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1.
The plasma cloud mechanism of spoke formation in Saturn's rings, proposed by Goertz and Morfill in 1983, is revisited in the light of new data and the criticisms raised by Farmer and Goldreich [Farmer, A.J., Goldreich, P., 2005. Icarus. This issue]. It is concluded that the plasma cloud model satisfies all available observational and physical constraints. 相似文献
2.
As part of a long-term study of Saturn's rings, we have used the Hubble Space Telescope's (HST) Wide Field and Planetary Camera (WFPC2) to obtain several hundred high resolution images from 1996 to 2004, spanning the full range of ring tilt and solar phase angles accessible from the Earth. Using these multiwavelength observations and HST archival data, we have measured the photometric properties of spokes in the B ring, visible in a substantial number of images. We determined the spoke particle size distribution by fitting the wavelength-dependent extinction efficiency of a prominent, isolated spoke, using a Mie scattering model. Following Doyle and Grün (1990, Icarus 85, 168-190), we assumed that the spoke particles were sub-micron size spheres of pure water ice, with a Hansen-Hovenier size distribution (Hansen and Hovenier, 1974, J. Atmos. Sci. 31, 1137-1160). The WFPC2 wavelength coverage is broader than that of the Voyager data, resulting in tighter constraints on the nature of spoke particles. The effective particle size was reff=0.57±0.05 μm, and the size distribution was quite narrow with a variance of b=0.09±0.03, very similar to the results of Doyle and Grün (1990, Icarus 85, 168-190), and consistent with predictions of plasma cloud models for spoke production from meteoritic impacts (Goertz and Morfill, 1983, Icarus 53, 219-229; Goertz, 1984, Adv. Space Res. 4, 137-141). In all, we identified 36 spokes or spoke complexes, predominantly on the morning (east) ansa. The photometric contrast of the spokes is strongly dependent on effective ring opening angle, Beff. Spokes were clearly visible on the north face of the rings in 1994, just prior to the most recent ring plane crossing (RPX) epoch, and on the south face shortly after RPX. However, spokes were both less abundant and fainter as the rings opened up, and no spokes were detected after 18 October 1998 (Beff=−15.43°), when a single faint spoke was seen on the morning ansa. The high resolution and photometric quality of the WFPC2 images enabled us to set a detection limit of ?1% in fractional brightness contrast for spokes for the post-1998 observations. We compare the observed trend of spoke contrast with Beff to radiative transfer calculations based on three models of the distribution of spoke material. In the first, the spoke “haze” is uniformly mixed with macroscopic B ring particles. No variation in spoke contrast is predicted for single-scattering, in this case, and only a modest decrease in contrast with Beff is predicted when multiple scattering is taken into account. In the second model, the spoke dust occupies an extended layer that is thicker than the B ring, which gives virtually identical results to a third case, when the haze layer lies exclusively above the ring. Multiple-scattering Monte Carlo calculations for these two extended haze models match the trend of spoke contrast exceptionally well. We compute the predicted spoke contrast for a wide variety of viewing geometries, including forward- and backscattering. Based on these results, spokes should be easily detectable during the Cassini mission when the rings are viewed at relatively small (|B|?10°) ring opening angles. 相似文献
3.
Orbital velocities and relative reflectances of a preliminary sample of 15 Saturnian spokes recorded in the Voyager 2 low-resolution ring movie have been examined. While 13 spokes exhibit the expected Keplerian velocities, 2 anomalous spokes deviate from this motion. For approximately 2 hr after their formation these spokes exhibit corotational motion and, only then, accelerate to Keplerian speeds. Only 1 of the 2 accelerating spokes is within view of the Voyager cameras throughout its lifetime. When first seen this spoke appears on the morning ansa of the B ring with a 0.02 contrast; it gains in contrast throughout its corotational phase, reaching a maximum of 0.09 during its velocity transition. The spoke then loses contrast during its Keplerian phase, dropping to 0.02 in the last visible image. Thus a correlation between the contrast and the anomalous dynamical phases of this feature is observed. The radial reflectance profile, measured when the spoke is approximately 1 hr old, suggests discrete sources for spoke material in regions of maximum contrast within the B ring; a lower limit of 3 × 1011 g can be established for the mass at this point. The behavior of this spoke seems to be explained by the plasma cloud model of Goertz and Morfill (1983, Icarus 55, 111–123). The atypical dynamics of these 2 spokes suggest that they are generated by plasma clouds of unusually high charge density, while the contrast of these features appears to depend more on ring particulate concentration than on plasma cloud density. 相似文献
4.
