On the damping of the bending waves in Saturn’s ring

We study numerically the motion of a single particle in the bending wave of finite thickness in Saturn’s ring. We include the forcing due to the planet, a moon, the coriolis force and the self gravity of the ring. In particular, we compute the variation of the velocity arising due to the variation of the amplitude and the phase of the epicyclic motion across the local vertical height of the ring. We suggest that the dissipation of energy due to the collision of ring particles in this shear layer damps out the bending wave of Saturn’s ring at the 5:3 vertical resonance of Mimas within a distance of 150 km from the site of its launching as is observed in Voyager data.     

HST observations of azimuthal asymmetry in Saturn's rings

From 378 Hubble Space Telescope WFPC2 images obtained between 1996-2004, we have measured the detailed nature of azimuthal brightness variations in Saturn's rings. The extensive geometric coverage, high spatial resolution (), and photometric precision of the UBVRI images have enabled us to determine the dependence of the asymmetry amplitude and longitude of minimum brightness on orbital radius, ring elevation, wavelength, solar phase angle, and solar longitude. We explore a suite of dynamical models of self-gravity wakes for two particle size distributions: a single size and a power law distribution spanning a decade in particle radius. From these N-body simulations, we calculate the resultant wake-driven brightness asymmetry for any given illumination and viewing geometry. The models reproduce many of the observed properties of the asymmetry, including the shape and location of the brightness minimum and the trends with ring elevation and solar longitude. They also account for the “tilt effect” in the A and B rings: the change in mean ring brightness with effective ring opening angle, |Beff|. The predicted asymmetry depends sensitively on dynamical ring particle properties such as the coefficient of restitution and internal mass density, and relatively weakly on photometric parameters such as albedo and scattering phase function. The asymmetry is strongest in the A ring, reaching a maximum amplitude A∼25% near a=128,000 km. Here, the observations are well-matched by an internal particle density near 450 kg m⁻³ and a narrow particle size distribution. The B ring shows significant asymmetry (∼5%) in regions of relatively low optical depth (τ∼0.7). In the middle and outer B ring, where τ?1, the asymmetry is much weaker (∼1%), and in the C ring, A<0.5%. The asymmetry diminishes near opposition and at shorter wavelengths, where the albedo of the ring particles is lower and multiple-scattering effects are diminished. The asymmetry amplitude varies strongly with ring elevation angle, reaching a peak near |Beff|=10° in the A ring and at |Beff|=15-20° in the B ring. These trends provide an estimate of the thickness of the self-gravity wakes responsible for the asymmetry. Local radial variations in the amplitude of the asymmetry within both the A and B rings are probably caused by regional differences in the particle size distribution.     

Voyager observations of the azimuthal brightness variations in Saturn's rings

This paper presents some Voyager observations of the azimuthal brightness variations in Saturn's ring A. Measurements in reflected light are in general agreement with Earth-based studies. The unique contribution of Voyager—images of the rings in light transmitted through them—shows the brightness variations also to be present, but they have a decidedly greater amplitude and differ in phase by ∼65° from those seen in reflexion. The photometric behavior on both sides can probably be qualitatively explained by the extensive presence of particle wakes in ring A.     

Partial redistribution effects on line polarization in the presence of velocity fields

Velocity fields in line formation regions strongly affect the line polarization. The conventionally used observer's frame method of solving the polarized transfer equation becomes expensive and inaccurate for partial redistribution problems, when large amplitude velocity fields have to be considered in the observer's frame. An alternative method of solution is the comoving frame method. Partial redistribution problems are solved using comoving frame formalism for line polarization caused by resonance scattering.     

Constraints on the C ring parameters of Saturn at the Titan −1:0 resonance

A one-armed spiral bending wave in Saturn's rings excited by Titan's −1:0 inner vertical resonance is one of the most prominent oscillatory features observed by Voyager 1 . We study detailed dynamics of the particles inside the ring, and show that one of the main causes of the complete dissipation of the bending wave within a distance of ∼85 km from the resonance site could be as a result of the presence of a strong shear caused by radial velocity variation along the vertical direction. Assuming this to be the only source, Voyager data would suggest that if the surface density of matter is around 0.45 g cm⁻² and the amplitude of the bending wave is around 1200 m, then the upper limit of total vertical thickness of the C ring near this resonance is around 40 m.     

Solar Cycle Variations of Large-Scale Flows in the sun

Using data from the Michelson Doppler Imager (MDI) instrument on board the Solar and Heliospheric Observatory (SOHO), we study the large-scale velocity fields in the outer part of the solar convection zone using the ring diagram technique. We use observations from four different times to study possible temporal variations in flow velocity. We find definite changes in both the zonal and meridional components of the flows. The amplitude of the zonal flow appears to increase with solar activity and the flow pattern also shifts towards lower latitude with time.     

