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At least 505 records · Page 28
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

A quantitative assessment of the role of the post-shock turbulent region in the formation of Forbush decreases

We present results of a survey of the relation between Forbush decreases, magnetic clouds, and interplanetary shocks during the period August 1978 to November 1982. We have used data from the ISEE-3 study of bidirectional ions associated with magnetic structures or clouds of Marsden et al. (1987), and ground-based observations of Forbush decreases from several neutron monitors. We use the two-step model of a Forbush decrease. We assume that the first step is due to the passage of the postshock turbulent region, and that the second is due to the passage of the magnetic cloud or structure which usually follows the postshock turbulent region. To determine the effectiveness of the postshock turbulent region in causing a Forbush decrease, we have evaluated the radial diffusion coefficient of the postshock turbulent region for the eight largest events during the above period using observations of the magnetic field. We have made a quantitative assessment of the relative importance of the postshock turbulent region in the formation of the Forbush decrease, concluding that the postshock turbulent region alone is not sufficient to cause a Forbush decrease.

Sanderson, T. R.↗
NASA NTRS1979

Visualization of turbulence in flow about a bluff body

A visual diagnostic investigation of turbulence in flow about a circular cylinder was conducted in order to gain a physical insight into the model for the structure of turbulence in flow around a bluff body advanced by the vorticity-amplification theory. A frame-by-frame qualitative and quantitative examination of selected movie strips over time intervals up to more than 800 ms was performed. The selective stretching of cross-vortex tubes, their streamwise tilting, the emergence of an organized turbulent flow pattern near the stagnation zone and the growth of a turbulent boundary layer were clearly revealed by the visualization. In particular, the visualization indicated that the cross-vortex tubes conveyed by the diverging stagnation flow constitute a coherent substructure within the overall turbulent flow that is triggered to its fullest manifestation by the stretching mechanism.

Sadeh, W. Z.↗
NASA NTRS1984

A comparison of specularly reflected gyrating ion orbits with observed shock foot thicknesses

ISEE 1 and 2 flux gate magnetometer data has been used to measure the length or thickness of the 'foot' of 60 quasi-perpendicular supercritical bow shocks along the shock normal. These bow shock crossings are contrasted with 10 quasi-perpendicular subscritical shocks which do not exhibit the precursor foot. The extent of the foot structure is compared to the orbit of an ion having solar wind velocity and undergoing simple specular reflection about the shock normal. Good quantitative agreement between the measured value of the foot and the predicted value of about 0.7 ion gyroradii is evident. The foot thickness normalized to this predicted value is compared to the ratio of fast Mach number to critical fast Mach number. There is no dependence of the normalized foot thickness on Mach number other than at the transition point between subcritical and supercritical regimes. No dependence of the normalized foot thickness on upstream ion Beta is noted.

Livesey, W. A.↗
NASA NTRS1985

Verification of computer-aided designs of traveling-wave tubes utilizing novel dynamic refocusers and graphite electrodes for the multistage depressed collector

A computational procedure for the design of TWT-refocuser-MDC systems was used to design a short dynamic refocusing system and highly efficient four-stage depressed collector for a 200-W, 8- to 18-GHz, TWT. The computations were carried out with advanced, multidimensional computer programs which model the electron beam as a series of disks of charge and follow their trajectories from the RF input of the TWT, through the slow-wave structure and refocusing section, to their points of impact in the depressed collector. Secondary emission losses in the MDC were treated semi-quantitatively by injecting a representative beam of secondary electrons into the MDC analysis at the point of impact of each primary beam. A comparison of computed and measured TWT and MDC performance showed very good agreement. The electrodes of the MDC were fabricated from a particular form of isotropic graphite that was selected for its low secondary electron yield, ease of machinability, and vacuum properties. This MDC was tested (at CW) for more than 1000 hr with negligible degradation in TWT and MDC performances.