The expansion and ionization of vapor produced by impacts of meteorites on Saturn's rings is described. There is an “impact plasma” produced in the initial collision, and a “secondary plasma” produced by subsequent ionization of the neutral gas ejecta. The dynamics of these plasma clouds, their size, density, and life time are calculated. It is suggested that large clouds, produced by meter-sized meteorites (or a collection of such clouds produced, e.g., by the impact of a swarm of meteorites) may lead to the formation of spokes by the mechanism discussed in Goertz, C. K., and Morfill, G. E. (Icarus53, 219–229, 1983). 相似文献
5.
The pattern of radial growth of an extended spoke recorded in the Voyager 2 low-resolution ring movie has been examined. This feature is atypical in that it orbits Saturn at the corotational rate for 1½hr after the onset of its formation and then undergoes a 40-min acceleration to sustained Keplerian velocities. The spoke exhibits two modes of radial growth. The first is a diffusion of material radially inward (at a rate of 101 ± 4m sec?1) and outward (at a rate of 40 ± 5m sec?1) that occurs throughout the feature's life. The second mode is a short-lived, rapid outward propagation of material (at a rate of 730 ± 70m sec?1) that occurs for approximately the first 50 min of the spoke's Keplerian dynamical phase. This rapid propagation, presumably driven by Lorentz forces acting on the negatively charged spoke particulates, allows charge-to-mass ratios for these particulates to be calculated at the onset of the propagation (?60 ± 1C kg?1), the termination of the propagation (?22 ± 2C kg?1), and the final dissipation of the spoke (?1.5 ± 0.2C kg?1). The value at the termination of the rapid propagation can be taken as an upper limit on the particulate charge-to-mass ratios of spokes which exhibit Keplerian velocities throughout their lifetimes. A correlation between the dynamical phases and the radial growth modes of the atypical spoke is observed, one that seems consistent with the plasma cloud model of spoke formation and evolution proposed by C. K. Goertz and G. Morfill (1983, Icarus53, 219–229), taken in the limit of high charge density. 相似文献
6.
Pre-Cassini models of Saturn’s E ring [Horányi, M., Burns, J., Hamilton, D., 1992. Icarus 97, 248-259; Juhász, A., Horányi, M., 2002. J. Geophys. Res. 107, 1-10] failed to reproduce its peculiar vertical structure inferred from Earth-bound observations [de Pater, I., Martin, S.C., Showalter, M.R., 2004. Icarus 172, 446-454]. After the discovery of an active ice-volcanism of Saturn’s icy moon Enceladus the relevance of the directed injection of particles for the vertical ring structure of the E ring was swiftly recognised [Juhász, A., Horányi, M., Morfill, G.E., 2007. Geophys. Res. Lett. 34, L09104; Kempf, S., Beckmann, U., Moragas-Klostermeyer, G., Postberg, F., Srama, R., Economou, T., Schmidt, J., Spahn, F., Grün, E., 2008. Icarus 193, 420-437]. However, simple models for the delivery of particles from the plume to the ring predict a too small vertical ring thickness and overestimate the amount of the injected dust.Here we report on numerical simulations of grains leaving the plume and populating the dust torus of Enceladus. We run a large number of dynamical simulations including gravity and Lorentz force to investigate the earliest phase of the ring particle life span. The evolution of the electrostatic charge carried by the initially uncharged grains is treated selfconsistently. Freshly ejected plume particles are moving in almost circular orbits because the Enceladus orbital speed exceeds the particles’ ejection speeds by far. Only a small fraction of grains that leave the Hill sphere of Enceladus survive the next encounter with the moon. Thus, the flux and size distribution of the surviving grains, replenishing the ring particle reservoir, differs significantly from the flux and size distribution of the particles freshly ejected from the plume. Our numerical simulations reproduce the vertical ring profile measured by the Cassini Cosmic Dust Analyzer (CDA) [Kempf, S., Beckmann, U., Moragas-Klostermeyer, G., Postberg, F., Srama, R., EconoDmou, T., Smchmidt, J., Spahn, F., Grün, E., 2008. Icarus 193, 420-437]. From our simulations we calculate the deposition rates of plume particles hitting Enceladus’ surface. We find that at a distance of 100 m from a jet a 10 m sized ice boulder should be covered by plume particles in 105-106 years. 相似文献
7.