Structure of the Earth’s circumsolar dust ring

Interplanetary dust particles from comets and asteroids pervade the Solar System and become temporarily trapped into orbital resonances with Earth, leading to a circumsolar dust ring. Using the unique vantage point of the Spitzer Space Telescope from its Earth-trailing solar orbit, we have measured for the first time the azimuthal structure of the Earth’s resonant dust ring. There is a relative paucity of particles within 0.1 AU of the Earth, followed by an enhancement in a cloud that is centered 0.2 AU behind Earth with a width of 0.08 AU along the Earth’s orbit. The North ecliptic pole is ∼3% brighter at 8 μm wavelength when viewed from inside the enhancement. The presence of azimuthal asymmetries in debris disks around other stars is considered strong evidence for planets. By measuring the properties of the Earth’s resonant ring, we can provide “ground truth” to models for interactions of planets and debris disks, possibly leading to improved predictions for detectability of life-bearing planets. The low amplitude of the azimuthal asymmetry in the Earth’s circumsolar ring suggests significant contributions to the zodiacal light from particles that are large (>30 μm) or have large orbital eccentricity that makes capture into mean motion resonances inefficient.     

A numerical model of cohesion in planetary rings

We present a numerical method that incorporates particle sticking in simulations using the N-body code pkdgrav to study motions in a local rotating frame, such as a patch of a planetary ring. Particles stick to form non-deformable but breakable aggregates that obey the (Eulerian) equations of rigid-body motion. Applications include local simulations of planetary ring dynamics and planet formation, which typically feature hundreds of thousands or more colliding bodies. Bonding and breaking thresholds are tunable parameters that can approximately mimic, for example, van der Waals forces or interlocking of surface frost layers. The bonding and breaking model does not incorporate a rigorous treatment of internal fracture; rather the method serves as motivation for first-order investigation of how semi-rigid bonding affects the evolution of particle assemblies in high-density environments.We apply the method to Saturn’s A ring, for which laboratory experiments suggest that interpenetration of thin, frost-coated surface layers may lead to weak cohesive bonding. These experiments show that frost-coated icy bodies can bond at the low impact speeds characteristic of the rings. Our investigation is further motivated by recent simulations that suggest a very low coefficient of restitution is needed to explain the amplitude of the azimuthal brightness asymmetry in Saturn’s A ring, and the hypothesis that fine structure in Saturn’s B ring may in part be caused by large-scale cohesion.This work presents the full implementation of our model in pkdgrav, as well as results from initial tests with a limited set of parameters explored. We find a combination of parameters that yields aggregate size distribution and maximum radius values in agreement with Voyager data for ring particles in Saturn’s outer A ring. We also find that the bonding and breaking parameters define two strength regimes in which fragmentation is dominated either by collisions or other stresses, such as tides. We conclude our study with a discussion of future applications of and refinements to our model.     

Low-frequency electric field fluctuations excited by ring current protons

Energetic protons haying ring type distributions are shown to generate low-frequency electrostatic waves, propagating nearly transverse to the geomagnetic field lines, in the ring current region by exciting Mode 1 arid Mode 2 nonresonant instabilities and a resonant instability. Mode 1 nonresonant instability has frequencies around ~4 Hz with transverse wavelengths of ~(8–80) km, and it is likely to occur in the region L = (7–8). Mode 2 nonresonant instability can generate frequencies ~(850–1450) Hz with transverse wavelengths ~(2–20) km. The typical frequencies and transverse wavelengths associated with the resonant instability are (950–1250) Hz and (30–65) km. Both the Mode 2 nonresonant instability and the resonant instability can occur in the ring current region with L = (4–6). The low-frequency modes driven by energetic protons could attain maximum saturation electric field amplitude varying from 0.8 mV/m to 70 mV/m. It is suggested that the turbulence produced by the low-frequency modes may cause pitch angle scattering of ring current protons in the region outside the plasmapause resulting in the ring current decay.     

Observations of Pg pulsations in the Northern Auroral zone and at lower latitude conjugate regions

An analysis is made of giant pulsation (Pg) data recorded at ground stations in the Northern Auroral Zone in Scandanavia (mainly at Tromsø, L = 6.4 and Kiruna, L = 5.5) during the period September 1976 to December 1977. They are shown to have a meridional variation of amplitude and polarization consistent with a field line resonance structure and their vertical component behaviour suggests that they also have a rapid azimuthal phase variation. Limited data from conjugate stations at L = 4.4 are used to show that Pg's are odd mode oscillations of the field line. Pg's are equated to the observation of a unique compressional wave in space at synchronous orbit and it is suggested that they result from the drift wave instability of the compressional Alfven wave at the outer edge of the quiet time ring current.     

