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At least 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Excitation of solar p-modes

We investigate the rates at which energy is supplied to individual p-modes as a function of their frequencies nu and angular degrees l. The observationally determined rates are compared with those calculated on the hypothesis that the modes are stochastically excited by turbulent convection. The observationally determined excitation rate is assumed to be equal to the product of the mode's energy E and its (radian) line width Gamma. We obtain E from the mode's mean square surface velocity with the aid of its velocity eigenfuction. We assume that Gamma measures the mode's energy decay rate, even though quasi-elastic scattering may dominate true absorption. At fixed l, E(Gamma) arises as nu(exp 7) at low nu, reaches a peak at nu approximately equal 3.5 mHz, and then declines as nu(exp 4.4) at higher nu . At fixed nu, E(Gamma) exhibits a slow decline with increasing l. To calculate energy input rates, P(sub alpha), we rely on the mixing-length model of turbulent convection. We find entropy fluctuations to be about an order of magnitude more effective than the Reynolds stress in exciting p-modes . The calculated P(sub alpha) mimic the nu(exp 7) dependence of E(Gamma) at low nu and the nu(exp -4.4) dependence at high nu. The break of 11.4 powers in the nu-dependence of E(Gamma) across its peak is attributed to a combination of (1) the reflection of high-frequency acoustic waves just below the photosphere where the scale height drops precipitously and (2) the absence of energy-bearing eddies with short enough correlation times to excite high-frequency modes. Two parameters associated with the eddy correlation time are required to match the location and shape of the break. The appropriate values of these parameters, while not unnatural, are poorly constrained by theory. The calculated P(sub alpha) can also be made to fit the magnitude of E(Gamma) with a reasonable value for the eddy aspect ratio. Our resutls suggest a possible explanation for the decline of mode energy with increasing l at fixed nu. Entropy fluctuations couple to changes in volume associated with the oscillation mode. These decrease with decreasing n at fixed nu, becoming almost zero for the f-mode.

Goldreich, Peter↗
NASA NTRS1994

The effects of scattering on solar oscillations

Acoustic modes are scattered by turbulent velocity fluctuations in the solar convection zone. The strongest scattering occurs near the top of the acoustic cavity where the mode changes character from propagating to evanescent. This layer is located at depth z(sub 1) approximately g/omega(exp 2) below the photosphere. The scattering optical depth tau(sub s) is of order M(sub 1)(exp 2), where M(sub 1) is the Mach number of the energy-bearing eddies at z(sub 1). The corresponding contribution to the line width is gamma(exp s) is approximately (omega) M(sub 1)(exp 2)/(pi)(n+1), where n is the mode's radial order. At the top of the acoustic cavity the correlation time of energy-bearing eddies is much longer than omega(exp -1). Also, the pressure scale height H and the eddy correlation length Lambda are comparable to omega/c, where c is the sound speed. Thus scattering couples modes of similar omega and all l and has little effect on the sum of their energies. Observations show that mode energies decline with decreasing n (increasing l) at fixed omega. Consequently, scattering damps p-modes and excites f-modes.

Goldreich, Peter↗
NASA NTRS1995

The origin of N III lambda 990 and C III lambda 977 emission in AGN narrow-line region gas

We discuss implications of Hopkins Ultraviolet Telescope (HUT) detections of C III lambda 977 and N III lambda 990 emission from the narrow-line region of the Seyfert 2 galaxy NGC 1068. In their discovery paper Kriss et al. showed that the unexpectedly great strength of these lines implies that the emitting gas must be shock-heated if the lines are collisionally excited. Here we investigate other processes which excite these lines in photoionization equilibrium. Recombination, mainly dielectronic, and continuum fluorescence are strong contributors to the line. The resulting intensities are sensitive to the velocity field of the emitting gas and require that the turbulence be of the same order of magnitude as the observed line width. We propose optical observations that will decide whether the gas is collisionally or radiatively heated.

