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NASA NTRS1984

The rings of Uranus

The nine narrow rings of Uranus have been observed at 3.5-km spatial resolution during 13 stellar occultations from 1977 to 1983. Sharp edges are characteristic of the ring structures, which have optical depths of 0.5 to more than 1.5. A Keplerian model, which includes five orbital elements for each ring, has been successfully fitted to the occultation data. Interior models of Uranus can be constrained by the gravitational harmonic coefficients yielded by the values of the orbit solution. IR imaging observations confirm the derived recession rate for the epsilon-Ring, and place limits on the optical depth of interring material. Ice-covered particles are ruled out by the geometric albedo of the ring spectra.

Elliot, J. L.↗

NASA NTRS1972

Electrostatic instability of ring current protons beyond the plasmapause during injection events

The stability of ring current protons with an injection spectrum modeled by an m = 2 mirror distribution function was examined for typical ring current parameters. It was found that the high frequency loss cone mode can be excited at wave numbers K lambda sub Di about = to 0.1 to 0.5, at frequencies omega about = to (0.2 to 0.6) omega sub pi and with growth rates up to gamma/omega about = to 0.03. These waves interact with the main body of the proton distribution and propagate nearly perpendicular to the local magnetic field. Cold particle partial densities tend to reduce the growth rate so that the waves are quenched at or near to the plasmapause boundary. Wave e-folding lengths are comparable to 0.1 R sub e, compared to the value of about 4 R sub e found for ion cyclotron waves at the same plasma conditions.

Coroniti, F. V.↗

NASA NTRS2011

Modeling the Superstorm in November 2003

The superstorm on 20.21 November 2003 was the largest geomagnetic storm in solar cycle 23 as measured by Dst, which attained a minimum value of .422 nT. We have simulated this storm to understand how particles originating from the solar wind and ionosphere get access to the magnetosphere and how the subsequent transport and energization processes contribute to the buildup of the ring current. The global electromagnetic configuration and the solar wind H+ distribution are specified by the Lyon-Fedder-Mobarry (LFM) magnetohydrodynamics model. The outflow of H+ and O+ ions from the ionosphere are also considered. Their trajectories in the magnetosphere are followed by a test-particle code. The particle distributions at the inner plasma sheet established by the LFM model and test-particle calculations are then used as boundary conditions for a ring current model. Our simulations reproduce the rapid decrease of Dst during the storm main phase and the fast initial phase of recovery. Shielding in the inner magnetosphere is established at early main phase. This shielding field lasts several hours and then breaks down at late main phase. At the peak of the storm, strong penetration of ions earthward to L shell of 1.5 is revealed in the simulation. It is surprising that O+ is significant but not the dominant species in the ring current in our calculation for this major storm. It is very likely that substorm effects are not well represented in the models and O+ energization is underestimated. Ring current simulation with O+ energy density at the boundary set comparable to Geotail observations produces excellent agreement with the observed symH. As expected in superstorms, ring current O+ is the dominant species over H+ during the main to mid-recovery phase of the storm.

Fok, Mei-Ching↗

NASA NTRS2016

Preparation for Cassini Grand Finale - Validation of Saturn Ring Heating in TSS

Thermal analysis of spacecraft at Saturn have previously been restricted to three environmental heat fluxes: solar, Saturn planet IR, and Saturn planet albedo. Heat flux from Saturn’s rings were not available in any commercially available thermal analysis tools. While preparing for the Cassini Grand Finale, during which Cassini will travel extremely close to the rings of Saturn, it was determined the existing thermal model required updating to accurately predict heat fluxes from the rings. This is particularly important for Cassini’s Visual and Infrared Mapping Spectrometer infrared spectrometer (VIMS-IR), as it is cooled by a passive radiator which operates below the temperature of Saturn’s rings. This paper discusses the validation of the heat flux from solar, Saturn, and Saturn’s rings in Spacedesign’s Thermal Synthesizer System (TSS). The incorporation of heating from Saturn’s rings also led to the use of SPICE geometry to ensure accurate spacecraft positioning and pointing. Thermal model results are compared against historical heating events as well as predictive Grand Finale sequences.

Swanson, Kirsten S.↗

NASA NTRS1987

Collisional dynamics of perturbed particle disks in the solar system

Investigations of the collisional evolution of particulate disks subject to the gravitational perturbation of a more massive particle orbiting within the disk are underway. Both numerical N-body simulations using a novel collision algorithm and analytical kinetic theory are being employed to extend our understanding of perturbed disks in planetary rings and during the formation of the solar system. Particular problems proposed for investigation are: (1) The development and testing of general criteria for a small moonlet to clear a gap and produce observable morphological features in planetary rings; (2) The development of detailed models of collisional damping of the wavy edges observed on the Encke division of Saturn's A ring; and (3) The determination of the extent of runaway growth of the few largest planetesimals during the early stages of planetary accretion.