We present a detailed analysis of the H+3 intensity and velocity profiles crossing Saturn's auroral/polar region, as described by Stallard et al. [Stallard, T., Miller, S., Melin, H., Lystrup, M., Dougherty, M., Achilleos, N., 2007. Icarus 189, 1-13], with a view to understanding the magnetospheric processes with which they are connected. The data are not consistent with the theory that Saturn's main auroral oval is associated with corotation enforcement currents in the middle magnetosphere. This implies that the main auroral oval can be associated with the open-closed field line boundary [Cowley, S.W.H., Bunce, E.J., O'Rourke, J.M., 2004. J. Geophys. Res. 109. A05212]; a third model, by Sittler et al. [Sittler, E.C., Blanc, M.F., Richardson, J.D., 2006. J. Geophys. Res. 111. A06208] associates the main oval with centrifugal instabilities in the outer magnetosphere, but does not make predictions about ionospheric plasma flows with which we can compare our data. We do, however, tentatively identify emission at latitudes lower than the main auroral oval which may be associated with the corotation enforcement currents in the middle magnetosphere. We also find that at latitudes higher than the main auroral oval there is often a region of the ionosphere that is in rigid corotation with the planet. We suggest that this region corresponds to field lines embedded in the centre of the magnetotail which are shielded from the solar wind such that their rotation is controlled only by the neutral atmosphere. 相似文献
8.
C. Paranicas D.G. Mitchell D.C. Hamilton N. Krupp R.E. Johnson T.P. Armstrong 《Icarus》2008,197(2):519-525
We present Cassini data revealing that protons between a few keV and about 100 keV energy are not stably trapped in Saturn's inner magnetosphere. Instead these ions are present only for relatively short times following injections. Injected protons are lost principally because the neutral gas cloud converts these particles to energetic neutral atoms via charge exchange. At higher energies, in the MeV to GeV range, protons are stably trapped between the orbits of the principal moons because the proton cross-section for charge exchange is very small at such energies. These protons likely result from cosmic ray albedo neutron decay (CRAND) and are lost principally to interactions with satellite surfaces and ring particles during magnetospheric radial diffusion. A main result of this work is to show that the dominant energetic proton loss and source processes are a function of proton energy. Surface sputtering by keV ions is revisited based on the reduced ion intensities observed. Relatively speaking, MeV ion and electron weathering is most important closer to Saturn, e.g. at Janus and Mimas, whereas keV ion weathering is most important farther out, at Dione and Rhea. 相似文献
9.