Ferguson, J. W.↗
NASA NTRS1994

Modeling radiative transfer in molecular clouds. 1: HCO(+) in the star-forming region W49A North

A new general multilevel, non-Local Thermodynamic Equilibrium (LTE) radiative transfer code, valid for any velocity field, is applied to HCO(+) observations of W49A North. Three classes of collapse models are considered: free-fall collapse (v proportional to 1/sq. root of r), rho proportional to r(exp -3/2) throughout the molecular cloud, successfully reproduces the features of the observations and gives the best fit to the J = 1-0 and J = 3-2 profiles both toward the prominent H II component G of W49A North and off the center. In addition to a slow radial fall-off of density, the theoretical modeling implies the following for the molecular cloud: the large line widths result from motions occurring within the inner 1 pc, and there are probably one or more fragments with peculiar velocities within this same region.

Dickel, Helene R.↗
NASA NTRS1995

A high spectral resolution VLA search for H I absorption towards A496, A1795, and A2584

In this paper, we present the results of a Very Large Array (VLA) search for H I absorption with high spectral resolution (1.6 km/s) towards A496, A1795, A2584, and A2597. These observations are well matched to the properties of cold, optically thick H I clouds, where the line width is given by the width of an individual cloud rather than the dispersion in an ensemble of clouds. We do not detect any H I absorption with narrow linewidths in these clusters. Our limits mainly apply to clouds which are larger than a few tenths parsec-i.e., if the clouds are much smaller than the background radio source and have a low covering factor in velocity space, they could still escape detection. The estimated limits on column density (for clouds in this regime of parameter space) are 2-3 orders of magnitude less than the 10(exp 21)/sq cm required to explain the x-ray absorption seen in some cooling flow clusters. The combination of our high spectral resolution H I absorption searches with the existing lower spectral resolution H I absorption searches and the searches for H I emission makes it unlikely that atomic hydrogen is the dominant component of the cold x-ray absorbing gas in the inter-cloud medium (ICM).

O'Dea, Christopher P.↗
NASA NTRS1995

The development of a tunable, single-frequency ultraviolet laser source for UV filtered Rayleigh scattering

We present the development of a flexible, high power, narrow line width, tunable ultraviolet source for diagnostic application. By frequency tripling the output of a pulsed titanium-sapphire laser, we achieve broadly tunable (227-360 nm) ultraviolet light with high quality spatial and spectral resolution. We also present the characterization of a mercury vapor cell which provides a narrow band, sharp edge absorption filter at 253.7 nm. These two components form the basis for the extension of the Filtered Rayleigh Scattering technique into the ultraviolet. The UV-FRS system is comprised of four pieces: a single frequency, cw tunable Ti:Sapphire seeding source; a high-powered pulsed Ti:Sapphire oscillator; a third harmonic generator system; and an atomic mercury vapor filter. In this paper we discuss the development and characterization of each of these elements.

Finkelstein, N.↗
NASA NTRS1995

A light-induced microwave oscillator

We describe a novel oscillator that converts continuous light energy into sta ble and spectrally pure microwave signals. This light-induced microwave oscillator (LIMO) consists of a pump laser and a feedback circuit, including an intensity modulator, an optical fiber delay line, a photodetector, an amplifier, and a filter. We develop a quasilinear theory and obtain expressions for the threshold condition, the amplitude, the frequency, the line width, and the spectral power density of the oscillation. We also present experimental data to compare with the theoretical results. Our findings indicate that the LIMO can generate ultrastable, spectrally pure microwave reference signals up to 75 GHz with a phase noise lower than -140 dBc/Hz at 10 kHz.

