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At least 919 records · Page 51

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NASA NTRS1981

MAGSAT anomaly map and continental drift

Anomaly maps of high quality are needed to display unambiguously the so called long wave length anomalies. The anomalies were analyzed in terms of continental drift and the nature of their sources is discussed. The map presented confirms the thinness of the oceanic magnetized layer. Continental magnetic anomalies are characterized by elongated structures generally of east-west trend. Paleomagnetic reconstruction shows that the anomalies found in India, Australia, and Antarctic exhibit a fair consistency with the African anomalies. It is also shown that anomalies are locked under the continents and have a fixed geometry.

Lemouel, J. L.↗

NASA NTRS1981

Sensitivity tests of two-dimensional model predictions of corridor effects

Future aerospace-vehicle systems, such as supersonic transport fleets, the Space Shuttle (SS), and the Heavy-Lift Launch Vehicle (HLLV) system will inject substantial amounts of pollutants into the stratosphere. It is, therefore, pertinent to ask whether the operation of these systems will lead to deleterious effects in the atmosphere. The current investigation is concerned with the development of criteria to assess the likelihood of a detectable corridor effect being caused by the long-term deposition of pollutants at a single latitude. The sources are assumed to operate continuously and at a uniform rate for periods of many years. It is found that transport by meridional winds and by eddy processes acts to diminish the corridor effect by advecting the pollutants out of the region of injection and by mixing them with the ambient air. Attention is given to the altitude for which a detectable corridor effect can be expected for the hypothetical launching of 400 HLLV's per year for 10 years.

Borucki, W. J.↗

NASA NTRS1983

Millisecond time variations in hard X-ray solar flares

The results of a search for fast spikes in 2830 hard X-ray solar flares as observed with the hard X-ray burst spectrometer on the Solar Maximum Mission (SMM) are presented. Hundreds of fast spikes with durations of less than 1 sec have been detected at time resolutions of 128 msec and 10 msec. Fast spikes have been detected with rise and decay times as short as 20 msec and with widths as short as 45 msec. They are the fastest hard X-ray variations yet seen from the sun. The observations of such fast variations place new constraints on the physical nature of the source, and these observations and constraints are discussed in terms of nonthermal and thermal models of flares.

Kiplinger, A. L.↗

NASA NTRS1986

Transfer function analysis of thermospheric perturbations

Applying perturbation theory, a spectral model in terms of vectors spherical harmonics (Legendre polynomials) is used to describe the short term thermospheric perturbations originating in the auroral regions. The source may be Joule heating, particle precipitation or ExB ion drift-momentum coupling. A multiconstituent atmosphere is considered, allowing for the collisional momentum exchange between species including Ar, O2, N2, O, He and H. The coupled equations of energy, mass and momentum conservation are solved simultaneously for the major species N2 and O. Applying homogeneous boundary conditions, the integration is carred out from the Earth's surface up to 700 km. In the analysis, the spherical harmonics are treated as eigenfunctions, assuming that the Earth's rotation (and prevailing circulation) do not significantly affect perturbations with periods which are typically much less than one day. Under these simplifying assumptions, and given a particular source distribution in the vertical, a two dimensional transfer function is constructed to describe the three dimensional response of the atmosphere. In the order of increasing horizontal wave numbers (order of polynomials), this transfer function reveals five components. To compile the transfer function, the numerical computations are very time consuming (about 100 hours on a VAX for one particular vertical source distribution). However, given the transfer function, the atmospheric response in space and time (using Fourier integral representation) can be constructed with a few seconds of a central processing unit. This model is applied in a case study of wind and temperature measurements on the Dynamics Explorer B, which show features characteristic of a ringlike excitation source in the auroral oval. The data can be interpreted as gravity waves which are focused (and amplified) in the polar region and then are reflected to propagate toward lower latitudes.

Mayr, H. G.↗

NASA NTRS1986

Estimating the vibration level of an L-shaped beam using power flow techniques

The response of one component of an L-shaped beam, with point force excitation on the other component, is estimated using the power flow method. The transmitted power from the source component to the receiver component is expressed in terms of the transfer and input mobilities at the excitation point and the joint. The response is estimated both in narrow frequency bands, using the exact geometry of the beams, and as a frequency averaged response using infinite beam models. The results using this power flow technique are compared to the results obtained using finite element analysis (FEA) of the L-shaped beam for the low frequency response and to results obtained using statistical energy analysis (SEA) for the high frequencies. The agreement between the FEA results and the power flow method results at low frequencies is very good. SEA results are in terms of frequency averaged levels and these are in perfect agreement with the results obtained using the infinite beam models in the power flow method. The narrow frequency band results from the power flow method also converge to the SEA results at high frequencies. The advantage of the power flow method is that detail of the response can be retained while reducing computation time, which will allow the narrow frequency band analysis of the response to be extended to higher frequencies.

