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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 235 records · Page 13
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NASA NTRS1988

Spectral observations of the extreme ultraviolet astronomical background radiation

Observations in the FUV and soft X-ray bands suggest that the ISM contains several components of high-temperature gas (100,000-1 million K). If large volumes of local interstellar space are filled with this hot plasma, emission lines will be produced in the EUV. Diffuse EUV radiation, however, has only been detected with photometric instruments; no spectral measurements exist below 520 A. A unique grazing-incidence spectrometer to study the diffuse emission between 80 and 650 A with resolution 10-30 A was successfully flown on a sounding rocket in April 1986, and a preliminary analysis reveals several features. In addition to the expected interplanetary He I 584 A emission and the geocoronal He II 304 A emission, other features appear which may orginate in the hot ionized interstellar gas.

Labov, S.↗
NASA NTRS1989

High-energy radiation background in space

The radiation environment of near-earth space and its effects on biological and hardware systems are examined in reviews and reports. Sections are devoted to particle interactions and propagation, data bases, instrument background and dosimetry, detectors and experimental progress, biological effects, and future needs and strategies. Particular attention is given to angular distributions and spectra of geomagnetically trapped protons in LEO, bremsstrahlung production by electrons, nucleon-interaction data bases for background estimates, instrumental and atmospheric background lines observed by the SMM gamma-ray spectrometer, the GRAD high-altitude balloon flight over Antarctica, space protons and brain tumors, a new radioprotective antioxidative agent, LEO radiation measurements on the Space Station, and particle-background effects on the Hubble Space Telescope and the Lyman FUV Spectroscopic Explorer.

Rester, A. C., Jr.↗
NASA NTRS1990

Far-infrared spectroscopy of the DR 21 star formation region

Far-IR line and continuum observations of the DR 21 star formation region are presented. It is shown that the extended emission in the 63 micron forbidden O I, 35 micron forbidden Si II, and 158 micron forbidden C II lines is most likely produced in dense, warm photodissociation regions on the surfaces of atomic and molecular clumps of size smaller than 0.6 pc. The gas temperatures in these photoelectrically heated, predominantly atomic layers are 250-500 K and are maintained by FUV fluxes 10,000 or more times the average interstellar radiation field. Gas densities in the surface layers are in the range 10,000-50,000/cu cm. The gas phase abundance of Si(+) is inferred to be about 5 x 10 to the -6th relative to hydrogen, or about 0.15 of its solar abundance. The mass of atomic gas is at least 200 solar masses.

Lane, Adair P.↗
NASA NTRS1990

Performance of compact multilayer coated telescopes at soft X-ray/EUV and far ultraviolet wavelengths

Compact soft X-ray/EUV (XUV) and far-ultraviolet (FUV) multilayer coated telescopes have been developed for the study of the solar chromosphere corona and corona/solar wind interface. Because they operate at short wavelengths (between about 40 and 1550 A), the modest apertures of 40 to 127 mm allow observations at very high angular resolution (0.1 to 0.7 arcsec). In addition to permitting traditional normal incidence optical configurations such as Cassegrain, Ritchey-Chretien, and Herschelian to be used at XUV wavelengths, multilayer coatings also allow a narrow wavelength band to be selected for imaging. The resulting telescopes provide a very powerful and flexible diagnostic instrument for the study of both the fine scale structure of the chromosphere/corona interface and the large scale structure of the corona and corona/solar wind interface. A new solar rocket payload, the Multi-Spectral Solar Telescope Array (MSSTA), composed of 17 of these compact telescopes, was developed. This paper reports on the performance of seven MSSTA Ritchey-Chretien telescopes.

