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NASA NTRS1985

A synchronous fiber optic ring local area network for multigigabit/s mixed-traffic communication

A synchronous-ring fiber optic local area network is reported that facilitates the simultaneous transmission of packet and real-time traffic at gigabit/s rates, minimizes the amount of high-speed logic, and simplifies the user interface to the network. The novelty of the technique is based on (1) suspending in transit around the ring's circumference an integral number of data frames and (2) achieving this condition by skewing the frame clock rate a small amount. Rather than use the whole data frame as one packet destined to a specific user, many individual channels are instead time-multiplexed into the data frame. This technique only becomes feasible for local networks as data rates approach the Gbit/s range. This departure from other synchronous rings results in several advantages both in terms of system performance and hardware simplicity.

Bergman, L. A.↗

NASA NTRS1986

Photometry from Voyager 2 - Initial results from the Uranian atmosphere, satellites, and rings

The results of Voyager 2 photopolarimetry (PPS) surveys of Uranus, the ring system, occultation experiments and observations of the Uranian moons are reported. Dual-channel photometry and polarimetry data obtained of the atmosphere at various emission and phase angles are delineated and compared with characteristics of the Saturn and Jupiter atmospheres. The results of temperature, UV absorption and density profile calculations are also discussed. Extensive ring dimensional data, based on two occultation experiments, are provided in tabular form noting that the rings contain no dust. Finally, the geometric albedos of the five major moons and the phase curve of the moon Titania are presented. The latter data indicate that Titanian surface features are not the result of recent events such as volcanism or ice slurry outflows.

Lane, A. L.↗

NASA NTRS1986

Towards a theory for Neptune's arc rings

It is proposed that the incomplete rings of Neptune consist of a number of short arcs centered on the corotation resonances of a single satellite. The satellite must have a radius of the order of 100 km or more and move on an inclined orbit. Corotation resonances are located at potential maxima. Thus, mechanical energy dissipated by interparticle collisions must be continually replenished to prevent the arcs from spreading. It is shown that each corotation resonance is associated with a nearby Lindblad resonance, which excites the ring particles' orbital eccentricity, thus supplying the energy required to maintain the arc. The ultimate energy reservoir is the satellite's orbital energy. Therefore, interaction with the arcs damps the satellite's orbital inclination. The self-gravity of the arcs limits their contraction and enforces a relation between arc length and mass. The estimated arc masses are so small, of the order of 10 to the 16th g, that the satellite's orbital inclination suffers negligible decay over the age of the solar system. The inferred surface mass densities are comparable to those found in the major rings of Saturn and Uranus.

Goldreich, P.↗

NASA NTRS1986

Experimental evaluation of the ring focus test for X-ray telescopes using AXAF's technology mirror assembly, MSFC CDDF Project No. H20

A test method particularly suited for X-ray telescopes was evaluated experimentally. The method makes use of a focused ring formed by an annular aperture when using a point source at a finite distance. This would supplement measurements of the best focus image which is blurred when the test source is at a finite distance. The telescope used was the Technology Mirror Assembly of the Advanced X-ray Astrophysis Facility (AXAF) program. Observed ring image defects could be related to the azimuthal location of their sources in the telescope even though in this case the predicted sharp ring was obscured by scattering, finite source size, and residual figure errors.

Zissa, D. E.↗

NASA NTRS1986

The ionization structure of planetary nebulae. Part 7: New observations in the ring nebula

New optical spectrophotometric observations of emission-line intensities have been made in eight positions in the Ring Nebula corresponding to those observed previously with the Ultraviolet Explorer satellite; the total coverage is therefore 1400 to 7200 A. The intensities are in generally good agreement with those found previously in corresponding positions. The 0(++) and Balmer continuum electron temperatures agree well on the average and, like the N(+) electron temperatures, decrease with increasing distance from the central star. As found previously for the Ring Nebula and for other planetaries in this series, the lambda 4267 C 2 line intensity near the central star implies a C(++) abundance that is higher than that determined from the lambda 1906, 1909 C 3 lines. The discrepancy again decreases with increasing distance from the central star and vanishes from the outermost positions, again suggesting that the excitation mechanism from the lambda 4267 line is not understood. Standard equations used to correct for the existence of elements in other than the optically observable ionization stages give results that are consistent and in appropriate agreement with abundances calculated using UV lines. The rather high abundances of O, N, and C, and, to some extent N, indicate that some mixing of CNO processed material into the nebular shell may have occurred in the Ring Nebula.

