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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 217 records · Page 12
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NASA NTRS1985

Photodissociation regions. I - Basic model. II - A model for the Orion photodissociation region

A theoretical parameter study of the temperature and chemical structure of dense photodissociation regions and their resultant spectrum is presented. Models are discussed which are relevant not only to the dust and gas between molecular clouds and H II regions, but also apply to any neutral cloud illuminated by intense FUV fluxes. The models relate observed line and continuum emission from these regions to physical parameters such as the gas density and temperature, the elemental and chemical abundances, the local radiation field, and the grain properties. The results are applied to observational data from the OMC-1 region. The model shows that the observed high brightness temperature of the C I 609 microns line can be explained by emission from the C(+)/C/CO transition region. This difference with previous chemical models is due to a higher gas phase elemental abundance of carbon, to the charge exchange reactions of C(+) with S and SiO, and to carbon self-shielding.

Tielens, A. G. G. M.↗
NASA NTRS1985

A reexamination of important N2 cross sections by electron impact with application to the dayglow - The Lyman-Birge-Hopfield band system and N I (1199.99 nm)

The emission cross section (cascade plus direct excitation) and predissociation cross section for the Lyman-Birge-Hopfield band systems is experimentally measured. The results are compared to electron energy loss measurements of the direct excitation cross section. An analytic approximation to the measured cross section is obtained which demonstrates the extreme simplicity of the collision strengths at energies above the cross-section peak. Cross sections are also obtained for the FUV atomic nitrogen multiplets. Available cross sections for N I emission at 119.99 nm are discussed in detail.

Ajello, J. M.↗
NASA NTRS1986

Spectroscopy of the extreme ultraviolet background radiation

Observations in the FUV and soft X-ray bands suggest that the interstellar medium contains several components of high-temperature gas and should be emitting in the EUV. Indeed, diffuse radiation has been detected in the EUV with photometric instruments, but no special measurements exist below 520 A. A unique grazing-incidence spectrometer has been designed to study the diffuse emission between 80 and 650 A with 10 to 20-A resolution. The instrument was launched on a Black Brant sounding rocket from White Sands Missile Range on April 22, 1986. Preliminary analysis shows the expected geocoronal and interplanetary He I 584-A emission, and possibly other features which may originate in the hot ionized interstellar gas. Flux limits to these possible emission lines are compatible with previous broadband measurements.

Labov, Simon↗
NASA NTRS1987

Observation of the O I ultraviolet intercombination emissions in the terrestrial dayglow

The first measurement of the 1173-A O I intercombination line in emission in the terrestrial dayglow was performed using the EUV-FUV spectrometer designed to be flown with the Astro-1 observatory. The atmospheric conditions and viewing geometry were such as to suppress nitrogen emission relative to those from atomic oxygen, permitting the identification and measurement of the weak O I 1173-A intercombination multiplet and the 1641-A line as well as the strong 989-A and 1304-A emissions. At lower altitudes, the 1173-A emission rate increased while the 989-A emission, from the same upper level, decreased. This behavior is consistent with the radiative entrapment model of Meier (1982) and the laboratory value of the 1173-A/989-A branching ratio measured by Morrison (1985). The 1641-A/1304-A emission ratio is also consistent with a radiative entrapment model.

Bowers, C. W.↗
NASA NTRS1987

The peculiar early-type emission line supergiant S 18/SMC - An optical and ultraviolet study

Spectroscopic observations of S 18/SMC, obtained in the visible at CTIO, the AAT, and ESO and in the UV with the IUE SWP and LWR instruments during 1978-1983, are reported. The data are presented in tables and spectra and characterized in detail. The variability of the object in He II, C IV, N IV, N V, and Si IV and the lack of detectable photometric variation between 120 and 1000 nm are discussed in terms of a stellar-envelope model with mass-loss rate greater than or equal to 0.00001 solar mass/yr and N abundance 3-5 times the solar value, corresponding to a transitional phase between extreme mass loss (near the Humphrey-Davidson limit) and the presupernova stage. The FUV flux is tentatively attributed to an extremely hot He-star or neutron-star companion.

