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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

A survey of interstellar H I from L-alpha absorption measurements. II

This paper presents interstellar H I column densities, mean H I space densities, total neutral hydrogen column densities, and total mean hydrogen space densities toward 100 stars surveyed in the L-alpha region with the U2 detector of the Copernicus satellite. The data for 47 stars are compared with OAO 2 results, and ratios of gas to E(B-V) color excess are computed for 30 'intercloud' stars, 45 'cloud' stars, and the combined sample of 75 stars. A definitive value of 5.8 x 10 to the 21st power atoms/sq cm per mag is obtained for the mean ratio of total neutral hydrogen to color excess, indicating that this ratio seems to be a constant, independent of location in the sky, except possibly toward stars with anomalous extinction. The anomalous ratio determined for Rho Oph is examined, and the galactic distribution of neutral hydrogen is evaluated. The following values are estimated for matter in the galactic plane within 500 pc of the sun: a total mean hydrogen space density of 1.15 atoms/cu cm, a mean H2 space density of 0.143 molecule/cu cm (possibly a lower limit), and a mean H I space density of 0.86 atom/cu cm (a possible overestimate).

Bohlin, R. C.↗
NASA NTRS1973

Physical and dynamical studies of meteors

Interplanetary distributions from a sample of 20,000 radar meteor observations are presented. These distributions are freed from all known selection effects with the exception of a possible bias against fragmenting meteors which has not yet been adequately assessed. These data thus represent the largest and most accurate collection of radar meteor distributions. Both general average distribution and the distribution of meteor streams with their comet and asteroid associations are presented. Sporadic space density and space density of meteor streams are also included.

Southworth, R. B.↗
NASA NTRS1981

Phase space variations of near equatorially mirroring ring current ions

Observations of near equatorially mirroring ring current ions before and after a magnetic storm are presented in the form of phase space densities with respect to the first adiabatic invariant. Particle densities were obtained from the medium energy particles instrument covering the energy range 24-2081 keV on ISEE 1 at L values between 3 and 8 earth radii and ratios of the magnetic field at the satellite position to the magnetic field at the magnetic equator less than 1.2. Analysis of the phase space densities through the magnetosphere reveals a well-defined high magnetic moment peak in the prestorm near-equatorial ring current ion phase space density distribution, with the magnetic storm resulting from an enhancement of phase space densities at magnetic moment values below the peak and phase space densities remaining constant above the peak. Results are found to be in good agreement with those obtained by Explorer 45 six years previously, indicating that the observed phase space density variations are characteristic of energetic ion behavior during magnetic storms.

Williams, D. J.↗
DOE OSTI2024

Improving the precision of forces in real-space pseudopotential density functional theory

The high-order finite difference real-space pseudopotential density functional theory (DFT) approach is a valuable method for large-scale, massively parallel DFT calculations. A significant challenge in the approach is the oscillating “egg-box” error introduced by aliasing associated with a coarse grid spacing. To address this issue while minimizing computational cost, we developed a finite difference interpolation (FDI) scheme [Roller et al., J. Chem. Theory Comput. 19, 3889 (2023)] as a means of exploiting the high resolution of the pseudopotential to reduce egg-box effects systematically. Here, we show an implementation of this method in the PARSEC code and examine the practical utility of the combination of FDI with additional methods for improving force precision and/or reducing its computational cost, including orbital-based forces, compensating charges (namely, adding and subtracting a judiciously chosen charge density such that the total density is unaltered), and a modified spatial domain in which the real-space grid is defined. Using selected small molecules, as well as metallic Li, as test cases, we show that a combination of all four aspects leads to a significant reduction in computational cost while retaining a high level of precision that supports accurate structures and vibrational spectra, as well as stable and accurate molecular dynamics runs.

Chemistry↗
NASA NTRS1973

Luminosity functions for K giant stars derived from the two-micron sky survey.