Yao, X. S.↗
NASA NTRS1995

Driven acoustic oscillations within a vertical magnetic field

The effects of a vertical magnetic field on p-mode frequencies, line widths, and eigenfunctions, are examined. A solar model, consisting of a neutrally stable polytropic interior matched to an isothermal chromosphere, is applied. The p-modes are produced by a spatially distributed driver. The atmosphere is threaded by a constant vertical magnetic field. The frequency shifts due to the vertical magnetic field are found to be much smaller than the shifts caused by horizontal fields of similar strength. A large vertical field of 2000 G produces shifts of several nHz. It is found that the frequency shifts decrease with increasing frequency and increase with field strength. The coupling of the acoustic fast mode to the escaping slow modes is inefficient. Constant vertical magnetic field models are therefore incapable of explaining the high level of absorption observed in sunspots and plage.

Hindman, Bradley W.↗
NASA NTRS1996

Demonstration and Analysis of Filtered Rayleigh Scattering Flow Field Diagnostic System

Filtered Rayleigh Scattering (FRS) is a diagnostic technique which measures velocity, temperature, and pressure by determining Doppler shift, total intensity, and spectral line shape of laser induced Rayleigh-Brillouin scattering. In the work reported here, this is accomplished by using a narrow line width, injection seeded Nd-YAG laser sheet to induce Rayleigh-Brillouin scattering from a gas flow. This light is passed through an optical notch filter, and transmitted light is imaged onto an intensified charge coupled display (CCD) camera. By monitoring the grayscale value at a particular pixel while the laser frequency is tuned, the convolution between the Rayleigh-Brillouin scattering profile and the filter transmission profile is attained. Since the filter profile can be independently measured, it can be deconvolved from the measuring signal, yielding the Rayleigh-Brillouin scattering profile. From this profile, flow velocity, temperature, and pressure are determined. In this paper the construction and characterization of the optical notch filter and a newly developed frequency apparatus are discussed.

Forkey, Joseph N.↗
NASA NTRS1997

Positron annihilation processes update

The present knowledge concerning the positron annihilation processes is reviewed, with emphasis on the data of the cross sections of the various processes of interest in astrophysical applications. Recent results are presented including results on reaction rates and line widths, the validity of which is verified.

Guessoum, Nidhal↗
NASA NTRS1996

Chip-on-Board Technology 1996 Year-end Report (Design, Manufacturing, and Reliability Study)

The major impetus for flight qualifying Chip-On-Board (COB) packaging technology is the shift in emphasis for space missions to smaller, better, and cheaper spacecraft and satellites resulting from the NASA New Millenium initiative and similar requirements in DoD-sponsored programs. The most important benefit that can potentially be derived from miniaturizing spacecraft and satellites is the significant cost saving realizable if a smaller launch vehicle may be employed. Besides the program cost saving, there are several other advantages to building COB-based space hardware. First, once a well-controlled process is established, COB can be low cost compared to standard Multi-Chip Module (MCM) technology. This cost competitiveness is regarded as a result of the generally greater availability and lower cost of Known Good Die (KGD). Coupled with the elimination of the first level of packaging (chip package), compact, high-density circuit boards can be realized with Printed Wiring Boards (PWB) that can now be made with ever-decreasing feature size in line width and via hole. Since the COB packaging technique in this study is based mainly on populating bare dice on a suitable multi-layer laminate substrate which is not hermetically sealed, die coating for protection from the environment is required. In recent years, significant improvements have been made in die coating materials which further enhance the appeal of COB. Hysol epoxies, silicone, parylene and silicon nitride are desirable because of their compatible Thermal Coefficient of Expansion (TCE) and good moisture resistant capability. These die coating materials have all been used in the space and other industries with varying degrees of success. COB technology, specifically siliconnitride coated hardware, has been flown by Lockheed on the Polar satellite. In addition, DARPA has invested a substantial amount of resources on MCM and COB-related activities recently. With COB on the verge of becoming a dominant player in DoD programs, DARPA is increasing its support of the availability of KGDs which will help decrease their cost. Aside from the various major developments and trends in the space and defense industries that are favorable to the acceptance and widespread use of'COB packaging technology, implementing COB can be appealing in other aspects. Since the interconnection interface is usually the weak link in a system, the overall circuit or system reliability may actually be improved because of the elimination of a level of interconnect/packaging at the chip. With COB, mixing packaging technologies is possible. Because some devices are only available in commercial plastic packages, populating a multi-layer laminate substrate with both bare dice and plastic-package parts is inevitable. Another attractive feature of COB is that re-workability is possible if die coating is applied only on the die top. This method allows local replacement of individual dice that were found to be defective instead of replacing an entire board. In terms of thermal management, unpackaged devices offer a shorter thermal resistance path than their packaged counterparts thereby improving thermal sinking and heat removal from the parts.