Hoover, Richard B.↗
NASA NTRS1990

Study of electron impact excitation of argon in the extreme ultraviolet - Emission cross section of resonance lines of Ar I, Ar II

In a crossed-beam experiment under optically thin conditions the EUV spectrum of argon produced by electron impact excitation is studied. The cross sections of the resonance lines of Ar I and II are measured. The resonance lines of Ar I at 104.8 nm and 106.7 nm, and of Ar II at 91.96 nm and 93.21 nm are the most prominent features of the EUV spectrum between 40 and 110 nm. The relative-flow technique is used to measure the absolute cross sections of these lines at 200 eV. The measurements are compared with previous estimates. The measured emission cross section values at 200 eV for the Ar I lines at 104.8 nm and 106.7 nm, when compared to the electron energy loss estimates of the direct excitation cross sections, establish that cascading is larger for the Ar I resonance lines than previous emission experiments have indicated. In addition, all the emission cross sections for the Ar I and II Rydberg series in the EUV are measured at 0.5 nm resolution. The FUV spectrum is also surveyed and found to consist of Ar II multiplets from simultaneous ionization-excitation.

Ajello, Joseph M.↗
NASA NTRS1991

Absolute extinction and the influence of environment - Dark cloud sight lines toward VCT 10, 30, and Walker 67

The range of validity of the average absolute extinction law (AAEL) proposed by Cardelli et al. (1988 and 1989) is investigated, combining published visible and NIR data with IUE UV observations for three lines of sight through dense dark cloud environments with high values of total-to-selective extinction. The characteristics of the data sets and the reduction and parameterization methods applied are described in detail, and the results are presented in extensive tables and graphs. Good agreement with the AAEL is demonstrated for wavelengths from 3.4 microns to 250 nm, but significant deviations are found at shorter wavelengths (where previous studies of lines of sight through bright nebulosity found good agreement with the AAEL). These differences are attributed to the effects of coatings on small-bump and FUV grains.

Cardelli, Jason A.↗
NASA NTRS1991

Aberration corrected aspheric grating for far ultraviolet spectrographs - Conventional approach

Two approaches to reducing optical aberrations of concave grating spectrographs have been used, holographically controlling the groove curvature and spacing and reshaping the optical substrate while ruling the grooves conventionally. The latter approach, slightly deforming an ellipsoidal grating blank, can lead to diffraction-limited performance at a single FUV wavelength. When such a grating is used in a slitted Rowland circle spectrograph, the result is an extremely efficient spectrograph with spectral resolving power of about 30,000 and low astigmatism. Optical fabrication technology has advanced to the point where these exotic surface gratings are becoming practical.

Content, David↗
NASA NTRS1991

Far-ultraviolet mapping of the Cygnus Loop with the Voyager 2 Ultraviolet Spectrometer

FUV maps of the Cygnus Loop made using spectroscopic data from the Voyager 2 Ultraviolet Spectrometer are presented. Emission line features at about 980 and about 1035 A dominate the 500-1700 A spectra of the Cygnus Loop as observed with Voyager. Maps were generated in the light of these two features. The 980 and 1035 A maps are compared with X-ray and optical images of the Cygnus Loop that have been sampled in the same manner. From this comparison it is clear that the 980 A feature arises mainly from regions of bright optical emission, whereas the 1035 A emission more closely resembles the X-ray emission. This general impression can be confirmed by inspection of the spectra as a function of position. Spectra extracted from positions corresponding to the primary shock front show the 1035 A feature to be relatively strong, while the 980 A feature is brighter when substantial optical emission is present within the aperture.

Blair, William P.↗
NASA NTRS1990

Strategy for contamination control to improve Wide-Field/Planetary Camera far-ultraviolet performance

A multifaceted contamination control strategy has been developed for the second generated Wide-Field and Planetary Camera (WFPC-2) to improve the FUV stability by several orders of magnitude, compared to the first camera (WFPC-1). The strategy involves: improved on-orbit boil-off capability of the detector optics, added internal shielding and instrument venting, in-process subassembly vacuum bakeout at elevated temperatures, material substitution, sample testing in ultraclean vacuum facility, and internal instrument contamination-transport modeling. A science performance goal of 1 percent photometric accuracy at 1470 A over an extended time (of at least 30 days) has been established as a contamination control target for WFPC-2. The WFPC-2 is currently planned to be launched by the Shuttle in mid-1993 and replace the WFPC-1 which was recently launched with the HST.

