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At least 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
NASA NTRS1996

The Age-Related Properties of the HD 98800 System

We present optical spectroscopy of the field K star system HD 98800, which has been found to have significant infrared emission from circumstellar material. The lithium abundances of the stars in HD 98800 are well above those of Pleiades of similar color, but activity levels and rotation in these stars are at or below Pleiades level. Thus, it is not yet possible to say whether HD 98800 is or is not a pre-main-sequence system, and it is possible that its components are on or near the zero-age main sequence. However, the two visible objects that make up HD 98800 both have high levels of lithium and activity, strongly suggesting that they are physically related to one another. As shown by Torres and coworkers, having these stars physically tied implies that their relative orbit is highly eccentric and highly inclined to our line of sight, and it also means that we are viewing the HD 98800 system at an unusual time in its orbit.

Soderblom, David R.↗
NASA NTRS2005

Channel alignment and radiometry in hyperspectral atmospheric infrared sounders

The Atmospheric Infrared Sounder (AIRS) is a hyper-spectral infrared sounder which covers the 3.7 to 15,4 micron region with 2378 spectral channels. The AIRS instrument specification called for spatial co-registration of all channels to better than 2% of the field of view. Pre-launch testing confirmed that this requirement was met, since the standard deviations in the centroids was about 1% of the 13.5 km IFOV in scan and 3% in track. Detailed analysis of global AIRS data show that the typical scene gradient in 10 micron window channels is about I .3K/km rms. The way these gradients, which are predominantly caused by clouds, manifest themselves in the data depends on the details of the instrument design and the way the spectral channels are used in the data analysis, AIRS temperature and moisture retrievals use 328 of the 2378 channels from 17 independent arrays. As a result, the effect of the boresight misalignment averages to zero mean. Any increase in the effective noise is less than 0.2K. Also, there is no discernable performance degradation of products at the 45 km spatial resolution in the presence of partially cloudy scenes with up to 80% cloudiness. Single pixel radiometric differences between channels with boresight alignment differences can be appreciable and can affect scientific investigations on a single 15km footprint scale, particularly near coastlines, thunderstorms and surface emissivity inhomogeneities.

sounders↗
NASA NTRS1964

Theoretical Current-Voltage Curve in Low-Pressure Cesium Diode for Electron-Rich Emission

Although considerable interest has been shown in the space-charge analysis of low-pressure (collisionless case) thermionic diodes, there is a conspicuous lack in the presentation of results in a way that allows direct comparison with experiment. The current-voltage curve of this report was, therefore, computed for a typical case within the realm of experimental interest. The model employed in this computation is shown in Fig. 1 and is defined by the limiting potential distributions [curves (a) and (b)]. Curve (a) represents the potential V as a monotonic function of position with a slope of zero at the anode; curve (b) is similarly monotonic with a slope of zero at the cathode. It is assumed that by a continuous variation of the anode voltage, the potential distributions vary continuously from one limiting form to the other. Although solutions for infinitely spaced electrodes show that spatically oscillatory potential distributions may exist, they have been neglected in this computation.

Coldstein, C. M.↗
NASA NTRS1976

Correlation between microwave scattering and emission from land and sea at 13.9 GHz

Correlations have been calculated between active and passive microwave responses received by the S-193 radiometer-scatterometer on Skylab. Over both land and sea, the correlations between polarizations are high, but the correlation between radiometer and scatterometer response at 30-deg incidence is negligible. This suggests that multipolarization instruments with this crude resolution (greater than 10 km in all cases) are redundant, but a combination of radiometer and scatterometer is useful. Correlations among the responses at different angles suggest that the five angles used in Skylab are more than would be useful between zero and 48 deg in future instruments, but that instruments with three angles of measurement would be useful. In small areas, conclusions based on these large-data-set averages may need to be modified.