Description of a method for determining either the space density or the luminosity function from star counts covering large areas of sky. Space density is assumed to vary only in the direction perpendicular to the galactic plane. The method extends that derived for use with the star counts in the Selected Areas by allowing for an integration over a wide and continuous range of galactic latitudes, and is therefore applicable to surveys where the number of stars per square degree is small but the area surveyed is an appreciable fraction of the sky. The catalog (IRC) produced from the 2-micron sky survey at Caltech is such a survey. Application of the method to a selection of IRC stars dominated by K giants shows that if these stars obey Oort's determination of their normalized space density perpendicular to the galactic plane than the dispersion of their 2.2-micron luminosity distribution must be large - i.e., on the order of plus or minus 1.0 min within a single spectral subtype. This result is in accord with conclusions recently set forth by Jung (1970).

Hughes, E. E., Jr.↗
NASA NTRS1983

Interaction of ring current ions with the magnetopause

An analysis of phase space density observations of 30-130 keV ring current protons made during a magnetopause crossing of ISEE 1 indicates that the phase space densities of 30-65 keV protons within the magnetosphere are higher for particles streaming parallel to the field than antiparallel to the field. In the magnetosheath, phase space densities parallel to the field are higher than the densities of both outward- and inward-flowing particles parallel to the field within the magnetosphere. The observations are discussed in the terms of reflected and transmitted ring current protons' acceleration in the magneteopause current layer.

Scholer, M.↗
NASA NTRS1988

Plasma electrons as tracers of distant magnetotail structure - ISEE-3

This paper compares the electron spectra and phase space densities measured concurrently by ISEE-3 at 200 R(E), with those measured by DMSP at low altitudes. The field-aligned lobe electron phase space densities above 200 eV at ISEE were found to agree well with the DMSP-measured polar rain phase space densities near the polar cap; the spectral slopes above 200 eV were also similar. Below 100-200 eV, a thermal electron population was measured by ISEE in the distant tail, which arose from local entry of plasma through the distant magnetopause, which is not present at DMSP altitudes. These data show that the suprathermal tail lobe electrons are essentially a test particle population which can move freely along field lines to form polar rain; in contrast, the thermal electrons are bound to the tailward-flowing lobe ion population far down the tail and, thus, cannot reach the polar cap regions.

Baker, D. N.↗
NASA NTRS1993

Electron distributions in the inner Jovian magnetosphere: Voyager 1 observations

Using several improvements in the analysis of the observations of the Low Energy Charged Particle (LECP) experiment on Voyager 1, electron phase space densities in the inner Jovian magnetosphere (5 - 10 R(sub J) were first calculated at constant first and second invariants (represented by mu and K, respectively), based on the LECP measurements. The calculated electron phase space density profiles show that in the inner Jovian magnetosphere there exist evident time and longitude variations, energetic electron injections, and present radial transport and distributed losses. To study the radial and pitch angle diffusions of Jovian electrons, we have calculated the phase space densities in the K-L space. It is found that the electron population in the inner Jovian magnetosphere seems to consist of two components: electrons radially diffusing from a main external source and electrons generated from local sources. The radially diffusing electrons have a relatively time stationary and isotropic distribution, while the locally created electrons mainly concentrate around the equatorial plane and have relatively lower energies, in comparison with the inward diffusing electrons. Consequently, the sources of precipitation losses to the ionosphere must be primarily electrons transported from outer sources, and the major precipitations occur in the inner magnetosphere (L less than 7.5 R(sub J). In the inner Jovian magnetosphere (L = 5 to approximately 10 R(sub J)) it is estimated that for electrons with magnetic moment mu = 300 MeV/G, the diffusion coefficient D is roughly 10(exp -8) to approximately 10(exp -6) R(exp 2)(sub J)/s, and the lifetime against the diffusion losses is of the order of 10(exp 4) to approximately 10(exp 6) s.

