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At least 937 records · Page 52

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NASA NTRS1998

Folding and translocation of the undecamer of poly-L-leucine across the water-hexane interface. A molecular dynamics study

The undecamer of poly-L-leucine at the water-hexane interface is studied by molecular dynamics simulations. This represents a simple model relevant to folding and insertion of hydrophobic peptides into membranes. The peptide, initially placed in a random coil conformation on the aqueous side of the system, rapidly translocates toward the hexane phase and undergoes interfacial folding into an alpha-helix in the subsequent 36 ns. Folding is nonsequential and highly dynamic. The initially formed helical segment at the N-terminus of the undecamer becomes transiently broken and, subsequently, reforms before the remainder of the peptide folds from the C-terminus. The formation of intramolecular hydrogen bonds during the folding of the peptide is preceded by a dehydration of the participating polar groups, as they become immersed in hexane. Folding proceeds through a short-lived intermediate, a 3(10)-helix, which rapidly interconverts to an alpha-helix. Both helices contribute to the equilibrium ensemble of folded structures. The helical peptide is largely buried in hexane, yet remains adsorbed at the interface. Its preferred orientation is parallel to the interface, although the perpendicular arrangement with the N-terminus immersed in hexane is only slightly less favorable. In contrast, the reversed orientation is highly unfavorable, because it would require dehydration of C-terminus carbonyl groups that do not participate in intramolecular hydrogen bonding. For the same reason, the transfer of the undecamer from the interface to the bulk hexane is also unfavorable. The results suggest that hydrophobic peptides fold in the interfacial region and, simultaneously, translocate into the nonpolar side of the interface. It is further implied that peptide insertion into the membrane is accomplished by rotating from the parallel to the perpendicular orientation, most likely in such a way that the N-terminus penetrates the bilayer.

NASA Discipline Exobiology↗

NASA NTRS1995

RY Scuti: Infrared and radio observations of the mass-loss wind of a massive binary star system

We report infrared (IR) imaging, IR photometry, IR spectroscopy, optical/IR photopolarimetry, and Very Large Array (VLA) radio observations of the peculiar binary star RY Scuti. These observations provide an unprecedented view of the detailed spatial structure of the equatorial mass-loss wind of a massive, luminous, 'overcontact' binary system. The binary star (0.43 AU separation) is surrounded by a flattened equatorial disk with an outer radius of approximately = 3 x 10(exp 16) cm (2000 AU) that emits strongly in the IR and radio. The inside of the disk is ionized and emits free-free radiation from hydrogen and 12.8 micrometers forbidden-line emission from (Ne II); the outside of the disk emits thermal radiation from silicate dust. Radio continuum emission is also produced in a compact H II region surrounding the binary. The dust may have a polycyclic aromatic hydrocarbon (PAH) component. We use a rudimentary geometric model in which the thermal IR and radio emission from the disk are assumed to arise in a pair of concentric toroidal rings to estimate the physical properties of the disk. The mean radius of the ionized gas toroid is approximately = 1.3 x 10(exp 16) cm (870 AU), and the mean radius of the dust toroid is approximately = 2.2 x 10(exp 16) cm (1470 AU). RY Scuti has a small intrinsic polarization, with the electric vector perpendicular to the equatorial disk, that is probably caused by electron scattering from hot gas close to the central binary. We conclude that neon in the nebula is overabundant with respect to hydrogen and helium by a factor of between 1.6 and 10. Our IR/radio image data suggest that the circumstellar disk is part of an extensive radiation driven mass-loss outflow that is strongly confined to the equatorial plane of the binary system. The sharp spatial separation of the outer dust torous from the inner ionized gas torus confirms earlier suggestions that dust formation in the circumstellar ejecta of very hot stars must occur in regions that are shielded from the hard radiation field.

