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At least 433 records · Page 24
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NASA NTRS1988

IUE program SUGSD: Varability time scale of H Ly-alpha from Uranus

The scientific motivation for this program was to determine the time scale for the variability of the H Lyman-alpha emission from Uranus. The purpose of this series of observation is to determine its variability on shorter time scales. A series of observations was carried out in coordination with ESA to cover as completely as possible one 24 hour period of the Uranian H Lyman-alpha emission. The observations were obtained on April 23 and 24. Two additional ovservations on April 25 and 26 were obtained to search for longer term trends. A small modulation in the brightness was observed and the results presented.

Durrance, Samuel T.↗
NASA NTRS1988

C-smithing of Voyager 2 non-imaging instrument pointing information at Uranus

The development of a family of techniques, collectively called C-smithing, for improving spacecraft nonimaging instrument pointing knowledge is discussed. C-smithing studies using data from the Voyager 2 Uranus Encounter show that significant improvements in pointing knowledge for nonimaging instruments can be achieved with these techniques. This improved pointing information can be used to regenerate instrument viewing geometry parameters for the encounter, which can then be made available to science investigators.

Wang, Tseng-Chan↗
NASA NTRS1988

Near-infrared stellar-occultation predictions for Uranus and Neptune - 1987-1990

Stellar-occultation predictions are presented for Uranus and Neptune covering the period 1987-1990, based on automated scans of R and I plates taken with the UK Schmidt Telescope in Australia. These scans have revealed many late-type or highly reddened stars suitable for stellar-occultation observations at 2.2 microns that were missed in previous searches of yellow-sensitive astrographic plates (Mink and Klemola, 1985). Comparisons involving several hundred stars measured by Mink and Klemola (1985) indicate that the internal random errors in the stellar positions estimated here are less than about 0.2 arcsec, comparable to the precision reached using conventional astrographic methods. In addition, JHK photometry was obtained for many of the Neptune stars, which is used to establish an empirical (I-K) vs (R-I) calibration for the remaining occultation candidates, and thus provide estimates of their K magnitudes.

Nicholson, Philip D.↗
NASA NTRS1987

Uranus satellites - Densities and composition

Homogeneous and core-differentiated silicate/ice models of the Uranian satellites Miranda, Ariel, Umbriel, Titania, and Oberon are examined in the light of imaging observations and mass and density determinations obtained during the Voyager 2 encounter with Uranus in January 1986. The data and model predictions are compared in extensive tables and graphs and discussed in detail. The mass fractions of silicates in Oberon and Titania are found to be between 0.42 and 0.65, about the same as the average for the satellites of Jupiter and Saturn but significantly higher than that for the smaller Saturnian satellites or that predicted by current solar-nebula models. It is suggested that the satellites formed by accretion of material from their primary planets' outer envelopes. The observed rock/ice fractions are attributed to solar-nebula CO and solid-organics abundances and to preferential dissolution of H2O in outer-envelope planetesimals.

Johnson, Torrence V.↗
NASA NTRS1987

Satellites of Uranus - Disk-integrated photometry from Voyager imaging observations

Voyager 2 imaging observations over a wide range of phase angles are used to determine the fundamental photometric parameters for the five largest satellites of Uranus. Over the spectral range covered by Voyager cameras (approximately 350-600 nm) the disk-averaged colors are moderately gray (no redder than the spectrum of Saturn's satellite Phoebe). Geometric albedos range from 0.19 for Umbriel to 0.40 for Ariel. Phase coefficients determined generally between phase angles of 10 and 60 deg vary from 0.021 mag/deg for Ariel to 0.028 mag/deg for Miranda. Phase integrals lie in the range of 0.5-0.65. The Bond albedos are about 0.1 for Umbriel and about 0.2 for the other satellites.

Veverka, J.↗
NASA NTRS1987

Effects of charged particles on the surfaces of the satellites of Uranus

Measurements of the ion and electron fluxes in the Uranian magnetosphere made by the low-energy charged-particle (LECP) instrument on the Voyager 2 spacecraft are used to discuss possible particle-induced modifications of the moons and rings of Uranus. The energy spectra of the orbit-integrated particle dosages expected on the major moons are derived from the LECP measurements of particle intensities and pitch-angle distributions. Laboratory-derived results on charged-particle-induced chemical and physical modifications of H2O, CH4, CO, and CO2 ions are used. The erosion rates of water ice and the darkening of organic ices on the surfaces of the moons are estimated from the orbit-integrated fluxes. Significant darkening of fresh organic ices to depths of the order of 1 micron is expected to occur for times as short as a few thousand years. Darkening at deeper depths will occur at increasingly longer times. The implications of these results for the interpretation of remote-sensing data are discussed.