Roberts, W. W.↗

NASA NTRS1992

A photometric study of Saturn's F Ring

The Saturn F Ring's radially integrated brightness has been measured over a wide range of phase angles from Voyager images; in order to model the resultant phase curve, the ring population has been divided into a dust regime and one of larger bodies, and while single scattering properties of small particles are modeled by semiempirical nonspherical/randomly oriented particles, those of large bodies are based on the photometric behavior of satellites. It is suggested that the dust in the envelope arises from micrometeoroid impacts into the large core particles, and then migrates inward.

Showalter, Mark R.↗

NASA NTRS2010

Dst Index in the 2008 GEM Modeling Challenge - Model Performance for Moderate and Strong Magnetic Storms

The GEM 2008 modeling challenge efforts are expanding beyond comparing in-situ measurements in the magnetosphere and ionosphere to include the computation of indices to be compared. The Dst index measures the largest deviations of the horizontal magnetic field at 4 equatorial magnetometers from the quiet-time background field and is commonly used to track the strength of the magnetic disturbance of the magnetosphere during storms. Models can calculate a proxy Dst index in various ways, including using the Dessler-Parker Sckopke relation and the energy of the ring current and Biot-Savart integration of electric currents in the magnetosphere. The GEM modeling challenge investigates 4 space weather events and we compare models available at CCMC against each other and the observed values of Ost. Models used include SWMF/BATSRUS, OpenGGCM, LFM, GUMICS (3D magnetosphere MHD models), Fok-RC, CRCM, RAM-SCB (kinetic drift models of the ring current), WINDMI (magnetosphere-ionosphere electric circuit model), and predictions based on an impulse response function (IRF) model and analytic coupling functions with inputs of solar wind data. In addition to the analysis of model-observation comparisons we look at the way Dst is computed in global magnetosphere models. The default value of Dst computed by the SWMF model is for Bz the Earth's center. In addition to this, we present results obtained at different locations on the Earth's surface. We choose equatorial locations at local noon, dusk (18:00 hours), midnight and dawn (6:00 hours). The different virtual observatory locations reveal the variation around the earth-centered Dst value resulting from the distribution of electric currents in the magnetosphere during different phases of a storm.

Rastaetter, Lutz↗

NASA NTRS1977

Far-infrared spectral observations of Saturn and its rings

The spectrum of Saturn and its rings between 45 and 115 microns has been measured at an average resolving power of 14 from the NASA Lear Jet. The combined brightness temperature of the rings and planetary disk decreases beyond 65 microns, in disagreement with previous results. A brightness temperature of 65 + or - 10 K is obtained for the planetary disk in the 80-110-micron wavelength range if a large-particle, constant-emissivity model is assumed for the rings. The possible effects of small particles in the rings are briefly considered.

Ward, D. B.↗

NASA NTRS2016

Do We Know the Actual Magnetopause Position for Typical Solar Wind Conditions?

We compare predicted magnetopause positions at the subsolar point and four reference points in the terminator plane obtained from several empirical and numerical MHD (magnetohydrodynamics) models. Empirical models using various sets of magnetopause crossings and making different assumptions about the magnetopause shape predict significantly different magnetopause positions (with a scatter greater than 1 Earth radius (R (sub E)) even at the subsolar point. Axisymmetric magnetopause models cannot reproduce the cusp indentations or the changes related to the dipole tilt effect, and most of them predict the magnetopause closer to the Earth than non axisymmetric models for typical solar wind conditions and zero tilt angle. Predictions of two global non axisymmetric models do not match each other, and the models need additional verification. MHD models often predict the magnetopause closer to the Earth than the non axisymmetric empirical models, but the predictions of MHD simulations may need corrections for the ring current effect and decreases of the solar wind pressure that occur in the foreshock. Comparing MHD models in which the ring current magnetic field is taken into account with the empirical Lin et al. model, we find that the differences in the reference point positions predicted by these models are relatively small for B (sub z) equals 0 (note: B (sub z) is when the Earth's magnetic field points north versus Sun's magnetic field pointing south). Therefore, we assume that these predictions indicate the actual magnetopause position, but future investigations are still needed.