Sobti, A.↗
NASA NTRS1981

Two-dimensional model studies of the effect of supersonic aircraft operations on the stratospheric ozone content

For a fleet of 250 aircraft, the change in the ozone column is predicted to be very close to zero; in fact, the ozone overburden may actually increase as a result of show that above 25 to 30 km the ozone abundance decreases via catalytic destruction, but at lower heights it increases, mainly as a result of coupling with odd hydrogen species. Water vapor released in the engine exhaust is predicted to cause ozone decreases; for the hypothetical engines used in the study, the total column ozone changes due to water vapor emission largely offset the predicted ozone increases due to NOx emission. The actual effect of water vapor may be less than calculated because present models do not include thermal feedback. Feedback refers to the cooling effect of additional water vapor that would tend to slow the NOx reactions which destroy ozone.

Whitten, R. C.↗
NASA NTRS2012

Temperature Induced Voltage Offset Drifts in Silicon Carbide Pressure Sensors

We report the reduction of transient drifts in the zero pressure offset voltage in silicon carbide (SiC) pressure sensors when operating at 600 C. The previously observed maximum drift of +/- 10 mV of the reference offset voltage at 600 C was reduced to within +/- 5 mV. The offset voltage drifts and bridge resistance changes over time at test temperature are explained in terms of the microstructure and phase changes occurring within the contact metallization, as analyzed by Auger electron spectroscopy and field emission scanning electron microscopy. The results have helped to identify the upper temperature reliable operational limit of this particular metallization scheme to be 605 C.

Okojie, Robert S.↗
NASA NTRS1972

Diffusion models for Jupiter's radiation belt

Solutions are given for the diffusion of trapped particles in a planetary magnetic field in which the first and second adiabatic invariants are preserved but the third is not, using as boundary conditions a fixed density at the outer boundary (the magnetopause) and a zero density at an inner boundary (the planetary surface). Losses to an orbiting natural satellite are included and an approximate evaluation is made of the effects of the synchrotron radiation on the energy of relativistic electrons. Choosing parameters appropriate to Jupiter, the electrons required to produce the observed synchrotron radiation are explained. If a speculative mechanism in which the diffusion is driven by ionospheric wind is the true explanation of the electrons producing the synchrotron emission it can be concluded that Jupiter's inner magnetosphere is occupied by an energetic proton flux that would be a serious hazard to spacecraft.

Jacques, S. A.↗
NASA NTRS1993

Observations of the gaseous Galactic halo toward 3C 273 with the Goddard High Resolution Spectrograph

An analysis of observations of 24 absorption lines produced near zero redshift by Milky Way disk and halo gas along the sight line to 3C 273 is presented. The reduced spectra have a resolution of about 20 km/s (FWHM) with broad wings and an SNR of 13 to 20. The species detected include C II, C II*, C IV, O I, Mg II, Si II, Si IV, S II, Mn II, Fe II, and Ni II. The O I absorption is blended with earth atmospheric emission, and one of the Si IV doublet lines is blended with an intergalactic Lyman-alpha line. Column densities are determined for C II*, C IV, Si II, Si IV, N V, S II, Mn II, and Ni II using standard curve-of-growth techniques and the apparent optical depth method. The column density for S II and a measure of N(H I) from FOS observations of Lyman-alpha reveal a 45-percent overabundance of sulfur compared with neutral hydrogen which is probably due to the presence of S II in ionized gas toward 3C 273.

