Engineering topics

Caldwell, J.

Publications and source records attributed to Caldwell, J..

At least 37 records · Page 2

NASA NTRS1983

Real line strength distributions for random band models

An improved random band-model method, which makes allowance for the real line-strength distribution, is proposed. The model is shown to be useful for low-resolution, infrared observational data of the outer solar system. The method can be used as easily as conventional random band calculations. In the illustrative examples cited here, the variation of line width with J, the rotational quantum number, is small. Other effects which can, in principle, cause the model to deviate from laboratory observations are discussed. These include the assumption that line positions are random, ignoring the effects of the Lorentz wings of lines immediately outside the specific interval for which the mean transmission is calculated, and ignoring the effects of instrumental slit functions.

Kim, S. J.↗

NASA NTRS1983

Further observations of 8-micron polar brightenings of Jupiter

North-to-south scans of Jupiter at 7.8-micron wavelength in early 1981 confirm polar brightening events that correlate with LCM (III), such that a polar limb is bright when the corresponding magnetic pole is tilted earthward. The correlation with magnetic features of the planet suggests that the energy source for the brightenings is magnetospheric particles incident upon the polar regions of the atmosphere. The northern polar events are more prominent and more regular than the southern ones. The polar emission may be indirectly related to the ultraviolet absorber observed near the poles by Voyager 2.

Caldwell, J.↗

NASA NTRS1983

New observational constraints on the temperature inversions of Uranus and Neptune

The presence of a temperature inversion in the lower stratospheres of both Uranus and Neptune is confirmed by the 20-micron photometric data presented. It is found that the brightness temperature difference between 17.8 and 19.6 microns is 0.8 + or - 0.5 K for Uranus and 1.8 + or - 0.6 K for Neptune, implying that the temperature inversions of both planets are weaker than previously thought. Comparisons with model atmospheres suggested by Appleby (1980) imply that these temperature inversions may be understood as a consequence of heating through CH4 and aerosol absorption of sunlight. The stratospheric CH4 mixing ratio of Neptune must, however, be higher than that at the temperature minimum.

Tokunaga, A. T.↗

NASA NTRS1983

Observing Comet Halley with Space Telescope

The NASA Space Telescope (ST) to be launched into LEO by STS in late 1985 is characterized, and its potential use for observations of Comet Halley shortly after the perihelion passage in February, 1986, is discussed. The ST comprises a 2.4-m MgF2-coated primary reflector (with maximum field of view 2.7 x 2.7 arcmin, wavelength coverage 120-1100 nm, and maximum tracking rate 0.21 arcsec/sec) and five first-generation scientific instruments (wide-field planetary camera, faint-object camera, high-resolution and faint-object spectrographs, and high-speed photometer). Planned ST observations of Halley include periods of continuous observation much longer than can be obtained from the ground, provision of supplementary data and navigation information to Giotto and other deep-space missions, emission spectroscopy, UV polarimetry, and possible detection of 124-nm H2O absorption. Before March 11, 1986, earth occultation or similar procedures will be required to observe Halley because it will be within the ST 50-deg solar-elongation-distance limit.

Caldwell, J.↗

NASA NTRS1982

The abundance of CH3D in the atmosphere of Titan, derived from 8- to 14-micron thermal emission

The 8.6-micron emission feature of Titan's infrared spectrum was analyzed using the Voyager temperature-pressure profile. Although both C3H8 and CH3D have bands at that wavelength, it is shown that CH3D dominates the observed emission on Titan. A CH3D/CH4 mixing ratio is derived using this band and the strong CH4 band at 7.7 microns. The corresponding D/H ratio is 4.2(+2 -1.5) x 10 to the -4th, neglecting deuterium fractionation with other molecules. The main uncertainty in this value comes from the continuum emission characteristics. The D/H ratio is apparently significantly enhanced on Titan with respect to published values for Saturn.

Kim, S. J.↗

NASA NTRS1982

Current NASA studies for a Far Ultraviolent Spectrographic Explorer (FUSE)

The NASA plans for FUSE, a satellite which obtains spectra with resolutions between 100,000 and 100 in the spectral regions from 912 to 1216A and 100 to 912A, are outlined. Scientific problems which can be tackled by FUSE, but not by IUE or the Space Telescope, are discussed. A grazing incidence echelle and a hybrid echelle design are presented. They have high throughput, large simultaneous spectral range, and low background photon counting statistics. The satellite operational organization is similar to that of IUE.