Barker, Timothy↗

NASA NTRS1987

Investigations of planetary ring phenomena

Faint planetary rings, their dynamical behavior and physical properties, were the main focus of the research efforts. The motion of weakly-charged dust through the gravitational and magnetic fields of Jupiter were examined. Several topics concerning features of Saturn's rings were addressed. The origin and fate of the Uranian ring dust is presently being studied.

Burns, Joseph A.↗

NASA NTRS1986

Magnetic connection for Saturn's rings and atmosphere

Latitudinal variations in images of Saturn's disk, upper atmospheric temperatures, and ionospheric electron densities are found in magnetic conjugacy with features in Saturn's ring plane. It is proposed that these latitudinal variations are the result of a variable influx of water transported along magnetic field lines from sources in Saturn's ring plane. These features are thus the surface expression of an electromagnetic erosion mechanism which transports water (in the form of high charge-to-mass ratio particles) from the rings to the atmosphere.

Connerney, J. E. P.↗

NASA NTRS1986

Detection of two satellites in the Cassini division of Saturn's rings

The presence of two about 10-km-radius satellites within a 100-km-wide gap in the Cassini division of Saturn's rings is inferred from the observation of perturbations of nearby ring material. The perturbations are detected as regular fluctuations in the optical depth profile measured during Voyager radio occultation by the rings and are used to estimate the satellite's orbits and masses. The two satellites appear to 'shepherd' a 30-km-wide ringlet within the gap.

Marouf, E. A.↗

NASA NTRS1986

Performance characteristics of ring-cusp thrusters with xenon propellant

The performance characteristics and operating envelope of several 30-cm ring-cusp ion thrusters with xenon propellant were investigated. Results indicate a strong performance dependence on the discharge chamber boundary magnetic fields and resultant distribution of electron currents. Significant improvements in discharge performance over J-series divergent-field thrusters were achieved for large throttling ranges, which translate into reduced cathode emission currents and reduced power dissipation which should be of significant benefit for operation at thruster power levels in excess of 10 kW. Mass spectrometer of the ion beam was documented for both the ring-cusp and J-series thrusters with xenon propellant for determination of overall thruster efficiency, and lifetime. Based on the lower centerline values of doubly charged ions in the ion beam and the lower operating discharge voltage, the screen grid erosion rate of the ring-cusp thruster is expected to be lower than the divergent-field J-series thruster by a factor of 2.

Patterson, M. J.↗

NASA NTRS1987

Observations of the 22 April 1982 stellar occultation by Uranus and the rings

The 1.5 m telescope of Teide Observatory at Teneriffe was used to observe an occultation of the stellar object KME 14 by the Uranus ring system. The telescope was equipped with a two-channel IR flux collector, with one passband centered at 4490 A and the other near 8800 A. Data processing procedures that were used are detailed. The ring profiles were analyzed with a square-well model, which accounted for diffraction and finite source size while treating each ring as sharp-edged and uniformly translucent. An emersion lightcurve was obtained and inverted to yield a temperature profile of the upper atmosphere of Uranus, which displayed a temperature maximum (at 8 microbar) near the equator.

Millis, Robert L.↗

NASA NTRS1987

Features in Saturn's rings

A systematic, uniform search of Voyage 2 photopolarimeter system (PSS) data set for all significant features of Saturn's rings is described. On August 25, 1981, the PSS observed the occultation of the star Delta Scorpii by the rings of Saturn, and the timing of the data taking was rapid enough that the spatial resolution in the radial direction in the ring plane was better than 100 m. Tabular information and figures for 216 significant features that were found are presented.

Esposito, Larry W.↗

NASA NTRS1987

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

The present Voyagers I and II measurements of Saturn A ring azimuthal brightness variations in reflected light are noted to be in general agreement with earth-based measurements. Voyager images of the rings in light transmitted through them also indicate the presence of these brightness variations, but with greater amplitude and an about 65-deg phase discrepancy with those seen in reflection. These differences in photometric behavior are qualitatively accounted for in terms of the widespread presence of particle wakes in the A ring.

Franklin, F. A.↗

NASA NTRS1987

Interaction of a decaying vortex ring with a rotational background flow bounded by a solid wall

The interaction between a vortex ring of finite strength and an axisymmetric rotational background flow bounded by a solid wall is studied by a singular perturbation method. The analysis is carried out by combining a composite solution of a vortex ring and an unsteady Navier-Stokes solution for the background rotational flow. Using the method of averaging, numerical scheme is developed to obtain a Navier-Stokes solution in which the grid and time-step sizes depend solely on the length and velocity scales of the background flow. Numerical results are presented to illustrate the separation of the boundary layer on a solid wall and its interaction with the vortex ring.