Savage, Blair D.↗
NASA NTRS2003

Asymmetric Stokes-V Profiles at the Penumbral Boundary of a Sunspot

We present the spectropolarimetric measurements of a sunspot in the active region NOAA 6958 (15S03W), situated near the central meridian disk passage. The follower polarity sunspot was somewhat symmetrically round shaped with an elongated penumbra. There were several opposite polarity magnetic elements at, and beyond the penumbral boundary. The H-alpha images of the sunspot show the bright emission regions near the penumbral boundary towards the sun-center, which was of opposite polarity with respect to the main spot. The net-circular polarization (NCP) map shows that NCP is negative in the inner part of the spot and positive at the penumbral boundary and near the H-alpha plage. The Doppler velocities were determined by measuring the center-of-gravity (COG) of the Stokes-I profile and zero-crossing (ZC) wavelength of the Stokes-V profiles. The COG velocity map in general agrees with the Evershed flow. In addition, it shows the up flow in the penumbral region. The ZC velocities show the strong down flow at the penumbral boundary. Double-lobed Stokes-V profiles are observed at the locations, where the penumbral fibrils terminate coinciding the H-alpha plage. The Double lobed profiles had an unshifted component similar to the Stokes-V profiles of the sunspot penumbra and a shifted component with a velocity of about 5 km/s. The amplitude of the second component increases along the penumbral fibril as a function of the distance from the center of the sunspot. In this paper we discuss the role of emerging flux in generating the observed double lobed profiles. Based on our present observations, we propose to observe with the Solar-B Spectropolarimeter for understanding the nature of emerging flux near the sunspots.

Choudhary, Debi Prasad↗
NASA NTRS1986

Optical and electrical characterizations of laser-chemical-vapor-deposited aluminum oxynitride films

Composition-dependent optical and electrical properties of L-CVD (Al2O3)1-c-(AlN)c composite films are examined. The optical properties exhibit a continuous change from Al2O3 to AlN with increasing value of the composition parameter c from zero to one. There is a critical concentration value of about 0.3 below and above which Al2O3- and AlN-like optical properties are observed. The impurity absorption regions are 5.1-6.7 eV for Al2O32 and 4.6-6.0 for AlN, while the energy gap is about 6.7 eV for Al2O3 and 6.0 eV for AlN. The electrical properties of Al-insulator n-Si MIS structures display externally programmable states when bulk-controlled electron emission occurs from the traps. In the high-field region a persistent polarization field is established within the insulator. C-V characteristics of the samples are more sensitive to the concentration factor c than the I-V characteristics. A large clockwise hysteresis is observed for AlN, but a smaller counterclockwise one is observed for Al2O3.

Demiryont, H.↗
NASA NTRS1991

Equatorial thermospheric wind changes during the solar cycle - Measurements at Arequipa, Peru, from 1983 to 1990

Near-equatorial thermospheric wind velocities at Arequipa, Peru, are determined over about two-thirds of a solar cycle using Fabry-Perot interferometer measurements of Doppler shifts in the nightglow 630-nm emission line. Mean monthly nocturnal variations in the meridional and zonal wind components are calculated from the nightly data to remove short-term (day-to-day) variability as well as any additional changes introduced by the progression of the solar cycle. For most of the years, at the winter solstice, there is a weak (more than 100 m/s) transequatorial flow from the summer to the winter hemisphere in the early and the late night, with essentially zero velocities in between. At the equinoxes, an early-night poleward (southward) flow at solar minimum (1986) is replaced by an equatorward (northward) flow at solar maximum (1989-1990).

Biondi, M. A.↗
NASA NTRS2019

KELT-24b: A 5M(J) Planet on a 5.6 day Well-aligned Orbit around the Young V=8.3 F-star HD 93148

We present the discovery of KELT-24 b, a massive hot Jupiter orbiting a bright (V=8.3 mag, K=7.2 mag) young F-star with a period of 5.6 days. The host star, KELT-24 (HD 93148), has a Teff=-+65094950K, a mass of M*=+1.4600.0590.055Me, a radius of R*=1.506±0.022Re, and an age of +0.780.420.61Gyr. Its planetary companion (KELT-24 b) has a radius of RP=1.272±0.021RJ and a mass of MP=-+5.180.220.21MJ, and from Doppler tomographic observations, we find that the planet’s orbit is well aligned to its host star’s projected spin axis (l=-+2.63.65.1). The young age estimated for KELT-24 suggests that it only recently started to evolve from the zero-age main sequence. KELT-24 is the brightest star known to host a transiting giant planet with a period between 5 and 10 days. Although the circularization timescale is much longer than the age of the system, we do not detect a large eccentricity or significant misalignment that is expected from dynamical migration. The brightness of its host star and its moderate surface gravity make KELT-24b an intriguing target for detailed atmospheric characterization through spectroscopic emission measurements since it would bridge the current literature results that have primarily focused on lower mass hot Jupiters and a few brown dwarfs.