Saturn’s rings host two known moons, Pan and Daphnis, which are massive enough to clear circumferential gaps in the ring around their orbits. Both moons create wake patterns at the gap edges by gravitational deflection of the ring material (Cuzzi, J.N., Scargle, J.D. [1985]. Astrophys. J. 292, 276-290; Showalter, M.R., Cuzzi, J.N., Marouf, E.A., Esposito, L.W. [1986]. Icarus 66, 297-323). New Cassini observations revealed that these wavy edges deviate from the sinusoidal waveform, which one would expect from a theory that assumes a circular orbit of the perturbing moon and neglects particle interactions. Resonant perturbations of the edges by moons outside the ring system, as well as an eccentric orbit of the embedded moon, may partly explain this behavior (Porco, C.C., and 34 colleagues [2005]. Science 307, 1226-1236; Tiscareno, M.S., Burns, J.A., Hedman, M.M., Spitale, J.N., Porco, C.C., Murray, C.D., and the Cassini Imaging team [2005]. Bull. Am. Astron. Soc. 37, 767; Weiss, J.W., Porco, C.C., Tiscareno, M.S., Burns, J.A., Dones, L. [2005]. Bull. Am. Astron. Soc. 37, 767; Weiss, J.W., Porco, C.C., Tiscareno, M.S. [2009]. Astron. J. 138, 272-286). Here we present an extended non-collisional streamline model which accounts for both effects. We describe the resulting variations of the density structure and the modification of the nonlinearity parameter q. Furthermore, an estimate is given for the applicability of the model. We use the streamwire model introduced by Stewart (Stewart, G.R. [1991]. Icarus 94, 436-450) to plot the perturbed ring density at the gap edges.We apply our model to the Keeler gap edges undulated by Daphnis and to a faint ringlet in the Encke gap close to the orbit of Pan. The modulations of the latter ringlet, induced by the perturbations of Pan (Burns, J.A., Hedman, M.M., Tiscareno, M.S., Nicholson, P.D., Streetman, B.J., Colwell, J.E., Showalter, M.R., Murray, C.D., Cuzzi, J.N., Porco, C.C., and the Cassini ISS team [2005]. Bull. Am. Astron. Soc. 37, 766), can be well described by our analytical model. Our analysis yields a Hill radius of Pan of 17.5 km, which is 9% smaller than the value presented by Porco (Porco, C.C., and 34 colleagues [2005]. Science 307, 1226-1236), but fits well to the radial semi-axis of Pan of 17.4 km. This supports the idea that Pan has filled its Hill sphere with accreted material (Porco, C.C., Thomas, P.C., Weiss, J.W., Richardson, D.C. [2007]. Science 318, 1602-1607). A numerical solution of a streamline is used to estimate the parameters of the Daphnis-Keeler gap system, since the close proximity of the gap edge to the moon induces strong perturbations, not allowing an application of the analytic streamline model. We obtain a Hill radius of 5.1 km for Daphnis, an inner edge variation of 8 km, and an eccentricity for Daphnis of 1.5 × 10−5. The latter two quantities deviate by a factor of two from values gained by direct observations (Jacobson, R.A., Spitale, J., Porco, C.C., Beurle, K., Cooper, N.J., Evans, M.W., Murray, C.D. [2008]. Astron. J. 135, 261-263; Tiscareno, M.S., Burns, J.A., Hedman, M.M., Spitale, J.N., Porco, C.C., Murray, C.D., and the Cassini Imaging team [2005]. Bull. Am. Astron. Soc. 37, 767), which might be attributed to the neglect of particle interactions and vertical motion in our model. 相似文献
10.
Ana C. Barbosa Aguiar Peter L. Read Robin D. Wordsworth Tara Salter Y. Hiro Yamazaki 《Icarus》2010,206(2):755-763
A hexagonal structure has been observed at ∼76°N on Saturn since the 1980s (Godfrey, D.A. [1988]. Icarus 76, 335-356). Recent images by Cassini (Baines, K., Momary, T., Roos-Serote, M., Atreya, S., Brown, R., Buratti, B., Clark, R., Nicholson, P. [2007]. Geophys. Res. Abstr. 9, 02109; Baines, K., Momary, T., Fletcher, L., Kim, J., Showman, A., Atreya, S., Brown, R., Buratti, B., Clark, R., Nicholson, P. [2009]. Geophys. Res. Abstr. 11, 3375) have shown that the feature is still visible and largely unchanged. Its long lifespan and geometry has puzzled the planetary physics community for many years and its origin remains unclear. The measured rotation rate of the hexagon may be very close to that of the interior of the planet (Godfrey, D.A. [1990]. Science 247, 1206-1208; Caldwell, J., Hua, X., Turgeon, B., Westphal, J.A., Barnet, C.D. [1993]. Science 206, 326-329; Sánchez-Lavega, A., Lecacheux, J., Colas, F., Laques, P. [1993]. Science 260, 329-332), leading to earlier interpretations of the pattern as a stationary planetary wave, continuously forced by a nearby vortex (Allison, M., Godfrey, D.A., Beebe, R.F. [1990]. Science 247, 1061-1063). Here we present an alternative explanation, based on an analysis of both spacecraft observations of Saturn and observations from laboratory experiments where the instability of quasi-geostrophic barotropic (vertically uniform) jets and shear layers is studied. We also present results from a barotropic linear instability analysis of the saturnian zonal wind profile, which are consistent with the presence of the hexagon in the North Pole and absence of its counter-part in the South Pole. We propose that Saturn’s long-lived polygonal structures correspond to wavemodes caused by the nonlinear equilibration of barotropically unstable zonal jets. 相似文献
11.
