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At least 613 records · Page 34

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NASA NTRS1977

Explorer 33 entry layer observations

Low fluxes of stagnant or irregularly flowing plasma along the inside of the magnetopause labeled as the entry layer by Haerendel and Paschmann, are observed on Explorer 33 orbits which cross the dayside magnetopause and on one orbit which skims the dawn flank. The data indicate that the entry layer is a permanent magnetospheric feature which extends across the dayside and along the flanks to large distances down the tail on dipolelike field lines between extensions of the northern and southern polar cusps. Pressure anisotropies are present in the entry layer but are often weaker and less frequent than those in the adjacent magnetosheath. These anisotropies give clues about the process of entry layer formation. The sense of anisotropy when it is present on both sides of the magnetopause is the same for magnetosheath fields with a southward component and opposite for fields with a northward component. These correlations are consistent with entry layer filling by bulk flow along field lines opened by magnetic merging in the one case and by diffusion onto closed field lines in the other.

Crooker, N. U.↗

NASA NTRS1979

The mean free path of low rigidity cosmic rays

A simple heuristic argument is presented which suggests that the large rigidity independent mean free paths observed for low rigidity solar and galactic cosmic rays can be understood in terms of weak turbulence diffusion theory if one assumes that the interplanetary magnetic turbulence consists of a combination of Alfven waves propagating with constant field magnitude and a small (approximately 5-10%) admixture of compressive fluctuations.

Goldstein, M. L.↗

NASA NTRS1980

Electromagnetic precipitation and ducting of particles in turbulent boundary layers

A method for analyzing magnetic migration of particles in turbulent flows is applied to the prediction of particle trajectories and densities in turbulent aerodynamic boundary layers. Results for conditions typical of aircraft with 30-40 micron particles indicate a large upstream collection and a 5% loss of particles during one pass through the boundary layer. The capacity of the magnetic field to achieve a balance with turbulent diffusion in confining the particles to the boundary layer is discussed.

Davey, K. R.↗

NASA NTRS1981

The electromagnetic ion beam instability upstream of the earth's bow shock

The linear theory of the electromagnetic ion beam instability for arbitrary angles of propagation has been studied. The parameters considered in the theory are typical of the solar wind upstream of the earth's bow shock when a 'reflected' proton beam is present. Maximum growth occurs for propagation parallel to the ambient magnetic field B, but this instability also displays significant growth at wave-vectors oblique to B. Oblique, unstable modes seem to be the likely source of the compressive magnetic fluctuations recently observed in conjunction with the 'diffuse' ion population. An energetic ion beam does not directly give rise to linear growth of either ion acoustic or whistler mode instabilities.

Gary, S. P.↗

NASA NTRS1980

Solar modulation of galactic cosmic rays

The current theory for the solar modulation of galactic cosmic rays is reviewed briefly. The four basic mechanisms which are important for modulation-diffusion, effects associated with the large-scale magnetic field, convection, and energy change - are discussed in simple terms. The equation which describes cosmic-ray behavior in the heliosphere is presented and some sample solutions are considered. It is pointed out that, although the basic mechanisms which describe the cosmic-ray behavior are understood, the actual cause of the variation in the cosmic-ray flux over the solar cycle, or over longer intervals, is beyond current comprehension.

Fisk, L. A.↗

NASA NTRS1988

Compact, variable sources on the sun at 2 centimeter wavelength

VLA observations of compact transient sources on the sun at 2 cm wavelength are presented. These sources have angular sizes of theta of about 5-25 arcsec, brightness temperatures of about 1-3 x 10 to the 5th K, and lifetimes ranging between a few minutes to several hours. The emission originates in regions of diffuse plage and quiet sun, where the photospheric magnetic fields are relatively weak (H of not greater than 100 G). In some cases the 2 cm radiation may be explained as the thermal bremsstrahlung of a dense plasma in the transition region. For other sources, the relatively high circular polarization suggests a nonthermal emission mechanism, such as the gyrosynchrotron radiation of mildly relativistic electron with a power-law spectrum.

