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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 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Stochastic histories of dust grains in the interstellar medium

The purpose is to study an evolving system of refractory dust grains within the Interstellar Medium (ISM). This is done via a combination of Monte Carlo processes and a system of partial differential equations, where refractory dust grains formed within supernova remnants and ejecta from high mass loss stars are subjected to the processes of sputtering and collisional fragmentation in the diffuse media and accretion within the cold molecular clouds. In order to record chemical detail, the authors take each new particle to consist of a superrefractory core plus a more massive refractory mantle. The particles are allowed to transfer to and fro between the different phases of the interstellar medium (ISM) - on a time scale of 10(exp 8) years - until either the particles are destroyed or the program finishes at a Galaxy time of 6x10(exp 9) years. The resulting chemical and size spectrum(s) are then applied to various astrophysical problems with the following results. For an ISM which has no collisional fragmentation of the dust grains, roughly 10 percent by mass of the most refractory material survives the rigors of the ISM intact, which leaves open the possibility that fossilized isotopically anomalous material may have been present within the primordial solar nebula. Stuctured or layered refractory dust grains within the model cannot explain the observed interstellar depletions of refractory material. Fragmentation due to grain-grain collisions in the diffuse phase plus the accretion of material in the molecular cloud phase can under certain circumstances cause a bimodal distribution in grain size.

Liffman, Kurt↗
NASA NTRS1976

Magnetospheric dynamics and wave-particle interactions

It has been demonstrated that two general classes of wave-particle interactions are of great importance for magnetospheric dynamics. Electromagnetic and electrostatic plasma instabilities give rise to relatively narrow-banded spontaneous emissions (e.g., ELF hiss, chorus, three-halves noise, ion cyclotron and ion-plasma-frequency turbulence) that can scatter trapped particles into the loss cone, leading to modified pitch-angle distributions, stable trapping limits, diffuse aurora, proton precipitation events, etc. The current-driven plasma instabilities give rise to impulsive ion acoustic or Buneman mode turbulence that provides very effective energy transfer (via the anomalous conductivity mechanism) at the bow shock and in regions where strong field-aligned currents are observed. We review these interactions and identify significant open questions that must be investigated during the IMS.

Scarf, F. L.↗
NASA NTRS2000

Hot HB Stars in Globular Clusters: Physical Parameters and Consequences for Theory: Radiative Levitation Versus Helium Mixing - 5

Atmospheric parameters (T(sub eff), log g), masses and helium abundances are derived for 42 hot horizontal branch (HB) stars in the globular cluster NGC6752. For 19 stars we derive magnesium and iron abundances as well and find that iron is enriched by a factor of 50 on average with respect to the cluster abundance whereas the magnesium abundances are consistent with the cluster abundance. Radiation pressure may levitate heavy elements like iron to the surface of the star in a diffusive process. Taking into account the enrichment of heavy elements in our spectroscopic analyses we find that high iron abundances can explain part, but not all, of the problem of anomalously low gravities along the blue HB. The blue HB stars cooler than about 15,100 K and the sdB stars (T(sub eff) greater than or = 20,000 K) agree well with canonical theory when analysed with metal-rich ([M/H] = +0.5) model atmospheres, but the stars in between these two groups remain offset towards lower gravities and masses. Deep Mixing in the red giant progenitor phase is discussed as another mechanism that may influence the position of the blue HB stars in the (T(sub eff), log g)-plane but not their masses.

Moehler, S.↗
NASA NTRS1981

The ultraviolet interstellar extinction curve in the Pleiades

The wavelength dependence of ultraviolet extinction in the Pleiades dust clouds has been determined from IUE observations of HD 23512, the brightest heavily reddened member of the Pleiades cluster. There is evidence for an anomalously weak absorption bump at 2200 A, followed by an extinction rise in the far ultraviolet with an essentially normal slope. A relatively weak absorption band at 2200 A and a weak diffuse absorption band at 4430 A seem to be common characteristics of dust present in dense clouds. Evidence is presented which suggests that the extinction characteristics found for HD 23512 are typical for a class of extinction curves observed in several cases in the Galaxy and in the LMC.