Gehrz, R. D.↗

DOE OSTI2020

Out-of-plane carrier spin in transition-metal dichalcogenides under electric current

Significance Although out-of-plane current-induced spin polarization (CISP) is important for perpendicular-magnetization reorientation, and it has been realized by the crystalline symmetry breaking, its material examples are relatively fewer compared with in-plane CISP in Rashba systems. With the help of intrinsic spin–orbit coupling, an intriguing out-of-plane CISP is designed in transition-metal dichalcogenides by the symmetry breaking in the spin space, which provides opportunities for out-of-plane magnetization rotation and electric control of valley splitting. Moreover, the spin polarization is associated with valley-dependent responses to electric current and adds a dimension, valley degree of freedom, to the study of CISP. The symmetry argument and non-Rashba effective model also helps to illuminate these physics and broaden the scope of the CISP.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2022

Mutual spin-phonon driving effects and phonon eigenvector renormalization in nickel (II) oxide

The physics of mutual interaction of phonon quasiparticles with electronic spin degrees of freedom, leading to unusual transport phenomena of spin and heat, has been a subject of continuing interests for decades. Despite its pivotal role in transport processes, the effect of spin-phonon coupling on the phonon system, especially acoustic phonon properties, has so far been elusive. By means of inelastic neutron scattering and first-principles calculations, anomalous scattering spectral intensity from acoustic phonons was identified in the exemplary collinear antiferromagnetic nickel (II) oxide, unveiling strong spin-lattice correlations that renormalize the polarization of acoustic phonon. In particular, a clear magnetic scattering signature of the measured neutron scattering intensity from acoustic phonons is demonstrated by its momentum transfer and temperature dependences. The anomalous scattering intensity is successfully modeled with a modified magneto-vibrational scattering cross-section, suggesting the presence of spin precession driven by phonon. The renormalization of phonon eigenvector is indicated by the observed “geometry-forbidden” neutron scattering intensity from transverse acoustic phonon. Importantly, the eigenvector renormalization cannot be explained by magnetostriction but instead, it could result from the coupling between phonon and local magnetization of ions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2025

Demonstration of controlled spatial incoherence for beam smoothing

A concept for beam smoothing of high-energy laser systems is demonstrated. Multiple coherent beams are multiplexed so that their incoherent mix decreases the far-field nonuniformity after random spatial phase modulation introduced by a phase plate. Modeling shows that the incoherent mix effectively leads to the expected scaling in terms of the number of uncorrelated beams, and that spatial multiplexing can be combined with smoothing by spectral dispersion. The results of an experimental demonstration of this concept with up to six beams, leading to more than 40 modes with polarization multiplexing and angular dispersion, are consistent with expectations. The potential practical advantages of this smoothing scheme and future developments are discussed.

47 OTHER INSTRUMENTATION↗

NASA NTRS1993

Future operations of Pioneer 10 and 11

The Pioneers 10 and 11 spacecraft were launched on 2 March 1972 and 5 April 1973 and are now 53 and 35 AU from the sun. Pioneer 10 is now the most distant man-made object in our solar system and the in situ measurements of the gas and dust surrounding the sun have been obtained for almost two solar sunspot cycles. Plasma analyzer measurements out to 50 AU show that the mean velocity is about 430 km/sec, the mean density decreases as R exp -2, and that the terminal shock has not been encountered. The magnetic field is drawn out to form Archimedean spirals in the ecliptic place and dipolelike asymmetry in the polar directions as predicted by the Parker model. Galactic cosmic ray measurements of the intensity and radial gradient indicate a 'modulation boundary' between 70 to 100 AU from the sun. All measurements to date indicate that both spacecraft are within the heliosphere and proceeding toward the outer boundary where the modulating effect of solar activity on cosmic ray intensity ceases.

Dyal, P.↗

NASA NTRS1995

Observational studies of the clearing phase in proto-planetary disk systems

Progress in this study is summarized for its second year. An extensive program of high dispersion UV spectral studies using the IUE (International Ultraviolet Explorer) has resulted in acquisition of 40 Herbig Ae/Be star and related object spectra. We find that accreting, circumstellar gas is detected with velocities consistent with material in free-fall toward the stars in approximately 38% of the sample. With acquisition of optical measurements of the projected stellar rotational velocities, we find that the systems with accreting gas exhibit systematically higher projected rotational velocities than the systems showing signatures of outflowing material only. When combined with polarimetric and photometric data, the IUE spectra for these stars indicate that accretion in intermediate-mass pre-main sequence stars is confined to the plane of the circumstellar dust disk. This is in contrast to lower-mass PMS stars where accretion, persumably along magnetic field lines, is seen at polar latitudes. Our data also support significant clearing of the central regions of these circumstellar disks, as originally suggested. The model which most closely matches the observational data is accretion from a disk envelope, as developed by Calvet et al. (1994).

