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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

Bidirectional anisotropies in solar cosmic ray events - Evidence for magnetic bottles

During the period 1967-1973 data were obtained from five satellites in order to investigate bidirectional anisotropies in low-energy solar protons and to study electron events. Detailed consideration is given to the characteristics of two observed solar electron events. It is particularly noted that (1) the mean duration of bidirectional anisotropy is about 9 hours, (2) for most events the bidirectional anisotropy occurs at the minimum of the associated Forbush decrease, (3) the existence of bidirectional anisotropy in subrelativistic electrons associated with an interplanetary shock is not expected as anisotropies in impulsive solar electron events usually decay quickly within a time of not more than an hour at 1 AU, and (4) simultaneous observations show a significant difference in the nature of the angular distributions first between electrons and protons and second between different proton energies. The formation of bidirectional anisotropy by propagation and by energy change is also discussed.

Palmer, I. D.↗
NASA NTRS1978

Adiabatic modulation of equatorial pitch angle anisotropy

Particle measurements from the geostationary ATS 6 satellite reveal striking equatorial pitch-angle anisotropies. A study of seven days of data shows a diurnal variation in anisotropy with the particle flux at a pitch angle of 40 deg greater than that at 90 deg during times of low magnetic-field intensity on the nightside and vice versa during times of high magnetic-field intensity on the dayside. Six representative anisotropy events are studied in finer detail. The 32- to 51-keV electron anisotropies increase and decrease with the total magnetic-field intensity. The proton and higher-energy electron anisotropies do not show as much variation. The particle anisotropies are studied in light of adiabatic and cyclotron resonance theory; the results indicate that adiabatic effects are the dominant modulation mechanism of particle pitch-angle distributions in the outer radiation belt

Kaye, S. M.↗
NASA NTRS1985

Formulation of cosmic-ray solar daily variation and its seasonal variation, produced from generalized stationary anisotropy of solar origin

In previous papers, a formulation was presented of cosmic ray daily variations produced from solar anisotropies stationary through a year, and also of their annual (or seasonal) modulation caused by the annual variation of the rotation axis of the Earth relative to that of the Sun. These anisotropies are symmetric for an arbitrary rotation around an axis. From observations of the tri-diurnal variation, it has been suggested that solar anisotropies also contain some axis-asymmetric term of the third order with respect to the IMF-axis. This suggestion has recently found support in a theoretical study by Munakata and Nagashima. According to their results, the terms of axis-asymmetry with respect to IMF-axis appear also in the 2nd order anisotropy, together with some different kinds of axis-symmetric terms. The contribution of these anisotropies to the daily variation is different from that of those discussed previously. The above mentioned formulation is extended to a case of a generalized anisotropy.

Tatsuoka, R.↗
NASA NTRS1988

The regimes of the east-west and the radial anisotropies of cosmic rays in the heliosphere

Observations dating back to 1957 are used to show that there are two distinct physical states of the heliosphere. From 1957-1970 the diurnal anisotropy consisted of the azimuthal component only. This period is defined as the regime of the east-west anisotropy. The period 1971-1979 is characterized by the appearance of a radial anisotropy which attained a maximum amplitude in 1976, when the solar activity was minimum. It is suggested that there is an inverse correlation between the amplitude of the radial anisotropy and solar activity. The physical state of the heliosphere is very stable during the regime of east-west anisotropy and extremely dynamic during the radial anisotropy regime. It is suggested that the heliosphere switches from one physical state to another following the onset of the solar polar field reversal.