Eccentric features in Saturn's outer C ring

A systematic search has been made for as yet unrecognized eccentric and inclined features in Saturn's outer C ring. The radii of all sharp-edged features in the outer C ring were measured in Voyager data consisting of six high-resolution images, the Photopolarimeter occultation data, and the Radio Science λ3.6-cm occultation data corrected for the effects of diffraction. Besides the well-known Maxwell ringlet at 87,491 km (1.450R_s), whose eccentric shape and kinematics have already been studied, two other narrow ringlets at 88,716 km (1.470R_S), and 90,171 km (1.495R_S) have been found to be demonstrably eccentric. The former has a mean width of ∼16 km and is located within a gap ∼30 km wide. The latter has a mean width of ∼62 km and is only partially isolated: its outer edge is defined by a gap ∼15 km wide. Though a coincidence of these two gaps with the Mimas 3:1 inner vertical and inner Lindblad resonances has been noted by previous workers, we find that neither ringlet shows conclusive evidence for the anticipated resonantly forced distortions. The 1.495R_S ringlet is best fitted by a model describing a freely precessing Keplerian ellipse with a radial amplitude of 2.8 ± 0.5 km. Neither a resonant forcing nor a free precession model fitted to the 1.470R_S ringlet provides conclusive results, though the latter is marginally better, yielding an amplitude no larger than ∼2.2 km. These two newly identified eccentric ringlets are compared with the previously studied Titan and Maxwell ringlets (C. Porco, P. D. Nicholson, N. Borderies, G. E. Danielson, P. Goldreich, J. B. Holberg, and A. L. Lane, Icarus 60 (1984), 1–16) and with the Uranian α, β, and ϵ ring.     

Saturn's nonaxisymmetric ring edges at 1.95 Rs and 2.27 Rs

The outer edges of Saturn's A and B rings, at 2.27 R_s and 1.95 R_s, have been examined using data acquired by four Voyager experiments. The shapes and kinematics of these features are influenced by their proximity to strong low-order Lindblad resonances. The data for the A-ring edge are consistent with a seven-loded radial distortion of amplitude 6.7 ± 1.5 km which rotates with the mass-weighted mean angular velocity of the coorbital satellite system. The B-ring edge has essentially a double-lobed figure of radial amplitude 74 ± 9 km which rotates with the mean motion of Mimas, though there is an indication that it is not completely described withe a simple Saturn-centered ellipse. An upper limit of 10 m has been placed on the vertical thickness in the unperturbed region of the B ring.     

Cassini imaging of Saturn's rings: II. A wavelet technique for analysis of density waves and other radial structure in the rings

We describe a powerful signal processing method, the continuous wavelet transform, and use it to analyze radial structure in Cassini ISS images of Saturn's rings. Wavelet analysis locally separates signal components in frequency space, causing many structures to become evident that are difficult to observe with the naked eye. Density waves, generated at resonances with saturnian satellites orbiting outside (or within) the rings, are particularly amenable to such analysis. We identify a number of previously unobserved weak waves, and demonstrate the wavelet transform's ability to isolate multiple waves superimposed on top of one another. We also present two wave-like structures that we are unable to conclusively identify. In a multi-step semi-automated process, we recover four parameters from clearly observed weak spiral density waves: the local ring surface density, the local ring viscosity, the precise resonance location (useful for pointing images, and potentially for refining saturnian astrometry), and the wave amplitude (potentially providing new constraints upon the masses of the perturbing moons). Our derived surface densities have less scatter than previous measurements that were derived from stronger non-linear waves, and suggest a gentle linear increase in surface density from the inner to the mid-A Ring. We show that ring viscosity consistently increases from the Cassini Division outward to the Encke Gap. Meaningful upper limits on ring thickness can be placed on the Cassini Division (3.0 m at r∼118,800 km, 4.5 m at r∼120,700 km) and the inner A Ring (10-15 m for r<127,000 km).     

Cross calibration of telescope optical throughput efficiencies using reconstructed shower energies for the Cherenkov Telescope Array

For reliable event reconstruction of Imaging Atmospheric Cherenkov Telescopes (IACTs), calibration of the optical throughput efficiency is required. Within current facilities, this is achieved through the use of ring shaped images generated by muons. Here, a complementary approach is explored, achieving cross calibration of elements of IACT arrays through pairwise comparisons between telescopes, focussing on its applicability to the upcoming Cherenkov Telescope Array (CTA). Intercalibration of telescopes of a particular type using eventwise comparisons of shower image amplitudes has previously been demonstrated to recover the relative telescope optical responses. A method utilising the reconstructed energy as an alternative to image amplitude is presented, enabling cross calibration between telescopes of varying types within an IACT array. Monte Carlo studies for two plausible CTA layouts have shown that this calibration procedure recovers the relative telescope response efficiencies at the few per cent level.     

