| 1. |
- Gurnett, D. A., et al.
(författare)
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A plasmapause-like density boundary at high latitudes in Saturn's magnetosphere
- 2010
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 37, s. L16806-
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Tidskriftsartikel (refereegranskat)abstract
- Here we report the discovery of a well-defined plasma density boundary at high latitudes in Saturn's magnetosphere. The boundary separates a region of relatively high density at L less than about 8 to 15 from a region with densities nearly three orders of magnitude lower at higher L values. Magnetic field measurements show that strong field-aligned currents, probably associated with the aurora, are located just inside the boundary. Analyses of the anisotropy of energetic electrons show that the magnetic field lines are usually closed inside the boundary and open outside the boundary, although exceptions sometimes occur. The location of the boundary is also modulated at the similar to 10.6 to 10.8 hr rotational period of the planet. Many of these characteristics are similar to those predicted by Brice and Ioannidis for the plasmapause at a strongly magnetized, rapidly rotating planet such as Saturn.
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| 2. |
- Garnier, P., et al.
(författare)
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The detection of energetic electrons with the Cassini Langmuir probe at Saturn
- 2012
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 117, s. A10202-
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Tidskriftsartikel (refereegranskat)abstract
- The Cassini Langmuir probe, part of the Radio and Plasma Wave Science (RPWS) instrument, has provided a wealth of information about the cold and dense plasma in the Saturnian system. The analysis of the ion side current (current for negative potentials) measured by the probe from 2005 to 2008 reveals also a strong sensitivity to energetic electrons (250-450 eV). These electrons impact the surface of the probe, and generate a detectable current of secondary electrons. A broad secondary electrons current region is inferred from the observations in the dipole L Shell range of similar to 6-10, with a peak full width at half maximum (FWHM) at L = 6.4-9.4 (near the Dione and Rhea magnetic dipole L Shell values). This magnetospheric flux tube region, which displays a large day/night asymmetry, is related to the similar structure in the energetic electron fluxes as the one measured by the onboard Electron Spectrometer (ELS) of the Cassini Plasma Spectrometer (CAPS). It corresponds spatially to both the outer electron radiation belt observed by the Magnetosphere Imaging Instrument (MIMI) at high energies and to the low-energy peak which has been observed since the Voyager era. Finally, a case study suggests that the mapping of the current measured by the Langmuir probe for negative potentials can allow to identify the plasmapause-like boundary recently identified at Saturn, and thus potentially identify the separation between the closed and open magnetic field lines regions.
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| 3. |
- Jinks, S. L., et al.
(författare)
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Cassini multi-instrument assessment of Saturn's polar cap boundary
- 2014
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Ingår i: Journal of Geophysical Research - Space Physics. - 2169-9380 .- 2169-9402. ; 119:10, s. 8161-8177
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Tidskriftsartikel (refereegranskat)abstract
- We present the first systematic investigation of the polar cap boundary in Saturn's high-latitude magnetosphere through a multi-instrument assessment of various Cassini in situ data sets gathered between 2006 and 2009. We identify 48 polar cap crossings where the polar cap boundary can be clearly observed in the step in upper cutoff of auroral hiss emissions from the plasma wave data, a sudden increase in electron density, an anisotropy of energetic electrons along the magnetic field, and an increase in incidence of higher-energy electrons from the low-energy electron spectrometer measurements as we move equatorward from the pole. We determine the average level of coincidence of the polar cap boundary identified in the various in situ data sets to be 0.34 degrees 0.05 degrees colatitude. The average location of the boundary in the southern (northern) hemisphere is found to be at 15.6 degrees (13.3 degrees) colatitude. In both hemispheres we identify a consistent equatorward offset between the poleward edge of the auroral upward directed field-aligned current region of similar to 1.5-1.8 degrees colatitude to the corresponding polar cap boundary. We identify atypical observations in the boundary region, including observations of approximately hourly periodicities in the auroral hiss emissions close to the pole. We suggest that the position of the southern polar cap boundary is somewhat ordered by the southern planetary period oscillation phase but that it cannot account for the boundary's full latitudinal variability. We find no clear evidence of any ordering of the northern polar cap boundary location with the northern planetary period magnetic field oscillation phase.
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| 4. |
- Morooka, Michiko W., et al.
