and ring current ion acceleration will be shown here. So while some auroral forms seem to be associated with the open/closed field boundary (i.e. in the cusp-Bunce et al., JGR, 2008), we also demonstrate that under some magnetospheric conditions for which protons and oxygen ions are accelerated once per Saturn magnetosphere rotation at a preferred local time between midnight and dawn, simultaneous auroral observations by the HST reveal a close correlation between these dynamical magnetospheric events and dawn-side transient auroral brightenings. Likewise, many of the recurrent energetic neutral atom enhancements coincide closely with bursts of Saturn kilometric radiation, again suggesting a linkage with high latitude auroral processes. Finally, we will show some intriguing results of auroral movie sequences from the Cassini UVIS instrument with corresponding ring current movies from the Magnetospheric Imaging Instrument Ion and Neutral Camera (MIMI/INCA).
  AGU Fall Meeting Abstracts. 11/2008; -1:03.
• Article: The interior of Iapetus: Constraints provided by the solar wind interaction

  J. S. Leisner, C. T. Russell, R. J. Strangeway, N. Omidi, M. K. Dougherty, W. S. Kurth
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  ABSTRACT: Iapetus is Saturn's third largest and most distant regular satellite. This moon is best known for its unique hemispheric albedo asymmetry and the kilometers-high ridge circling its equator, but observations made during Cassini's 2007 flyby show another of the moon's unusual features. The spacecraft approached Iapetus from downstream and stayed to the side until it had passed upstream, never having entered the moon's geometric wake. The interplanetary magnetic field's (IMF) orientation was such that during the approach, Cassini's trajectory took it towards a region where it would be magnetically connected to the moon. During this time, the Cassini plasma wave spectrometer observed low-frequency waves that were unique to the Iapetus encounter and strongly correlated with the spacecraft's distance from the moon. When Cassini passed into the space where the IMF should have magnetically connected it to the moon, the magnetometer observed a strong perturbation of the magnetic field. When we extrapolate that perturbation towards Iapetus, we find that the IMF rotated to completely avoid intersecting the moon. This suggests that the solar wind plasma was deflected by Iapetus. We have analyzed the perturbation and we find that it can be caused by neither an inert body nor a mass-loading source. Such a perturbation could be caused by either an internal, electrically-conducting ocean or remanent magnetization. Iapetus' density of 1100 kg m-3 allows for the possibility of an interior ocean and current models of Iapetus' interior allow for the possibility of a rocky core 500-700 km across (Castillo-Rogez et al., 2008). That rocky core could support the needed remanent magnetization, but the difficulty is in finding a mechanism to provide the source magnetic field.
  AGU Fall Meeting Abstracts. 11/2008; -1:08.