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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Planetary rings

This review discusses each planetary ring system and describes recent theoretical developments and models of ring evolution. The distribution of the inner rings and satellites of all four giant planets reduced to a uniform scale of 'planetary units', normalized by the equatorial radius of each planet, is summarized. Only major ring subunits are indicated, with the density of cross-hatching representing in a crude way the relative optical depths of the different components of each system. One result of the spacecraft observations has been the discovery at each planet of several small (10-100 km in radius) satellites that appear to be intimately related to the rings, dynamically and possibly genetically as well.

Nicholson, Philip D.↗
NASA NTRS1978

Photometric confirmation of the Encke division in Saturn's ring A

A physical model of the behavior of Saturn's satellite Iapetus passing through the shadow of the rings is discussed. The model is examined via computer simulation to give photometric confirmation of the Encke division in ring A. The actual event was observed with a two-channel sky-chopping photometer. Simultaneous pulse-counter output from the red and blue channels was integrated for 10-ms intervals alternately on the sky and on Iapetus. In the computer simulation a solar limb-darkening coefficient was used in the blue region and Iapetus was considered two limb-darkened hemispheres of radii 800 km with the trailing hemisphere brighter than the preceding hemisphere by a factor of seven.

Reitsema, H. J.↗
NASA NTRS1982

Inclinations of the Uranian rings

A kinematic model of the orbits of the Uranian rings was fitted to stellar occultation data. The results reveal that seven of the nine rings have significant inclinations with respect to the equatorial plane of Uranus. The inclinations are typically a few hundredths of a degree.

French, R. G.↗
NASA NTRS1990

Queueing models for token and slotted ring networks

Currently the end-to-end delay characteristics of very high speed local area networks are not well understood. The transmission speed of computer networks is increasing, and local area networks especially are finding increasing use in real time systems. Ring networks operation is generally well understood for both token rings and slotted rings. There is, however, a severe lack of queueing models for high layer operation. There are several factors which contribute to the processing delay of a packet, as opposed to the transmission delay, e.g., packet priority, its length, the user load, the processor load, the use of priority preemption, the use of preemption at packet reception, the number of processors, the number of protocol processing layers, the speed of each processor, and queue length limitations. Currently existing medium access queueing models are extended by adding modeling techniques which will handle exhaustive limited service both with and without priority traffic, and modeling capabilities are extended into the upper layers of the OSI model. Some of the model are parameterized solution methods, since it is shown that certain models do not exist as parameterized solutions, but rather as solution methods.

Peden, Jeffery H.↗
NASA NTRS1985

Saturn's rings - Particle size distributions for thin layer model

A model incorporating limited interaction between the incident energy and particles in the ring is considered which appears to be consistent with the multiple scattering process in Saturn's rings. The model allows for the small physical thickness of the rings and can be used to relate Voyager 1 observations of 3.6- and 13-cm wavelength microwave scatter from the rings to the ring particle size distribution function for particles with radii ranging from 0.001 to 20 m. This limited-scatter model yields solutions for particle size distribution functions for eight regions in the rings, which exhibit approximately inverse-cubic power-law behavior.

Zebker, H. A.↗
NASA NTRS2010

Global Response to Local Ionospheric Mass Ejection

We revisit a reported "Ionospheric Mass Ejection" using prior event observations to guide a global simulation of local ionospheric outflows, global magnetospheric circulation, and plasma sheet pressurization, and comparing our results with the observed global response. Our simulation framework is based on test particle motions in the Lyon-Fedder-Mobarry (LFM) global circulation model electromagnetic fields. The inner magnetosphere is simulated with the Comprehensive Ring Current Model (CRCM) of Fok and Wolf, driven by the transpolar potential developed by the LFM magnetosphere, and includes an embedded plasmaspheric simulation. Global circulation is stimulated using the observed solar wind conditions for the period 24-25 Sept 1998. This period begins with the arrival of a Coronal Mass Ejection, initially with northward, but later with southward interplanetary magnetic field. Test particles are launched from the ionosphere with fluxes specified by local empirical relationships of outflow to electrodynamic and particle precipitation imposed by the MIlD simulation. Particles are tracked until they are lost from the system downstream or into the atmosphere, using the full equations of motion. Results are compared with the observed ring current and a simulation of polar and auroral wind outflows driven globally by solar wind dynamic pressure. We find good quantitative agreement with the observed ring current, and reasonable qualitative agreement with earlier simulation results, suggesting that the solar wind driven global simulation generates realistic energy dissipation in the ionosphere and that the Strangeway relations provide a realistic local outflow description.

Moore, T. E.↗
NASA NTRS1992

Ballistic transport in planetary ring systems due to particle erosion mechanisms. II - Theoretical models for Saturn's A- and B-ring inner edges

The present numerical simulations and analytic arguments show that many of the common morphological features of the Saturn A- and B-ring inner-edge regions are due to 'ballistic transport', or the net radial transport of mass and angular momentum generated by exchanges of meteoroid impact ejecta. It is suggested that the observed 100-km undulatory structure of the inner B-ring arises from ballistic transport echoing of the inner edge. A strongly prograde ejecta-distribution function is used to fit the edge-region features.