Ye, G.↗
NASA NTRS1995

Stormtime ring current and radiation belt ion transport: Simulations and interpretations

We use a dynamical guiding-center model to investigate the stormtime transport of ring current and radiation-belt ions. We trace the motion of representative ions' guiding centers in response to model substorm-associated impulses in the convection electric field for a range of ion energies. Our simple magnetospheric model allows us to compare our numerical results quantitatively with analytical descriptions of particle transport, (e.g., with the quasilinear theory of radial diffusion). We find that 10-145-keV ions gain access to L approximately 3, where they can form the stormtime ring current, mainly from outside the (trapping) region in which particles execute closed drift paths. Conversely, the transport of higher-energy ions (approximately greater than 145 keV at L approximately 3) turns out to resemble radial diffusion. The quasilinear diffusion coefficient calculated for our model storm does not vary smoothly with particle energy, since our impulses occur at specific (although randomly determined) times. Despite the spectral irregularity, quasilinear theory provides a surprisingly accurate description of the transport process for approximately greater than 145-keV ions, even for the case of an individual storm. For 4 different realizations of our model storm, the geometric mean discrepancies between diffusion coefficients D(sup sim, sub LL) obtained from the simulations and the quasilinear diffusion coefficient D(sup ql, sub LL) amount to factors of 2.3, 2.3, 1.5, and 3.0, respectively. We have found that these discrepancies between D(sup sim, sub LL) and D(sup ql, sub LL) can be reduced slightly by invoking drift-resonance broadening to smooth out the sharp minima and maxima in D(sup ql, sub LL). The mean of the remaining discrepancies between D(sup sim, sub LL) and D(sup ql, sub LL) for the 4 different storms then amount to factors of 1.9, 2.1, 1.5, and 2.7, respectively. We find even better agreement when we reduce the impulse amplitudes systematically in a given model storm (e.g., reduction of all the impulse amplitudes by half reduces the discrepancy factor by at least its square root) and also when we average our results over an ensemble of 20 model storms (agreement is within a factor of 1.2 without impulse-amplitude reduction). We use our simulation results also to map phase-space densities f in accordance with Liouville's theorem. We find that the stormtime transport of approximately greater than 145-keV ions produces little change in f-bar the drift-averaged phase-space density on any drift shell of interest. However, the stormtime transport produces a major enhancement from the pre-storm phase-space density at energies approximately 30-145 keV, which are representative of the stormtime ring current.

Lyons, Larry R.↗
DOE OSTI2026

Real-Space Constrained Density Functional Theory Investigation of Site-Specific, Interfacial Charge Recombination Dynamics Across the Au Nanoparticle/TiO 2 Heterojunction

Au nanoparticle (NP)/TiO 2 heterojunction is a representative system to study interfacial charge transfer in photocatalysis and photovoltaics, where suppressing recombination from TiO 2 to Au can enhance hot carrier extraction. We apply real-space constrained density functional theory (CDFT) with Marcus theory to quantify charge recombination time scales across Au/TiO 2 . This approach enables direct control and visualization of charge-separated states, aligning with site-specific probes like time-resolved X-ray photoelectron spectroscopy (trXPS). We find that the charge-separated state features a bipolaron, with recombination dominated by TiO 2 LUMO to Au HOMO transitions, primarily at interfacial Au sites. Marcus rate predictions are benchmarked with surface hopping methods, quantifying differences in time scales and computational efficiency. Lastly, we examine how the Au cluster size affects the free energy change (ΔG) and reorganization energy (λ), explaining trends in closed-shell systems and highlighting challenges for open-shell extrapolations. Overall, CDFT + Marcus theory provides efficient, mechanistically transparent interfacial charge transfer modeling, and we clearly defined its applicability and limitation.