Shore, S. N.↗
NASA NTRS1987

Chromospheres and transition regions

The properties of stellar chromospheres are examined, summarizing the results of observations obtained with the IUE satellite since its launch in 1978. Numerous sample spectra, graphs, and diagrams are presented and analyzed in detail. Consideration is given to 120-320-nm spectroscopy of main-sequence stars, giants, and supergiants; the global properties of main-sequence and post-main-sequence chromospheres; dynamic phenomena and structures (systematic flows, atmospheric inhomogeneities, and intrinsic variability); and structural and energy-balance modeling. A number of outstanding problems are listed, and the potential value of data from the Hubble Space Telescope and the Lyman FUV Spectroscopic Explorer in solving them is indicated.

Jordan, Carole↗
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 NTRS1988

Simple ultraviolet calibration source with reference spectra and its use with the Galileo orbiter ultraviolet spectrometer

A simple compact electron impact laboratory source of UV radiation has been developed and is used to obtain calibrated optically thin VUV spectra and synthetic spectral models for important molecular band systems of H2, N2, and the n 1P0 Rydberg series of He. The relative spectral intensity of the electron impact source can be used as the primary calibration standard for VUV instrumentation in the 80-230 nm range and with FWHM of not less than 0.4 nm. The accuracy of the present method is 10 percent in the FUV and 20 percent in the EUV. Results for calibration of the Galileo orbiter ultraviolet spectrometer have been compared with those obtained using other methods.

Shemansky, D. E.↗
NASA NTRS1988

Two lines of sight with exceedingly anomalous ultraviolet interstellar extinction

Low-resolution IUE data toward HD 62542 reveal an extinction curve with an extremely broad and weak UV extinction bump and the highest FUV extinction yet observed in the Milky Way. Parameters describing the central position and FWHM for weak bumps observed toward HD 62542 and HD 29647 are derived using analytical fitting techniques. In both cases, the bump central positions are shifted shortward of 2175 A with an initial wavelength of 2110 A for HD 62542 and an initial wavelength of 2128 A for HD 29647. The bump FWHM is found to be 1.29/micron and 1.62/micron for HD 62542 and HD 29647, respectively.

Cardelli, Jason A.↗
NASA NTRS1987

Far-ultraviolet observations of the Delta-Scuti variable Beta Cassiopeiae

FUV (115-200-nm) observations of Beta Cas, obtained at resolutions 5 and 0.15 A by the IUE during September 1984, are reported and analyzed. The data are presented graphically and compared with results for Procyon, a sharp-lined F star similar in temperature to Beta Cas. A sharp factor-of-two increase in H I Ly-alpha flux on a time scale comparable to its Delta Sct pulsation period was observed, but the data were insufficient to determine whether the increase was flarelike or pulsation-related. The outer edges of the Beta Cas emission are shown to be significantly wider than those of Procyon.

Ayres, Thomas R.↗
NASA NTRS1988

Readout techniques for photon-counting microchannel image systems

A comparative evaluation is made of such readout methods for the microchannel plates that are commonly used in EUV, FUV, and X-ray low light level image systems as the (1) phosphor-video, (2) phosphor and binary-mask encoder, (3) direct discrete-position encoder, (4) direct analog amplitude position-encoder systems, and (5) delay-line encoders. Relative advantages and limitations are discussed in the context of low light level space-based astronomy applications. The delay-line technique offers great promise for high-resolution applications where oversampling is mandatory, as in spectroscopy.

Lampton, Michael↗
NASA NTRS1988

WF/PC internal molecular contamination during system thermal-vacuum test

During the recent system thermal vacuum test of the Wide-Field/Planetary Camera (WF/PC), instrumentation was added to the WF/PC to characterize the internal molecular contamination and verify the instrument throughput down to 1470 angstroms. Analysis of data elements revealed two contaminants affecting the far-ultraviolet (FUV) performance of the WF/PC. The one contaminant (heavy volatile) is correlated with the electronic and housing temperature, and the contamination is significantly reduced when the electronics are operated below plus 8 degrees to plus 10 degrees C. The other contaminant (light volatile) is controlled by the heat pipe temperature, and the contamination is significantly reduced when the Thermal Electric Cooler (TEC) hot-junction temperature is below minus 40 degrees to minus 50 degrees C. The utility of contamination sensors located behind instruments during system tests was demonstrated.

Taylor, Daniel M.↗
NASA NTRS1988

Solar spectroscopy in the far-ultraviolet-X-ray wavelength regions - Status and prospects

Progress made by space experimentation in obtaining solar spectra satisfactory for extensive applications of plasma diagnostic techniques is reviewed. Particular attention is given to solar spectroscopy in the FUV-X-ray wavelength regions in the 1.7-2000 A range. The quality of the available spectra obtained in each region is evaluated, concentrating on spectral resolution, wavelength coverage and accuracy, and line intensity and line-profile information.