Willson, Robert F.↗

NASA NTRS1990

Electron heating in quasi-perpendicular shocks - A Monte Carlo simulation

To study the problem of electron heating in quasi-perpendicular shocks, under the combined effects of 'reversible' motion, in the shock electric potential and magnetic field, and wave-particle interactions a diffusion equation was derived, in the drift (adiabatic) approximation and it was solved by using a Monte Carlo method. The results show that most of the observations can be explained within this framework. The simulation has also definitively shown that the electron parallel temperature is determined by the dc electromagnetic field and not by any wave particle induced heating. Wave-particle interactions are effective in smoothing out the large gradients in phase space produced by the 'reversible' motion of the electrons, thus producing a 'cooling' of the electrons. Some constraints on the wave-particle interaction process may be obtained from a detailed comparison between the simulation and observations. In particular, it appears that the adiabatic approximation must be violated in order to explain the observed evolution of the perpendicular temperature.

Veltri, Pierluigi↗

NASA NTRS1994

Anisotropy in the angular broadening of Sagittarius A(*) at the galactic center

We present the results of a lambda = 20 cm (VLA) squared observation of the compact Galactic center radio source Sgr A(*). The scatter-broadened image is elongated in the east-west direction, with an axial ratio of 0.56 +/- 0.22 and a position angle of 82 deg +/- 1.8 deg. A similar shape and orientation has been found previously at shorter wavelengths using Very Long Base Interferometry (VLBI) and Very Long Baseline Array (VLBA). Both the major and minor axes follow the lambda squared law appropriate for scattering by turbulence in the intervening medium. We argue that the scattering occurs in thin ionized surface layers of molecular clouds lying in the central 100 pc of the Galaxy. The ionized regions are characterized by electron densities of 10(exp 4)/cu cm and milligauss magnetic field strengths, and are responsible for the diffuse free-free emission and the Faraday rotation in the region.

Yusef-Zadeh, Farhad↗

NASA NTRS2008

Radiation from Relativistic Jets

Nonthermal radiation observed from astrophysical systems containing relativistic jets and shocks, e.g., gamma-ray bursts (GRBs), active galactic nuclei (AGNs), and Galactic microquasar systems usually have power-law emission spectra. Recent PIC simulations of relativistic electron-ion (electron-positron) jets injected into a stationary medium show that particle acceleration occurs within the downstream jet. In the presence of relativistic jets, instabilities such as the Buneman instability, other two-streaming instability, and the Weibel (filamentation) instability create collisionless shocks, which are responsible for particle (electron, positron, and ion) acceleration. The simulation results show that the Weibel instability is responsible for generating and amplifying highly nonuniform, small-scale magnetic fields. These magnetic fields contribute to the electron's transverse deflection behind the jet head. The 'jitter' radiation from deflected electrons in small-scale magnetic fields has different properties than synchrotron radiation which is calculated in a uniform magnetic field. This jitter radiation, a case of diffusive synchrotron radiation, may be important to understand the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets, and supernova remnants.

Nishikawa, K.-I.↗

NASA NTRS2011

Magnetic Reconnection in a Compressible MHD Plasma

Using steady-state resistive MHD, magnetic reconnection is reinvestigated for conditions of high resistivity/low magnetic Reynolds number, when the thickness of the diffusion region is no longer small compared to its length. Implicit expressions for the reconnection rate and other reconnection parameters are derived based on the requirements of mass, momentum, and energy conservation. These expressions are solved via simple iterative procedures. Implications specifically for low Reynolds number/high resistivity are being discussed

Hesse, Michael↗

NASA NTRS1972

Galactic cosmic ray modulation from 1965-1970.