Linsky, J.↗

NASA NTRS1982

Solar analogs in the 2600 to 3200 A region

No criteria for the selection of solar analogs are found. At low dispersion, the spectra of 16 Cyg A, 16 Cyg B, 18 Sco and alpha Cen A look identical to each other and to sunlight reflected from Galilean satellites with the possible exception of Mg II lambda 2800 being stronger in the stars. The use of alpha Cen A or 16 Cyg B as solar substitutes for planetary photometry is expected to be far superior to using published solar irradiances.

Hardorp, J.↗

NASA NTRS1982

Uranus science with space telescope

The Space Telescope Observatory, scheduled for launch in 1985, is described. The advantages of the space environment and the consequent features of ST performance are given, with Uranus observations as examples. The first generation instruments, including two cameras, two spectrographs and a high speed photometer, are discussed. The Space Telescope Science Institute, which will manage the Observatory, is discussed briefly. The potential scientific interaction with the Voyager 2 encounter of Uranus is also considered.

Caldwell, J.↗

NASA NTRS1982

Observational constraints on the atmospheres of Uranus and Neptune from new measurements near 10 micron

Uranus was detected at 10.3, 11.6 and 12.5 micrometers approximately 1 micrometer spectral bandpasses, with respective brightness temperatures of 74.0 + 0.9 or -1.1, 67.6 + 0.5 or -0.7, and 65.5 + 0.6 or -0.7 K and the first detection of Neptune at 10.3 micrometers with a brightness temperature of 77.5 + 0.7 or -0.9 K. We also detected Neptune at 11.36 micrometers with 2% spectral resolution at 81.0 + 0.8 or -0.9 K. The 10 micrometers continuous of both Uranus and Neptune may in part be due to reflected solar radiation as well as thermal emission. If all of the observed flux is reflected light, then the maximum geometric albedo of Uranus is 0.115 + or - 0.020, and that of Neptune is 0.229 + or - 0.043. In the context of previous observations in this region, the maximum stratospheric C2H6 mixing ratio is found to be 3 x 10 to the -8 power for Uranus and 3 x 10 to the -6 power for Neptune. A value for the maximum mixing ratio in the stratosphere of Neptune on the order of 1 - 0.004 appears to be consistent with the available data.

Orton, G. S.↗

NASA NTRS1981

Latitudinal variations in Jovian stratospheric temperature

Ground-based observations of Jupiter show that the planet's stratospheric and tropospheric thermal emission are anticorrelated. The observations can possibly be explained by latitudinal variations in cloud altitude. These variations cause differential stratospheric heating by sunlight which is reflected off the clouds and then absorbed within the stratosphere by visible and near-infrared bands of methane.

Cess, R. D.↗

NASA NTRS1981

Observations of Uranus, Neptune, and Titan by the International Ultraviolet Explorer

The ultraviolet spectra of Uranus, Neptune, and Titan do not reveal absorption features at 7-A resolution from 2100 to 3200 A. Upper limits of from 1 to 3 A are set for the equivalent widths of narrow absorptions, which corresponds to a CO/H2 mixing ratio less than 2 x 10 to the -4th in the case of Uranus. The slopes of the continuum reflectivities of Uranus and Neptune are consistent with the semi-infinite Rayleigh-Raman scattering model of Cochran, while the absolute levels are matched only if solar photometry is modified within acceptable limits. An alternative, but less satisfactory explanation of the new data is that the UV reflectivities of both Uranus and Neptune are depressed uniformly by a continuum absorber. The suggestion by Savage et al (1980) that the albedo of Uranus decreases by approximately 20% from 2200 to 1800 A is not confirmed. For Titan, the albedo decreases monotonically toward shorter wavelengths down to 2300 A. New limits are set for the pressure level in the atmosphere of Titan up to which the real, but presently unidentified, UV absorber there must extend, and for the fraction of Titan that must be covered by this absorber

Caldwell, J.↗

NASA NTRS1981

Titan on the eve of Voyager encounter

A decade of scientific study of Titan is reviewed with emphasis on the composition and the extent of its atmosphere. Several viable models are briefly discussed including the inversion model of Danielsen et al. (1973), Hunten's model (1978) which includes an extensive troposphere and a warm surface, with nitrogen as the major component, and models with neon, primordial carbon monoxide, or various mixtures of hydrogen, helium, and CH4. Recent observations by Pioneer 11 and other satellites are examined.