Witt, A. N.↗
NASA NTRS1987

Latitudinal and radial gradients of anomalous and galactic cosmic rays in the outer heliosphere

Measurements from instruments on Voyagers 1 and 2 and Pioneer 10 are used to derive simultaneous radial and latitudinal gradients of anomalous cosmic-ray oxygen during the latter part of 1985. It is found that in the energy interval from 7.1-30.6 MeV/nucleon the latitudinal gradient is -3 to -4 percent/deg. The sign of the latitudinal gradient is opposite to that reported for the last solar cycle when the solar magnetic field polarity was reversed, as predicted by propagation models in which curvature and gradient drifts are significant. The ratios of the radial and latitudinal gradients of anomalous oxygen, anomalous helium, and galactic cosmic rays are similar, also in agreement with drift theory predictions. Based on drift theory, these observed ratios, which are near unity, lead to estimates of the perpendicular diffusion coefficient/particle velocity ratio in the range 3-8 x 10 to the 22nd sq cm/sec at 24.9 AU for particles with rigidities of about 2-3 GV.

Cummings, A. C.↗
DOE OSTI2026

Role of ion acoustic instability in magnetic reconnection

We report on a first-principles numerical study of magnetic reconnection in plasmas with different initial ion-to-electron temperature ratios. In cases where this ratio is significantly below unity, we observe intense wave activity in the diffusion region, driven by the ion-acoustic instability. Our analysis shows that the dominant macroscopic effect of this instability is to drive substantial ion heating. In contrast to earlier studies reporting significant anomalous resistivity, we find that anomalous contributions due to the ion-acoustic instability are minimal. These results shed light on the dynamical impact of this instability on reconnection processes, offering new insights into the fundamental physics governing collisionless reconnection.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1987

A new interpretation of plasma-line overshoot phenomena

The temporal evolution of Langmuir waves excited by high-power, high-frequency (HF) radio waves in the ionosphere is studied theoretically. This study is motivated by past observations made with the 450 MHz radar at Arecibo Observatory in Puerto Rico. Two kinds of nonlinear damping to the parametric decay instability are considered in the derivation of the rate equation for the spectral intensity of enhanced Langmuir waves. They are Langmuir wave cascading caused by nonlinear Landau damping and cross-field electron diffusion. The first damping process leads to the saturation of individual unstable Langmuir wave. The second process, which results from the incoherent scattering of electron orbits by the total excited Langmuir waves, yields anomalous damping that applies to each Langmuir wave. Consequently, Langmuir waves with smaller growth rates will be suppressed by those with larger growth rates. Such a mode competition process may cause the overshoot of the HF-enhanced plasma line observed with the Arecibo 430 MHz radar. Favorable agreement is obtained between theory and the Arecibo observations.

Kuo, S. P.↗
NASA NTRS1988

Computer simulation of the Farley-Buneman instability and anomalous electron heating in the auroral ionosphere

The nonlinear saturation of the Farley-Buneman instability in a collisional plasma is studied by a 2 1/2 dimensional electrostatic particle simulation which includes inelastic and elastic collisions of electrons and elastic collision of ions with neutrals. In the simulation, a uniform convection electric field is applied externally so that the relative velocity between the electrons and ions is greater than the ion sound speed and destabilizes the instability. A nonlinear frequency shift from higher to lower frequencies and diffusion of the wave spectrum in two dimensional wave number space are found. It is found that the dominant mechanism for electron heating is due to an enhanced effective electron collision frequency and hence enhanced resistive heating as suggested by Primdahl (1986) and not due to the heating of electrons by the electric field of the waves parallel to the magnetic field. For the ionospheric conditions discussed by Schlegel and St.-Maurice (1981)an anomalous heating rate of about 4 x 10 to the 7th W/cu cm is found.

Machida, S.↗
NASA NTRS1998

Analysis of earth albedo effect on sun sensor measurements based on theoretical model and mission experience

Analysis of flight data from previous missions indicates that anomalous Sun sensor readings could be caused by Earth albedo interference. A previous Sun sensor study presented a detailed mathematical model of this effect. The model can be used to study the effect of both diffusive and specular reflections and to improve Sun angle determination based on perturbed Sun sensor measurements, satellite position, and an approximate knowledge of attitude. The model predicts that diffuse reflected light can cause errors of up to 10 degrees in Coarse Sun Sensor (CSS) measurements and 5 to 10 arc sec in Fine Sun Sensor (FSS) measurements, depending on spacecraft orbit and attitude. The accuracy of these sensors is affected as long as part of the illuminated Earth surface is present in the sensor field of view. Digital Sun Sensors (DSS) respond in a different manner to the Earth albedo interference. Most of the time DSS measurements are not affected, but for brief periods of time the Earth albedo can cause errors which are a multiple of the sensor least significant bit and may exceed one degree. This paper compares model predictions with Tropical Rainfall Measuring Mission (TRMM) CSS measurements in order to validate and refine the model. Methods of reducing and mitigating the impact of Earth albedo are discussed. ne CSS sensor errors are roughly proportional to the Earth albedo coefficient. Photocells that are sensitive only to ultraviolet emissions would reduce the effective Earth albedo by up to a thousand times, virtually eliminating all errors caused by Earth albedo interference.