Grady, Carol A.↗

NASA NTRS2001

Substituent and Solvent Effects on Excited State Charge Transfer Behavior of Highly Fluorescent Dyes Containing Thiophenylimidazole-Based Aldehydes

The excited state charge transfer for a series of highly fluorescent dyes containing thiophenylimidazole moiety was investigated. These systems follow the Twisted Intramolecular Charge Transfer (TICT) model. Dual fluorescence was observed for each substituted dye. X-ray structures analysis reveals a twisted ground state geometry for the donor substituted aryl on the 4 and 5 position at the imidazole ring. The excited state charge transfer was modeled by a linear solvation energy relationship using Taft's pi and Dimroth's E(sub T)(30) as solvent parameters. There is linear relation between the energy of the fluorescence transition and solvent polarity. The degree of stabilization of the excited state charge transfer was found to be consistent with the intramolecular molecular charge transfer. Excited dipole moment was studied by utilizing the solvatochromic shift method.

Santos, Javier↗

NASA NTRS2003

Analysis of Properties of the North and South Polar Layered Deposits

One of the many questions of Martian exploration is to uncover the history of Mars, through analysis of the polar layered deposits (PLD). Martian polar ice caps hold most of the exposed water ice on the surface of Mars and yet their history and physical processes involved in their formation are unclear. We will attempt to contribute to our knowledge of the composition and stratigraphy of the PLD. In this work we present the latest imaging data acquired by the Mars Odyssey THermal EMission Imaging System (THEMIS) [1] and place it into context of the Mars Global Surveyor (MGS) data. We have discussed the North Polar data in [5]. This work concentrates on data acquired over the South pole of Mars and compares properties of North and South PLD. We are primarily interested in properties of the layers in both ice caps : their continuity, morphology and stratigraphy. These questions can be addressed by THEMIS VIS color images, along with MOC high resolution data and MOLA Digital Elevation Models (DEM). We will investigate thermophysical properties of the layered deposits employing THEMIS IR images. Based on the data obtained by the orbiting spacecraft and described here, we will attempt to expose major directions for modeling and further understanding of the physical processes involved in the formation of the polar layered terrain

Ivanov, A. B.↗

NASA NTRS2005

Solar Sail Roadmap Mission GN and C Challenges

The NASA In-Space Propulsion program is funding development work for solar sails to enhance future scientific opportunities. Key to this effort are scientific solar sail roadmap missions identified by peer review. The two near-term missions of interest are L1 Diamond and Solar Polar Imager. Additionally, the New Millennium Program is sponsoring the Space Technology 9 (ST9) demonstration mission. Solar sails are one of five technologies competing for the ST9 flight demonstration. Two candidate solar sail missions have been identified for a potential ST9 flight. All the roadmap missions and candidate flight demonstration missions face various GN&C challenges. A variety of efforts are underway to address these challenges. These include control actuator design and testing, low thrust optimization studies, attitude control system design and modeling, control-structure interaction studies, trajectory control design, and solar radiation pressure model development. Here we survey the various efforts underway and identify a few of specific recent interest and focus.

Heaton, Andrew F.↗

NASA NTRS2008

Electron Precipitation Parameters and Ionospheric Conductances Inferred from Auroral Images Acquired by the Visible Imaging Systems (VIS) on the Polar Spacecraft

The Visible Imaging System (VIS) on the polar spacecraft provided time sequences of auroral images at multiple wavelengths that yield information of auroral dynamics on a global scale with a spatial resolution of - 20 km and temporal resolution of approx. 1 minute. Time sequences of VIS images in which the aurora was highly dynamic are used to infer global maps for the electron precipitation parameters, energy flux and characteristic energies, and ionospheric conductances. The maps are inferred from the corresponding VIS images using an auroral model (Lumerzheim et al., 1987). The temporal and spatial resolution of the VIS inferred patterns are unprecedented. The inferred patterns are highly structured and vary significantly on a time scale of less than 5 minutes. These patterns can be very beneficial for global physics-based numerical models for the high-latitude ionosphere which previously had to rely on statistical models for the electron precipitation and ionospheric conductance.