Feldman, U.↗
NASA NTRS1988

Images in the rocket ultraviolet - UV fluxes of M31 globular clusters

Images obtained by a rocket-borne UV imaging telescope are used here to determine near-UV fluxes for 17 sources in M31 that are optical globular-cluster candidates and for the bright open cluster vdB0 in M31. Far-UV fluxes or flux limits are determined for the same clusters. The m(NUV)-V colors for M31 clusters are similar to those of Galactic clusters, except for the high-metallicity M31 cluster Bo 171. Four of the detected clusters have optical, m(NUV) - V, and m(FUV) - V colors indicating ages of about 100 million years. These four clusters are probably similar to the so-called 'blue globular' clusters of the LMC. The existence of young LMC-type blue globulars and the possible existence of middle-aged metal-rich globulars may indicate that M31 has continued to form globular clusters throughout its life.

Bohlin, Ralph C.↗
NASA NTRS1988

The Spartan-281 Far Ultraviolet Imaging Spectrograph

The U.S. Naval Research Laboratory's Far Ultraviolet Imaging Spectrograph (FUVIS), currently under development for flight as a Spartan shuttle payload, is designed to perform spectroscopy of diffuse sources in the FUV with very high sensitivity and moderate spatial and spectral resolution. Diffuse nebulae, the general galactic background radiation, and artificially induced radiation associated with the Space Shuttle vehicle are sources of particular interest. The FUVIS instrument will cover the wavelength range of 970-2000 A with selectable resolutions of 5 and 30 A. It is a slit imaging spectrograph having 3 arcmin spatial resolution along its 2.7 deg long slit.

Carruthers, George R.↗
NASA NTRS1988

The Berkeley extreme ultraviolet calibration facility

The vacuum calibration facilities of the Space Sciences Laboratory, University of California at Berkeley are designed for the calibration and testing of EUV and FUV spaceborne instrumentation (spectral range 44-2500 A). The facility includes one large cylindrical vacuum chamber (3 x 5 m) containing two EUV collimators, and it is equipped with a 4-axis manipulator of angular-control resolution 1 arcsec for payloads weighing up to 500 kg. In addition, two smaller cylindrical chambers, each 0.9 x 1.2 m, are available for vacuum and thermal testing of UV detectors, filters, and space electronics hardware. All three chambers open into class-10,000 clean rooms, and all calibrations are referred to NBS secondary standards.

Welsh, Barry Y.↗
NASA NTRS1989

Ionospheric dynamo theory for production of far ultraviolet emissions on Uranus

A model is presented to explain diffuse FUV emissions from the outer planets, specifically Uranus, in excess of those diffuse emissions that are currently explainable by scattering of sunlight and/or excitation by photoelectrons. These electroglow emissions in H Ly-alpha and H2 bands, which occur in the sunlit hemisphere slightly above the homopause, appear to require particle excitation in the 10- to 50-eV range. An in situ mechanism for accelerating photoelectrons (and ions is proposed, involving neutral wind dynamo generation of field-aligned currents analogous to what occurs in the earth's equatorial E and F regions. Sufficiently strong field-aligned currents are found in the model calculation for Uranus to produce a potential drop of about 100 eV or greater between the F peak and homopause, concentrated at lower altitudes, and capable in principle of accelerating photoelectrons (and ions) to the 10- to 50-eV energies required to explain the observed emissions. The fact that the excitation and ionization cross sections are larger than elastic scattering cross sections in an H2 atmosphere at these energies makes in situ acceleration feasible for the production of UV on the outer planets.

Hudson, M. K.↗
NASA NTRS1989

The correlation of C II 158 micron and CO (J = 1 - 0) line emission

The good correlation observed between the C II 158 micron and CO (J = 1 - 0) line emission from a large sample of Galactic H II regions, reflection nebulae, PN, and extragalactic nuclei is explained here by postulating a common origin to these two lines: photodissociation regions produced by the illumination of molecular clouds by FUV flux which are more than 100 times the average local interstellar field. Part of the correlation is due to a similar density dependence of I(C II) and I(CO) at high incident flux and part is due to beam dilution of Galactic and extragalactic sources. A method is presented for estimating a low limit to the beam filling factor of emitting gas and the incident UV radiation field.

Wolfire, Mark G.↗