Engineering topics

Elliot, J. L.

Publications and source records attributed to Elliot, J. L..

At least 73 records · Page 4

NASA NTRS1982

The upper atmosphere of Uranus - A critical test of isotropic turbulence models

Observations of the August 15, 1980, Uranus occultation of KM 12, obtained from Cerro Tololo InterAmerican Observatory, European Southern Observatory, and Cerro Las Campanas Observatory, are used to compare the atmospheric structure at points separated by approximately 140 km along the planetary limb. The results reveal striking, but by no means perfect correlation of the light curves, ruling out isotropic turbulence as the cause of the light curve spikes. The atmosphere is strongly layered, and any acceptable turbulence model must accommodate the axial ratios of greater than about 60 which are observed. The mean temperature of the atmosphere is 150 plus or minus 15 K for the region near number density 10 to the 14th per cu cm. Derived temperature variations of vertical scale approximately 130 km and amplitude plus or minus 5 K are in agreement for all stations, and correlated spikes correspond to low-amplitude temperature variations with a vertical scale of several kilometers.

French, R. G.↗

NASA NTRS1982

Inclinations of the Uranian rings

It is theoretically shown that at least seven of the nine rings of Uranus have orbits inclined to the equatorial plane of the planet. To the existing model of the Uranian rings, free parameters for the inclination of the rings and for the angle between the line of nodes of the ring orbit and a common reference line in the equatorial plane were added. An available set of occultation timings and expressions for the precession and nodal regression were combined, and an excellent fit was obtained.

French, R. G.↗

NASA NTRS1982

Studies of planetary upper atmospheres through occultations

The structure, composition, dynamics and energy balance of planetary upper atmospheres through interpretation of steller occultation data from Uranus is discussed. The wave-optical problem of modelling strong scintillation for arbitrary turbulent atmospheres is studied, as well as influence of turbulence. It was concluded that quasi-global features of atmospheric structure are accurately determined by numerical inversion. Horizontally inhomogeneous structures are filtered out and have little effect on temperature profiles.

Elliot, J. L.↗

NASA NTRS1982

Rings of Uranus - A review of occultation results

The structure and orbits of the nine confirmed rings of Uranus are reviewed based on occultation observations from ring discovery in March 1977 through April 1981. Observations of stellar occultations by Uranus have revealed the rings to be characterized by narrow widths (up to 100 km) and sharp edges, with possible internal structure, and to have orbits fitting the coplanar precession model, and showing a decreasing eccentricity with semimajor axis. Occultation results have also allowed the determination of the J2 and J4 coefficients of the Uranian gravity potential, the coordinates of the ring plane pole, planetary equatorial radius and ellipticity and the rotation period. The sharp edges and narrow widths of the rings may be explained by small satellites orbiting near the rings, and the uniform precession of three of the rings is attributed to self-gravity. Future plans for Uranus ring observations include further occultation studies, the Voyager 2 fly-by, and Space Telescope data.

Elliot, J. L.↗

NASA NTRS1982

The rings of Uranus

Stellar occultation data on the Uranian rings are summarized. Spatial resolution is 0.1 km, for positions of ring segments, 4 km for structural details. The system consists of 9 narrow rings with average geometric albedo of 0.02. The data are well fitted by a kinematic model that describes the rings as ellipses, inclined to the equatorial plane of Uranus and precess due to harmonic terms in the Uranian potential fields. The main characteristics of the Uranian ring system that need a confirmed theoretical explanation are: narrow rings; sharp edges; uniform orbital precession; origin of the eccentricities and inclinations; the structure of the epsilon ring; the structure of the eta ring; and the origin of the ring system.

Elliot, J. L.↗

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 NTRS1981

No evidence of rings around Neptune

The results of two observations of stellar occultations of Neptune to determine if the planet has a ring system are reported. The sightings were made from Mt. Stromlo, Mauna Kea, and Cerro Tololo, noting that an equatorial ring would subtend only two arcsec of view. An upper accretion limit was defined to set the region around Neptune where rings, rather than satellites, could form. The intensities of the starlight from the two selected stars were recorded by photometers on magnetic tape during the occultation period. One of the stars did not occult, but passed through the entire region where a ring system might be present. No definitive evidence for rings was found, although an optical depth for a Neptunian ring was calculated at 0.07, with a width of more than 5 km and a radius of 31,400 km.