Galactic oscillations

A stable galaxy, if excited above its ground state, oscillates about that ground state. If it is reasonably robust, it can support oscillations of large amplitude. Normal mode oscillations, with surprisingly large amplitudes, have been seen in numerical experiments. Observational evidence shows that real galaxies also oscillate. Galaxies ring like a bell in the experiments, and ringing continues undamped long after initial transients have died out. Their total kinetic energy oscillates with an amplitude as large as 10% of the mean. A fundamental mode dominates. It is an homologous expansion/contraction of the entire galaxy (no nodes). Inward or outward velocities due to this mode are sufficiently large in the outer reaches of a galaxy to account for kinematic warps in observed velocity fields. A second spherically symmetrical mode has one node and is important near the center of the galaxy. It may be the driving force behind bulges in spiral galaxies. Two other normal modes have been identified as well. This appears to be the first experimental demonstration of normal mode oscillations within stable galaxy models.     

A Thin Ring Model for the Oh Megamaser in Iiizw35

We present a model for the OH megamaser emission in the starburst galaxy IIIZw35. The observed diffuse and compact OH maser components in this source are explained by a single phase of unsaturated clumpy gas distributed in a thin ring structure and amplifying background continuum. We emphasize the importance of clumpiness in the OH masing medium, an effect that has not been fully appreciated previously.The model explains why multiple bright spots are seen only at the ring tangents while smoother emission is found elsewhere. Both the observed velocity gradients and the line to continuum ratios around the ring enquire a geometry where most of the seed photons come from a continuum emission which lies outside the OH ring. To explain both the OH and continuum brightness, free-free absorbing gas is required along the ring axis to partially absorb the far side of the ring. It is proposed that the required geometry arises from an inwardly propagating ring of starburst activity.     

Optical Gravitational Lensing Experiment Cepheids Have Lower Amplitudes in the Small Magellanic Cloud than in the Large Magellanic Cloud

We selected Cepheids from the Optical Gravitational Lensing Experiment database for the Magellanic Clouds in the period range of 101.1相似文献   

Radar imaging of Saturn's rings

We present delay-Doppler images of Saturn's rings based on radar observations made at Arecibo Observatory between 1999 and 2003, at a wavelength of 12.6 cm and at ring opening angles of 20.1°?|B|?26.7°. The average radar cross-section of the A ring is ∼77% relative to that of the B ring, while a stringent upper limit of 3% is placed on the cross-section of the C ring and 9% on that of the Cassini Division. These results are consistent with those obtained by Ostro et al. [1982, Icarus 49, 367-381] from radar observations at |B|=21.4°, but provide higher resolution maps of the rings' reflectivity profile. The average cross-section of the A and B rings, normalized by their projected unblocked area, is found to have decreased from 1.25±0.31 to 0.74±0.19 as the rings have opened up, while the circular polarization ratio has increased from 0.64±0.06 to 0.77±0.06. The steep decrease in cross-section is at variance with previous radar measurements [Ostro et al., 1980, Icarus 41, 381-388], and neither this nor the polarization variations are easily understood within the framework of either classical, many-particle-thick or monolayer ring models. One possible explanation involves vertical size segregation in the rings, whereby observations at larger elevation angles which see deeper into the rings preferentially see the larger particles concentrated near the rings' mid-plane. These larger particles may be less reflective and/or rougher and thus more depolarizing than the smaller ones. Images from all four years show a strong m=2 azimuthal asymmetry in the reflectivity of the A ring, with an amplitude of ±20% and minima at longitudes of 67±4° and 247±4° from the sub-Earth point. We attribute the asymmetry to the presence of gravitational wakes in the A ring as invoked by Colombo et al. [1976, Nature 264, 344-345] to explain the similar asymmetry long seen at optical wavelengths. A simple radiative transfer model suggests that the enhancement of the azimuthal asymmetry in the radar images compared with that seen at optical wavelengths is due to the forward-scattering behavior of icy ring particles at decimeter wavelengths. A much weaker azimuthal asymmetry with a similar orientation may be present in the B ring.     

Inductive electric fields in the magnetosphere

A quantitative estimate of the electric fields induced by the time dependent ring current is made incorporating the drifts and induced electric fields in a self-consistent manner. It has been shown that in the ring current region, the results of the self-consistent calculations deviate substantially from the first order estimates hitherto obtained. Since for a rapidly varying ring current, the induced electric field can be of the same order as the convection electric field in the magnetosphere, these deviations have to be taken into account in substorm studies.     

Dust in jupiter''s magnetosphere: Origin of the ring

A model for the production of the Jovian ring is proposed. The ‘visible’ ring particles are micron-sized and produced by erosive collisions between an assumed population of km-sized parent bodies and sub-micron sized magnetospheric dust particles. These small dust particles are ejected by volcanoes from Io. The observed topology of the ring is described quite well with the theory, and properties of the parent bodies are deduced.