Lanzerotti, L. J.↗
NASA NTRS1987

Micron-sized particle impacts detected near Uranus by the Voyager 2 plasma wave instrument

The characteristics and distribution of the small particles detected by the plasma-wave and radio-astronomy instruments during the Voyager 2 encounter with Uranus in January 1986 are examined. The nature of the observational data and the analysis techniques employed are discussed, and the results are presented in graphs. Particles of rms mass about 2.6 ng and radii of a few microns were found in an impact region of thickness 3840 km, with a maximum number density of 1600/cu m about 280 km from the equatorial plane, shortly after Voyager 2 crossed the plane of the Uranian rings. The possible origin of the particles in the rings or in small satellites outside the rings is considered.

Gurnett, D. A.↗
NASA NTRS1987

The atmosphere of Uranus - Results of radio occultation measurements with Voyager 2

The Uranian atmosphere is investigated on the basis of S-band and X-band occultation observations (including measurements of Doppler frequency perturbations) obtained during the Voyager 2 encounter with Uranus in January 1986. The data are presented in extensive tables and graphs and characterized in detail. The atmosphere is assumed to have an H2/He abundance ratio of about 85/15, but also to contain small amounts of CH4 at above-cloud relative humidity 30 percent, cloud-base relative humidity 78 percent, and below-cloud mixing ratio 2.3 percent by number density. Other parameters estimated include magnetic-field rotation period 17.24 h, 1-bar equatorial radius 25,559 + or - 4 km, polar radius 24,973 + or - 20 km, equatorial acceleration of gravity 8.69 + or - 0.01 m/sec sq, and atmospheric temperature 76 + or - 2 K (assuming 85 + or - 3 percent H2).

Lindal, G. F.↗
NASA NTRS1987

Voyager infrared observations of Uranus' atmosphere - Thermal structure and dynamics

The temperature structure of the Uranus atmosphere is investigated on the basis of 325/cm and 225/cm Voyager 2 IRIS observations of a layer between 60 and 200 mbar (including the tropopause). The data are presented in graphs and analyzed in detail. The latitudinal variations of the temperature near the tropopause and the inferred thermal winds are shown to be in good agreement with the findings reported for lower altitudes by Hanel et al. (1986), although greater in amplitude. A linear zonally symmetric circulation model with no solar and condensation heating, a frictional damping time 1-2 times the radiative damping time, a subrotating atmosphere at low latitudes, and zonal winds decaying with altitude is proposed to account for the observed structures.

Flasar, F. M.↗
NASA NTRS1987

The upper atmosphere of Uranus - EUV occultations observed by Voyager 2

EUV (52-170 nm) solar and stellar occultation observations of the Uranian atmosphere from 500 microbar to about 1 pbar, obtained with the UV spectrometer on Voyager 2 during its encounter with Uranus in January 1986, are reported. The data are presented in extensive tables and graphs and characterized in detail. The atmosphere is found to be dominated by H2 (with very small hydrocarbon mixing ratios) out to about 1.25 Uranian radii, where atomic H becomes important. Also noted are a small (relative to Jupiter and Saturn) homopause eddy-diffusion coefficient, little difference between the atmospheres of the day and night hemispheres, high temperatures (800 + or - 100 K) above about 1-10 nbar, and number densities of several hundred H/cu cm at 2 Uranian radii. The implications of the latter finding for ring dynamics and plasma populations are explored.

Herbert, Floyd↗
NASA NTRS1987

The excitation of the far ultraviolet electroglow emissions on Uranus, Saturn, and Jupiter

Data from Voyager 2 observations of FUV electroglow in the sunlit atmospheres of Jupiter, Saturn, and Uranus are compiled in tables and graphs and analyzed theoretically, and an excitation mechanism is proposed. This mechanism is based on local acceleration of photoelectrons and ions by the electric field (consisting of field-aligned potentials) due to an ionospheric dynamo. Anomalous resistivity and plasma instabilities induced by field-aligned currents in the ionosphere are invoked to explain the field-aligned potentials, and the processes which might give rise to such currents are further explored.

