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At least 469 records · Page 26

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NASA NTRS1989

Electrostatic waves in the bow shock at Uranus

Electrostatic emissions measured by the Voyager 2 plasma wave detector (PWS) during the inbound crossing of the Uranian bow shock are shown to differ in some aspects from the waves measured during bow shock crossings at Jupiter and Saturn. The wave amplitudes in the foot of the bow shock at Uranus are in general much lower than those detected at the other outer planets due to the unusually enhanced solar wind ion temperature during the crossing. This reduces the effectiveness of wave-particle interactions in heating the incoming electrons. Strong wave emissions are observed in the shock ramp that possibly arise from currents producing a Buneman mode instability. Plasma instrument (PLS) and magnetometer (MAG) measurements reveal a complicated shock structure reminiscent of computer simulations of high-Mach number shocks when the effects of anomalous resistivity are reduced, and are consistent with high ion temperatures restricting the growth of electrostatic waves.

Moses, S. L.↗

NASA NTRS1989

Latitudinal variations of ammonia in the atmosphere of Uranus - An analysis of microwave observations

A 2-cm wavelength VLA map of Uranus is found upon analysis to display strong limb darkening and a lack of symmetry about the subearth point, perhaps due to a latitudinally varying NH3 mixing ratio. The change in NH3 abundance occurs near -45 deg latitude, and the calculated NH3 molar mixing ratios in the 5-20 bar region average 4.9 + or - 0.7 x 10 to the -7th at -90 to -45 deg latitudes, and 1.3 + or - 0.4 x 10 to the -6th at -45 to -15 deg latitudes. The model is in agreement both with previous work indicating the depletion of ammonia relative to solar abundance and a preliminary analysis of 6-cm data.

Hofstadter, Mark D.↗

NASA NTRS1989

Uranus deep atmosphere revealed

The present examination of the radio spectrum and latitudinal radio brightness temperature variation of Uranus leads to a ejection of thermochemical equilibrium models and the adoption of an atmospheric model characterized by a low ammonia volume-mixing ratio that is uniformly distributed over a wide altitude range. The elemental ratios derivable from this model support the planetary accretion theory of Pollack and Bodenheimer (1989). It is noted that while the equatorial and midlatitude values of Uranian radio brightness temperature are explainable by condensation theories, the polar value can only be accounted for through the invocation of strong, dry air downdrafts.

De Pater, Imke↗

NASA NTRS1990

The ultraviolet continuum albedo of Uranus

A radiative transfer code explicitly treating the Raman scattering of solar protons by H2 is presently used to analyze the Uranus geometric albedo in the 2000-5000 A range. The Baines and Bergstralh (1986) baseline model used reproduces the geometric albedo peak produced by Raman scattering filling of solar absorption line cores, but is found to be excessively bright for wavelengths below 2400 A. This discrepancy is resolvable through inclusion of an absorbing stratospheric haze layer, and results are thereby obtained which are consistent with the Pollack et al. (1987) model, in which aerosols are generated stratospherically through photochemical effects on hydrocarbons.

Cochran, William D.↗

NASA NTRS1990

Ray tracing of broadband bursty radio emissions from Uranus

To determine the source position of the broadband bursty emission, rays of X-mode emissin were traced from source positions along magnetic field lines with footprints that form a large grid centered approximately on the south magnetic pole of Uranus. For large wave normal angles, source regions different from those producing b-smooth emission were found. The emission observed prior to closest approach has a source along field lines that are distinct from those which generate emissions observed after closest approach.

Curran, D. B.↗

NASA NTRS1990

Source location of the narrowbanded radio bursts at Uranus - Evidence of a cusp source

While Voyager 2 was inbound to Uranus, radio bursts of narrow bandwidth (less than 5 kHz) were detected between 17-116 kHz. These R-X mode bursts, designated n-bursts, were of short duration, tended to occur when the north magnetic pole tipped toward the spacecraft, and increased in occurrence with increasing solar wind density. An explicit determination of the burst source location is presented, based upon fitting the region of detection at high and low frequencies to field-aligned, symmetric cones. The region of good fits was located between the north magnetic pole and the rotational pole, corresponding approximately to the northern polar cusp.