(författare)
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Dusty plasma in the vicinity of Enceladus
- 2011
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Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 116
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Tidskriftsartikel (refereegranskat)abstract
- We present in situ Cassini Radio Plasma Wave Science observations in the vicinity of Enceladus and in the E ring of Saturn that indicate the presence of dusty plasma. The four flybys of Enceladus in 2008 revealed the following cold plasma characteristics: (1) there is a large plasma density (both ions and electrons) within the Enceladus plume region, (2) no plasma wake effect behind Enceladus was detected, (3) electron densities are generally much lower than the ion densities in the E ring (n(e)/n(i) < 0.5) as well as in the plume (n(e)/n(i) < 0.01), and (4) the average bulk ion drift speed is significantly less than the corotation speed and is instead close to the Keplerian speed. These signatures result from half or more of the electrons being attached to dust grains and by the interaction between the surrounding cold plasma and the predominantly negatively charged submicrometer-sized dust grains. The dust and plasma properties estimated from the observations clearly show that the dust-plasma interaction is collective. This strong dust-plasma coupling appears not only in the Enceladus plume but also in the Enceladus torus, typically from about 20 R(E) (similar to 5000 km) north and about 60 R(E) (similar to 15,000 km) south of Enceladus. We also suggest that the dust-plasma interaction in the E ring is the cause of the planetary spin-modulated dynamics of Saturn's magnetosphere at large.
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| 5. |
- Roussos, E., et al.
(författare)
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Energetic electron observations of Rhea's magnetospheric interaction
- 2012
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Ingår i: Icarus. - : Elsevier BV. - 0019-1035 .- 1090-2643. ; 221:1, s. 116-134
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Tidskriftsartikel (refereegranskat)abstract
- Saturn's moon Rhea is thought to be a simple plasma absorber, however, energetic particle observations in its vicinity show a variety of unexpected and complex interaction features that do not conform with our current understanding about plasma absorbing interactions. Energetic electron data are especially interesting, as they contain a series of broad and narrow flux depletions on either side of the moon's wake. The association of these dropouts with absorption by dust and boulders orbiting within Rhea's Hill sphere was suggested but subsequently not confirmed, so in this study we review data from all four Cassini flybys of Rhea to date seeking evidence for alternative processes operating within the moon's interaction region. We focus on energetic electron observations, which we put in context with magnetometer, cold plasma density and energetic ion data. All flybys have unique features, but here we only focus on several structures that are consistently observed. The most interesting common feature is that of narrow dropouts in energetic electron fluxes, visible near the wake flanks. These are typically seen together with narrow flux enhancements inside the wake. A phase-space-density analysis for these structures from the first Rhea flyby (R1) shows that Liouville's theorem holds, suggesting that they may be forming due to rapid transport of energetic electrons from the magnetosphere to the wake, through narrow channels. A series of possibilities are considered to explain this transport process. We examined whether complex energetic electron drifts in the interaction region of a plasma absorbing moon (modeled through a hybrid simulation code) may allow such a transport. With the exception of several features (e.g. broadening of the central wake with increasing electron energy), most of the commonly observed interaction signatures in energetic electrons (including the narrow structures) were not reproduced. Additional dynamical processes, not simulated by the hybrid code, should be considered in order to explain the data. For the small scale features, the possibility that a flute (interchange) instability acts on the electrons is discussed. This instability is probably driven by strong gradients in the plasma pressure and the magnetic field magnitude: magnetometer observations show clearly signatures consistent with the (expected) plasma pressure loss due to ion absorption at Rhea. Another potential driver of the instability could have been gradients in the cold plasma density, which are, however, surprisingly absent from most crossings of Rhea's plasma wake. The lack of a density depletion in Rhea's wake suggests the presence of a local cold plasma source region. Hybrid plasma simulations show that this source cannot be the ionized component of Rhea's weak exosphere. It is probably related to accelerated photoelectrons from the moon's negatively charged surface, indicating that surface charging may play a very important role in shaping Rhea's magnetospheric interaction region. (C) 2012 Elsevier Inc. All rights reserved.
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| 6. |
- Sakai, S., et al.
(författare)
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Dust-plasma interaction through magnetosphere-ionosphere coupling in Saturn’s plasma disk
- 2012
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Ingår i: European Planetary Science Congress 2012, held 23-28 September, 2012 in Madrid, Spain. http://meetings.copernicus.org/epsc2012, id. EPSC2012-433.