Durisen, Richard H.↗
NASA NTRS2006

Recent Highlights in Ring Structure, Dynamics, and Composition

While the Cassini orbiter has been in a near-equatorial orbit for most of the time since the last DPS meeting, the science teams have turned to preliminary analysis of many aspects of data taken over the first year of the mission. New understanding has been gained of several different aspects of ring structure and dynamics - especially the small-scale ring vertical structure related to marginal gravitational instabilities, and the diverse roles played by small moonlets sprinkled around and within the rings. In addition, a closer look is being taken at how ring composition varies locally and regionally and how this variation compares with models of ring compositional evolution. Cassini will re-emerge from the equator plane in late July, and new ring observations will again be available. In this talk we will review progress over the last year, highlight key outstanding issues and problems, and provide a context for the new observations reported at this meeting.

Cuzzi, Jeffrey N.↗
NASA NTRS2010

Modeling of the Convection and Interaction of Ring Current, Plasmaspheric and Plasma Sheet Plasmas in the Inner Magnetosphere

Distinctive sources of ions reside in the plasmasphere, plasmasheet, and ring current regions at discrete energies constitute the major plasma populations in the inner/middle magnetosphere. They contribute to the electrodynamics of the ionosphere-magnetosphere system as important carriers of the global current system, in triggering; geomagnetic storm and substorms, as well as critical components of plasma instabilities such as reconnection and Kelvin-Helmholtz instability at the magnetospheric boundaries. Our preliminary analysis of in-situ measurements shoves the complexity of the plasmas pitch angle distributions at particularly the cold and warm plasmas, vary dramatically at different local times and radial distances from the Earth in response to changes in solar wind condition and Dst index. Using an MHD-ring current coupled code, we model the convection and interaction of cold, warm and energetic ions of plasmaspheric, plasmasheet, and ring current origins in the inner magnetosphere. We compare our simulation results with in-situ and remotely sensed measurements from recent instrumentation on Geotail, Cluster, THEMIS, and TWINS spacecraft.

Fok, Mei-Ching↗
NASA NTRS1984

New model of Saturn's ionosphere with an influx of water from the rings

A radically different model of Saturn's ionosphere is proposed in which water plays a major role as a minor constituent present by downward diffusion from an external source. The model ionosphere is a classical F2 type layer resulting from the photodissociative production of H(+) from H2 and rapid chemical loss by a series of charge exchange reactions with water. A planet-wide influx of about 4 x 10 to the 7th molecules/sq cm/s of water from the rings is consistent with the observed ionospheric electron densities. An enhanced influx of water occurs at latitudes (-38, +44 deg) connected magnetically at the inner edge of Saturn's B ring, where an electromagnetic erosion process takes place. The present-day influx at these latitudes may be as large as 2 x 10 to the 9th molecules/sq cm/s.

Connerney, J. E. P.↗
NASA NTRS2002

Wave-Particle Interactions Associated with Nongyrotropic Distribution Functions: A Hybrid Simulation Study

Nongyrotropic plasma distribution functions can be formed in regions of space where guiding center motion breaks down as a result of strongly curved and weak ambient magnetic fields. Such are the conditions near the current sheet in the Earth's middle and distant magnetotail, where observations of nongyrotropic ion distributions have been made. Here a systematic parameter study of nongyrotropic proton distributions using electromagnetic hybrid simulations is made. We model the observed nongyrotropic distributions by removing a number of arc length segments from a cold ring distribution and find significant differences with the results of simulations that initially have a gyrotropic ring distribution. Model nongyrotropic distributions with initially small perpendicular thermalization produce growing fluctuations that diffuse the ions into a stable Maxwellian-like distribution within a few proton gyro periods. The growing waves produced by nongyrotropic distributions are similar to the electromagnetic proton cyclotron waves produced by a gyrotropic proton ring distribution in that they propagate parallel to the background magnetic field and occur at frequencies on the order of the proton gyrofrequency, The maximum energy of the fluctuating magnetic field increases as the initial proton distribution is made more nongyrotropic, that is, more highly bunched in perpendicular velocity space. This increase can be as much as twice the energy produced in the gyrotropic case.

Convery, P. D.↗
NASA NTRS1982

A model containing blobs for the A ring of Saturn

Observations of the brightness distribution of Saturn's ring A from four separate years, including azimuthal variations, are calibrated and interpreted through the use of a scattering model. In this model the A ring is composed of elongated density enhancements (blobs) embedded in a ring of uniform density. The blobs are approximated by biaxial ellipsoids. Intensity observations at one tilt angle can be used in the model to predict the intensity at other tilt angles. Analysis shows that there must be matter between blobs characterized by an optical thickness = 0.45 and single scattering albedo = 0.70. A rough estimate, based on what appears to be shadowing of one blob by another, gives the ring area occupied by blobs as 5%.