Cuschieri, J. M.↗

NASA NTRS1987

The VISTA spacecraft: Advantages of ICF (Inertial Confinement Fusion) for interplanetary fusions propulsion applications

Inertial Confinement Fusion (ICF) is an attractive engine power source for interplanetary manned spacecraft, especially for near-term missions requiring minimum flight duration, because ICF has inherent high power-to-mass ratios and high specific impulses. We have developed a new vehicle concept called VISTA that uses ICF and is capable of round-trip manned missions to Mars in 100 days using A.D. 2020 technology. We describe VISTA's engine operation, discuss associated plasma issues, and describe the advantages of DT fuel for near-term applications. Although ICF is potentially superior to non-fusion technologies for near-term interplanetary transport, the performance capabilities of VISTA cannot be meaningfully compared with those of magnetic-fusion systems because of the lack of a comparable study of the magnetic-fusion systems. We urge that such a study be conducted.

Orth, Charles D.↗

NASA NTRS1987

Distribution of whistler mode bursts at Venus

Several thousand impulsive whistler mode noise bursts were detected by the Pioneer Venus wave instrument during the first 10 seasons with nightside traversals at low altitudes. The altitude distribution for these events shows that essentially all of the bursts were detected when the orbiter was less than 2000 km above the planet, suggesting that the varying plasma conditions could not maintain coherent whistler mode field-aligned guidance over greater distances. Within the 2000-km range, the distribution of the number of events versus altitude shows that there are two distinct subregions. These results are interpreted in terms of two types of whistler mode propagation from sources below the ionosphere.

Scarf, F. L.↗

NASA NTRS1987

Microwave emission from steady and moving sunspots

Force-free extrapolations of photospheric magnetic field observations from Marshall Space Flight Center have been used to compute the total intensity and circular polarizaton of sunspot associated emission from active region 2502 in the period June 13 to 15, 1980. The computed maps were compared to high resolution observations of the same active region obtained with the Westerbork Synthesis Radio Telescope. The most interesting aspect of the active region was the development of a new spot between the preceding and the following spots on June 14, which subsequently merged with the preceding with the preceding spot. The new spot was associated with enhanced microwave emission with a peak brightness temperature in excess of 4 x 10 to the 6th K. It is shown that unrealistic values of the conductive flux are required for the interpretation of the emission of the new sunspot in terms of thermal processes. It is suggested that this source is due to gyrosynchrotron radiation from mildly relativistic electrons accelerated by resistive instabilities in the evolving magnetic field.

Alissandrakis, C. E.↗

NASA NTRS1991

Linear ion trap based atomic frequency standard

In order to develop a trapped ion-based fieldable frequency standard with stability 1 x 10 to the -13th/sq rt tau for averaging times tau greater than 10,000 s, a hybrid RF/DC linear ion trap was developed which permits storage of large numbers of ions with reduced susceptibility to the second-order Doppler effect caused by the RF confining fields. The authors have confined Hg-199(+) ions in this trap and have measured very high Q transitions with good SNRs. In preliminary measurements they obtained stabilities of 1.6 x 10 to the -13th/sq rt tau (tau between 50 and 800 s) with a 160-mHz wide atomic resonance linewidth and a signal-to-noise ratio of 40 for each measurement cycle. Atomic resonance lines as narrow as 30 mHz on the 40.5-GHz clock transition have been measured with no appreciable reduction in the ion signal. A stability of 7 x 10 to the -14th/sq rt tau is made possible by the signal-to-noise and line Q of this measured transition. Analysis of fundamental sources of frequency instability indicates that a long-term stability of 2 x 10 to the -16th is feasible for this device with existing technology for tau = 10 to the 6th s or more.