Farrell, W. M.↗

NASA NTRS1990

Absorption coefficients for the 6190-A CH4 band between 290 and 100 K with application to Uranus' atmosphere

A novel laser intracavity photoacoustic spectroscopy method allowing high sample control accuracy due to the small sample volume required has been used to obtain absorption coefficients for the CH4 6190 A band as a function of temperature, from 290 to 100 K. The peak absorption coefficient is found to increase from 0.6 to 1.0/cm, and to be accompanied by significant band shape changes. When used to further constrain the Baines and Bergstrahl (1986) standard model of the Uranus atmosphere, the low-temperature data yield an excellent fit to the bandshape near the 6190 A band's minimum.

Smith, Wm. Hayden↗

NASA NTRS1990

New arcs associated with the smooth high-frequency radiation from Uranus

New vertex-early kilometric arcs have been found to be associated with the dominant smooth high-frequency radio emission from Uranus. These arc features had vertices at about 300 kHz and bandwidths of about 200 kHz. Based on the fact that they exhibited a different temporal occurrence, spectral character, source location, and beaming pattern compared to the more dominant smooth component, it is concluded that they are a separate and independent radio component, different from those previously identified for this planet.

Farrell, W. M.↗

NASA NTRS1990

Comparison of plasma wave measurements in the bow shocks at Earth, Jupiter, Saturn, Uranus and Neptune

Plasma wave measurements from the Voyager 2 crossing of Neptune's bow shock are presented and compared with measurements from the bow shocks of Earth, Jupiter, Saturn, and Uranus. The wave amplitudes above 0.01fp, when normalized to the solar wind ion thermal energy density at each planet, are significantly higher at the outer planets than at Earth. Despite the differences in amplitude, the shock spectra of all the planets can be fitted to curves of similar form in this frequency range. The total normalized electric field energy densities exhibit an exponential dependence on ion thermal Mach number, Magnetosheath wave energies are comparable at all of the planets when normalized to the downstream plasma pressure.

Moses, S. L.↗

NASA NTRS1990

Satellite sweeping of electrons at Neptune and Uranus

Knowledge of satellite sweeping parameters at Neptune and Uranus, and of their functional dependences on particle energy and pitch angle, can be critical in the proper identification of parent absorbers for observed absorption signatures in regions where OTD (offset, tilted dipole) models are valid representations of the measured magnetic fields. In this paper, critical electron energies are calculated for longitudinal drift resonance, snowplow (i.e., strong) absorption, leapfrog, and corkscrew effects, using a reduced version of OTD that neglects nonaxial dipole offsets. Earlier analytic work on sweeping rates is extended to give the radial dependence of these rates within the minimum-L region and to set limits on diffusion of electrons with the simplifying approximation that leapfrog effects are ignored.

Cooper, John F.↗

NASA NTRS1991

Vertical mixing and methane photochemistry in the atmosphere of Uranus: Analysis of Voyager UVS occultation experiments

Extensive capabilities were developed in the analysis of ultraviolet spectrometer (UVS) absorptive lightcurves. The application of these capabilities to the Voyager UVS data sets from Uranus and Neptune has provided significant findings regarding the stratospheres of these planets. In particular, the direct comparison between photochemical models and UVS measurements accomplished by these efforts is unique, and it helps to guarantee that the information returned by the Voyager 2 spacecraft is being used to the fullest extent possible.

Bishop, James↗

NASA NTRS1990

Shock temperature measurements of planetary ices - NH3, CH4, and 'synthetic Uranus'

Shock temperature measurements have been performed on several materials which have relevance to the modeling of the outer planets. These materials are methane, ammonia and a mixture of water, ammonia, and isopropanol known as synthetic Uranus. Temperatures have been measured in these materials over the pressure range 33-76 GPa for which there also exists measurements of equation of state and electrical conductivity. The temperatures are found to agree well with available calculations, with small discrepancies between data and theory ascribed to energy absorbing processes such as dissociation and molecular ionization.