Leschly, Kim↗
NASA NTRS1992

The ultraviolet albedo of Mars

Ultraviolet observations have proven successful at studying the composition and structure of the upper atmospheres of the giant planets and Venus. In the case of Mars, the analysis of UV data has concentrated on the spatial distribution and temporal behavior of ozone. The potential of far ultraviolet (FUV) data (shortward of 200 nm) for measuring the atmospheric structure, in particular the vertical distribution of aerosols, is demonstrated. For illustrative purposes, a simple two layer model (atmosphere and surface) is computed using the atmospheric composition measured at the Viking landing sites. This model is then compared to the albedo observed with the International Ultraviolet Explorer (IUE). This comparison clearly shows that a clear atmosphere with the nominal Viking mixing ratios of CO2, O2, and H2O is incompatible with the IUE data shortward of 200 nm. To obtain a better match between data and model, an aerosol has to be introduced that effectively scatters at these wavelengths. The required reduction in effective CO2 pathlength is a factor of 5. The comparison also points out the inadequacy of the IUE to provide data in the critical wavelength region between 195 nm and 210 nm. Such data will be forthcoming from the Hubble Space Telescope and will provide the opportunity to infer the vertical distribution of aerosols on Mars on a global scale.

Wagener, R.↗
NASA NTRS1991

Molecular line emission models of Herbig-Haro objects. I - H2 emission

A comprehensive model for molecular hydrogen emssion in Herbig-Haro objects that are associated with the heads of radiative stellar jets is presented by using a simple representation of the jet head as a comprising a leading bow shock and a trailing jet shock, separated by a dense layer of cool shocked gas. Attention is given to collisional excitation in a nondissociative shock and formation pumping in the molecular reformation zone behind a dissociative shock, employing detailed shock and photodissociation-region emission models that incorporate most of the relevant atomic physics and chemistry. The conditions under which each of these excitation mechanisms may be expected to contribute to the observed emission are discussed, and a general diagnostic scheme for discriminating among them is constructed. Applying this scheme to the HH 1-2 system, strong evidence for excitation by the radiation field of a fast shock is found. It is inferred that FUV pumping contributes a significant fraction of the H2 line emission, and it is shown that this can occur only if the UV pump lines are not strongly self-shielded.

Wolfire, Mark G.↗
NASA NTRS1991

Peeking through the picket fence - What astrophysical surprises may be present in the 100-200 A region?

In anticipation of more sensitive EUV and FUV spectroscopic instruments, spectra are simulated including interstellar absorption of solarlike, RS CVn, and flare stars as folded through the instrument parameters of the EUVE, Lyman/FUSE Phase A, and a desirable next-generation spectrometer. Even the relatively insensitive EUVE spectrometer is able to detect sufficient spectral lines from many active binary and dMe stars to determine their coronal-emission-measure distributions. The Lyman/FUSE or next-generation spectrometers are needed to study solar-type stars or flaring stars with high time resolution. The high throughput and effective area of a nex-generation spectrometer is needed for Doppler imaging studies, stellar wind and downflow measurements, and high time and spectral resolution of stellar flares.

Linsky, Jeffrey L.↗
NASA NTRS1991

Fundamental and practical limitations of FUV/EUV photon-counting image detectors

In EUV and FUV space-astronomy applications, the best contemporary detector designs are based on the use of microchannel plates due to their ability to deliver photon-counting performance with high efficiency. The major unresolved issue is the choice of position-readout system for the individual photoevents. Electrical event-readout systems are divided into two classes: the discrete wire anodes that perform coordinate digitization by wire-group selection, and the continuous centroid-position encoders for which coordinates are digitized in the associated electronics. The centroid-position encoder techniques are discussed in terms of how they overcome the four chief limitations of the discrete-wire readouts - their limited format size, their flat focal surface, their fundamental hex-channel vs squared-pixel moire pattern, and their image undersampling. With these limitations overcome, microchannel based image systems can deliver the performance demanded by the forthcoming generation of applications in space astronomy.