Le, Binh Q.↗
NASA NTRS1998

Recent Advances in Visualizing 3D Flow with LIC

Line Integral Convolution (LIC), introduced by Cabral and Leedom in 1993, is an elegant and versatile technique for representing directional information via patterns of correlation in a texture. Although most commonly used to depict 2D flow, or flow over a surface in 3D, LIC methods can equivalently be used to portray 3D flow through a volume. However, the popularity of LIC as a device for illustrating 3D flow has historically been limited both by the computational expense of generating and rendering such a 3D texture and by the difficulties inherent in clearly and effectively conveying the directional information embodied in the volumetric output textures that are produced. In an earlier paper, we briefly discussed some of the factors that may underlie the perceptual difficulties that we can encounter with dense 3D displays and outlined several strategies for more effectively visualizing 3D flow with volume LIC. In this article, we review in more detail techniques for selectively emphasizing critical regions of interest in a flow and for facilitating the accurate perception of the 3D depth and orientation of overlapping streamlines, and we demonstrate new methods for efficiently incorporating an indication of orientation into a flow representation and for conveying additional information about related scalar quantities such as temperature or vorticity over a flow via subtle, continuous line width and color variations.

Interrante, Victoria↗
NASA NTRS1996

An Acoustic Charge Transport Imager for High Definition Television Applications: Low-Voltage SAW Amplifiers on Multilayer GaAs/ZnO Substrates

This thesis addresses the acoustoelectric issues concerning the amplification of surface acoustic waves (SAWs) and the reflection of SAWs from slanted reflector gratings on GaAs, with application to a novel acoustic charge transport (ACT) device architecture. First a simple model of the SAWAMP was developed, which was subsequently used to define the epitaxially grown material structure necessary to provide simultaneously high resistance and high electron mobility. In addition, a segmented SAWAMP structure was explored with line widths on the order of an acoustic wavelength. This resulted in the demonstration of SAWAMPS with an order of magnitude less voltage and power requirements than previously reported devices. A two-dimensional model was developed to explain the performance of devices with charge confinement layers less then 0.5 mm, which was experimentally verified. This model was extended to predict a greatly increased gain from the addition of a ZnO overlay. These overlays were experimentally attempted, but no working devices were reported due to process incompatibilities. In addition to the SAWAMP research, the reflection of SAWs from slanted gratings on GaAs was also studied and experimentally determined reflection coefficients for both 45 deg grooves and Al stripes on GaAs have been reported for the first time. The SAWAMp and reflector gratings were combined to investigate the integrated ring oscillator for application to the proposed ACT device and design parameters for this device have been provided.

Hunt, W. D.↗
NASA NTRS1997

Determination of 3D coronal structures from UVCS/SOHO synoptic observations

Daily coronal synoptic observations with the ultraviolet coronagraph spectrometer (UVCS) on SOHO are used to create Carrington maps of line of sight intensities and spectral line widths for H I Lyman alpha and the O 6 1032/1037 doublet. These 2D maps can then be used as inputs to a tomographic inversion routine to produce 3D models of the corona out to the 2.5 solar radius in the polar regions (3.0 solar radius in equatorial regions). The initial results for the determination of O(5+) outflow velocities as a function of heliographic latitude is presented.