Glenna, Drew M. [Univ. of Idaho, Idaho Falls, ID (↗
NASA NTRS2006

Pickup Ion Phase Space Distributions at Titan in a Three Dimensional Exosphere

The composition and structure of neutral exospheres imbedded in moving plasmas can be determined by measurements of the velocity distributions of their pickup ion progeny. In turn, the velocity distributions are dependent on the spatial structure of the neutral source gases. Since Titan's neutral exosphere extends into the Saturn's magnetosphere (or solar wind) and well above its ionopause, it serves as a good place to analyze such characteristics. They are analyzed using pickup ion measurements made by the Cassini Plasma Spectrometer (CAPS) at Titan [e.g., Hartle et al., 2006] and an ion kinetic model. An early version of the model [Hartle and Sittler, 2007] is an expression describing the phase space density of pickup ions, which is derived from the Vlasov equation with an ion source that explicitly accounts for the velocity and spatial variation of the exosphere source gases. The current version used here includes exosphere source gases in three dimensions. A fundamental parameter of the phase space densities is the ratio of the gyroradius to the neutral scale height alpha, = r(sub g)/H. Titan's exosphere structure yields pickup ions whose phase space distributions are beam-like when alpha >> 1 and fluid-like when alpha << 1. Downstream from the source peak, the light pickup ions, with alpha << 1, are easily observed because their phase space densities are almost uniform over the orbit phases. On the other hand, the phase space distributions of the heavier ions, with alpha >> 1, peak over narrow velocity and spatial ranges. This beam-like nature makes it considerably more difficult to observe heavy ions because their downstream positions and viewing directions are narrowly constrained. Examples of these extremes will be discussed.

Hartle, Richard↗
NASA NTRS1994

Simulations of phase space distributions of storm time proton ring current

We use results of guiding-center simulations of ion transport to map phase space densities of the stormtime proton ring current. We model a storm as a sequence of substorm-associated enhancements in the convection electric field. Our pre-storm phase space distribution is an analytical solution to a steady-state transport model in which quiet-time radial diffusion balances charge exchange. This pre-storm phase space spectra at L approximately 2 to 4 reproduce many of the features found in observed quiet-time spectra. Using results from simulations of ion transport during model storms having main phases of 3, 6, and 12 hr, we map phase space distributions from the pre-storm distribution in accordance with Liouville's theorem. We find stormtime enhancements in the phase space densities at energies E approximately 30-160 keV for L approximately 2.5 to 4. These enhancements agree well with the observed stormtime ring current. For storms with shorter main phases (approximately 3 hr), the enhancements are caused mainly by the trapping of ions injected from open night side trajectories, and diffusive transport of higher-energy (greater than or approximately 160 keV) ions contributes little to the stormtime ring current. However, the stormtime ring current is augmented also by the diffusive transport of higher-energy ions (E greater than or approximately 160 keV) durinng stroms having longer main phases (greater than or approximately 6 hr). In order to account for the increase in Dst associated with the formation of the stormtime ring current, we estimate the enhancement in particle-energy content that results from stormtime ion transport in the equatorial magnetosphere. We find that transport alone cannot account for the entire increase in absolute value of Dst typical of a major storm. However, we can account for the entire increase in absolute value of Dst by realistically increasing the stormtime outer boundary value of the phase space density relative to the quiet-time value. We compute the magnetic field produced by the ring current itself and find that radial profiles of the magnetic field depression resemble those obtained from observational data.

Chen, Margaret W.↗
NASA NTRS1994

Radial diffusion models of energetic electrons and Jupiter's synchrotron radiation. 2: Time variability

We used a radial diffusion code for energetic electrons in Jupiter's magnetosphere to investigate variations in Jupiter's radio emission due to changes in the electron phase space density at L shells between 6 and 50, and due to changes in the radial diffusion parameters. We suggest that the observed variations in Jupiter's radio emission are likely caused by changes in the electron phase space density at some boundary L(sub 1) is greater than 6, if the primary mode of transport of energetic electrons is radial diffusion driven by fluctuating electric and/or magnetic fields induced by upper atmospheric turbulence. We noticed an excellent empirical correlation, both in phase and relative amplitude, between changes in the solar wind ram pressure and Jupiter's synchrotron radiation if the electron phase space density at the boundary L(sub 1) (L(sub 1) is approximately equal to 20-50) varies linearly with the square root of the solar wind ram pressure, f is approximately (N(sub s)nu(exp 2 sub s))(exp 1/2). The calculations were carried out with a diffusion coefficient D(sub LL) = D(sub n)L(exp n) with n = 3. The diffusion coefficient which best fit the observed variations in Jupiter's synchrotron radiation D(sub 3) = 1.3 +/- 0.2 x 10(exp -9)/s is approximately 0.041/yr, which corresponds to a lagtime of approximately 2 years. We further show that the observed short term (days-weeks) variations in Jupiter's radio emission cannot be explained adequately when radial diffusion is taken into account.

De Pater, Imke↗