Numerical solutions of the cosmic-ray equation of transport within the solar cavity and including the effects of diffusion, convection, and energy losses due to adiabatic deceleration, have been used to reproduce the modulation of galactic electrons, protons, and helium nuclei observed during the period from 1965 to 1970. Kinetic energies between 10 and 10,000 MeV/nucleon are considered. Computed and observed spectra are given for the years 1965, 1968, 1969, and 1970 together with the diffusion coefficients. These diffusion coefficients are assumed to be of separable form in rigidity and radial dependence, and are consistent with the available magnetic-field power spectra. The force-field solutions are given for these diffusion coefficients and galactic spectra and are compared with the numerical solutions. It is shown that the energy losses and convection lead to near-earth nuclei spectra at kinetic energies less than or equal to 100 MeV/nucleon in which the differential intensity is proportional to the kinetic energy with little dependence on the form of the galactic spectrum. This dependence is in agreement with the observed spectra of all species of atomic nuclei and it is argued that this provides strong observational evidence for the presence of energy losses in the propagation process, and for the exclusion of low-energy galactic nuclei from near earth.

Urch, I. H.↗

DOE OSTI2022

Structural magnetic glassiness in the spin ice Dy 2 Ti 2 O 7

The origin and nature of glassy dynamics presents one of the central enigmas of condensed-matter physics across a broad range of systems ranging from window glass to spin glasses. The spin-ice compound Dy 2 Ti 2 O 7 , which is perhaps best known as hosting a three-dimensional Coulomb spin liquid with magnetically charged monopole excitations, also falls out of equilibrium at low temperature. How and why it does so remains an open question. Based on an analysis of low-temperature diffuse neutron-scattering experiments employing different cooling protocols alongside recent magnetic noise studies, combined with extensive numerical modeling, we argue that upon cooling, the spins freeze into what may be termed a “structural magnetic glass,” without an a priori need for chemical or structural disorder. Specifically, our model indicates the presence of frustration on two levels, first producing a near-degenerate constrained manifold inside which phase ordering kinetics is in turn frustrated. A remarkable feature is that monopoles act as sole annealers of the spin network and their pathways and history encode the development of glass dynamics, allowing the glass formation to be visualized. Our results suggest that spin ice Dy 2 Ti 2 O 7 provides one prototype of magnetic glass formation specifically and a setting for the study of kinetically constrained systems more generally.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

NASA NTRS1988

Magnetic alignment of grains

This paper reviews mechanisms that have been proposed to account for alignment of dust grains in diffuse clouds and in dense clouds. The mechanisms that have proved inadequate are considered, including alignment by nonmagnetic and magnetic processes. The results thus far favor the Davis-Greenstein mechanism, in which paramagnetic relaxation of spinning grains removes components of rotation perpendicular to the magnetic field. Polarization measurements showing the alignment of grains in cool dense clouds are discussed.

Hildebrand, Roger H.↗

DOE OSTI2024

Structural Studies of Alloyed and Nanoparticle Transition Metal Dichalcogenides by Selenium-77 Solid-State Nuclear Magnetic Resonance Spectroscopy

Layered transition metal dichalcogenides (TMDCs) such as MoS 2 , MoSe 2 , and WSe 2 are under intense investigation because they are atomically thin semiconductors with photophysical properties that can be tuned by changing their composition or morphology. Mechanochemical processing has been proposed as a method to obtain alloyed TMDCs in the series Mo 1–x W x S y Se 2–y (x = 0–1; y = 0–2). However, elucidating the chemical transformations occurring at the atomic scale following mechanochemical processing can be challenging because the products are often amorphous or microcrystalline. To address this challenge, we probe TMDC mixing and alloying by using a combination of powder X-ray diffraction, Raman spectroscopy, diffuse reflectance spectroscopy, 77 Se solid-state nuclear magnetic resonance (SSNMR) spectroscopy, and planewave density functional theory (DFT) calculations. The nature of the milling material and reaction atmosphere are shown to be essential factors in limiting the formation of undesired oxide byproducts. We demonstrate acquisition of 77 Se SSNMR spectra using different combinations of Carr-Purcell Meiboom-Gill acquisition (CPMG) pulse sequences, magic angle spinning (MAS), and MAS dynamic nuclear polarization. Further, the combination of SSNMR with the other characterization methods clearly demonstrates that high energy impact ball milling induces molecular level alloying of Mo, W and chalcogen atoms in the family Mo 1–x W x S y Se 2–y . Gauge including projector augmented wave DFT calculations yield accurate 77 Se chemical shift (CS) tensor components. 77 Se SSNMR spectroscopy was also applied to study the structure of WSe 2 nanocrystals intercalated with ethylenediamine. The intercalated WSe 2 nanocrystals exhibit a more positive isotropic 77 Se CS as compared to bulk WSe 2 , however, the 77 Se CS anisotropy is the same, confirming the WSe 2 layers have a similar structure as in their bulk counterparts.

