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At least 235 records · Page 13

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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 influence of NO and ClO variations at twilight on the interpretation of solar occultation measurements

Measurement of short-lived photochemically-produced species in the stratosphere by solar occultation is difficult because the rapid variation of such species near the terminator introduces ambiguities in interpreting the measured absorption in terms of meaningful atmospheric abundances. These variations produce tangent path concentrations that are asymmetric relative to the tangent point, as opposed to the symmetrical distribution usually assumed in most inversion algorithms. Neglect of this asymmetry may yield an inverted profile that deviates significantly from the true sunset/sunrise profile. In the present paper, the influence of this effect on solar occultation measurements of ClO and NO is examined. The results show that average inhomogeneity factors, which measure the concentration variation along the tangent path and which can be calculated from a photochemical model, can indicate which species require more careful data analysis.

Boughner, R.↗

NASA NTRS1980

Radio scintillations during occultations by turbulent planetary atmospheres

The radio occultation experiment which uses the radio link between the earth and spacecraft passing behind planets has proven to be an important method for remote sensing turbulence in planetary atmospheres. The effects of defocusing and anisotropic irregularities on the turbulence-induced fluctuations of the radio occultation signal are examined. Rytov's method along with geometrical optics is employed to study the frequency spectra and coherences of the log amplitude and phase fluctuations of spherical waves operating at one as well as two frequencies. Comparison with the Mariner 5 2.3-GHz measurements shows good agreement with the theoretical results.

Woo, R.↗

NASA NTRS1980

Vertical structure of the atmosphere of Venus from Pioneer Venus orbiter radio occultations

Between December 1978 and February 1979 the Pioneer Venus orbiter spacecraft returned some 140 S band and X band radio occultation measurements of the Venus atmosphere. Results from 13 measurements, covering diverse latitudes from near equatorial to polar, are presented in this paper in the form of vertical profiles of temperature. The temperature profiles show a pronounced inversion at the tropopause for latitudes higher than about 50 deg, with the deepest inversions occurring between the latitudes of 60 deg and 70 deg, the latitudes at which the cold collar cloud feature was observed by the Pioneer Venus Vortex instrument. A comparison of the temperature profiles derived from radio occultation measurements with the stratospheric infrared temperature soundings of the Vortex instrument and the in situ measurements of tropospheric temperature and pressure by the North, Sounder, and Day probes indicates excellent agreement.

Kliore, A. J.↗

NASA NTRS1981

Radiance profile enhancement for solar occultation measurements

It is pointed out that distortions in received signals in solar occultation measurements made from space derive from the systematic motion of the electrooptical sensor. In a wide range of cases, the motion-related signal distortions brought about by the instrument's full modulation transfer function are as important as those from the optics. It is shown through simulation that existing techniques in the field of digital-image processing can be applied directly to solar occultation experiments to correct for these distortions. The resulting enhanced radiation profile is found to be much closer to the true radiance profile than is the sampled output of the instrument.

Gomberg, R. I.↗

NASA NTRS1982

The diameter of 88 Thisbe from its occultation of SAO 187124

The 7 October, 1981 occultation of SAO 187124 by 88 Thisbe was observed at twelve sites. The occultation observations, together with information about the asteroid's light curve, gives a mean diameter for Thisbe of 232 + or - 10 km. This value is 10 percent larger than the previously published radiometric diameter of Thisbe.

Millis, R. L.↗

NASA NTRS1984

The occultation of AG+29 deg 398 by 98 Minerva

On 22 Nov. 1982, the asteroid 93 Minerva occulted AG-29 deg 398 (= SAO 76O17A), a seventh magnitude star of AO spectral type. The data were best fitted by a circular limb profile having a diameter of 170.8 + or - 1.4 km, a value that agrees well with the published radiometric diameter for this asteroid. However, evidence of significant departure from a spherical shape is found in the occultation observations and in photometric measurements of Minerva. Additional observations are required to specify difinitively the three dimensional figure of Minerva.