Elliot, J. L.↗

NASA NTRS1981

Orbits of nine Uranian rings

Observations of a stellar occultation by Uranus and its nine rings are presented and used to examine the structures and kinematics of the rings. The observations of the occultation of the K giant star KM 12 were obtained in the K band with the 4-m CTIO telescope at a signal-to-noise ratio higher than any previously obtained. Ring occultation profiles reveal the alpha ring to possibly have a double structure and less abrupt boundaries than the gamma ring, which exhibits diffraction fringes, while the eta ring is a broad ring with an unresolved narrow component at its inner edge. The present timing data, as well as previous occultation timings, are fit to a kinematic model in which all nine rings are treated as coplanar eclipses of zero inclination, precessing due to the zonal harmonics of the Uranian gravitational potential to obtain solutions for the ring orbits. Analysis of the residuals from the fitted orbits reveals that the proposed model is a good representation of ring kinematics. The reference system defined by the orbit solutions has also been used to obtain a value of 0.022 + or - 0.003 for the ellipticity of Uranus and a Uranian rotation period of 15.5 h.

Elliot, J. L.↗

NASA NTRS1981

The 20 March 1980 occultation by the Uranian rings

On March 20, 1980, observations of occultations by rings alpha, beta, gamma, delta, and epsilon were observed from Cerro Tololo and Sutherland. A model ring profile was fit to the data to obtain the midtimes of the occultations and the widths of the profiles. A 14-parameter kinematic model, including J sub 2, J sub 4, and some of the orbital elements for rings 4, alpha, beta, and epsilon as free parameters, was fit to these data and previous ring occultation timings. The results yield J sub 2 = (3.396 + or - 0.020) x 10 to the -3 and a 2 sigma upper limit, absolute value of J sub 4 less than 7 x 10 to the -5th. Uniform precession models for rings alpha, beta are established and the dispersion in the values of semimajor axes and eccentricities for rings alpha, beta, and epsilon have been obtained. If self-gravity is the cause of the uniform precession, the mass of the beta ring is estimated at approximately 4 x 10 to the 16th g; the mass of the alpha ring probably is also near this value.

Elliot, J. L.↗

NASA NTRS1981

Predicted occultations by Neptune - 1981-1984

Predictions are presented for eight occultations by and appulses to Neptune from 1981 through 1984. Of special interest are a near-central occultation of Hyd - 22 deg 58794 on June 15, 1983, and a July 22, 1984 occultation of SAO 186001.

Mink, D. J.↗

NASA NTRS1981

Predicted occultations by Uranus - 1981-1984

Predictions are presented for 11 occultations by and appulses to Uranus and its ring system for ten stars from 1981 through 1984. The brightest stars are occulted on April 26, 1981 (BD - 19 deg 4222) and on April 22, 1982 (Hyd - 20 deg 51699). The ring system occults the same star twice during March 1983 (Hyd - 21 deg 64352).

Klemola, A. R.↗

NASA NTRS1980

The radius and ellipticity of Uranus from its occultation of SAO 158687

From occultation timings obtained from the Kuiper Airborne Observatory and from Cape Town for Mar. 10, 1977 occultation of SAO 158687 by Uranus, the equatorial radius, Re, of the planet has been determined to be 26,228 + or - 30 km and its ellipticity epsilon = 1 - Rp/Re = 0.033 + or - 0.007. These values refer to the 1.0 x 10 to the 14th/cu cm number-density level, under the assumption that the upper atmosphere is composed of H2 and He with a mean molecular weight mu = 2.20. The dominant source of uncertainty is the position of the center of the ring system, which was used to define the center of Uranus in our analysis. A rotation rate of 12.8 + or - 1.7 hours for the planet is implied by our value for the ellipticity, under the assumption that Uranus is in hydrostatic equilibrium below the 1.0 x 10 to the 14th/cu cm number density level.

