Engineering topics

Caldwell, J.

Publications and source records attributed to Caldwell, J..

At least 19 records

NASA NTRS1993

The ultraviolet absorption spectrum of CO - Applications to planetary atmospheres

Laboratory gas-phase photoabsorption cross sections of the CO Cameron 0-0 band and the underlying pseudocontinuum have been measured at a temperature of 147 K and pressures of about 200 mbar, conditions similar to ambient in various planetary and satellite stratospheres in the solar system. A theoretical modeling program has also been used to calculate the band's spectrum. Agreement between the theoretical and the experimental spectra is very good. Models suggest that the observations of the CO Cameron band using the Hubble Space Telescope will be straightforward for Mars, but marginal for Titan.

White, H. P.↗

NASA NTRS1992

The Unified Radio and Plasma wave investigation

The scientific objectives of the Ulysses Unified Radio and Plasma wave (URAP) experiment are twofold: (1) the determination of the direction, angular size, and polarization of radio sources for remote sensing of the heliosphere and the Jovian magnetosphere and (2) the detailed study of local wave phenomena, which determine the transport coefficients of the ambient plasma. A brief discussion of the scientific goals of the experiment is followed by a comprehensive description of the instrument. The URAP sensors consist of a 72.5 m electric field antenna in the spin plane, a 7.5-m electric field monopole along the spin axis of a pair of orthogonal search coil magnetic antennas. The various receivers, designed to encompass specific needs of the investigation, cover the frequency range from dc to 1 MHz. A relaxation sounder provides very accurate electron density measurements. Radio and plasma wave observations are shown to demonstrate the capabilities and limitations of the URAP instruments: radio observations include solar bursts, auroral kilometric radiation, and Jovian bursts; plasma waves include Langmuir waves, ion acousticlike noise, and whistlers.

Stone, R. G.↗

NASA NTRS1986

Theoretical and observational planetary physics

This program supports NASA's deep space exploration missions, particularly those to the outer Solar System, and also NASA's Earth-orbital astronomy missions, using ground-based observations, primarily with the NASA IRTF at Mauna Kea, Hawaii, and also with such instruments as the Kitt Peak 4 meter Mayall telescope and the NRAO VLA facility in Socorro, New Mexico. An important component of the program is the physical interpretation of the observations. There were two major scientific discoveries resulting from 8 micrometer observations of Jupiter. The first is that at that wavelength there are two spots, one near each magnetic pole, which are typically the brightest and therefore warmest places on the planet. The effect is clearly due to precipitating high energy magnetospheric particles. A second ground-based discovery is that in 1985, Jupiter exhibited low latitude (+ or - 18 deg.) stratospheric wave structure.

Caldwell, J.↗

NASA NTRS1986

Infrared radiation models for atmospheric methane

Mutually consistent line-by-line, narrow-band and broad-band infrared radiation models are presented for methane, a potentially important anthropogenic trace gas within the atmosphere. Comparisons of the modeled band absorptances with existing laboratory data produce the best agreement when, within the band models, spurious band intensities are used which are consistent with the respective laboratory data sets, but which are not consistent with current knowledge concerning the intensity of the infrared fundamental band of methane. This emphasizes the need for improved laboratory band absorptance measurements. Since, when applied to atmospheric radiation calculations, the line-by-line model does not require the use of scaling approximations, the mutual consistency of the band models provides a means of appraising the accuracy of scaling procedures. It is shown that Curtis-Godson narrow-band and Chan-Tien broad-band scaling provide accurate means of accounting for atmospheric temperature and pressure variations.

Cess, R. D.↗

NASA NTRS1986

The geometric albedos of Uranus and Neptune between 2100 and 3350 A

Results are reported from large aperture IUE observations of the stratospheres of Uranus and Neptune. The data were collected using 16 Cyg A and B as solar analog stars. The spectra observed were similar to geometric albedos of the predicted in terms of Rayleigh-Raman scattering in the 2200-2800 A region. A falloff was observed near visible wavelengths, a factor which might be attributable to low-altitude haze absorbing in the 3000-4000 A interval. The data also indicated that the Uranus stratosphere does not strongly absorb UV radiation, and is not a good heating source. The stratospheric haze constituents over Uranus could therefore be condensates of CH4 and H2 photochemistry.