Ishii, K.↗

NASA NTRS1987

Proof that timing requirements of the FDDI token ring protocol are satisfied

The fiber distributed data interface (FDDI) is an ANSI draft proposed standard for a 100 Mbit/s fiber-optic token ring. The FDDI timed token access protocol provides dynamic adjustment of the load offered to the ring, with the goal of maintaining a specified token rotation time and of providing a guaranteed upper bound on time between successive arrivals of the token at a station. FDDI also provides automatic recovery when errors occur. The bound on time between successive token arrivals is guaranteed only if the token rotates quickly enough to satisfy timer requirements in each station when all ring resources are functioning properly. Otherwise, recovery would be initiated unnecessarily. The purpose of this paper is to prove that FDDI timing requirements are satisfied, i.e., the token rotates quickly enough to prevent initiation of recovery unless there is failure of a physical resource or unless the network management entity within a station initiates the recovery process.

Johnson, Marjory J.↗

NASA NTRS1987

Energetic neutral atom image of a storm-time ring current

Energetic neutral atom (ENA) images of a storm-time ring current produced from direct measurements and model simulations are presented and analyzed. An ENA image of the storm-time ring current was obtained with the Medium Energy Particles Instrument (MEPI) mounted on ISEE 1 on September 29, 1978, and a zero-order model of the ring current ion was employed to simulate the ENA image. The simulated instrument response is compared with actual ENA measurements. It is observed that the spatial distribution of singly charged ions detected from the ENA images displays equatorial midnight/noon asymmetry in differential ion intensity with a midnight/noon ratio of greater than or equal to 20:1; peak ion fluxs, H(+) and O(+), in the midnight sector are greater than 10 to the 6th/(sq cm s sr keV); and ENA fluxes from the brighter areas of the image exceed greater than 1000/(sq cm s sr keV). It is noted that the ENA image from MEPI data correlates with the morphology of the simulated response of MEPI calculated from the model.

Roelof, Edmond C.↗

NASA NTRS1987

Studies of storm-time ring current from the AMPTE/CCE MEPA measurements

The evolution of the ring current for the September 4-7, 1984 storm is studied with data from the Medium Energy Particle Analyzer on the AMPTE/CCE spacecraft. From an examination of particle pressures and current densities, it is found that the leading and trailing edges of particle injections are associated with depressions and enhancements of westward current densities, respectively. Based on the ion composition measurements which cover the bulk of the ring current population, the contribution of the O(+) ions to the westward ring current density is estimated to be no more than about 25 percent for this magnetic storm.

Lui, A. T. Y.↗

NASA NTRS1987

The ionization structure of planetary nebulae. VII. New observations of the Ring Nebula

New optical spectrophotometric observations of emission-line intensities have been made in eight positions in the Ring Nebula corresponding to those observed previously with the Ultraviolet Explorer satellite; the total coverage is therefore 1400 to 7200 A. The intensities are in generally good agreement with those found previously in corresponding positions. The Oand Balmer continuum electron temperatures agree well on the average and, like the Nelectron temperatures, decrease with increasing distance from the central star. As found previously for the Ring Nebula and for other planetaries in this series, the lambda 4267 C 2 line intensity near the central star implies a Cabundance that is higher than that determined from the lambda 1906, 1909 C 3 lines. The discrepancy again decreases with increasing distance from the central star and vanishes from the outermost positions, again suggesting that the excitation mechanism from the lambda 4267 line is not understood. Standard equations used to correct for the existence of elements in other than the optically observable ionization stages give results that are consistent and in appropriate agreement with abundances calculated using UV lines. The rather high abundances of O, N, and C, and, to some extent N, indicate that some mixing of CNO processed material into the nebular shell may have occurred in the Ring Nebula.

Barker, Timothy↗

NASA NTRS1988

Propagation of vortex rings and starting plumes in high and low g

The propagation of vortex rings and starting plumes during crystal growth in supercooled solutions was investigated in variable gravity environment created by acceleration-deceleration routine of a NASA's KC-135 flight. A specially designed crystal growth cell was used to study convection around crystals growing in supersaturated solutions of Na2SO4 and NaCl aboard the NASA KC-135. The results of vertical velocity measurements have shown that a continuously fed plume attains a higher velocity than the individual vortex ring. The results also indicated that the vortex ring decelerates as it propagates, and slows down much more rapidly than the starting plume, indicating a less efficient transport. It is suggested that inertial effects and buoyancy effects on vortex and plume propagation can be separated in the controlled environment of a Space Station borne centrifuge.

Hallett, J.↗