36 MATERIALS SCIENCE↗

NASA NTRS2007

Short-Term Forecasting of Radiation Belt and Ring Current

A computer program implements a mathematical model of the radiation-belt and ring-current plasmas resulting from interactions between the solar wind and the Earth s magnetic field, for the purpose of predicting fluxes of energetic electrons (10 keV to 5 MeV) and protons (10 keV to 1 MeV), which are hazardous to humans and spacecraft. Given solar-wind and interplanetary-magnetic-field data as inputs, the program solves the convection-diffusion equations of plasma distribution functions in the range of 2 to 10 Earth radii. Phenomena represented in the model include particle drifts resulting from the gradient and curvature of the magnetic field; electric fields associated with the rotation of the Earth, convection, and temporal variation of the magnetic field; and losses along particle-drift paths. The model can readily accommodate new magnetic- and electric-field submodels and new information regarding physical processes that drive the radiation-belt and ring-current plasmas. Despite the complexity of the model, the program can be run in real time on ordinary computers. At present, the program can calculate present electron and proton fluxes; after further development, it should be able to predict the fluxes 24 hours in advance

Fok, Mei-Ching↗

NASA NTRS1987

Particle acceleration at shocks - A Monte Carlo method

A Monte Carlo method is presented for the problem of acceleration of test particles at relativistic shocks. The particles are assumed to diffuse in pitch angle as a result of scattering off magnetic irregularities frozen into the fluid. Several tests are performed using the analytic results available for both relativistic and nonrelativistic shock speeds. The acceleration at relativistic shocks under the influence of radiation losses is investigated, including the effects of a momentum dependence in the diffusion coefficient. The results demonstrate the usefulness of the technique in those situations in which the diffusion approximation cannot be employed, such as when relativistic bulk motion is considered, when particles are permitted to escape at the boundaries, and when the effects of the finite length of the particle mean free path are important.

Kirk, J. G.↗

DOE OSTI2022

On heat conduction in an irregular magnetic field. Part 1

Anisotropic heat conduction in a plasma embedded in a magnetic field with irregular, possibly chaotic, field lines is discussed. If the collisional mean free path exceeds the electron gyroradius, the heat conductivity is much larger along the field lines than across them, and this enhances the transport across a domain where good flux surfaces do not exist. Recognising that anisotropic heat conduction may be cast in a variational form, and by constructing increasingly sophisticated trial functions that are based on invariant and almost-invariant structures under the magnetic field-line flow, bounds are derived on this enhancement and on the temperature variation along the magnetic field. In this way, remarkably accurate approximations for the temperature can be rapidly constructed without solving the diffusion equation, even in the small perpendicular-diffusion limit when the solution for the temperature is dominated by the fractal structure the magnetic field lines.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2020

Heat Diffusion in High-Cp Nb3Sn Composite Superconducting Wires

A major focus of Nb3Sn accelerator magnets is on significantly reducing or eliminating their training. Demonstration of an approach to increase the Cp of Nb3Sn magnets using new materials and technologies is very important both for particle accelerators and light sources. It would improve thermal stability and lead to much shorter magnet training, with substantial savings in machines’ commissioning costs. Both Hypertech and Bruker-OST have attempted to introduce high-Cp elements in their wire design. This paper includes a description of these advanced wires, the finite element model of their heat diffusion properties as compared with the standard wires, and whenever available, a comparison between the minimum quench energy (MQE) calculated by the model and actual MQE measurements on wires.

43 PARTICLE ACCELERATORS↗