Strachan, L.↗
NASA NTRS1998

Characterization of Thin Film Microstrip Lines on Polyimide

This paper presents an in depth characterization of thin film microstrip (TFMS) lines fabricated on Dupont PI-2611 polyimide. Measured attenuation and effective dielectric constant is presented for TFMS lines with thicknesses from 2.45-7.4 microns and line widths from 5-34.4 microns over the frequency range of 1-110 GHz. The attenuation is separated into conductor and dielectric losses to determine the loss tangent of Dupont PI-2611 polyimide over the microwave frequency range. In addition, the measured characteristics are compared to closed form equations for a and eff from the literature. Based on the comparisons, recommendations for the best dosed form design equations for TFMS are made.

Ponchak, George E.↗
NASA NTRS2000

Evanescent Microwave Probes Using Coplanar Waveguide and Stripline for Super-Resolution Imaging of Materials

An evanescent field microwave imaging probe based on half-wavelength, microwave transmission line resonators is described. Optimization of the probe tip design, the coupling gap, and the data analysis has resulted in images of metal lines on semiconductor substrates with 2.6 microns spatial resolution and a minimum detectable line width of 0.4 microns at 1 GHz.

Ponchak, G. E.↗
NASA NTRS2000

Sub-Optical Lithography With Nanometer Definition Masks

Nanometer feature size lithography represents a major paradigm shift for the electronics and micro-electro-mechanical industries. In this paper, we discuss the capacity of dynamic focused reactive ion beam (FIB) etching systems to undertake direct and highly anisotropic erosion of thick evaporated gold coatings on boron-doped silicon X-ray mask membranes. FIB offers a new level of flexibility in micro fabrication, allowing for fast fabrication of X-ray masks, where pattern definition and surface alteration are combined in the same step which eliminates the whole lithographic process, in particular resist, resist development, electro-deposition and resist removal. Focused ion beam diameters as small as 7 nm can be obtained enabling fabrication well into the sub-20 nm regime. In preliminary demonstrations of this X-ray mask fabrication technique 22 nm width lines were milled directly through 0.9 microns of gold and a miniature mass spectrometer pattern was milled through over 0.5 microns of gold. Also presented are the results of the shadow printing, using the large depth of field of synchrotron high energy parallel X-ray beam, of these and other sub-optical defined patterns in photoresist conformally coated over surfaces of extreme topographical variation. Assuming that electronic circuits and/or micro devices scale proportionally, the surface area of devices processed with X-ray lithography and 20 nm critical dimension X-ray masks would be 0.5% that of contemporary devices (350 nm CD). The 20 CD mask fabrication represents an initial effort - a further factor of three reduction is anticipated which represents a further order-of-magnitude reduction in die area.

Hartley, Frank T.↗
NASA NTRS2000

Little Homunculus with in the Homunculus of Eta Carinae

The famous HST/WFPC2 images of Eta Carinae provide a two-dimensional projection of the bipolar nebula that is really a three-dimensional structure. Much is hidden in subtle, projected details that a velocity-tuned instrument can pull apart. We have used the HST/STIS with a 52" x 0.1" aperture and with about 5000 spectral resolving power to examine the kinetic information contained within emission/absorption features. By velocity tuning, we can translate this information into spatial structures. The spectroscopic datasets have been transformed to a set of images, spaced at half instrumental line width steps, 15 - 20 km/s , and with a spatial resolution of 0.1 x 0.1 arcsec near Balmer beta and 0.25 x 0.1 arcsec near Balmer alpha. We examined these narrow-band images and individual spectra to characterize the nature of an internal nebula (formerly known as the Integral nebula). The shape of this nebulosity is an bipolar nebula, deeply embedded within the Homunculus, the well-known bipolar nebula surrounding Eta Carinae. The internal nebula is shaped nearly identically to the Homunculus. It is best described as a "little Homunculus within the Homunculus". Indeed, it is reminiscent of the Russian dolls, known as Matryoshka dolls, that successively nest within each larger doll. For that reason, we call this internal nebula the Matryoshka nebula. This was performed as one of the STIS GTO key projects and was funded by the HST project. Observations were done through the STScI.

Ishibashi, Kazunori↗