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Konferensbidrag (refereegranskat)abstract
- The ion bulk speeds in the equatorial region of Saturn’s inner magnetosphere, according to data from the Langmuir Probe (LP) on board the Cassini spacecraft, are about 60% of the ideal co-rotation speed. These findings suggest that sub-micrometer negatively charged E ring dust contributes to the plasma dynamics in the plasma disk. We calculated the ion speeds by using multicomponent MHD equations, taking into account dust interactions to investigate the effects of ion-dust coulomb collision, mass loading, as well as taking into account magnetosphere-ionosphere coupling to investigate the effect of the magnetospheric electric field. The results show that the ion speeds can be significantly reduced by the electric fields generated by the ion-dust collisions when the dust density is high and the thickness of dust distribution is large. We also show that the ion speeds from our model are consistent with the LP observations when the maximum density of dust is larger than ~10\^5 m\^-3.
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| 7. |
- Farrell, W. M., et al.
(författare)
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An estimate of the dust pickup current at Enceladus
- 2014
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Ingår i: Icarus. - : Elsevier BV. - 0019-1035 .- 1090-2643. ; 239, s. 217-221
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate that the acceleration of submicron dust originating at Enceladus by a reduced co-rotating E-field is capable of creating a dust pickup current perpendicular to the magnetic field with values ranging from 3 to 15 kA (depending upon the effective grain charge). Such a current represents a new contribution to the total pickup current in the region. As such, we suggest that dust pickup currents, along with ion and electron pickup currents, are all active within the plume.
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| 8. |
- Farrell, W. M., et al.
(författare)
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Modification of the plasma in the near-vicinity of Enceladus by the enveloping dust
- 2010
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 37:20, s. L20202-
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Tidskriftsartikel (refereegranskat)abstract
- The plasma near Saturn's equator is quasi-corotating, but those fluid elements entering the near-vicinity of the moon Enceladus become uniquely modified. Besides the solid body, the Moon has a surrounding dust envelop that we show herein to be detected similar to 20 Enceladus radii (1 R-E = 252 km) both north and south of the body. Previous reports indicate that corotating plasma slows down substantially in the near-vicinity of Enceladus. We show herein that the commencement of this plasma slow down matches closely with Cassini's entry into the dense portions of the enveloping dust in the northern hemisphere above the Moon. We also examine in detail the source of the dust about 400 km above the south polar fissures. We find that a large positive potential must exist between the south pole of the moon and the spacecraft to account for ions streaming away from the pole on connecting magnetic field lines.
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| 9. |
- Farrell, William M., et al.
(författare)
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The electromagnetic pickup of submicron-sized dust above Enceladus's northern hemisphere
- 2012
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Ingår i: Icarus. - : Elsevier BV. - 0019-1035 .- 1090-2643. ; 219:1, s. 498-501
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Tidskriftsartikel (refereegranskat)abstract
- As the saturnian magnetoplasma sweeps past Enceladus, it experiences both a decrease in electron content and sharp slowdown in the northern hemisphere region within similar to 5 Enceladus Radii (R-e). This slowdown is observed by Cassini in regions not obviously associated with the southern directed plume-originating ions. We suggest herein that the decrease in northern hemisphere electron content and plasma slowdown could both be related to the presence of fine dust grains that are being accelerated by the Lorentz force created within the saturnian magnetic field system.
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| 10. |
- Garnier, P., et al.
(författare)
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Mapping 300 eV electrons at Saturn with the Cassini RPWS Langmuir probe
- 2011
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Ingår i: EPSC-DPS Joint Meeting 2011.
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Konferensbidrag (refereegranskat)abstract
- The Cassini Langmuir probe (onboard RPWS experiment) has provided wealth of information about the kronian cold plasma environment since the Saturn Orbit Insertion in 2004. The usage of the Langmuir probe is based on the fitting of the currentvoltage curve which brings information on several plasma parameters in cold and dense plasma regions. The ion part of the I-V curve may however be influenced by energetic particles hitting the probe, leading to an enhanced ion current measured. We report here the influence of 300 eV electrons on the probe current, with a current belt observed between Dione and Rhea.
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| 11. |
- Gurnett, D. A., et al.