Wagener, R.↗

NASA NTRS1986

Constraints on the NH3 and PH3 distributions in the Great Red Spot

Medium resolution (10 A) UV spectra were obtained for the Great Red Spot (GRS) and South Tropical Zone (STZ) of Jupiter using the low dispersion mode of the IUE spectrometers at wavelengths from 1900-2200 A. The scans were carried out to determine the coloring agent for the GRS to improve the database for developing photochemical models of the feature. The wavelengths were selected to cover the absorption features of NH3 and PH3. The resulting data were interpreted using a vertically inhomogeneous Rayleigh scattering radiative transfer model. Various NH3 concentrations were explored in an effort to fit the data, taking into account changes which would occur at different atmospheric pressure levels and due to the projected temperature fields. A forbidden NH3/forbidden H2 mixing ratio that was calculated at the 80-125 mbar pressure level in the GRS was enhanced by 3-10 percent relative to the STZ. An upper limit was obtained for the mixing ratio of PH3 in the GRS that is significantly lower than previously predicted concentrations, implying that vertical transport in the GRS is not much greater than in adjacent regions.

Wagener, R.↗

NASA NTRS1985

Infrared polar brightening on Jupiter. III - Spectrometry from the Voyager 1 IRIS experiment

Spectra from the Voyager 1 IRIS experiment confirm the existence of enhanced infrared emission near Jupiter's north magnetic pole in March 1979. The spectral characteristics of the enhanced emission are consistent with a Planck source function. A temperature-pressure profile is derived for the region near the north magnetic pole, from which quantitative abundance estimates of minor species are made. Some species previously detected on Jupiter, including CH3D, C2H2, and C2H6, have been observed again near the pole. Newly discovered species, not previously observed on Jupiter, include C2H4, C3H4, and C6H6. All of these species except CH3D appear to have enhanced abundances at the north polar region with respect to midlatitudes. Upper limits are determined for C4H2 and C3H8. The quantitative results are compared with model calculations based on ultraviolet results from the IUE satellite. The plausibility of the C6H6 identification is discussed in terms of the literature on C2H2 polymerization. The relation of C6H6 to cuprene is also discussed.

Kim, S. J.↗

NASA NTRS1985

The Jovian stratosphere in the ultraviolet

Models of the spectral reflectivity at the center of the disk of Jupiter from 1450 to 3150 angstroms are presented. The reflectivity was computed from 30 low-dispersion IUE spectra taken during the 1978-1980 solar maximum. A vertically inhomogeneous radiative transfer program was used to compute model reflectivities of various stratospheric compositions for comparison. Ammonia and acetylene are well determined because they show narrow absorption bands in the ultraviolet. Possible compositions to improve the fit to the data below 1800 angstroms are suggested. The data are too noisy to detect possible CO Cameron band absorption near 2000 angstroms.

Wagener, R.↗

NASA NTRS1985

Space telescope observations of the earth's upper atmosphere by stellar occultations

Stellar occultations provide a useful means of measuring the trace gas composition of the earth's mesosphere with a sensitivity of order one part per billion. The operational details will differ from those of other astronomical observations by ST, because of the difficulties in guiding near the earth's limb. Two specific trace gases of interest to atmospheric studies, Cl and ClO, are discussed in this paper.

Caldwell, J.↗

NASA NTRS1984

The Jovian Stratosphere in the ultraviolet

The center-of-disk reflectivity of Jupiter in the wavelength range from 1500 A to 3000 A was computed from 31 low dispersion IUE spectra taken during solar maximum in 1979/80. The spectra were normalized to a reflectivity scale with the improved solar spectrum of July, 1980. Consideration of wavelength shifts between different IUE spectra and within the solar spectrum improved the apparent noise, especially longward of 2000 A. Six out of seven ammonia bands between 1900 A and 2200 A were detected. A vertically inhomogeneous radiative transfer program is used to compute model reflectivities for various stratospheric compositions. In addition to ammonia, the abundance of acetylene is also well determined because these molecules show narrow absorption bands in the ultraviolet. The abundances of the other molecules in the models (C3H4, C2H4, C4H2, C2H6, C3H6) are very uncertain and therefore are quoted only as upper limits. The best model fit is consistent with infrared observations by Voyager IRIS.

Wagener, R.↗

NASA NTRS1984

Ultraviolet observations of Uranus and Neptune below 3000 A

From 2000 A to 3000 A, both Uranus and Neptune have albedos that are about two times higher than Jupiter or Saturn's, implying that the outer giants have stratospheres that are relatively free of aerosol absorption. Uncertainties in the absolute calibration procedure allow discrepancies of order 15% between conservative models and the observations. A small amount of aerosol absorption is therefore possible. Below 2000 A, the derived albedo is highly dependent on the solar spectrum source used in the data reduction. The most recent result for Uranus is consistent with a secular change in C2H2 mixing ratio from approximately 3 x (10 to the -8 power) in 1980 to or = 10 to the -9 power in 1983. These values are approximately 2 orders of magnitude less than the mixing ratios of this gas on Saturn, and comparable to the amount on Jupiter.