Ahluwalia, H. S.↗
NASA NTRS1990

Characteristics of trapped proton anisotropy at Space Station Freedom altitudes

The ionizing radiation dose for spacecraft in low-Earth orbit (LEO) is produced mainly by protons trapped in the Earth's magnetic field. Current data bases describing this trapped radiation environment assume the protons to have an isotropic angular distribution, although the fluxes are actually highly anisotropic in LEO. The general nature of this directionality is understood theoretically and has been observed by several satellites. The anisotropy of the trapped proton exposure has not been an important practical consideration for most previous LEO missions because the random spacecraft orientation during passage through the radiation belt 'averages out' the anisotropy. Thus, in spite of the actual exposure anisotropy, cumulative radiation effects over many orbits can be predicted as if the environment were isotropic when the spacecraft orientation is variable during exposure. However, Space Station Freedom will be gravity gradient stabilized to reduce drag, and, due to this fixed orientation, the cumulative incident proton flux will remain anisotropic. The anisotropy could potentially influence several aspects of Space Station design and operation, such as the appropriate location for radiation sensitive components and experiments, location of workstations and sleeping quarters, and the design and placement of radiation monitors. Also, on-board mass could possible be utilized to counteract the anisotropy effects and reduce the dose exposure. Until recently only omnidirectional data bases for the trapped proton environment were available. However, a method to predict orbit-average, angular dependent ('vector') trapped proton flux spectra has been developed from the standard omnidirectional trapped proton data bases. This method was used to characterize the trapped proton anisotropy for the Space Station orbit (28.5 degree inclination, circular) in terms of its dependence on altitude, solar cycle modulation (solar minimum vs. solar maximum), shielding thickness, and radiation effect (silicon rad and rem dose).

Armstrong, T. W.↗
NASA NTRS1993

Energetic ion anisotropies in the geomagnetic tail. I - A statistical survey. II - Magnetic field and substorm characteristics

A comprehensive data set from the ISEE 2 spacecraft is used here to perform a statistical study of events in the earth's central magnetotail that are characterized by high anisotropies of energetic ions. In about 75 percent of the cases the anisotropy vector deviates no more than 45 deg from the tidal axis. High-anisotropy samples within 45 deg of the tidal axis are dominated by the earthward fraction. High ion anisotropies are observed continuously for longer than 1 min only in a few cases. The probability of observing high ion anisotropies is significantly enhanced beyond about 16 R(E) downtail distance within a few earth radii of the neutral sheet and on the duskside of the magnetotail. The analysis is extended with respect to the local magnetic field and to the relationship between energetic ion anisotropies and substorm phases. It is found that the events can be well organized in terms of substorm expansion phase and substorm recovery. These relations and the magnetic field characteristics during the events support the notion that the near-earth source of tailward streaming ions is identical with a substorm neutral line.

Kettmann, Georg↗
DOE OSTI2025

Optomechanical Tuning of Second Harmonic Generation Anisotropy in Janus MoSSe/MoS 2 Heterostructures

Symmetry breaking in van der Waals materials enables the realization of quantum states and advanced device functionalities. Janus transition-metal dichalcogenides (TMDs) exhibit distinctive nonlinear optical properties due to their broken out-of-plane mirror symmetry. However, the dynamic control of second harmonic generation (SHG) anisotropy and resonance behavior via optical excitation remains elusive. Here, in this work, we investigate the SHG response of Janus MoSSe/MoS 2 heterostructures with 2H and 3R stackings. We can tune the SHG response by varying the incident photon wavelength from 800 to 1000 nm, which shows a resonance-dependent enhancement in intensity and a deviation from 6-fold symmetry, indicating wavelength-dependent anisotropy. The ratio between maximum and minimum intensity in the armchair directions, associated with the SHG anisotropy, reaches a value of 1.73 at the excitation wavelength of 1000 nm. Group theory analysis and first-principles calculations reveal that the observed anisotropy arises from optically induced strain. Our findings highlight the role of symmetry breaking and optical resonance contributing to the optomechanical tuning of SHG anisotropy, offering opportunities for developing Janus TMD-based photonic devices for frequency conversion, light generation, and optical switching.

Janus transition metal dichalcogenides↗
NASA NTRS1971

Magnetic stress anisotropy field in plated cylindrical Permalloy films.

An analysis is made of the magnetic stress anisotropy field (Hks) arising from internal and external stress sources in plated-wire memory elements. The analysis takes into consideration circumferential composition variation and cylindrical geometry of the Permalloy film. Expressions are derived relating Hks to uniaxial film stress, average composition, and amplitude of composition variation. A result of particular importance is that even for average zeromagnetostrictive composition (ZMC) films, Hks may still make an appreciable contribution to the total anisotropy field if the composition is not uniform. Calculated Hks characteristics are shown to correlate with anisotropy field changes observed in annealing experiments. Examples are given to show the importance of composition uniformity in determining the stability of the anisotropy field. The utility of the analysis is extended by the inclusion of data expressing the inverse relation between anisotropy field and easy-axis dispersion in the film.