Ramins, P.↗
NASA NTRS1986

Variable angle of incidence spectroscopic ellipsometry Application to GaAs-Al(x)Ga(1-x)As multiple heterostructures

The sensitivity of spectroscopic ellipsometry data to multilayer model parameters is shown to be a strong function of the angle of incidence. A quantitative study of sensitivity versus angle of incidence is performed for a GaAs-Al(x)Ga(1-x)As-GaAs substrate structure, showing that maximum sensitivity to layer thicknesses and AlGaAs composition occurs near the wavelength-dependent principal angle. These results are verified by experimental measurements on two molecular-beam epitaxy grown samples.

Snyder, Paul G.↗
NASA NTRS1992

Quantitative analysis of surface characteristics and morphology in Death Valley, California using AIRSAR data

The Jet Propulsion Laboratory Airborne Synthetic Aperture Radar (JPL-AIRSAR) is used to collect full polarimetric measurements at P-, L-, and C-bands. These data are analyzed using the radar analysis and visualization environment (RAVEN). The AIRSAR data are calibrated using in-scene corner reflectors to allow for quantitative analysis of the radar backscatter. RAVEN is used to extract surface characteristics. Inversion models are used to calculate quantitative surface roughness values and fractal dimensions. These values are used to generate synthetic surface plots that represent the small-scale surface structure of areas in Death Valley. These procedures are applied to a playa, smooth salt-pan, and alluvial fan surfaces in Death Valley. Field measurements of surface roughness are used to verify the accuracy.

Kierein-Young, K. S.↗
NASA NTRS1994

Experimental and numerical study of the intermittency exponent mu

After publication of the Kolmogorov refined similarity hypotheses, the small-scale intermittency of the energy dissipation field became a central problem in fully developed turbulence (FDT). This phenomenon has been studied in many different ways, e.g. by searching for corrections to scaling exponents in the inertial range velocity structure functions. A direct measure of this intermittency is, however, available by studying the local rate of energy dissipation, and it may be quantitatively characterized by the intermittency exponent mu. As far as we know, nobody has posed an obvious question: Is the intermittency exponent mu a unique constant, i.e., are the values mu(sub kappa), mu(sub epsilon), mu(sub r), mu(sub b), and mu(sub e) the same at high Reynolds numbers, or do they create a set of different (and perhaps independent) exponents? This paper addresses the above question using the high Reynolds number experiments.

Praskovsky, Alexander↗
NASA NTRS1997

Fitting a 3-D analytic model of the coronal mass ejection to observations

The application of an analytic magnetohydrodynamic model is presented to observations of the time-dependent explusion of 3D coronal mass ejections (CMEs) out of the solar corona. This model relates the white-light appearance of the CME to its internal magnetic field, which takes the form of a closed bubble, filled with a partly anchored, twisted magnetic flux rope and embedded in an otherwise open background field. The density distribution frozen into the expanding CME expanding field is fully 3D, and can be integrated along the line of sight to reproduce observations of scattered white light. The model is able to reproduce the three conspicuous features often associated with CMEs as observed with white-light coronagraphs: a surrounding high-density region, an internal low-density cavity, and a high-density core. The model also describes the self-similar radial expansion of these structures. By varying the model parameters, the model can be fitted directly to observations of CMEs. It is shown how the model can quantitatively match the polarized brightness contrast of a dark cavity emerging through the lower corona as observed by the HAO Mauna Loa K-coronameter to within the noise level of the data.

Gibson, S. E.↗
NASA NTRS2000

Mineralogical Studies of Experimentally Shocked Dolomite: Implications for the Outgassing of Carbonates