Numerical simulation of energetic electron microsignature drifts at Saturn: Methods and applications
Many recent studies show that energetic electron microsignatures are a powerful tool for characterizing key aspects of Saturn’s magnetospheric configuration and dynamics. In all previous investigations, however, analysis of these features was performed through the use of a series of simplifying assumptions (e.g. dipole field model). Furthermore, typical observable parameters of microsignatures (e.g. energy dependent location) have only been discussed qualitatively and a clear understanding about how microsignatures evolve in the magnetosphere is currently lacking. In this study we present a numerical simulation that we developed in order to describe the apparent motion of microsignatures in Saturn’s magnetosphere, under the influence of arbitrary magnetic and electric field models. Our simulations reproduce successfully some typical microsignature properties (energy–time dispersion, high/low lifetime at low/high electron energies). They also indicate how simplifying assumptions used in analytical methods introduce several systematic errors. We demonstrate that, depending on the application and under certain conditions these errors can be neglected, like for instance for small pitch angles and at regions that the dipole approximation is sufficient (inside the orbit of Dione) or for electron energies below few hundred keV. For higher electron energies, systematic errors amplify significantly and existing analytical methods cannot be used. Our model can reconstruct the energy dependent position of microsignatures observed by the MIMI/LEMMS detector with high accuracy, allowing the inference of non-corotational flows (or electric fields) that can be as low as few tens of m s−1. Since, however the calculation of such flows is indirect, the accuracy of such a determination can be reduced by more than an order of magnitude, if some of these free parameters involved in the simulation cannot be sufficiently constrained. One way to provide such constraints is through inputs (e.g. instantaneous plasma moments) from various Cassini instruments and updated magnetospheric field models. In that case, microsignature analysis may prove to be one of the best methods for attempting to measure or to at least constrain the magnitude of the very slow and global plasma outflow in Saturn’s magnetosphere that is driven by mass loading at Enceladus. 相似文献
12.
Carlos E. Chavez 《Icarus》2009,203(1):233-237
In this article we explore the aspect of the F ring with respect to the anti-alignment configuration between the ring and Prometheus. We focus our attention on the shape of the F ring’s azimuthal channels which were first reported by Porco et al. (Porco, C.C., Baker, E., Barbara, J., Beurle, K., Brahic, A., Burns, J.A., Charnoz, S., Cooper, N., Dawson, D.D., Del Genio, A.D., Denk, T., Dones, L., Dyudina, U., Evans, M.W., Giese, B., Grazier, K., Helfenstein, P., Ingersoll, A.P., Jacobson, R.A., Johnson, T.V., McEwen, A., Murray, C.D., Neukum, G., Owen, W.M., Perry, J., Roatsch, T., Spitale, J., Squyres, S., Thomas, P., Tiscareno, M., Turtle, E., Vasavada, A.R., Veverka, J., Wagner, R., West, R. [2005] Science, 307, 1226-1236) and numerically explored by Murray et al. (Murray, C.D., Chavez, C., Beurle, K., Cooper, N., Evans, M.W., Burns, J.A., Porco, C.C. [2005] Nature 437, 1326-1329) who found excellent agreement between Cassini’s ISS reprojected images and their numerical model via a direct comparison. We find that for anti-alignment the channels are wider and go deeper inside the ring material. From our numerical model we find a new feature, an island in the middle of the channel. This island is made up of the particles that have been perturbed the most by Prometheus and only appears when this satellite is close to apoapsis. In addition, plots of the anti-alignment configuration for different orbital stages of Prometheus are obtained and discussed here. 相似文献
13.