Brasoveanu, Dan↗
NASA NTRS1983

Ion-beam-driven electrostatic ion cyclotron instabilities

Results are presented of a particle simulation study of the electrostatic ion-cyclotron (EIC) instability driven by a parallel ion beam. The results of this simulation study demonstrate the nonlinear consequences of nonresonant EIC waves destabilized by an ion beam parallel to the magnetic field for the case of a large beam velocity. As a consequence of the instability, it is shown that the beam ions are heated strongly in the perpendicular direction and suffer a strong anomalous friction via EIC waves which leads to the beam slowing down. Simulation results indicate that the anomalous slowing down of beam ions by EIC waves is much larger than that from the classical electron-drag, and the perpendicular collision frequency measured from perpendicular beam heating is as large as that from Bohm diffusion. It is concluded that the ion beam driven EIC wave is a viable mechanism for the transfer of ion parallel beam energy to the ion perpendicular energy.

Okuda, H.↗
NASA NTRS1984

Ultraviolet spectroscopy of hot young stars with infrared excesses - NGC 2264-W46, W90, and W100

New ultraviolet data are combined with optical data to investigate the wavelength-dependent extinction of the stars W46, W90, and W100 in the young galactic cluster NGC 2264. The observations of W90 confirm the existence of anomalous (circumstellar) dust extinction with a ratio of total to selective extinction, R greater than 7. This extinction is most easily explained by a graphite-silicate mixture with larger mean sizes than are present in the diffuse interstellar medium. For W46 and W100, the grains are likely to be gray, but the possibility of nongray extinction cannot be excluded because of uncertainties arising from the very small ultraviolet optical depths of the circumstellar dust.

Meade, M. R.↗
NASA NTRS1987

Facilitation of the growth of protein crystals by heterogeneous/epitaxial nucleation

A preliminary exploration has been made of the effect of inorganic crystals on the nucleation of protein crystals. Four representative proteins were crystallized by the vapor diffusion technique in the presence of fifty species of inorganic crystals. A substantial decrease in critical supersaturation was caused by thirty nucleants for canavalin and by seven to fifteen nucleants for the other proteins. Several cases of anomalous morphology were observed, as well as one instance of epitaxy, i.e., lysozyme on apophyllite. It has thus been shown that inorganic crystals can be used as heterogeneous nuclei or epitaxial substrates for facilitating reliable and controlled nucleation in automated protein crystal growth experiments.

Mcpherson, Alexander↗
NASA NTRS1992

Diffusive shock acceleration - Acceleration rate, magnetic-field direction and the diffusion limit

This paper reviews the concept of diffusive shock acceleration, showing that the acceleration of charged particles at a collisionless shock is a straightforward consequence of the standard cosmic-ray transport equation, provided that one treats the discontinuity at the shock correctly. This is true for arbitrary direction of the upstream magnetic field. Within this framework, it is shown that acceleration at perpendicular or quasi-perpendicular shocks is generally much faster than for parallel shocks. Paradoxically, it follows also that, for a simple scattering law, the acceleration is faster for less scattering or larger mean free path. Obviously, the mean free path can not become too large or the diffusion limit becomes inapplicable. Gradient and curvature drifts caused by the magnetic-field change at the shock play a major role in the acceleration process in most cases. Recent observations of the charge state of the anomalous component are shown to require the faster acceleration at the quasi-perpendicular solar-wind termination shock.

Jokipii, J. R.↗
NASA NTRS1995

Hybrid Simulations of Interstellar Pickup Ion Acceleration at the Solar Wind Termination Shock

Hybrid (kinetic ion/fluid electron) simulations have been used to study self-consistently the injection and initial stages of acceleration of interstellar pickup ions at the solar wind termination shock. Results are presented from one-dimensional simulations of high Mach number oblique (40 deg-50 deg) shocks with a 10% population of interstellar pickup hydrogen. In these simulations, the pickup ions, the solar wind ions, the shock fields and the waves are all treated self-consistently. Pickup ions reflected by the shock excite large amplitude (DELTA.B / B approximately 0.3) upstream magnetosonic waves. These waves, in turn, scatter the pickup hydrogen, as expected in the diffusive shock acceleration process. The spectrum of excited waves broadens in time. We find that, for the parameters studied, the termination shock efficiently injects and accelerates the interstellar pickup hydrogen, and thus this work represents a microscopic verification of this anomalous cosmic ray injection mechanism. Comparison of injected fluxes from the simulations with Voyager fluxes extrapolated to the termination shock suggests that injection at the termination shock is probably greater than injection by interplanetary shocks. A study of accelerated pickup ion orbits shows that the energy gain comes predominantly from shock drift acceleration in the shock front, with the upstream waves aiding the acceleration by allowing multiple encounters with the shock.