Common rock-forming rhombohedral carbonates - calcite and dolomite - constitute a considerable fraction of terrestrial sediments that may be shocked during hypervelocity impacts, such as during the Cretaceous-Tertiary (K/T) event or at the Haughton structure. The relatively modest temperatures needed to decompose carbonates and to release their CO2 are easily attained during such impacts. However, detailed and quantitative understanding of the CO2 release of carbonates as a function of shock stress is still the subject of controversy, as are a number of other reactions and phase transitions that were suggested for carbonates. The first devolatilization studies of carbonates in the early 1980s suggested that incipient devolatilization of calcite and/or dolomite commences at modest pressures, in the range < 10-18 GPa (amounting to 0.03-0.3% CO2 loss), with massive decarbonation (30-40% CO2 loss) occurring at shock pressure of 20 GPa and above. However, most later studies revealed that both calcite and dolomite are unexpectedly stable under shock conditions and no significant outgassing has been observed at pressures as high as 40 GPa for calcite and 60 GPa for dolomite. Despite these uncertainties, the calculations of O'Keefe and Ahrens (and others) serve to illustrate that the K/T bolide liberated sufficient CO2 that a substantial temperature increase of the global atmosphere is possible due to CO2-triggered greenhouse effects. The quantitative understanding of the devolatilization of carbonates as a function of shock stress is obviously critical to refine such calculations. Additional information is contained in the original extended abstract.

Skala, R.↗
NASA NTRS2004

Computed Tomography and its Application for the 3D Characterization of Coarse Grained Meteorites

With judicious selection of parameters, computed tomography can provide high precision density data. Such data can lead to a non-destructive determination of the phases and phase distribution within large solid objects. Of particular interest is the structure of the Mundrabilla meteorite, which has 25 volumes, percent of a sulfide within a metallic meteorite. 3D digital imaging has enabled a quantitative evaluation of the distribution and contiguity of the phases to be determined.

Gillies, Donald C.↗
NASA NTRS2020

Cluster Analysis of Thermal Icequakes Using the Seismometer to Investigate Ice and Ocean Structure (SIIOS): Implications for Ocean World Seismology

Ocean Worlds are of high interest to the planetary community due to the potential habitability of their subsurface oceans. Over the next few decades several missions will be sent to ocean worlds including the Europa Clipper, Dragonfly, and possibly a Europa lander. The Dragonfly and Europa lander missions will carry seismic payloads tasked with detecting and locating seismic sources. The Seismometer to Investigate Ice and Ocean Structure (SIIOS) is a NASA PSTAR funded project that investigates ocean world seismology using terrestrial analogs. The goals of the SIIOS experiment include quantitatively comparing flight-candidate seismometers to traditional instruments, comparing single-station approaches to a small-aperture array, and characterizing the local seismic environment of our field sites. Here we present an analysis of detected local events at our field sites at Gulkana Glacier in Alaska and in Northwest Greenland approximately 80 km North of Qaanaaq, Greenland. Both field sites passively recorded data for about two weeks. We deployed our experiment on Gulkana Glacier in September 2017 and in Greenland in June 2018. At Gulkana there was a nearby USGS weather station which recorded wind data. Temperature data was collected using the MERRA satellite. In Greenland we deployed our own weather station to collect temperature and wind data. Gulkana represents a noisier and more active environment. Temperatures fluctuated around 0°C, allowing for surface runoff to occur during the day. The glacier had several moulins, and during deployment we heard several rockfalls from nearby mountains. In addition to the local environment, Gulkana is located close to an active plate boundary (relative to Greenland). This meant that there were more regional events recorded over two weeks, than in Greenland. Greenland’s local environment was also quieter, and less active. Temperatures remained below freezing. The Greenland ice was much thicker than Gulkana (~850 m versus ~100 m) and our stations were above a subglacial lake. Both conditions can reduce event detections from basal motion. Lastly, we encased our Greenland array in an aluminum vault and buried it beneath the surface unlike our array in Gulkana where the instruments were at the surface and covered with plastic bins. The vault further insulated the array from thermal and atmospheric events.

Marusiak, A. G.↗
NASA NTRS1980

Theoretical foundations for quantitative paleogenetics. III - The molecular divergence of nucleic acids and proteins for the case of genetic events of unequal probability

Theoretical equations are derived for molecular divergence with respect to gene and protein structure in the presence of genetic events with unequal probabilities: amino acid and base compositions, the frequencies of nucleotide replacements, the usage of degenerate codons, the distribution of fixed base replacements within codons and the distribution of fixed base replacements among codons. Results are presented in the form of tables relating the probabilities of given numbers of codon base changes with respect to the original codon for the alpha hemoglobin, beta hemoglobin, myoglobin, cytochrome c and parvalbumin group gene families. Application of the calculations to the rabbit alpha and beta hemoglobin mRNAs and proteins indicates that the genes are separated by about 425 fixed based replacements distributed over 114 codon sites, which is a factor of two greater than previous estimates. The theoretical results also suggest that many more base replacements are required to effect a given gene or protein structural change than previously believed.