Eberhard Grün Gregor E. Morfill Richard J. Terrile Torrence V. Johnson Gerhard Schwehm 《Icarus》1983,54(2):227-252
The optical appearance of spokes was studied in high resolution (?200 km/lp) images obtained by Voyager 2. Spokes are classified into three categories. (1) Extended spokes are seen in the distance interval of 100,000 to 112,000 km from Saturn's center. They have diffuse edges and are slightly wedge shaped. Their width at the base (towards Saturn) is about 20,000 km. Their active times (during which they increase in width) range from 4000 to 12,000 sec. (2) Narrow spokes are found in the distance range 104,000 to 116,000 km, have sharply defined edges, and are narrowest at the corotation distance (112, 300 km). Their typical radial extension and width is 6000 and 2,000 km, respectively. (3) Filamentary spokes are found outside 110,000 km mostly joined with a wider spoke further in. They are typically 3000 km in length and 500 km in width. Their active time is less than 1000 sec. Several narrow spokes were observed during formation along radial lines in the sunlit portion of the ring. The formation time is typically ?5 min for a 6000-km-long spoke. The rate of spoke formation is highest at the morning ansa outside Saturn's shadow. Several spokes have been found where one edge revolves with Keplerian speed whereas the other edge stays radial. Recurrent spoke patterns have been observed at the period of Saturn's rotation. From edge-on views of the ring system, an upper limit for the height of spokes of 80 km is derived. 相似文献
14.
During 2004 the Cassini/RPWS (Radio and Plasma Wave Science) instrument recorded about 5400 SEDs (Saturn Electrostatic Discharges), which were organized in 4 storm systems and 95 episodes. A computer algorithm with different intensity thresholds was applied to extract the SEDs from the RPWS data, and a statistical analysis on the main characteristics of these SEDs is performed. Compared to the SEDs recorded by the Voyagers in the early 1980s, some characteristics like SED rate, intensity, signal duration, or power spectrum are similar, but there are also remarkable differences with regard to time occurrence and frequency range: The first appearance of SEDs (storm 0) was recorded by RPWS from a distance of more than 300 Saturn radii at the end of May 2004, followed by storm A in mid-July, storm B at the beginning of August, and the most prominent storm C throughout most of September. There were also significant intervals of time with no detectable SED activity, e.g., SEDs were practically absent from October 2004 until June 2005. No clear indication for SEDs below a frequency of 1.3 MHz could be found. We suggest that the SED storms A, B, C, and possibly also storm 0 originate from the same storm system residing at a latitude of 35° South, which lasted for several months, waxed and waned in strength, and rotated with the Voyager radio period of Saturn. The SED source might be located in the updrafting water clouds beneath the visible cloud features detected in the Cassini images. 相似文献
15.
The spatial distribution of N+ in Saturn's magnetosphere obtained from Cassini Plasma Spectrometer (CAPS) data can be used to determine the spatial distribution and relative importance of the nitrogen sources for Saturn's magnetosphere. We first summarize CAPS data from 15 orbits showing the spatial and energy distribution of the nitrogen component of the plasma. This analysis re-enforces our earlier discovery [Smith, H.T., Shappirio, M., Sittler, E.C., Reisenfeld, D., Johnson, R.E., Baragiola, R.A., Crary, F.J., McComas, D.J., Young, D.T., 2005. Geophys. Res. Lett. 32 (14). L14S03] that Enceladus is likely the dominant nitrogen source for Saturn's inner magnetosphere. We also find a sharp enhancement in the nitrogen ion to water ion ratio near the orbit of Enceladus which, we show, is consistent with the presence of a narrow Enceladus torus as described in [Johnson, R.E., Liu, M., Sittler Jr., E.C., 2005. Geophys. Res. Lett. 32. L24201]. The CAPS data and the model described below indicate that N+ ions are a significant fraction of the plasma in this narrow torus. We then simulated the combined Enceladus and Titan nitrogen sources using the CAPS data as a constraint. This simulation is an extension of the model we employed earlier to describe the neutral tori produced by the loss of nitrogen from Titan [Smith, H.T., Johnson, R.E., Shematovich, V.I., 2004. Geophys. Res. Lett. 31 (16). L16804]. We show that Enceladus is the principal nitrogen source in the inner magnetosphere but Titan might account for a fraction of the observed nitrogen ions at the largest distances discussed. We also show that the CAPS data is consistent with Enceladus being a molecular nitrogen source with a nitrogen to water ratio roughly consistent with INMS [Waite, J.H., and 13 colleagues, 2006. Science 311 (5766), 1419-1422], but out-gassing of other nitrogen-containing species, such as ammonia, cannot be ruled out. 相似文献
16.