Caldwell, J.↗

NASA NTRS1981

Planetary science with Space Telescope

The capabilities of the Space Telescope observatory which will comprise a 2.4 m primary mirror scheduled for launch into low earth orbit in late 1983 by the Shuttle are discussed. Attention is given to the first generation of science instruments, including two imaging systems, two complementary spectrographs, a high-speed photometer, and a fine guidance/astrometry system. A specific observing program, an intercomparison of Uranus and Neptune, is examined in detail.

Caldwell, J.↗

NASA NTRS1980

Radius and brightness temperature observations of Titan at centimeter wavelengths by the Very Large Array

Brightness and radius measurements of the surface of Titan at 6, 2, and 1.3 cm wavelengths obtained with the Very Large Array radio interferometer are presented. Combined results for the three wavelengths indicate that the radius is 2400 + or - 250 km, implying a density of 2.4 + or - 0.7 g/cu cm, and that the brightness temperature is 87 + or - 9 K. The surface temperature may be somewhat higher if the emissivity is less than unity. The new data do not permit a choice between an inversion model for the atmosphere of Titan that predicts a surface temperature of 78 K and a model with both a stratospheric temperature inversion and a modest greenhouse effect that would increase the surface temperature by 10-40 K.

Jaffe, W.↗

NASA NTRS1980

Possible infrared aurorae on Jupiter

Infrared brightenings near the poles of Jupiter at 8 microns were observed in early 1980 by the NASA 3-m Infrared Telescope Facility at Mauna Kea and the Mayall 4-m telescope at the Kitt Peak National Observatory. It is suggested that these brightenings are related to the auroral zones which are determined by the magnetic mapping of the magnetotail onto the atmosphere, rather than by the Io flux tube. They were present in both hemispheres in January, present only in the north in February, and probably absent in the south in March. When visible, they were only seen in the hemisphere where the auroral zone was oriented toward the earth and absent otherwise.

Caldwell, J.↗

NASA NTRS1980

The 20-micron brightness temperature of the unilluminated side of Saturn's rings

Observations of the 20-micron brightness temperature of the unilluminated (north) side of Saturn's rings are presented and discussed in terms of models of B ring heating. Observations centered on the B ring ansae were made at 19.8 microns by the 3-m IR telescope facility on Mauna Kea during a period when the unilluminated side of the rings could be observed from earth. Flux measurements indicate a 20-micron brightness temperature of 56 + or - 1 K, in agreement with that measured at 45 microns by Pioneer 11. Under the assumptions that the emission of the Cassini division is negligible and that the ring brightness temperature is close to the actual temperature, it is shown that the brightness temperature can be accounted for by heating by the disk of Saturn, and is proportional to the sine of the Saturnicentric declination of the sun.

Tokunaga, A. T.↗

NASA NTRS1980

A model of Saturn's seasonal stratosphere at the time of the Voyager encounters

A polar-equatorial climate model by Cess and Cladwell (1979) is extended to all latitudes of Saturn. Modifications for solar absorption within the stratosphere are made in the extension to intermediate latitudes. The latitudinally and temporally variable effects of sunlight absorption by the rings of Saturn are included in the model. Temperature-latitude profiles for several levels within the Saturn stratosphere are presented for January 1981, which should be directly comparable with Saturn Voyager observations in November 1980 and August 1981. Temperature-pressure profiles for latitudes +30 degrees and -25 degrees are provided, which correspond to the planned points at which radio equipment aboard Voyager II will produce occultation observations.

Carlson, B. E.↗

NASA NTRS1980

An upper limit to the global SO2 abundance on Io

An upper limit to the global abundance of SO2 gas in the atmosphere of Io has been derived from observations of near-UV reflectivity made from earth orbit. Spectra in the range 2900-3100 A were obtained by the long-wavelength spectrograph of the IUE, and compared with the convolved solar spectrum of Broadfoot (1972). From the apparent solar origin of all the spectral features, an upper limit of 0.008 cm atm was calculated for the abundance of SO2, which was confirmed by ratio spectra of Vesta. Discrepancies between the upper limit obtained and the detection of a column abundance of 0.2 cm atm by the Voyager 1 IRIS experiment have a number of possible explanations, the most likely of which is that the SO2 atmosphere in Io is patchy, being confined to regions over solid deposits and volcanic vents.

Butterworth, P. S.↗