Liewer, P. C.↗
NASA NTRS1990

Molecular dynamics of liquid SiO2 under high pressure

The molecular dynamics of pure SiO2 liquids was investigated up to pressures of 20 GPa at 4000 K using 252, 498, 864, and 1371 particles. The results obtained suggest that the pressure-induced maxima in the self-diffusion coefficients of both oxygen and silicon are dependent on the system size. In the case of larger systems, the maximum decreases and shifts to lower pressures. Changes in the velocity autocorrelation function with increasing pressure are described. The populations of anomalously coordinated silicon and oxygen are then discussed as a function of pressure and system size.

Rustad, James R.↗
NASA NTRS2012

NPP VIIRS and Aqua MODIS RSB Comparison Using Observations from Simultaneous Nadir Overpasses (SNO)

Suomi NPP (National Polar-orbiting Partnership) satellite (http://npp.gsfc.nasa.gov/viirs.html) began to daily collect global data following its successful launch on October 28, 2011. The Visible Infrared Imaging Radiometer Suite (VIIRS) is a key NPP sensor. Similar to the design of the OLS, SeaWiFS and MODIS instruments, VIIRS has on-board calibration components including a solar diffuser (SD) and a solar diffuser stability monitor (SDSM) for the reflective solar bands (RSB), a V-groove blackbody for the thermal emissive bands (TEB), and a space view (SV) port for background subtraction. Immediately after the VIIRS nadir door s opening on November 21, 2011, anomalously large degradation in the SD response was identified in the near-IR wavelength region, which was unexpected as decreases in the SD reflectance usually occur gradually in the blue (~0.4 m) wavelength region based on past experience. In this study, we use a well-calibrated Aqua MODIS as reference to track and evaluate VIIRS RSB stability and performance. Reflectances observed by both sensors from simultaneous nadir overpasses (SNO) are used to determine VIIRS to MODIS reflectance ratios for their spectral matching bands. Results of this study provide an immediate post-launch assessment, independent validation of the anomalous degradation observed in SD measurements at near-IR wavelengths and initial analysis of calibration stability and consistency.

Xiong, X.↗
DOE OSTI2025

Tools for probing new physics with newly discovered gamma-ray targets

Here, we present a computational tool, TweedleDEE, for empirically modeling diffuse gamma-ray background emission in a 1° region of the sky, using publicly available gamma-ray data off-axis from the region of interest. This background model allows a user to perform a purely data-driven search for anomalous localized sources of gamma-ray emission, including new physics. A major application of this tool would be in searching for dark matter annihilation in newly discovered astrophysical targets. For this purpose, we derive a scaling relation for determining velocity-dependent 𝐽-factors using only the stellar parameters, which can be broadly applied to obtain dark matter constraints from new targets. As an application of these tools, we use TweedleDEE and MADHATv2 to perform the first search for dark matter annihilation in the newly discovered Leo VI dwarf spheroidal galaxy, and present model constraints for a variety of choices of the annihilation channel and velocity dependence of the cross section.

dark matter↗
NASA NTRS1983

Anomalous bulk viscosity of two-phase fluids and implications for planetary interiors

The irreversible entropy production is calculated for the imposition of a pressure perturbation on a two-phase medium composed of a dilute suspension of droplets (or snowflakes) and a liquid phase of other materials. An absence of metastability is assumed, allowing the relaxation to be dominated by the solute finite diffusivity. The fluid medium was found to display a behavior suggestive of a bulk viscosity near 10 trillion P, a finding that is significant for studies of dissipation in planetary cores for tidal or seismic disturbances. A minimum quality factor for acoustic or tidal pressure oscillations and the accompanying frequency are calculated. An example is provided in terms of helium rain clouds in the deep interiors of giant planets. Additionally, a tidal quality factor of 10 to the 15th is found necessary to account for Io volcanism and resurfacing on Enceladus.

Stevenson, D. J.↗