Sigwarth, John B.↗

NASA NTRSDate not supplied

Improved Characterization of PSC Processes Derived from a Third-Generation CALIOP and MLS Detection and Composition Classification Algorithm

The new 3-year CloudSat and CALIPSO Science Team project described in this poster will use a unique combination of data from the Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) instrument on CALIPSO and the Microwave Limb Sounder (MLS) on Aura, in conjunction with supporting meteorological information and detailed modeling studies, to advance our understanding of polar stratospheric cloud (PSC) processes and their role in ozone depletion. We will develop a third-generation (Gen3) PSC detection and composition algorithm that incorporates a new, more robust two-dimensional, multi-channel CALIOP feature detection scheme (2D-McDA). We will also devise and implement an improved two-dimensional PSC composition classification scheme that utilizes multiple parameters (e.g., CALIOP 532-nm parallel and perpendicular scattering ratios, CALIOP 1064-nm total scattering ratio, MLS HNO3 and H2O, ambient temperature, and temperature histories) in a Bayesian approach to determine the most likely PSC composition and help constrain solid PSC particle number density and size/shape. The combined CALIOP/MLS analyses will allow us to study in detail the full life cycle of PSCs and their resulting impact on gas-phase HNO3 and H2O, which should lead to improved parameterizations of PSC microphysics in global chemistry-climate models (CCMs) where detailed particle information is not available. The Gen3 CALIOP PSC algorithm will be a natural stepping-stone toward the analysis of data collected during future spaceborne lidar missions, such as NASA’s Atmosphere Observation System (AOS) mission currently scheduled for launch late in this decade. We will also investigate possible trends in PSC occurrence and composition over the entire CALIOP data record and through further comparisons with the Stratospheric Aerosol Measurement (SAM) II solar occultation PSC record from 1979-1989. Finally, we will validate the mountain-wave parameterization and PSC schemes used in the UM-UKCA (Unified Model coupled to the United Kingdom Chemistry and Aerosol module) CCM through detailed comparisons with earlier CALIOP PSC data products and those developed under this proposal.

CALIPSO↗

NASA NTRS1992

External calibration of polarimetric radar images using distributed targets

A new technique is presented for calibrating polarimetric synthetic aperture radar (SAR) images using only the responses from natural distributed targets. The model for polarimetric radars is assumed to be X = cRST where X is the measured scattering matrix corresponding to the target scattering matrix S distorted by the system matrices T and R (in general T does not equal R(sup t)). To allow for the polarimetric calibration using only distributed targets and corner reflectors, van Zyl assumed a reciprocal polarimetric radar model with T = R(sup t); when applied for JPL SAR data, a heuristic symmetrization procedure is used by POLCAL to compensate the phase difference between the measured HV and VH responses and then take the average of both. This heuristic approach causes some non-removable cross-polarization responses for corner reflectors, which can be avoided by a rigorous symmetrization method based on reciprocity. After the radar is made reciprocal, a new algorithm based on the responses from distributed targets with reflection symmetry is developed to estimate the cross-talk parameters. The new algorithm never experiences problems in convergence and is also found to converge faster than the existing routines implemented for POLCAL. When the new technique is implemented for the JPL polarimetric data, symmetrization and cross-talk removal are performed on a line-by-line (azimuth) basis. After the cross-talks are removed from the entire image, phase and amplitude calibrations are carried out by selecting distributed targets either with azimuthal symmetry along the looking direction or with some well-known volume and surface scattering mechanisms to estimate the relative phases and amplitude responses of the horizontal and vertical channels.

Yueh, Simon H.↗

NASA NTRS1997

Solar and Magnetic Attitude Determination for Small Spacecraft

During the Phase B development of the NASA New Millennium Program (NMP) Earth Orbiter-1 (EO-1) spacecraft, detailed analyses were performed for on-board attitude determination using the Sun and the Earth's magnetic field. This work utilized the TRMM 'Contingency Mode' as a starting point but concentrated on implementation for a small spacecraft without a high performance mechanical gyro package. The analyses and simulations performed demonstrate a geographic dependence due to diurnal variations in the Earth magnetic field with respect to the Sun synchronous, nearly polar orbit. Sensitivity to uncompensated residual magnetic fields of the spacecraft and field modeling errors is shown to be the most significant obstacle for maximizing performance. Performance has been evaluated with a number of inertial reference units and various mounting orientations for the two-axis Fine Sun Sensors. Attitude determination accuracy using the six state Kalman Filter executing at 2 Hz is approximately 0.2 deg, 3-sigma, per axis. Although EO-1 was subsequently driven to a stellar-based attitude determination system as a result of tighter pointing requirements, solar/magnetic attitude determination is demonstrated to be applicable to a range of small spacecraft with medium precision pointing requirements.