We suggest that spokes consist of charged micron-sized dust particles elevated from the rings by radially moving dense plasma columns created by meteor impacts on the ring. Dense plasma causes electrostatic wall-sheaths at the ring and charging of the ring with electric fields strong enough to overcome the gravitational force on small dust particles. Under “ordinary” conditions only very few dust particles will be elevated as the probability of a dust particle having at least one excess electronic charge is very low. Dense plasma raises this probability significantly. The radial motion of the plasma column is due to an azimuthal polarization electric field built up by the relative motion between the corotating plasma and the negatively charged dust particles which move with a Keplerian speed. 相似文献
17.
New global maps of the five inner midsize icy saturnian satellites, Mimas, Enceladus, Tethys, Dione, and Rhea, have been constructed in three colors (UV, Green and near-IR) at resolutions of 1 km/pixel. The maps reveal prominent global patterns common to several of these satellites but also three major color features unique to specific satellites or satellite subgroups. The most common features among the group are first-order global asymmetries in color properties. This pattern, expressed on Tethys, Dione and Rhea, takes the form of a ∼1.4-1.8 times enhancement in redness (expressed as IR/UV ratio) of the surface at the center of the trailing hemisphere of motion, and a similar though significantly weaker IR/UV enhancement at the center of the leading hemisphere. The peak in redness on the trailing hemisphere also corresponds to a known decrease in albedo. These double hemispheric asymmetries are attributable to plasma and E-ring grain bombardment on the trailing and leading hemispheres, respectively, for the outer three satellites Tethys, Dione and Rhea, whereas as E-ring bombardment may be focused on the trailing hemisphere of Mimas due to its orbital location interior to Enceladus. The maps also reveal three major deviations from these basic global patterns. We observe the previously known dark bluish leading hemisphere equatorial band on Tethys but have also discovered a similar band on Mimas. Similar in shape, both features match the surface patterns expected for irradiation of the surface by incident MeV electrons that drift in a direction opposite to the plasma flow. The global asymmetry on Enceladus is offset ∼40° to the west compared to the other satellites. We do not consider Enceladus in detail here, but the global distribution of bluish material can be shown to match the deposition pattern predicted for plume fallback onto the surface (Kempf, S., Beckmann, U., Schmidt, S. [2010]. Icarus 206, 446-457. doi:10.1016/j.icarus.2009.09.016). E-ring deposition on Enceladus thus appears to mask or prevent the formation of the lenses and hemispheric asymmetries we see on the other satellites. Finally, we observe a chain of discrete bluish splotches along the equator of Rhea. Unlike the equatorial bands of Tethys and Mimas, these splotches form a very narrow great circle ?10-km wide (north-to-south) and appear to be related to surface disruption, exposing fresh, bluish ice on older crater rims. This feature is unique to Rhea and may have formed by impact onto its surface of orbiting material. 相似文献
18.