Caldwell, J.↗

NASA NTRS1984

Ultraviolet Observations of Uranus and Neptune Below 3000 Angstrom

From 2000 to 3000 A, both Uranus and Neptune have albedos that are about two times higher than Jupiter or Saturn's, implying that the outer giants have stratospheres that are relatively free of aerosol absorption. Uncertainties in the absolute calibration procedure allow discrepancies of order 15% between conservative models and the observations. A small amount of aerosol absorption is therefore possible. Below 2000 A the derived albedo is highly dependent on the solar spectrum source used in the data reduction. The most recent result for Uranus, first reported here, is consistent with a secular change in C2H2 mixing ratio from approximately three times ten to the minus eight in 1980 to less than or equal to ten to the minus ninth in 1983. These values are approximately 2 orders of magnitude less than the mixing ratios of this gas on Saturn, and comparable to the amount on Jupiter.

Caldwell, J.↗

NASA NTRS1984

Uranus - Microwave images

Observations of Uranus at wavelengths of 2 and 6 centimeters with the Very Large Array were made in 1980 and 1981. The resulting maps of brightness temperature show a subsolar symmetry at 2 centimeters but a near-polar symmetry at 6 centimeters. The 6-centimeter maps show an increase in temperature from equator to pole with some evidence for a warm 'ring' surrounding the north pole. The disk-average temperatures (147 + or - 5 K and 230 + or - 6 K at 2 and 6 centimeters, respectively) are distinctly lower than recently reported values; these results suggest that the secular increase in temperature reported during the last 15 years has been reversed. The variations in brightness temperature probably reflect variations in ammonia abundance in the planet's atmosphere, but the mechanism driving these variations is still unclear.

Jaffe, W. J.↗

NASA NTRS1984

Chemical evolution on the giant planets and Titan

The atmospheres of Jupiter, Saturn, Neptune and Uranus, and Titan are characterized chemically in a review of recent observational and theoretical investigations. Compositions are indicated in tables, and special emphasis is given to the formation of HCN on Jupiter, the differentiation of polar and equatorial zones in the Jovian atmosphere, the mechanisms responsible for the color of the Great Red Spot, and the possible origin of the N2-dominated atmosphere of Titan.

Caldwell, J.↗

NASA NTRS1983

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

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 percent 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. Previously announced in STAR as N83-29155

Tokunaga, A. T.↗

NASA NTRS1983

The ISPM unified radio and plasma wave experiment

Hardware for the International Solar Polar Mission (ISPM) Unified Radio and Plasma (URAP) wave experiment is presented. The URAP determines direction and polarization of distant radio sources for remote sensing of the heliosphere, and studies local wave phenomena which determine the transport coefficients of the ambient plasma. Electric and magnetic field antennas and preamplifiers; the electromagnetic compatibility plan and grounding; radio astronomy and plasma frequency receivers; a fast Fourier transformation data processing unit waveform analyzer; dc voltage measurements; a fast envelope sampler for the solar wind, and plasmas near Jupiter; a sounder; and a power converter are described.

Stone, R. G.↗

NASA NTRS1983

Tentative confirmation of an aurora on Uranus

There have been three recent reports of the detection of Lyman-alpha radiation (due to atomic hydrogen, H, at wavelength 1,216 A) from Uranus by means of the International Ultraviolet Explorer (IUE) satellite. The interpretations of these results differ. Two reports conclude that there is a strong aurora on Uranus, but the third concludes that the source is resonance scattering of solar Lyman-alpha. This paper reports the detection of emission features due to molecular hydrogen, H2, near 1,600 A. This detection is near the limit of the IUE sensitivity. If it is real, the detection of H2 emission strongly supports the conclusion that Uranus has an aurora comparable in strength with those of the inner two giant planets, Jupiter and Saturn. The first published IUE spectra of the Saturn aurora are also presented.

Caldwell, J.↗

NASA NTRS1983

A determination of the composition of the Saturnian stratosphere using the IUE

Ultraviolet spectra of Saturn from the IUE satellite was reduced to produce a geometric albedo of the planet from 1500 to 3000 A. By matching computer models to the albedo a chemical composition consistent with the data was determined. This model includes C2H2 and C2H6 with mixing ratios and distributions of 9 + or - 3 x 10 to the -8th in the top 20 mbar of the atmosphere with none below for C2H2 and 6 + or 1 x 10 to the -6th also in the top 20 mbar with none below for C2H6. The C2H2 and C2H6 distributions and the C2H6 mixing ratio are taken directly from the Voyager IRIS model (R. Courtin et al., 1981). The Voyager IRIS model also includes PH3, which is not consistent with the UV albedo from 1800 to 2400 A. This model requires a previously unidentified absorber to explain the albedo near 1600 A. After considering several candidates, it is found that the best fit to the data is obtained with H2O, having a column density of 6 + or - 1 x 10 to the -3 cm-am.

Winkelstein, P.↗