Lutes, O. S.↗
NASA NTRS1978

Quiet time interplanetary cosmic ray anisotropies observed from Pioneer 10 and 11

Cerenkov counters on the Pioneer 10 and 11 spacecraft, capable of detecting alpha particles and protons with energies up to 480 MeV, and nucleons and electrons with energies up to 6 MeV, have yielded data on cosmic ray anisotropies during periods of low solar activity. Observations from Pioneer 11 place east-west anisotropy at 0.41 plus or minus 0.11%, and the north-south anisotropy at near zero; Pioneer 10 results show east-west anisotropy to be approximately 0.59 plus or minus 0.18%, and the north-south component at 0.25 plus or minus 0.08%. It is noted that the Pioneer 10 observations were obtained at the 6 AU range, while those from Pioneer 11 originated closer to the sun (1.1 to 2.7 AU). Attention is given to the ratio of the perpendicular to parallel components of the diffusion coefficient, and to the large north-south anisotropy reported by Pioneer 10, an effect due possibly to gradient drift, and to an additional streaming independent of the magnetic field polarity.

Ip, W.-H.↗
NASA NTRS1985

Comparative study of the phase of diurnal anisotropy on quiet and disturbed days on a long term basis up to recent period

The data from the world wide grid of neutron monitoring stations was analyzed for a comparative study of the phase of diurnal anisotropy on quiet and disturbed days on a long term basis up to recent period. It has been observed that the phase of the diurnal anisotropy on disturbed days where the value of the Ap-index is higher, is found to shift towards earlier hours in comparison to the phase of the diurnal anisotropy on quiet days where the value of Ap-index is lower on all the stations from 1965 to 71. Such a trend is not observable for the later period. This affect is found to be more pronounced on equatorial stations, in particular, in comparison to high latitude stations. It was derived from these observational facts that the relationship between Ap-index and the phase of the diurnal anisotropy is not invariant throughout the period of consideration. Furthermore, the exact cause of such a drastic change is not known, but it demonstrates very clearly that the interplanetary conditions which are responsible for both, diurnal anisotropy of cosmic ray intensity and the geomagnetic Ap-index variation, have drastically changed during the period 1971 and onwards.

Kumar, S.↗
NASA NTRS1984

Correlation between proton anisotropy and magnetic field direction in the distant geotail

A statistical analysis has been conducted of the anisotropy of suprathermal protons and the polarity of the magnetic field during April 10-16, 1983. At this time, ISEE-3 was at lunar distances in the geomagnetic tail of the earth, and well within the nominal magnetopause. The first-order anisotropy is presently correlated with the latitude angle and the z-component of the magnetic field. The anisotropy direction's frequency distribution is strongly peaked in the earthward and tailward direction, indicating fast earthward and tailward flows. For large anisotropies, and within 5 earth radii of the nominal neutral sheet position, a strong correlation is found between the earthward-streaming suprathermal protons and the northward polarity of the magnetic field; large tailward anisotropies are generally correlated with southward magnetic field polarity. This correlation is most simply interpreted in terms of a neutral line or reconnection model.

Klecker, B.↗
NASA NTRS1988

Observational and theoretical evidence for anisotropies in Saturn's magnetosphere

A study of plasma anisotropies in Saturn's magnetosphere was conducted using data from the Voyager 1 plasma science (PLS) instrument together with an improved Voyager plasma data analysis routine of Sands (1987) and Sands and McNutt (1988). First, the degree of anisotropy required to reconcile differences in the inbound and outbound Voyager 1 plasma observations was determined. The anisotropies derived from the PLS plasma data were then compared with those required to reconcile inbound-outbound differences in plasma flux shell content. It was found that better fits to the data could be obtained if temperature anisotropies were allowed and that the magnitude of the anisotropies was roughly consistent with those required to provide azimuthal symmetry in the plasma flux tube content.