Clarke, John T.↗
NASA NTRS1987

The sources of Uranus' dominant nightside radio emissions

The broad-bandwidth radio emission detected by Voyager 2 over the nightside of Uranus is examined. It is concluded that the source location of the smooth component is consistent with emission originating near the electron gyrofrequency from a small set of field lines whose foot points lie near the Uranomagnetic southern (dark) pole. The source centroid is at L = 11.5, and extends in latitude between about L = 8 and L = 25. This deduced source region is primarily on closed field lines that pass through the outer radiation belt and have their opposite foot points near the Uranomagnetic northern pole (near the present epoch terminator). The source location of the bursty component is less well defined but is consistent with the set of open field lines which map down to the region surrounding the planet's south magnetic dipole tip.

Kaiser, M. L.↗
NASA NTRS1987

The radiation belt origin of Uranus' nightside radio emission

On the basis of the location of the source field lines of the smooth nightside component of Uranus kilometric radiation, the most likely free energy source is the outer radiation belts. As the terminator sweeps over the magnetic north polar region, precipitation of electrons generated by solar heating of the upper atmosphere and submergence of the electron mirror points deeper in the atmosphere will create a backscattered electron distribution with an enhanced population at large pitch angles. The clocklike radio emission turns out to be a direct consequence of the terminator's control of the emission process.

Curtis, S. A.↗
NASA NTRS1987

Bursty radio emissions from Uranus

Uranus emits a bursty form of radio emission in the range from 70 kHz to 1.2 MHz from a region near the negative magnetic pole. The emission is beamed into a flat equatorial sheet which appears to rotate with the magnetic field. The bursts typically have a duration of tens to hundreds of milliseconds, with a bandwidth of less than 20 kHz. A deep modulation of approximately 30 Hz is evident in the bursts.

Evans, David R.↗
NASA NTRS1987

Electrostatic waves in the magnetosphere of Uranus

Observations of electrostatic waves in the magnetosphere of Uranus are discussed with attention focused on the Bernstein emissions. The most intense Bernstein waves were observed near the magnetic equator of the planet, even though the tilt of the Uranian magnetic moment is very large relative to the rotational axis. In addition to the Bernstein modes, a number of highly sporadic emissions were detected in the vicinity of the Miranda L shell.

Kurth, W. S↗
NASA NTRS1987

Magnetic field and current structures in the magnetosphere of Uranus

Voyager 2 magnetic field and plasma data are compared with theoretical model calculations for the magnetosphere of Uranus to derive a global picture from the limited set of measurements. The results suggest that Voyager 2 entered the Uranian magnetosphere during a relaxation phase which followed a compression of the entire magnetosphere. The plasma beta values in the Uranian neutral sheet were found to be smallar by a factor of 3-8 compared to corresponding beta values in the average terrestrial neutral sheet. The excellent agreement found between observed and calculated magnetic tail lobe field strengths indicates that the Uranian magnetosphere reaches the state of quasi-static (i.e., slowly time-dependent MHD equilibrium).

Voigt, G.-H.↗
NASA NTRS1987

The magnetotail of Uranus

The paper describes the characteristics of Uranus' magnetic tail as deduced from Voyager 2 magnetic field and plasma observations, both low energy in the plasma science investigation and high-energy in the low-energy charged particle investigation. The large-scale geometry of the tail is illustrated and the structure and characteristics of the plasma sheet are considered. Included is a discussion of the geometry of the current sheet embedded in the plasma sheet and an estimate of cross-tail current density in the neutral sheet region.

Behannon, K. W.↗
NASA NTRS1987

The auroral kilometric radiation from Uranus and its magnetospheric implications

The Gulkis and Carr (1987) emission beam model for the kilometric radio emission from Uranus and types of information regarding the Uranian inner magnetosphere that may be derivable from its application are discussed. A modeled electron density profile is presented. The manner in which the hollow-cone beam from each element of the distributed auroral-zone source varies with frequency is determined. It is suggested that the observed effects might be due to refraction by a smooth component of the electron density variation with distance, together with multiple small refractions by electron density inhomogeneities along the ray path.

Carr, Thomas D.↗