Millis, R. L.↗

NASA NTRS1983

Voyager photopolarimeter stellar occultation of Saturn's rings

On Aug. 25, 1981, the Voyager 2 photopolarimeter system observed a stellar occultation by Saturn's rings. A brief description of this experiment along with details of the data reduction are presented. The occultation results are given in tabular and graphical form at a resolution of 60 km. Histograms of the frequency of optical depth show dominantly unimodal distributions in each of the classical ring elements. The frequency distribution of the entire ring system shows three modes at tau values of about 0.08, 0.5, and more than 2.5.

Esposito, L. W.↗

NASA NTRS1983

Saturn's upper atmosphere from the Voyager 2 EUV solar and stellar occultations

The temperature and composition of the upper atmosphere of Saturn have been inferred from Voyager 2 ultraviolet-spectrometer-occultation measurements made by observing the sun and the star Delta Scorpii while they were being occulted by Saturn. The observations analyzed here provide atmospheric parameters from 2900 km down to 960 km above the 1-bar level referred to the equator. The temperature in the model simulation of the data is 420 + or - 30 K down to about 1600 km. Below 1600 km the temperature decreases with a variable lapse rate down to 120 + or - 30 K near the methane homopause located at 1010 + or - 40 km. A constant temperature at 120 K is applied in the model for the region of the methane homopause down to 960 km where the present analysis terminates. Column amounts of H2 and H were measured from 2900 km down to about 1100 km, giving respective densities of about 1.0 x 10 to the 8th/cu cm and about 5.5 x 10 to the 6th/cu cm near the exobase at 2500 km. Near the methane homopause the H2 density is = 1.2 x 10 to the 12th/cu cm with a CH4 number-density mixing ratio of 0.00006. The eddy-diffusion coefficient in the vicinity of the methane homopause is estimated to be 5.0 x 10 to the 6th sq cm/sec. The atomic hydrogen density profile suggests a downward H flux of 1.8 x 10 to the 9th sq cm/sec from near the exobase down to a terminal boundary of about 1200 km.

Smith, G. R.↗

NASA NTRS1983

The rings of Uranus - Occultation profiles from three observatories

Occultation profiles for the nine confirmed Uranian rings obtained from Las Campanas, the European Southern Observatory, and Cerro Tololo on 15-16 August 1980 are compared. The alpha ring shows a 'double-dip' structure; the eta ring shows a broad and narrow component (similar to Saturn's F ring); and the epsilon ring shows six features that appear in the data from all three observatories. Diffraction fringes appear at the edges of several of the occultation profiles.

Elias, J. H.↗

NASA NTRS1984

The diameter of 375 Ursula from its occultation of AG + 39 deg 303

A value of 216 + or - 10 km for the mean diameter of Ursula is given, based on photometric measurements and observations of the occultation of AG + 39 deg 303, a K-type star of visual magnitude 8.8, by 375 Ursula on November 15, 1982. From the occultation diameter and the absolute visual magnitudes presented, the visual geometric albedo of Ursula is determined to be 0.032, similar to the value typical of C-type asteroids.

Millis, R. L.↗

NASA NTRS1984

Saturn's rings - Voyager 1 radio occultation experiment results

The Voyager 1 encounter with Saturn in 1980 included the first radio occultation measurements of the ring system. Opacity of and scattering by Saturn's rings were measured at 3.6- and 13-cm wavelengths using transmissions from the spacecraft to the earth. Detailed models of ring structure have been derived from these data at radial resolutions as small as 1 km, depending on the radio opacity of the region probed. Inverse power-law-type distributions of ring particle sizes with indices of about 3 and upper size cutoffs near 5 m are in good agreement with the data, but parameters vary throughout the rings. Occultation measurements have also provided information on ring dynamics and have led to a more precise definition of Saturn's pole vector.