Holmquist, R.↗
NASA NTRS1999

The Plasmasphere as "Seen" by the IMAGE Mission

The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) is the first mission designed exclusively to remotely measure the magnetosphere. As such, it will reveal the ring current, plasmasphere, polar cusp, and magnetopause as whole extended, interacting systems. For the first time, our impressions of the global magnetosphere, synthesized through many years of whistler and in situ measurement, will be replaced by images. The overall morphology of each system of plasma and the correspondence of changes between them in response to the sun and solar wind will become available. The Extreme Ultraviolet Imager (EUV) and the Radio Plasma Imager (RPI) are the two IMAGE instruments which will remotely measure and image the plasmasphere. What we expect to "see" from these instruments and how it may be interpreted is the subject of this presentation. The EUV instrument includes three optical cameras, with an almost 90 degree field of view, transverse to the spin axis. EUV is designed to see He+ ions in resonantly scatter solar light at 30.4rim. The IMAGE spacecraft will spin with a period of about 2 minutes, with its spin axis parallel to the orbit normal. The IMAGE orbit will be highly inclined, with a high latitude apogee at a geocentric distance of 8RE and perigee of about 1.2RE. The normal observing integration time of 10 minutes will easily see to the outer edge of the plasmasphere. The RPI instrument makes use of three orthoganal dipole antennas: two in the spin plane with a tip-to-tip length of 500m and one along the spin axis with a length of 20 meters. Using coded pulse transmissions, the RPI instrument will broadcast from 3kHz to 3MHz. With one minute resolution, plasma densities from about 0.1 cm(exp -3) to 100,000 cm(exp -3), along with line-of-sight bulk velocities and locations, will be obtained from all returned radio wave signals. When transmitting from the high latitude magnetospheric cavity, RPI will measure density profiles for the major plasma structures in the magnetosphere, including the magnetopause, polar cusp, and plasmasphere. RPI should also see isolated density irregularities and possibly the plasma sheet. Observations The EUV instrument will return line-of-sight integrated images through the optically thin helium medium of the plasmasphere and magnetosphere. A variety of techniques have been suggested for the translation of the images into physically useful data, such as plasmapause location and three dimensional density distribution. The RPI instrument will return quantitative density values and line-of-sight velocity as a function of position along reflecting wave propagation paths. How they may be used individually and together to study plasmaspheric dynamics and global structure will be discussed. Attention will also be given to the data products and how access to IMAGE data will be provided by the IMAGE team and the NSSDC.

Gallagher, D. L.↗
NASA NTRS2021

Ultraviolet line profiles of slowly rotating massive star winds using the ‘analytic dynamical magnetosphere’ formalism

Recent large-scale spectropolarimetric surveys have established that a small but significant percentage of massive stars host stable, surface dipolar magnetic fields with strengths on the order of kG. These fields channel the dense, radiatively driven stellar wind into circumstellar magnetospheres, whose density and velocity structure can be probed using ultraviolet (UV) spectroscopy of wind-sensitive resonance lines. Coupled with appropriate magnetosphere models, UV spectroscopy provides a valuable way to investigate the wind–field interaction, and can yield quantitative estimates of the wind parameters of magnetic massive stars. We report a systematic investigation of the formation of UV resonance lines in slowly rotating magnetic massive stars with dynamical magnetospheres. We pair the analytic dynamical magnetosphere (ADM) formalism with a simplified radiative transfer technique to produce synthetic UV line profiles. Using a grid of models, we examine the effect of magnetosphere size, the line strength parameter, and the cooling parameter on the structure and modulation of the line profile. We find that magnetic massive stars uniquely exhibit redshifted absorption at most viewing angles and magnetosphere sizes, and that significant changes to the shape and variation of the line profile with varying line strengths can be explained by examining the individual wind components described in the ADM formalism. Finally, we show that the cooling parameter has a negligible effect on the line profiles.