Thompson, W.↗
NASA NTRS1984

Saturn's rings - Voyager 1 radio occultation experiment results

The Voyager 1 encounter with Saturn in 1980 included the first radio occultation measurements of the ring system. Opacity of and scattering by Saturn's rings were measured at 3.6- and 13-cm wavelengths using transmissions from the spacecraft to the earth. Detailed models of ring structure have been derived from these data at radial resolutions as small as 1 km, depending on the radio opacity of the region probed. Inverse power-law-type distributions of ring particle sizes with indices of about 3 and upper size cutoffs near 5 m are in good agreement with the data, but parameters vary throughout the rings. Occultation measurements have also provided information on ring dynamics and have led to a more precise definition of Saturn's pole vector.

Simpson, R. A.↗
NASA NTRS1989

Spiral waves in Saturn's rings

Spiral density waves and spiral bending waves have been observed at dozens of locations within Saturn's rings. These waves are excited by resonant gravitational perturbations from moons orbiting outside the ring system. Modeling of spiral waves yields the best available estimates for the mass and the thickness of Saturn's ring system. Angular momentum transport due to spiral density waves may cause significant orbital evolution of Saturn's rings and inner moons. Similar angular momentum transfer may occur in other astrophysical systems such as protoplanetary disks, binary star systems with disks and spiral galaxies with satellites.

Lissauer, Jack J.↗
NASA NTRS1991

Assessment of microburst models for downdraft estimation

The effectiveness of three microburst models in estimating the downdraft from horizontal velocity measurements is assessed. The development of the models and their characteristics are discussed. The simplest model, a linear one, works well for altitudes below 200 m and near the microburst core. A ring-vortex model and an empirical model give the best overall results. The former is the most complex, requiring four variables to define it. The empirical model uses three variables.

Vicroy, Dan D.↗
NASA NTRS1996

Ring Current Development During Storm Main Phase

The development of the ring current ions in the inner magnetosphere during the main phase of a magnetic storm is studied. The temporal and spatial evolution of the ion phase space densities in a dipole field are calculated using a three dimensional ring current model, considering charge exchange and Coulomb losses along drift paths. The simulation starts with a quiet time distribution. The model is tested by comparing calculated ion fluxes with Active Magnetospheric Particle Tracer Explorers/CCE measurement during the storm main phase on May 2, 1986. Most of the calculated omnidirectional fluxes are in good agreement with the data except on the dayside inner edge (L less than 2.5) of the ring current, where the ion fluxes are underestimated. The model also reproduces the measured pitch angle distributions of ions with energies below 10 keV. At higher energy, an additional diffusion in pitch angle is necessary in order to fit the data. The role of the induced electric field on the ring current dynamics is also examined by simulating a series of substorm activities represented by stretching and collapsing the magnetic field lines. In response to the impulsively changing fields, the calculated ion energy content fluctuates about a mean value that grows steadily with the enhanced quiescent field.

Fok, Mei-Ching↗
NASA NTRS2010

On the Role of Global Magnetic Field Configuration in Affecting Ring Current Dynamics

Plasma and field interaction is one important aspect of inner magnetospheric physics. The magnetic field controls particle motion through gradient, curvature drifts and E cross B drift. In this presentation, we show how the global magnetic field affects dynamics of the ring current through simulations of two moderate geomagnetic storms (20 November 2007 and 8-9 March 2008). Preliminary results of coupling the Comprehensive Ring Current Model (CRCM) with a three-dimensional plasma force balance code (to achieve self-consistency in both E and B fields) indicate that inclusion of self-consistency in B tends to mitigate the intensification of the ring current as other similar coupling efforts have shown. In our approach, self-consistency in the electric field is already an existing capability of the CRCM. The magnetic self-consistency is achieved by computing the three-dimensional magnetic field in force balance with anisotropic ring current ion distributions. We discuss the coupling methodology and its further improvement. In addition, comparative studies by using various magnetic field models will be shown. Simulation results will be put into a global context by analyzing the morphology of the ring current, its anisotropy and characteristics ofthe interconnected region 2 field-aligned currents.

Zheng, Y.↗
NASA NTRS1993

Ion transport and loss in the earth's quiet ring current. I - Data and standard model

A study of the transport and loss of ions in the earth's quiet time ring current, in which the standard radial diffusion model developed for the high-energy radiation belt particles is compared with the measurements of the lower-energy ring current ions, is presented. The data set provides ionic composition information in an energy range that includes the bulk of the ring current energy density, 1-300 keV/e. Protons are found to dominate the quiet time energy density at all altitudes, peaking near L of about 4 at 60 keV/cu cm, with much smaller contributions from O(+) (1-10 percent), He(+) (1-5 percent), and He(2+) (less than 1 percent). A minimization procedure is used to fit the amplitudes of the standard electric radial diffusion coefficient, yielding 5.8 x 10 exp -11 R(E-squared)/s. Fluctuation ionospheric electric fields are suggested as the source of the additional diffusion detected.

Sheldon, R. B.↗