We present a forward modeling approach for determining, in part, the ring particle spatial distribution in the vicinity of sharp ring or ringlet edges. Synthetic edge occultation profiles are computed based on a two-parameter particle spatial distribution model. One parameter, h, characterizes the vertical extent of the ring and the other, δ, characterizes the radial scale over which the ring optical depth transitions from the background ring value to zero. We compare our synthetic occultation profiles to high resolution stellar occultation light curves observed by the Cassini Ultraviolet Imaging Spectrograph (UVIS) High Speed Photometer (HSP) for occultations by the Titan ringlet and Huygens ringlet edges.More than 100 stellar occultations of the Huygens ringlet and Titan ringlet edges were studied, comprising 343 independent occultation cuts of the edges of these two ringlets. In 237 of these profiles the measured light-curve was fit well with our two-parameter edge model. Of the remaining edge occultations, 69 contained structure that could only be fit with extremely large values of the ring-plane vertical thickness (h > 1 km) or by adopting a different model for the radial profile of the ring optical depth. An additional 37 could not be fit by our two-parameter model.Certain occultations at low ring-plane incidence angles as well as occultations nearly tangent to the ring edge allow the direct measurement of the radial scale over which the particle packing varies at the edge of the ringlet. In 24 occultations with these particular viewing geometries, we find a wide variation in the radial scale of the edge. We are able to constrain the vertical extent of the rings at the edge to less than ∼300 m in the 70% of the occultations with appropriate viewing geometry, however tighter constraints could not be placed on h due to the weaker sensitivity of the occultation profile to vertical thickness compared to its sensitivity to δ.Many occultations of a single edge could not be fit to a single value of δ, indicating large temporal or azimuthal variability, although the azimuthal variation in δ with respect to the longitudes of various moons in the system did not show any discernible pattern. 相似文献
19.
The Cassini spacecraft made a single flyby each of Saturn's icy moons Tethys and Rhea in late 2005. The magnetic field observations from these flybys provide unique portraits of the magnetic properties of these moons. These are the first observations of interactions of these inert moons with the sub-magnetosonic plasma of Saturn's magnetosphere. Because the upstream field and plasma conditions are extremely stable, we are able to observe the interaction in great detail. One of the major findings of this study is that the region of plasma depletion is greatly elongated along the field direction in a sub-magnetosonic interaction. Based on the consideration of field aligned velocities of thermal ions, we show that overlapping particle shadow wings form downstream of an inert moon such that in each of the particle shadow wings, particles of specific field aligned velocities are depleted. Other major findings of this study are: (1) Tethys and Rhea are devoid of any internal magnetic field; (2) No induction generated field was observed, as expected because of the extremely weak primary inducing (time varying) field; (3) There is no appreciable mass-loading of Saturn's magnetosphere from Tethys and Rhea; (4) We predict that wave particles interactions would be generated that smooth out the phase space holes created by the moon/plasma interaction. These waves serve to isotropize the plasma distribution function. 相似文献
20.
We have completed a series of local N-body simulations of Saturn’s B and A rings in order to identify systematic differences in the degree of particle clumping into self-gravity wakes as a function of orbital distance from Saturn and dynamical optical depth (a function of surface density). These simulations revealed that the normal optical depth of the final configuration can be substantially lower than one would infer from a uniform distribution of particles. Adding more particles to the simulation simply piles more particles onto the self-gravity wakes while leaving relatively clear gaps between the wakes. Estimating the mass from the observed optical depth is therefore a non-linear problem. These simulations may explain why the Cassini UVIS instrument has detected starlight at low incidence angles through regions of the B ring that have average normal optical depths substantially greater than unity at some observation geometries [Colwell, J.E., Esposito, L.W., Srem?evi?, M., Stewart, G.R., McClintock, W.E., 2007. Icarus 190, 127-144]. We provide a plausible internal density of the particles in the A and B rings based upon fitting the results of our simulations with Cassini UVIS stellar occultation data. We simulated Cassini-like occultations through our simulation cells, calculated optical depths, and attempted to extrapolate to the values that Cassini observes. We needed to extrapolate because even initial optical depths of >4 (σ > 240 g cm−2) only yielded final optical depths no greater than 2.8, smaller than the largest measured B ring optical depths. This extrapolation introduces a significant amount of uncertainty, and we chose to be conservative in our overall mass estimates. From our simulations, we infer the surface density of the A ring to be , which corresponds to a mass of . We infer a minimum surface density of for Saturn’s B ring, which corresponds to a minimum mass estimate of . The A ring mass estimate agrees well with previous analyses, while the B ring is at least 40% larger. In sum, our lower limit estimate is that the total mass of Saturn’s ring system is 120-200% the mass of the moon Mimas, but significantly larger values would be plausible given the limitations of our simulations. A significantly larger mass for Saturn’s rings favors a primordial origin for the rings because the disruption of a former satellite of the required mass would be unlikely after the decay of the late heavy bombardment of planetary surfaces. 相似文献