Richardson, John D.↗
NASA NTRS1994

A two-fluid approximation for calculating the cosmic microwave background anisotropies

We present a simplified treatment for calculating the cosmic microwave background anisotropy power spectrum in adiabatic models. It consists of solving for the evolution of a two-fluid model until the epoch of recombination and then integrating over the sources to obtain the cosmic microwave background (CMB) anisotropy power spectrum. The approximation is useful both for a physical understanding of CMB anisotropies as well as for a quantitative analysis of cosmological models. Comparison with exact calculations shows that the accuracy is typically 10%-20% over a large range of angles and cosmological models, including those with curvature and cosmological constant. Using this approximation we investigate the dependence of the CMB anisotropy on the cosmological parameters. We identify six dimensionless parameters that uniquely determine the anisotropy power spectrum within our approximation. CMB experiments on different angular scales could in principle provide information on all these parameters. In particular, mapping of the Doppler peaks would allow an independent determination of baryon mass density, matter mass density, and the Hubble constant.

Seljak, Uros↗
NASA NTRS1994

Inverse correlations between the ion temperature anisotropy and plasma beta in the Earth's quasi-parallel magnetosheath

Average proton parameters in the magnetosheath downstream from the quasi-perpendicular shock (the quasi-perpendicular magnetosheath) for high solar wind dynamic pressure conditions are observed to vary continuously from high-beta, low-temperature anisotropy to low-beta, high-temperature anisotropy. Observations and theory have shown that this inverse correlation is a direct consequence of pitch angle scattering by electromagnetic ion cyclotron (EMIC) waves, which regulate the anisotropy, restoring the plasma toward marginal stability. ALthough the previously documented spectral characteristics of EMIC waves are not evident downstream of the quasi-parallel bow shock, the inverse anisotropy-beta relation found in the quasi-perpendicular magnetosheath also holds in the quasi-parallel magnetosheath. This indicates that the EMIC instability regulates the ion anisotropy regardless of the shock geometry.

Fuselier, Stephen A.↗
NASA NTRS2022

Anisotropy Measurements from the Near-Threshold Photodissociation of the N2−NO Complex

We have used velocity map ion imaging to measure the angular anisotropy of the NO (A) products from the photodissociation of the N 2 −NO complex. Our experiment ranged from 108 to 758 cm −1 above the threshold energy to form NO (A)+N 2 (X) products, and these measurements reveal, for the first time, a strong angular anisotropy from photodissociation. At 108 cm −1 above the photodissociation threshold, we observed NO (A) photoproducts recoil preferentially perpendicular to the laser polarization axis with an average anisotropy parameter, β = −0.25; however, as the available energy was increased, the anisotropy increased, and at 758 cm −1 above the threshold energy, we found an average β = +0.28. The observed changes in the angular anisotropy of the NO (A) photoproduct are qualitatively similar to those observed for the photodissociation of the Ar−NO complex and likely result from changes in the region of the excited state potential energy surface accessed during the electronic excitation. At the lowest available energy, we also noted a large contribution from hot band excitation; however, this contribution decreased as the available energy increased. The outsized contribution at the lowest available energy may result from hot bands having better Franck−Condon overlap with the excited electronic state near threshold. Finally, we contrast the experimental center of mass translational energy distribution with a statistical energy distribution determined from phase space theory. The experimental and statistical distributions show pronounced disagreement, particularly at low kinetic energies, with the experimental one showing less dissociation resulting in high rotational levels of the fragments.

Energy↗
DOE OSTI2025

In situ deformation of antigorite-olivine two-phase mixtures: Implications for dynamics and seismic anisotropy in the mantle wedge