Woodham, Kurt↗

NASA NTRS2004

Regional Scale Meteorological Analysis and Prediction Using GPS Occultation and EOS Data

The main objective of the research under this award is to improve regional meteorological analysis and prediction for traditionally data limited regions, particularly over the Southern Ocean and Antarctica, using the remote sensing observations from current and upcoming GPS radio occultation missions and the EOS instrument suite. The major components of this project are: 1.Develop and improve the methods for retrieving temperature, moisture, and pressure profiles from GPS radio occultation data and EOS radiometer data. 2. Develop and improve a regional scale data assimilation system (MM5 4DVAR). 3. Perform case studies involving data analysis and numerical modeling to investigate the impact of different data for regional meteorological analysis and the importance of data assimilation for regional meteorological simulation over the Antarctic region. 4. Apply the findings and improvements from the above studies to weather forecasting experiments. 5. In the third year of the award we made significant progress toward the remaining goals of the project. The work included carefully evaluating the performance of an atmospheric mesoscale model, the Polar MM5 in Antarctic applications and improving the upper boundary condition.

Bromwich, David H.↗

NASA NTRS1995

A geochemical model for the formation of hydrothermal carbonates on Mars

It is often argued that substantially more carbon dioxide and water were degassed from the martian interior than can be found at present in the atmosphere, polar caps and regolith. Calculations have shown that atmospheric escape cannot account for all of the missing volatiles. Suggestions that carbon dioxide is stored as marine or lacustrine deposits, are challenged by Earth-based and spacecraft remote-sensing data. Moreover, recent modelling of the martian atmosphere suggests that rainfall or open bodies of water are in any case unlikely to have persisted for extended periods of time. Hydrothermal carbonates therefore provide a possible solution to this dilemma. Using an accessible terrestrial system (Iceland) as a guide to the underlying processes, and a host rock composition inferred from the least-altered martian meteorite, we present a geochemical model for the formation of carbonates in possible martian hydrothermal systems. Our results suggest that an extensive reservoir of carbonate minerals--equivalent to an atmospheric pressure of carbon dioxide of at least one bar--could have been sequestered beneath the surface by widespread hydrothermal activity in the martian past.

NASA Discipline Exobiology↗

NASA NTRS2009

NASA Applied Sciences Program

NASA's strategic Goals: a) Develop a balanced overall program of science, exploration, and aeronautics consistent with the redirection of human spaceflight program to focus on exploration. b) Study Earth from space to advance scientific understanding and meet societal needs. NASA's partnership efforts in global modeling and data assimilation over the next decade will shorten the distance from observations to answers for important, leading-edge science questions. NASA's Applied Sciences program will continue the Agency's efforts in benchmarking the assimilation of NASA research results into policy and management decision-support tools that are vital for the Nation's environment, economy, safety, and security. NASA also is working with NOAH and inter-agency forums to transition mature research capabilities to operational systems, primarily the polar and geostationary operational environmental satellites, and to utilize fully those assets for research purposes.

Estes, Sue M.↗

NASA NTRS2010

A First-Order Radiative Transfer Model for Microwave Radiometry of Forest Canopies at L-Band

In this study, a new first-order radiative transfer (RT) model is developed to more accurately account for vegetation canopy scattering by modifying the basic r-co model (the zero-order RT solution). In order to optimally utilize microwave radiometric data in soil moisture (SM) retrievals over moderately to densely vegetated landscapes, a quantitative understanding of the relationship between scattering mechanisms within vegetation canopies and the microwave brightness temperature is desirable. A first-order RT model is used to investigate this relationship and to perform a physical analysis of the scattered and emitted radiation from vegetated terrain. The new model is based on an iterative solution (successive orders of scattering) of the RT equations up to the first order. This formulation adds a new scattering term to the i-w model. The additional term represents emission by particles (vegetation components) in the vegetation layer and emission by the ground that is scattered once by particles in the layer. The new model is tested against 1.4 GHz brightness temperature measurements acquired over deciduous trees by a truck-mounted microwave instrument system called ComRAD in 2007. The model predictions are in good agreement with the data and they give quantitative understanding for the influence of first-order scattering within the canopy on the brightness temperature. The model results show that the scattering term is significant for trees and modifications are necessary to the T-w model when applied to dense vegetation. Numerical simulations also indicate that the scattering term has a negligible dependence on SM and is mainly a function of the angle and polarization of the microwave observation.

Kurum, Mehmet↗