Elliot, J. L.↗

NASA NTRS1980

The upper atmosphere of Uranus - Mean temperature and temperature variations

The number-density, pressure, and temperature profiles of the Uranian atmosphere in the pressure interval from 0.3 to 30 dynes/sq cm are derived from observations of the occultation of SAO 158687 by Uranus on 1977 March 10, observations made from the Kuiper Airborne Observatory and the Cape Town station of the South African Astronomical Observatory. The mean temperature is found to be about 95 K, but peak-to-peak variations from 10 K to 20 K or more exist on a scale of 150 km or 3 scale heights. The existence of a thermal inversion is established, but the inversion is much weaker than the analogous inversion on Neptune. The mean temperature can be explained by solar heating in the 3.3 micron methane band with a methane mixing ratio of 4 x 10 to the -6th combined with the cooling effect of ethane with a mixing ratio of not greater than 4 x 10 to the -6th. The temperature variations are probably due to a photochemical process that has formed a Chapman layer.

Dunham, E.↗

NASA NTRS1979

Temperature structure of the Uranian upper atmosphere

The temperature structure of the upper atmosphere of Uranus at two locations on the planet was determined from observations of the occultation of the star SAO158687 by Uranus on 10 March 1977, carried out at the Kuiper Airborne Observatory. The temperature-pressure relationships obtained from the immersion and emersion data for 7280 A channel show peak-to-peak variations of 45 K for immersion and 35 K for emersion. The mean temperature for both immersion and emersion profiles is about 100 K, which shows that Uranus has a temperature inversion between 0.001 mbar and the 100 mbar level probed by IR measurements. Both profiles show wavelike temperature variations, which may be due to dynamical or photochemical processes.

Elliot, J. L.↗

NASA NTRS1979

Occultation of Epsilon Geminorum by Mars. III - Temperature structure of the Martian upper atmosphere

High-quality photoelectric observations of the April 8, 1976, occultation of Epsilon Gem by Mars are analyzed which were obtained at five different observatories. Temperature profiles are derived by numerical inversion of the star's light curves and found to be in mutual agreement. This is taken as evidence for the presence of strong atmospheric waves in the Martial upper atmosphere between number densities of 10 to the 13th and 10 to the 15th per cu cm. It is estimated that the vertical wavelength is about 20 km, the horizontal wavelength is greater than 600 km, and the peak-to-peak wave amplitude is approximately 40 K. The results are shown to be qualitatively consistent with Zurek's (1976) model of thermally driven tidal modes. No compelling evidence for turbulence effects is found in the occultation data.

French, R. G.↗

NASA NTRS1979

The diameter of Pallas from its occultation of SAO 85009

The May 29, 1978, occultation of SAO 85009 by Pallas was observed photoelectrically at seven widely spaced sites. The observations are well represented by an elliptical apparent limb profile having semimajor and semiminor axes of 279.5 + or - 2.9 and 262.7 + or - 4.5 km, respectively. Combining these results with published information on the light curve and rotational pole position, Pallas's mean diameter is found to be 538 + or - 12 km, which yields a mean density for Pallas of 2.8 + or - 0.5 g/cu cm and a visual geometric albedo of 0.103 + or - 0.005. The diameter of Pallas as determined from this occultation is significantly smaller than the values derived by radiometric, polarimetric, and double-image techniques.

Wasserman, L. H.↗

NASA NTRS1979

Stellar occultation studies of the solar system

The paper covers the principles, observational procedures, and results relating to occultations of stars by solar system bodies other than the moon. Physical processes involved in occultations are presented including (1) extinction by ring material, (2) differential refraction by a planetary atmosphere, (3) extinction by a planetary atmosphere, and (4) Fresnel diffraction by sharp edges. It is noted that from a sufficient number of immersion and emersion timings of a stellar occultation, the radius and ellipticity of the occulting body can be accurately determined. From an occultation by a planet having an atmosphere, temperature, pressure, and number density profiles can be obtained along with information about the composition of the atmosphere and the extinction.

Elliot, J. L.↗

NASA NTRS1979

Direct determination of asteroid diameters from occultation observations

Of the available earth-based techniques for determining asteroid diameters, observation of stellar occultations involving asteroids is clearly the most direct. The high degree of accuracy achievable by this method has already been demonstrated in the case of Pallas, whose mean diameter has been measured with a standard error of plus or minus 2%. In this paper the problems, results and prospects of the stellar occultation technique are reviewed. It is shown that, with the use of a network of small, portable telescopes, the method is currently applicable to a large number of asteroids. The best results can be expected for asteroids of large angular diameter and regular shape. The potential of lunar occultation observations for asteroid diameter measurements is also briefly discussed.

Millis, R. L.↗