Transit photometry↗
NASA NTRS2013

Uncertainty of Passive Imager Cloud Optical Property Retrievals to Instrument Radiometry and Model Assumptions: Examples from MODIS

The optical and microphysical structure of clouds is of fundamental importance for understanding a variety of cloud radiation and precipitation processes. With the advent of MODIS on the NASA EOS Terra and Aqua platforms, simultaneous global-daily 1 km retrievals of cloud optical thickness (COT) and effective particle radius (CER) are provided, as well as the derived water path (WP). The cloud product (MOD06/MYD06 for MODIS Terra and Aqua, respectively) provides separate retrieval datasets for various two-channel retrievals, typically a VISNIR channel paired with a 1.6, 2.1, and 3.7 m spectral channel. The MOD06 forward model is derived from on a homogeneous plane-parallel cloud. In Collection 5 processing (completed in 2007 with a modified Collection 5.1 completed in 2010), pixel-level retrieval uncertainties were calculated for the following non-3-D error sources: radiometry, surface spectral albedo, and atmospheric corrections associated with model analysis uncertainties (water vapor only). The latter error source includes error correlation across the retrieval spectral channels. Estimates of uncertainty in 1 aggregated (Level-3) means were also provided assuming unity correlation between error sources for all pixels in a grid for a single day, and zero correlation of error sources from one day to the next. I n Collection 6 (expected to begin in late summer 2013) we expanded the uncertainty analysis to include: (a) scene-dependent calibration uncertainty that depends on new band and detector-specific Level 1B uncertainties, (b) new model error sources derived from the look-up tables which includes sensitivities associated with wind direction over the ocean and uncertainties in liquid water and ice effective variance, (c) thermal emission uncertainties in the 3.7 m band associated with cloud and surface temperatures that are needed to extract reflected solar radiation from the total radiance signal, (d) uncertainty in the solar spectral irradiance at 3.7 m, and (e) addition of stratospheric ozone uncertainty in visible atmospheric corrections. A summary of the approach and example Collection 6 results will be shown.

Cloud Properties Observations↗
NASA NTRS1984

The high-ionization and excited-state interstellar lines in the Carina Nebula - A giant H II region in absorption

The dominant component of the highly ionized and excited-state interstellar species seen in the ultraviolet toward stars within the Carina Nebula has been found to have a heliocentric radial velocity of about -30 km/s. This velocity is known to correspond to the near edge of the expanding H II region, as shown by the double optical nebular emission lines and the single He I lambda 3889 nebular absorption line. It is also seen as a (nondominant) component in both the optical and ultraviolet low-ionization interstellar lines, discussed previously. Hence, the Carina Nebula provides a clear case in which the principal high-ionization interstellar features are associated with the hot stars against which they are observed; and a considerable range of absorption lines can provide physical information about the H II region. Also, the weaker zero-velocity, line-of-sight component in the high-ionization species can be measured in this direction relatively free from confusion by the material associated with the stars. There is, in addition, a pervasive high-ionization feature with a velocity of -90 km/s in this region, which has the relatively large value of N(C IV)/N(Si IV) = 4.5.

Walborn, N. R.↗
NASA NTRS1980

The X-ray structure of two rich galaxy clusters and its implications for the detectability of microwave diminutions

The X-ray observations of the galaxy clusters Abell 2319 and 576 show a reduced temperature of the microwave background due to the Suniaev-Zeldovich effect. The X-ray maps indicate smooth emission in these clusters concentrated toward the optical center; the surface brightness in the 1-3 keV band falls with the inverse square of radius outside a small core, and fits the Hubble's law of optical surface brightness of elliptical galaxies. The low temperature indicates that in A576 (1) the core radius of the gravitational potential is two to three times larger than inferred from galaxy counts, (2) the temperature rises with radius over most of the cluster and falls to zero at large radii, and (3) the gas does not obey the equation of state.