Water released from hydrous minerals in subducting slabs reacts with the overlying plate, resulting in widespread serpentinization in the mantle wedge. Deformation of serpentinized peridotites has been invoked to explain forearc seismic anisotropy, yet studies of the mechanical properties and deformation behaviors of serpentine-bearing multiphase aggregates remain limited. Here we deformed olivine-antigorite mixtures containing 70, 50, and 20 vol.% of antigorite at 2.5 – 7.6 GPa, 673 K and strain rates of ∼10 –5 –10 –4 s –1 . Elasto-viscoplastic self-consistent simulations, constrained by synchrotron X-ray diffraction (XRD) data, were used to estimate lattice strain, stress–strain partitioning, crystallographic preferred orientations (CPO) development, and aggregate strength. Selected run products were also analyzed by electron backscatter diffraction for comparison with the CPO results obtained from XRD experiments. We found olivine transitions from A- or B-type to C-type when antigorite fraction drops to 20 vol.%, coinciding with a microstructural change from interconnected weak layers to a load-bearing framework (LBF). An additional run on a sample Atg50/Ol50 with preexisting microstructures suggested the formation of LBF was promoted by these microstructures, although the preexisting antigorite CPO has been overprinted at 20.8 % strain and could be erased completely by subsequent deformation in nature. Estimated viscosity of the two-phase mixtures suggests that low-degree serpentinization (≤20 %) in the mantle wedge may increase the strength of olivine-rich peridotite and hinder slab-mantle decoupling, whereas high-degree serpentinization (≥50–70 %) weakens the peridotite and favors decoupling if sufficient viscosity contrast (>10) develops. Seismic anisotropy shows a nonlinear dependence on antigorite fraction: antigorite CPO governs the anisotropy of the mixtures with ≥50 vol.% antigorite, whereas olivine CPO dominates at low fractions (∼20 vol.%). The presence of pre-existing microstructures reduces seismic anisotropy of the deformed mixtures, however the persistence of pre-existing CPO in actively subducting slabs remains uncertain, making their significance over geological timescales questionable.

Crystallographic preferred orientation↗
DOE OSTI2024

Impact of anisotropy on TRISO fuel performance

Manufacturing of tristructural isotropic (TRISO) particles involves the deposition of pyrolytic carbon (PyC) and silicon carbide (SiC) layers using the fluidized bed chemical vapor deposition (CVD) process. The CVD process is known to generate polycrystalline layers with crystallographic textures, which imparts anisotropic thermophysical properties to the layers. Past studies have shown the risk for particle failure increases with an increase in anisotropy. The limit beyond which the anisotropy of PyC layers becomes unacceptable due to failure risk has been identified as a high-priority knowledge gap. This work presents a first systematic study on the effects of anisotropic thermal and mechanical properties on TRISO fuel performance. This computational study, performed using the fuel performance code BISON, investigates how the anisotropy in elasticity and thermal properties affect the stresses, temperature, and failure of a TRISO particle. The influence of other factors, such as operating temperature and particle geometry on the anisotropy effects, also has been analyzed. The studies utilize the recently published anisotropic elasticity and thermal behavior models for TRISO PyC and SiC layers implemented using tensors with full anisotropic capability. The spherical TRISO particles with anisotropic properties were found to have greater maximum tensile stress and significantly higher failure probability than the spherical particles with isotropic properties. In conclusion, the fuel performance predicted using these recently developed models was found to be comparable with the performance obtained using the historical models.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
NASA NTRS2013

Daily MODIS 500 m Reflectance Anisotropy Direct Broadcast (DB) Products for Monitoring Vegetation Phenology Dynamics

Land surface vegetation phenology is an efficient bio-indicator for monitoring ecosystem variation in response to changes in climatic factors. The primary objective of the current article is to examine the utility of the daily MODIS 500 m reflectance anisotropy direct broadcast (DB) product for monitoring the evolution of vegetation phenological trends over selected crop, orchard, and forest regions. Although numerous model-fitted satellite data have been widely used to assess the spatio-temporal distribution of land surface phenological patterns to understand phenological process and phenomena, current efforts to investigate the details of phenological trends, especially for natural phenological variations that occur on short time scales, are less well served by remote sensing challenges and lack of anisotropy correction in satellite data sources. The daily MODIS 500 m reflectance anisotropy product is employed to retrieve daily vegetation indices (VI) of a 1 year period for an almond orchard in California and for a winter wheat field in northeast China, as well as a 2 year period for a deciduous forest region in New Hampshire, USA. Compared with the ground records from these regions, the VI trajectories derived from the cloud-free and atmospherically corrected MODIS Nadir BRDF (bidirectional reflectance distribution function) adjusted reflectance (NBAR) capture not only the detailed footprint and principal attributes of the phenological events (such as flowering and blooming) but also the substantial inter-annual variability. This study demonstrates the utility of the daily 500 m MODIS reflectance anisotropy DB product to provide daily VI for monitoring and detecting changes of the natural vegetation phenology as exemplified by study regions comprising winter wheat, almond trees, and deciduous forest.

Vegetation↗