Prestage, John D.↗

NASA NTRS1993

Galactic fine-structure lines - Morphologies of the warm ionized interstellar medium

Far-infrared ionic fine-structure emission lines associated with the large-scale structure of the Galactic ionized medium are interpreted in terms of a heuristic model in which three distinct source morphologies are considered: ionization-bounded H II regions, an extended, low-density medium, and externally ionized surfaces of neutral clouds. The model reveals that the overall luminosity of the Galaxy in the fine-structure lines of both C II and N II is due, in comparable proportions, to diffuse medium and the boundaries between ionized and neutral regions. Limits are set on the luminosity of the diffuse component in the plane of the Galaxy. Some individual lines of sight of the COBE FIRAS instrument are dominated, in these spectral lines, by H II regions, even with its 7 deg field of view. Average densities of both H II regions and photodissociation regions are estimated on a Galactic scale.

Petuchowski, S. J.↗

NASA NTRS1993

LDEF Materials Results for Spacecraft Applications

These proceedings describe the application of LDEF data to spacecraft and payload design, and emphasize where space environmental effects on materials research and development is needed as defined by LDEF data. The LDEF six years of exposure of materials has proven to be by far the most comprehensive source of information ever obtained on the long-term performance of materials in the space environment. The conference provided a forum for materials scientists and engineers to review and critically assess the LDEF results from the standpoint of their relevance, significance, and impact on spacecraft design practice. The impact of the LDEF findings on materials selection and qualification, and the needs and plans for further study, were addressed from several perspectives. Many timely and needed changes and modifications in external spacecraft materials selection have occurred as a result of LDEF investigations.

Whitaker, Ann F.↗

NASA NTRS1996

On the Retrieval of Lightning Radio Sources from Time-of-Arrival Data

We examine the problem of retrieving three-dimensional lightning locations from radio frequency Time-Of-Arrival (TOA) measurements. Arbitrary antenna locations are considered. By judiciously differencing measurements that are related to the location of the antennas and their excitation times, the problem is converted from the initial spherical nonlinear form to a system of linear equations. In the linear formalism, the source location and time-of-occurrence is viewed geometrically as an intersection of hyperplanes in the four-dimensional Minkowski space (x,y,z,t). The linear equations are solved to obtain explicit analytic expressions for the location and time variables. Retrieval errors are not interpreted with conventional Geometrical Dilution of Precision (GDOP) arguments as discussed by Holmes and Reedy (1951), but with more recent inversion analyses considered by Twomey (1977). Measurement errors are propagated analytically so that the specific effect of these errors on the solution is clarified. The sensitivity of the solution on the number of antennas used, antenna network geometry, source position, and measurement differencing schemes are discussed in terms of the eigenvalues of the linear system.

Koshak, William J.↗

NASA NTRS2000

Search for the Centrifugal Barrier in Transient X-Ray Pulsars

During the period 15 April 1997 through 15 April 2000 we have been supported in part by NASA grant NAG5-4115 to pursue a 'Search for the Centrifugal Barrier in Transient X-ray Pulsars' with the Rossi X-Ray Timing Explorer (RXTE). The RXTE observations associated with this research involved monitoring the X-ray intensities and pulse profiles of six transient X-ray pulsars on five different occasions. The goal was to detennine whether the so-called 'centrifugal barrier' effect exists for accretion-powered X-ray pulsars. In this long-postulated effect, accretion onto the neutron star should be inhibited when the accretion rate falls below the point where the magnetospheric boundary lies outside the neutron star's corotation radius. Our RXTE observations yielded detections of two of the six X-ray pulsars under study, but were insufficient, overall, to yield any new insight into the centrifugal barrier effect. The data from the two detected pulsars will be utilized by Professor Deepto Chakrabarty as part of his overall campaign to study the long-term behavior of accretion-powered X-ray sources.

Jefferson, Adrian↗

NASA NTRS2005

Prediction of Turbulence-Generated Noise in Unheated Jets: JeNo Technical Manual (Version 1.0) - Part 1

The model-based approach, used by the JeNo code to predict jet noise spectral directivity, is described. A linearized form of Lilley's equation governs the non-causal Green s function of interest, with the non-linear terms on the right hand side identified as the source. A Reynolds-averaged Navier-Stokes (RANS) solution yields the required mean flow for the solution of the propagation Green s function in a locally parallel flow. The RANS solution also produces time- and length-scales needed to model the non-compact source, the turbulent velocity correlation tensor, with exponential temporal and spatial functions. It is shown that while an exact non-causal Green s function accurately predicts the observed shift in the location of the spectrum peak with angle as well as the angularity of sound at low to moderate Mach numbers, the polar directivity of radiated sound is not entirely captured by this Green s function at high subsonic and supersonic acoustic Mach numbers. Results presented for unheated jets in the Mach number range of 0.51 to 1.8 suggest that near the peak radiation angle of high-speed jets, a different source/Green s function convolution integral may be required in order to capture the peak observed directivity of jet noise. A sample Mach 0.90 heated jet is also discussed that highlights the requirements for a comprehensive jet noise prediction model.