Simpson, R. A.↗

NASA NTRS1984

Results from the occultation of 14 Piscium by (51) Nemausa

Observations of the September 11, 1983 occultation of 14 Psc by the asteroid (51) Nemausa were obtained at six sites distributed across the northern two-thirds of the track. The observations are well represented by an elliptical limb profile with a semimajor axis of 84.9 + or - 2.0 km and oblateness (1-b'/a') 0.20 + or - 0.05. The mean radius of 74 + or - 4 km agrees with the 1979 occultation effective radius of 76.5 + or - 4 km and the published radiometric radius of 75.5 km (Morrison and Zellner, 1979).

Dunham, E. W.↗

NASA NTRS1985

The SO2 atmosphere and ionosphere of Io - Ion chemistry, atmospheric escape, and models corresponding to the Pioneer 10 radio occultation measurements

Some numerical models of the SO2 atmosphere and ionosphere of Io at the time of the Pioneer 10 fly-by are calculated. It is shown that the formation of the observed ionosphere in the downstream direction may have required the precipitation of electrons, and that solar EUV radiation alone cannot account for it. In a comparison with Pioneer 10 radio occultation measurements, an electron impact in the range 500-800 eV was found to agree with Pioneer 10 radio occultation data for a SO2 atmosphere with a surface density of 4 x 10 to the 10th per cu cm. The relatively narrow energy range and flux required for incident electrons suggests that a fraction of a torus plasma compresses the ionosphere. It is pointed out on the basis of the model calculations that a weak magnetic field may be associated with Io.

Kumar, S.↗

NASA NTRS1985

The occultation of AG + 29 deg 398 by 93 Minerva

The occultation of AG + 29 deg 398 by 93 Minerva on 22 November 1982 was successfully observed at 10 sites. The data are best fitted by a circular limb profile having a diameter of 170.8 + or - 1.4 km, a value that agrees well with the published radiometric diameter for this asteroid. However, evidence of significant departure from a spherical shape is found in the occultation observations and in photometric measurements of Minerva made at Lowell Observatory over several months. Additional observations are needed to specify definitively the three-dimensional figure of Minerva.

Millis, R. L.↗

NASA NTRS1985

Photometry of occultation candidate stars. I - Uranus 1985 and Saturn 1985-1991

Photometric observations of five stars to be occulted by the rings around Uranus are presented. The four stars to be occulted by Saturn or its rings during the period 1985-1991 were also observed. The observations were carried out with a CCD detector attached to the Kitt Peak McGraw-Hill 1.30-m telescope. Landolt standards of widely ranging V-I color indices were used to determine the extinction coefficients, transformation coefficients, and zero points of the stars. Mean extinction coefficients are given for each night of observation. K magnitudes for each star were estimated on the basis of the results of Johnson (1967). The complete photometric data set is given in a series of tables.

French, L. M.↗

NASA NTRS1984

Predictions of occultations of stars by Comet Crommelin

Predictions of occultation of stars are presented on the basis of star and radio-source catalogs; these are: the AGK3 catalog, the SAO catalog south of the celestial equator, and Perth 70 catalog, as well as the Master List of Radio Sources. The predictions are formulated under the assumption of the coma radius of 22,500 km and a very large cometary radius of 675,000 km for cometary ion tails that may be involved in radio-source occultations. The predictions are expected to change with time.

Bowell, E.↗

NASA NTRS1985

Radio occultation by Saturn's rings - Observations of structure and particle size with Voyager 1

Voyager 1 radio occultation study of Saturn's rings gives detailed information regarding the rings' radial structure and particle sizes. Structure within the rings is mapped to a radial resolution of few hundred m in the tenuous parts of ring C and the Cassini Division, and few km over most of ring A. Fine resolution profiles reveal extremely sharp edges, very narrow gaps, and a host of wave phenomena. Particle size distributions obtained from occultation data within several ring regions are roughly consistent with an inverse cube power law with upper size cutoff in the 5 to 10 m radius range.

Marouf, E. A.↗