Radousky, H. B.↗

NASA NTRS1991

Properties of scatterers in the troposphere and lower stratosphere of Uranus based on Voyager imaging data

Scalar and vector radiative transfer and microphysical models are presently constructed from photometrically and geometrically corrected Voyager images of Uranus defining spatially-resolved intensities over a range of phase angles for two latitude bands. The methane ice cloud occupying 1.2-1.3 bar is of 0.7 optical depth at 22.5 deg S, rising to 2.4 at 65 deg S; the volume absorption coefficient of the cloud particles is 50 percent greater at the low latitude than at the high, assuming constant mean cloud particle size. The scattering model also includes photochemically-produced stratospheric hydrocarbon ices in the upper troposphere and stratosphere, as well as an optically thick hydrogen sulfide cloud.

Rages, Kathy↗

NASA NTRS1991

Observations of Mars, Uranus, Neptune, Io, Europa, Ganymede, and Callisto at a wavelength of 2.66 mm

An accurate thermal model of Mars is presently used as a calibrator for 2.66-mm brightness temperature observations of Uranus, Neptune, and the Galilean satellites. While the measurements for Io, Europa, and Callisto, are consistent with thermal models of the lunar type, Ganymede is found to be anomalous. It appears that the near surface of Ganymede differs from that of the other satellites in some unknown fashion, perhaps by scattering more strongly at short wavelengths.

Muhleman, Duane O.↗

NASA NTRS1991

Interior structure of Neptune - Comparison with Uranus

Measurements of rotation rates and gravitational harmonics of Neptune made with the Voyager 2 spacecraft allow tighter constraints on models of the planet's interior. Shock measurements of material that may match the composition of Neptune, the so-called planetary 'ice', have been carried out to pressures exceeding 200 gigapascals (2 megabars). Comparison of shock data with inferred pressure-density profiles for both Uranus and Neptune shows substantial similarity through most of the mass of both planets. Analysis of the effect of Neptune's strong differential rotation on its gravitational harmonics indicates that differential rotation involves only the outermost few percent of Neptune's mass.

Hubbard, W. B.↗

NASA NTRS1991

Thermal stress tectonics on the satellites of Saturn and Uranus

Thermal stress histories of the Saturnian and Uranian satellites are investigated. To this end, the thermal evolution of an icy satellite subjected to accretional and radiogenic heating, thermal conduction, and solid-state convection is modeled, and changes in the internal stress that occur during satellite evolution are examined. Results show that internal temperature changes that occur during normal evolution of many of the satellites of Saturn and Uranus can be expected to generate large extensional stresses in the satellites' outer regions. These stresses arise from three sources: (1) radiogenic warming, causing thermal expansion of materials in the satellite's deep interior; (2) radiogenic warming in larger satellites that can induce a phase transition from ice II to ice I and to produce a volume increase in the deep interior; and (3) accretional heating depositing heat in the satellite'e outer regions.

Hillier, John↗

NASA NTRS1985

The Voyager Uranus Travel Guide

Its purpose is to explain in simple language, including numerous illustrations, the Voyager-2 plans to examine Uranus and its moons, rings, particles, and fields. The Guide will also contain a variety of interesting facts about the Voyager mission, both past and future.

Source record↗

NASA NTRS1991

An explanation for both the large inclination and eccentricity of the dipole-like field of Uranus and Neptune

It is shown that the offset tilted dipole model of Uranus and Neptune, deduced from the spherical harmonic analysis of the Voyager magnetic field observation, can be represented fairly well by the combined field of an axial and an auxiliary dipole; the latter is roughly oriented in the east-west direction and is located near the surface of the core in low latitude. The present dynamo theories of planetary magnetism consider an axial dipolar field as an essential element, since the planetary rotation plays a vital role in the dynamo process. On the other hand, the auxiliary dipoles may be a result of leakage of the toroidal field, like a pair of sunspots on the photosphere, which is also an essential part of the dynamo process.

Akasofu, S.-I.↗