C Erba↗
NASA NTRS1984

Stream dynamics between 1 AU and 2 AU: A detailed comparison of observations and theory

A radial alignment of three solar wind stream structures observed by IMP-7 and -8 (at 1.0 AU) and Voyager 1 and 2 (in the range 1.4 to 1.8 AU) in late 1977 is presented. It is demonstrated that several important aspects of the observed dynamical evolution can be both qualitatively and quantitatively described with a single-fluid 2-D MHD numerical model of quasi-steady corotating flow, including accurate prediction of: (1) the formation of a corotating shock pair at 1.75 AU in the case of a simple, quasi-steady stream; (2) the coalescence of the thermodynamic and magnetic structures associated with the compression regions of two neighboring, interacting, corotating streams; and (3) the dynamical destruction of a small (i.e., low velocity-amplitude, short spatial-scale) stream by its overtaking of a slower moving, high-density region associated with a preceding transient flow. The evolution of these flow systems is discussed in terms of the concepts of filtering and entrainment.

Burlaga, L. F.↗
NASA NTRS1985

Stream dynamics between 1 AU and 2 AU - A comparison of observations and theory

A radial alignment of three solar wind stream structures observed by IMP-7 and -8 (at 1.0 AU) and Voyager 1 and 2 (in the range 1.4 to 1.8 AU) in late 1977 is presented. It is demonstrated that several important aspects of the observed dynamical evolution can be both qualitatively and quantitatively described with a single-fluid 2-D MHD numerical model of quasi-steady corotating flow, including accurate prediction of: (1) the formation of a corotating shock pair at 1.75 AU in the case of a simple, quasi-steady stream; (2) the coalescence of the thermodynamic and magnetic structures associated with the compression regions of two neighboring, interacting, corotating streams; and (3) the dynamical destruction of a small (i.e., low velocity-amplitude, short spatial-scale) stream by its overtaking of a slower moving, high-density region associated with a preceding transient flow. The evolution of these flow systems is discussed in terms of the concepts of filtering and entrainment.

Burlaga, L. F.↗
NASA NTRS1999

Soot Aerosol Properties in Laminar Soot-Emitting Microgravity Nonpremixed Flames

The 0-g flame soot measurements reported in previous studies are extended by adding new 0-g data for different fuel flow rates and burner diameters. The new flame conditions allow more conclusive comparisons regarding the effect of characteristic flow residence times on soot field structure, the influence of fuel preheat on fuel pyrolysis rates near the flame centerline, and the premature cessation of soot growth along the soot annulus in 0-g when the fuel is preheated. The paper also reports on the implementation of thermophoretic soot sampling in a specific 0-g flame featuring burner exit velocities typical of buoyant flames and presents quantitative data on the radial variation of soot microstructure at a fixed height above the burner mouth.

FLAME STRUCTURE↗
NASA NTRS1993

CCD imaging of Comet Wilson (1987VII) - A quantitative coma analysis

Distinctive cometary components (dust, ions, and radicals) are studied on the basis of 2D, narrow-band CCD images of Comet Wilson (1987VII). The fact that Comet Wilson showed no significant structures in the neutral coma during its first perihelion passage is additional evidence for the hypothesis that dynamically new comets do not show a heterogeneous nucleus, but still have a relatively uniform surface. The deviations from the 1/rho law for the decrease of surface brightness as a function of nuclear distance are explained by a combination of short-term variations in the dust production and the effects of solar radiation pressure. The C2 production rate remains basically constant during the whole observational period, while the CN production rate decreases with increasing heliocentric distance. It is inferred that the formation of C2 might be due both to photolytic destruction of some parent molecules as well as to chemical reactions between other species.

Schulz, Rita↗