Samsonov, A. A.↗

NASA NTRS1983

Origin and evolution of the Saturn system

A review is provided of current concepts concerning the formation of the Saturn system and the subsequent history of the planet, its satellites, and rings. Emphasis is placed upon numerical models of Saturn's evolution and interior models of its satellites. Alternative theories are presented and assessed for the origins of the Saturn system, the rings of Saturn, and the atmosphere of Titan.

Pollack, J. B.↗

NASA NTRS2002

Planetary Rings: An Update on Structure and Properties

Over the last several years there has been steady progress in several areas related to the structure and composition of planetary rings. Much of the progress was related to Saturn's rings: new observations by Hubble Space Telescope (HST) and NASA's Infrared Telescope Facility (IRTF), theoretical modeling of the vertical structure of, and particle properties in the rings, discovery of intriguing structural/spectral variations on small spatial scales, new observations and analysis of thermal emission from the rings, theoretical and observational constraints on the quadrupole brightness asymmetry at visual and microwave wavelengths, and new studies of "clumps" in and near the F ring. In addition, new observations were obtained of Jupiter's rings by Cassini, and new models were put forth to explain the narrow edges and apse alignment of the Uranian rings (and presumably other narrow eccentric rings).

Cuzzi, Jeff↗

NASA NTRS2014

Saturn's Rings

The rings are changing before our eyes; structure varies on all timescales and unexpected things have been discovered. Many questions have been answered, but some answers remain elusive. Here we highlight the major ring science progress over the mission to date, and describe new observations planned for Cassini’s final three years. Ring Composition and particle sizes: The rings are nearly all water ice with no other ices – so why are they reddish? The C Ring and Cassini Division are “dirtier” than the more massive B and A Rings, as shown by near- IR and, recently, microwave observations. Particle sizes, from stellar and radio occultation's, vary from place to place. Ring structure, micro and macro: numerous spiral density waves and ubiquitous “self-gravity wakes” reveal processes which fostered planet formation in the solar system and elsewhere. However, big puzzles remain regarding the main ring divisions, the C Ring plateau structures, and the B Ring irregular structure. Moonlets, inside and out, seen and unseen: Two gaps contain sizeable moonlets, but more gaps seem to contain none; even smaller embedded “propeller” objects wander, systematically or randomly, through the A ring. Rubble pile ring moons just outside the rings may escaped from the rings, and the recently discovered “Peggy” may be trying this as we watch. Impact bombardment of the rings: Comet fragments set the rings to rippling on century-timescales, and boulders crash through hourly; meanwhile, the constant hail of in falling Kuiper belt material has a lower mass flux than previously thought. Origin and Age of the Rings: The ring mass and bombardment play key roles. The ring mass is well known everywhere but in the B Ring (where most of it is). New models suggest how tidal breakup of evolving moons may have formed massive ancient rings, of which the current ring is just a shadow. During its last three years, the Cassini tour profile will allow entirely new observations: direct measurement of the still-unknown ring mass; direct in-situ sampling of ring particle composition (targeting the iron- or carbon based red non-icy component); and radar backscattering observations.

Cassini↗

NASA NTRS1982

Two-component proton spectra in the inner Saturnian magnetosphere

Measurements obtained by the Pioneer-11 spacecraft in Saturn's inner magnetosphere revealed the presence of a high energy proton component, primarily confined within the orbits of satellites Enceladus and Mimas. The high energy component was interpreted as due to protons from cosmic ray neutron albedo decay interactions with the Saturnian rings and, secondarily, the planetary atmosphere, but without detailed knowledge of the differential or integral spectrum. The main objective of the present investigation is to examine in some detail the ion spectra in the considered region of the magnetosphere, and to provide some of the basic input necessary in evaluating various models of energetic particle-ring interactions. The conducted measurements clearly show the presence of low energy ions and electrons inside the orbit of Mimas and provide a measure of the evolution of the hot ion spectrum observed inside the orbit of Rhea into a two-component power law spectrum inside the orbits of Enceladus and Mimas.