Khavaran, Abbas↗

NASA NTRS2006

Miniaturized Mercury Ion Clock for Ultrastable Deep Space Applications

We have recently completed a prototype ion-clock physics package based on Hg ions shuttled between a quadrupole and a 16-pole rf trap. With this architecture we have demonstrated short-term stability ~2-3x10-13 at 1 second, averaging to 10-15 at 1 day. This development shows that H-maser quality stabilities can be produced in a small clock package, comparable in size to an ultra-stable quartz oscillator required for holding 1-2x10-13 at 1 second. This performance was obtained in a sealed vacuum configuration where only a getter pump was used to maintain vacuum. The vacuum tube containing the traps has now been under sealed vacuum conditions for nearly 1.5 years with no measurable degradation of ion trapping lifetimes or clock short-term performance. Because the tube is sealed, the Hg source and Neon buffer gas are held indefinitely, for the life of the tube. There is no consumption of Hg in this system unlike in a Cs beam tube where lifetime is often limited by Cs depletion.

mercury ion clocks↗

NASA NTRS2011

Multi-Decadal Change of Atmospheric Aerosols and their Effect on Surface Radiation

We present an investigation on multi-decadal changes of atmospheric aerosols and their effects on surface radiation using a global chemistry transport model, GOCART, along with the near-term to long-term data records. We focus on a 28-year time period of satellite era from 1980 to 2007 during which a suite of aerosol data from satellite observations, ground-based measurements, and intensive field experiments have become available. Particularly: (1) We compare the model calculated clear sky downward radiation at the surface with surface network data from BSRN and CMA (2) We compare the model and surface data with satellite derived downward radiation products from ISCCP and SRS (3) We analyze the long-term global and regional aerosol trends in major anthropogenic source regions (North America, Europe, Asia) that have been experiencing considerable changes of emissions during the three decades, dust and biomass burning regions that have large interannual variability, downwind regions that are directly affected by the changes in the source area, and remote regions that are considered to representing "background" conditions. The comparisons and methods from this study can be applied to multiple model analysis in the AeroCom framework.

Chin, Mian↗

NASA NTRS2012

Multi-decadal Change of Atmospheric Aerosols and their Effect on Surface Radiation

We present an investigation on multi-decadal changes of atmospheric aerosols and their effects on surface radiation using a global chemistry transport model GOCART along with the near-term to long-term data records. We focus on a 28-year time period of satellite era from 1980 to 2007 during which a suite of aerosol data from satellite observations, ground-based measurements, and intensive field experiments have become available. Particularly: (1) We compare the model calculated clear sky downward radiation at the surface with surface network data from Baseline Surface Radiation Network (BSRN) and CMA (2) We compare the model and surface data with satellite derived downward radiation products from ISCCP and SRB (3) We analyze the long-term global and regional aerosol trends in major anthropogenic source regions (North America, Europe, Asia) that have been experiencing considerable changes of emissions during the three decades, dust and biomass burning regions that have large interannual variability, downwind regions that are directly affected by the changes in the source area, and remote regions that are considered to representing "background" conditions.

Chin, Mian↗

NASA NTRS2011

The OLI Radiometric Scale Realization Round Robin Measurement Campaign

A round robin radiometric scale realization was performed at the Ball Aerospace Radiometric Calibration Laboratory in January/February 2011 in support of the Operational Land Imager (OLI) Program. Participants included Ball Aerospace, NIST, NASA Goddard Space Flight Center, and the University of Arizona. The eight day campaign included multiple observations of three integrating sphere sources by nine radiometers. The objective of the campaign was to validate the radiance calibration uncertainty ascribed to the integrating sphere used to calibrate the OLI instrument. The instrument level calibration source uncertainty was validated by quatnifying: (1) the long term stability of the NIST calibrated radiance artifact, (2) the responsivity scale of the Ball Aerospace transfer radiometer and (3) the operational characteristics of the large integrating sphere.

Cutlip, Hansford↗