(författare)
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The rotation of the plasmapause-like boundary at high latitudes in Saturn's magnetosphere and its relation to the eccentric rotation of the northern and southern auroral ovals
- 2011
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 38, s. L21203-
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Tidskriftsartikel (refereegranskat)abstract
- Here we present a study of the rotation of the plasmapause-like density boundary discovered by the Cassini spacecraft at high latitudes in the Saturnian magnetosphere, and compare the results with previously published studies of high-latitude magnetic field perturbations and the eccentric rotation of the auroral ovals. Near the planet the density boundary is located at dipole L values ranging from about 8 to 15, and separates a region of very low densities at high latitudes from a region of higher densities at lower latitudes. We show that the density boundary rotates at different rates in the northern and southern hemispheres, and that the periods are the same as the modulation periods of Saturn kilometric radiation in those hemispheres. We also show that the phase of rotation in a given hemisphere is closely correlated with the phase of the high-latitude magnetic field perturbations observed by Cassini in that hemisphere, and also with the phase of the eccentric rotation of the auroral oval observed by the Hubble Space Telescope.
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| 12. |
- Romanelli, N., et al.
(författare)
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Outflow and plasma acceleration in Titan's induced magnetotail : Evidence of magnetic tension forces
- 2014
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Ingår i: Journal of Geophysical Research - Space Physics. - 2169-9380 .- 2169-9402. ; 119:12
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Tidskriftsartikel (refereegranskat)abstract
- Cassini plasma wave and particle observations are combined with magnetometer measurements to study Titan's induced magnetic tail. In this study, we report and analyze the plasma acceleration in Titan's induced magnetotail observed in flybys T17, T19, and T40. Radio and Plasma Wave Science observations show regions of cold plasma with electron densities between 0.1 and a few tens of electrons per cubic centimeter. The Cassini Plasma Spectrometer (CAPS)-ion mass spectrometer (IMS) measurements suggest that ionospheric plasma in this region is composed of ions with masses ranging from 15 to 17 amu and from 28 to 31 amu. From these measurements, we determine the bulk velocity of the plasma and the Alfven velocity in Titan's tail region. Finally, a Walen test of such measurements suggest that the progressive acceleration of the ionospheric plasma shown by CAPS can be interpreted in terms of magnetic tension forces.
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| 13. |
- Sakai, S., et al.
(författare)
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Dust-plasma interaction through magnetosphere-ionosphere coupling in Saturn's plasma disk
- 2013
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Ingår i: Planetary and Space Science. - : Elsevier BV. - 0032-0633 .- 1873-5088. ; 75:1, s. 11-16
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Forskningsöversikt (refereegranskat)abstract
- The ion bulk speeds in the equatorial region of Saturn's inner magnetosphere, according to data from the Langmuir Probe (LP) on board the Cassini spacecraft, are about 60% of the ideal co-rotation speed; the ion speeds are between the co-rotation and Keplerian speeds (Holmberg et al.; Ion densities and velocities in the inner plasma torus of Saturn, Planetary and Space Science). These findings suggest that sub-micrometer negatively charged E ring dust contributes to the plasma dynamics in the plasma disk. We calculated the ion speeds by using a multi-species fluid model, taking into account dust interactions to investigate the effects of ion-dust coulomb collision, mass loading, as well as taking into account magnetosphere-ionosphere coupling to investigate the effect of the magnetospheric electric field. The results show that the ion speeds can be significantly reduced by the electric fields generated by the collisions between ions and dusts when the dust density is high and the thickness of dust distribution is large. We also show that the ion speeds from our model are consistent with the LP observations when the maximum density of dust is larger than ∼105 m-3.
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| 14. |
- Ye, S. -Y, et al.
(författare)
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Electron density inside Enceladus plume inferred from plasma oscillations excited by dust impacts
- 2014
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Ingår i: Journal of Geophysical Research-Space Physics. - 2169-9380. ; 119:5, s. 3373-3380
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Tidskriftsartikel (refereegranskat)abstract
- Enceladus' southern plume is one of the major discoveries of the Cassini mission. The water neutrals and water ice particles (dust) ejected by the cryovolcanic activity populate Saturn's E ring and the neutral torus, and they interact with the plasma environment of Saturn's magnetosphere. The plasma neutrality inside Enceladus' plume has been shown by the Langmuir probe measurement to be modified by the presence of the dust particles. We present an independent method of determining the electron density inside the plume. Sometimes, after dust impacts, plasma oscillations (ringing) were detected by the Cassini Radio and Plasma Wave Science instrument. The frequencies of these oscillations have been shown to be consistent with the local plasma frequency, thus providing a measurement of the local electron density.
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