Krimigis, S. M.↗

NASA NTRS2008

How Ionospheric Ions Populate the Magnetosphere during a Magnetic Storm

Ionospheric oxygen ions have been observed throughout the magnetosphere, from the plasma sheet to the ring current region. I t has been found that the O+ /H+ density ratio in the magnetosphere increases with geomagnetic activity and varies with storm phases. During the magnetic storm in late September to earIy October 2002, Cluster was orbiting in the plasma sheet and ring current regions. At prestorm time, Cluster observed high H+ density and low O+ density in the plasma sheet and lobes. During the storm main phase, 0+ density has increased by 10 times over the pre-storm level. Strong field-aligned beams of O+ were observed in the lobes. O+ fluxes were significantly reduced in the central plasma sheet during the storm recovery. However, 0+ was still evident on the boundaries of the plasma sheet and in the lobes. In order to interpret the Cluster observations and to understand how O+ ions populate the magnetosphere during a magnetic storm, we model the storm in early October 2002 using our global ion kinetic simulation (GIK). We use the LFN global simulation model to produce electric and magnetic fields in the outer magnetosphere, the Strangeway outflow scaling with Delcourt ion trajectories to include ionospheric outflows, and the Fok inner magnetospheric model for the plasmaspheric and ring current response to all particle populations. We find that the observed composition features are qualitatively reproduced by the simulations, with some quantitative differences that point to future improvements in the models.

Fok, Mei-Ching↗

NASA NTRS1980

Saturn's rings - Particle composition and size distribution as constrained by observations at microwave wavelengths. II - Radio interferometric observations

Theoretical models are presented of the brightness of Saturn's rings at microwave wavelengths (0.34-21.0 cm) including both intrinsic ring emission and diffuse scattering by the rings of the planetary emission. In addition, several previously existing sets of interferometric observations of the Saturn system at 0.83, 3.71, 6.0, 11.1, and 21.0 cm wavelengths are analyzed. A comparison of models and experimental data make it possible to establish improved constraints on the properties of the rings. In particular, it is found that (1) the maximum optical depths in the rings is 1.5 + or - 0.3 referred to visible wavelengths; (2) a significant decrease in ring optical depths from 3.7 to 21.0 cm makes it possible to rule out the possibility that more than 30% of the cross section of the rings is composed of particles larger than about a meter; and (3) the ring particles cannot be primarily of silicate composition (independently of particle size), and the particles cannot be primarily smaller than about 0.1 cm, independently of composition.

Cuzzi, J. N.↗

NASA NTRS2006

Non-Grey Radiation Modeling using Thermal Desktop/Sindaworks TFAWS06-1009

This paper provides an overview of the non-grey radiation modeling capabilities of Cullimore and Ring's Thermal Desktop(Registered TradeMark) Version 4.8 SindaWorks software. The non-grey radiation analysis theory implemented by Sindaworks and the methodology used by the software are outlined. Representative results from a parametric trade study of a radiation shield comprised of a series of v-grooved shaped deployable panels is used to illustrate the capabilities of the SindaWorks non-grey radiation thermal analysis software using emissivities with temperature and wavelength dependency modeled via a Hagen-Rubens relationship.

radiation modeling↗

NASA NTRS1998

Design-Oriented Analysis of Aircraft Fuselage Structures

A design-oriented analysis capability for aircraft fuselage structures that utilizes equivalent plate methodology is described. This new capability is implemented as an addition to the existing wing analysis procedure in the Equivalent Laminated Plate Solution (ELAPS) computer code. The wing and fuselage analyses are combined to model entire airframes. The paper focuses on the fuselage model definition, the associated analytical formulation and the approach used to couple the wing and fuselage analyses. The modeling approach used to minimize the amount of preparation of input data by the user and to facilitate the making of design changes is described. The fuselage analysis is based on ring and shell equations but the procedure is formulated to be analogous to that used for plates in order to take advantage of the existing code in ELAPS. Connector springs are used to couple the wing and fuselage models. Typical fuselage analysis results are presented for two analytical models. Results for a ring-stiffened cylinder model are compared with results from conventional finite-element analyses to assess the accuracy of this new analysis capability. The connection of plate and ring segments is demonstrated using a second model that is representative of the wing structure for a channel-wing aircraft configuration.

Giles, Gary L.↗

NASA NTRS1976

Vortex simulation of the pressure field of a jet

Fluctuations of the pressure field of a jet are simulated numerically by a flow model consisting of axisymmetric vortex rings with viscous cores submerged in an inviscid uniform stream. Vortex shedding time intervals, randomly created to imitate the time-history characteristics of the pressure signals of a jet, are generated based on a probability distribution of the intervals between successive pressure peaks obtained from experiments. It is found that, up to five diameters downstream of the jet exit, the characteristics of the pressure fluctuations and the most probable time intervals between experimental and numerical results show good qualitative agreements. The role played by the axisymmetric vortex model in pressure field as well as extensions of the model is also discussed.

Fung, Y. T.↗