Lampton, M.↗
NASA NTRS1991

Performance of the Multi-Spectral Solar Telescope Array. III - Optical characteristics of the Ritchey-Chretien and Cassegrain telescopes

The Multi-Spectral Solar Telescope Array (MSSTA), which is a sounding-rocket-borne observatory for investigating the sun in the soft X-ray/EUV and FUV regimes of the electromagnetic spectrum, utilizes single reflection multilayer coated Herschelian telescopes for wavelengths below 100 A, and five doubly reflecting multilayer coated Ritchey-Chretien and two Cassegrain telescopes for selected wavelengths in the EUV region between 100 and 1000 A. The paper discusses the interferometric alignment, testing, focusing, visible light testing, and optical performance characteristics of the Ritchey-Chretien and Cassegrain telescopes of MSSTA. A schematic diagram of the MSSTA Ritchey-Chretien telescope is presented together with diagrams of the system autocollimation testing.

Hoover, Richard B.↗
NASA NTRS1991

Solar EUV/FUV line polarimetry. I - Observational parameters and theoretical considerations. II - Instruments and methods

The paper discusses theoretical considerations and presents the observational parameters for the development of reflective FUV coronagraph/polarimeter and EUV imaging polarimeter instruments. New EUV/FUV polarimeter instrument concepts for solar research are described. One of these is a new coronagraph/polarimeter operating at 1215.6 A, which could observe the neutral hydrogen Ly-alpha line in the near solar corona and lead to the first direct measurements of both the strength and the direction of coronal magnetic fields. The other is a new multilayer EUV imaging polarimeter, operating at the wavelengths of strong He emission lines, which could observe impact polarization phenomena and provide information concerning the relative importance of thermal and nonthermal processes in solar flares.

Fineschi, Silvano↗
NASA NTRS1991

Design of narrow band XUV and EUV coronagraphs using multilayer optics

A novel approach to the study of the solar corona is described, which enables observation of the soft X-ray (XUV), EUV, and FUV emission of the highly ionized ions present in the corona. The principal advantage of this approach is a relatively low contrast between the innner corona and the solar disk at the wavelengths involved; the contrast between coronal visible light and the XUV or EUV light is about 1. The design and the anticipated performances of the XUV and EUV coronagraphs using normal incidence multilayer optics are described.

Walker, A. B. C., Jr.↗
NASA NTRS1992

The constitution of the atmospheric layers and the extreme ultraviolet spectrum of hot hydrogen-rich white dwarfs

An analysis is presented of the atmospheric properties of hot, H-rich, DA white dwarfs that is based on optical, UV, and X-ray observations aimed at predicting detailed spectral properties of these stars in the range 80-800 A. The divergences between observations from a sample of 15 hot DA white dwarfs emitting in the EUV/soft X-ray range and pure H synthetic spectra calculated from a grid of model atmospheres characterized by Teff and g are examined. Seven out of 15 DA stars are found to consistently exhibit pure hydrogen atmospheres, the remaining seven stars showing inconsistency between FUV and EUV/soft X-ray data that can be explained by the presence of trace EUV/soft X-ray absorbers. Synthetic data are computed assuming two other possible chemical structures: photospheric traces of radiatively levitated heavy elements and a stratified hydrogen/helium distribution. Predictions about forthcoming medium-resolution observations of the EUV spectrum of selected hot H-rich white dwarfs are made.

Vennes, Stephane↗
NASA NTRS1992

Hopkins ultraviolet telescope observations of the far-ultraviolet spectrum of NGC 4151

Observations of the FUV spectrum of the Seyfert galaxy NGC 4151 from 912 to 1860 A during the flight of Astro-1 aboard the space shuttle Columbia in December 1990 are reported. Broad emission lines with full-width at half-maximum of about 8500 km/s dominate the spectrum. Numerous absorption features modify the continuum shape, particularly at wavelengths shortward of Ly-alpha. The continuum turns over sharply below 1000 A and disappears by 924 A, well above the redshifted Lyman edge of NGC 4151 at 915 A. The absorption lines have intrinsic widths of about 1000 km/s and are blueshifted relative to the system velocity of NGC 4151 by 200-1300 km/s. Absorption of the continuum by the converging higher-order Lyman lines explains the sharp turnover of the continuum below 1000 A. The blueshifts of the absorption lines, their large intrinsic widths, and the inferred high densities are all consistent with outflowing material originating in the broad-line region.

Kriss, G. A.↗