White, S. D. M.↗
NASA NTRS1996

3C 279 Multiwavelength Monitoring: The Ground-Based Campaign - 2

The optically violently variable quasar 3C 279 was monitored simultaneously from radio to gamma-ray frequencies in 1992 December - 1993 January. We report a detailed study of the ground-based results from radio to optical wavelengths. These data show that 3C 279 has a typical blazar spectrum, slightly rising at radio frequency and then progressively steeper above a first turnover frequency between 37 and 90 GHz. In the millimeter wavelength region, a simple power law is not an adequate description of the spectrum. We suggest that the millimeter "shoulder" corresponds to an additional emission component, self-absorbed between 150 and 375 GHz, possibly associated with the detachment of a new Very Long Base Interferometry (VLBI) knot and with the start of radio flare. A flux increase of 20% over 20 days was observed at 37 and 90 GHz, while contemporaneously the R-band flux doubled in about two weeks. The lack of strong variability in contemporaneous X-ray light curves (possible X-ray variations are less than 30%) implies no direct (i.e., zero lag) correlation between the optical and X-ray fluxes. If X-rays are produced by inverse-Compton scattering of relativistic electrons on some seed photons, the above results exclude that the observed optical photons are the seeds and/or that the relativistic electrons radiating via synchrotron in the optical band are responsible for the scattering to X-ray energies. We suggest that the X-rays are instead produced through the inverse-Compton process by electrons of lower energy, which radiate via synchrotron in the radio to millimeter wave bands and which scatter either on the synchrotron photons themselves or on external photons.

Grandi, P.↗
NASA NTRS1983

High-velocity iron absorption lines in supernova remnant 1006

The very strong, broad absorption lines of a sdOB star continuum shown in the International Ultraviolet Explorer spectrum of the Schweizer-Middleditch star projected near the center of supernova remnant SNR 1006 are investigated. It is found that strong Fe(+) resonance absorption lines are present whose centers show zero radial velocity while their profiles are broadened by approximately 5000-6000 km/s. Also identified are redshifted Si(+), Si(2+), and Si(3+) lines at velocities of approximately 5000 km/s. Results show that the absorptions must occur in the ejecta of the supernova. The strength and symmetric width of the Fe(+) line indicates that the bulk of the ejecta is iron, in agreement with the current theory for the origin of Type I supernova. It is suggested that the previous failure to detect strong Fe emission lines in the X-ray spectra of this and other young Type I SNRs may be a result of the ejecta not having had time to interact significantly with the ambient medium. In addition, the presence of redshifted adsorption lines due to supernova ejecta in its spectrum shows that this star is located behind the SNR and is not physically associated with it.

Wu, C.-C.↗
NASA NTRS2002

Study of Lightweight Ni-Co Alloy Mirrors Obtained by Electroforming Techniques

One contribution in reducing the costs of optics in space can be provided by production of ultralight mirrors. The decrease in the weight of the primary mirror of a telescope is anticipated to lead to the possibility of increasing the size of the telescopes, therefore increasing the amount and distance from which information is received. An electroplating process of ultralight replica mirrors from nickel sulfamate solution will be described. Based on an experimental setup with cylindrical symmetry, flat mirrors with a diameter of 7 inches and thickness of 1.5 mm are made from a Ni-Co alloy. The composition of the resulting deposit is analyzed using Rutherford Backscattering Spectrometry (RBS) and Proton Induced X-ray Emission (PIXE). In order to resolve Ni and Co, 10 MeV nitrogen ions are used as projectiles in the RBS measurements. Solution parameters monitored during the deposition process using optical absorption and polarography will be correlated with the final concentration of Ni and Co in the deposit. Bath parameters like temperature, current density, agitation level and acidity are chosen at certain values and maintained constant from one sample to another throughout the deposition process. The purpose of the experiment is to obtain mirrors with near zero stress, and predetermined composition and hardness. This study is an intermediate step in obtaining through the same process, but with a larger scale setup, ultralight large aperture replica mirrors.

Jones, Ruth↗