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At least 829 records · Page 46

Narrow Scale Flow and a Weak Field by the Top of Earth's Core: Evidence from Orsted, Magsat and Secular Variation

As Earth's main magnetic field weakens, our magnetic shield against the onslaught of the solar wind thins. And the field strength needed to fend off battering by solar coronal mass ejections is decreasing, just when the delicate complexity of modem, vulnerable, electro-technological systems is increasing at an unprecedented rate. Recently, a working group of distinguished scientist from across the nation has asked NASA's Solid Earth and Natural Hazards program a key question: What are the dynamics of Earth s magnetic field and its interactions with the Earth system? Paleomagnetic studies of crustal rocks magnetized in the geologic past reveal that polarity reversals have occurred many times during Earth s history. Networked super-computer simulations of core field and flow, including effects of gravitational, pressure, rotational Coriolis, magnetic and viscous forces, suggest how this might happen in detail. And space-based measurements of the real, time-varying magnetic field help constrain estimates of the speed and direction of fluid iron flowing near the top of the core and enable tests of some hypotheses about such flow. Now scientists at NASA s Goddard Space Flight Center have developed and applied methods to test the hypotheses of narrow scale flow and of a dynamically weak magnetic field near the top of Earth s core. Using two completely different methods, C. V. Voorhies has shown these hypotheses lead to specific theoretical forms for the "spectrum" of Earth s main magnetic field and the spectrum of its rate of change. Much as solar physicists use a prism to separate sunlight into its spectrum, from long wavelength red to short wavelength blue light, geophysicists use a digital prism, spherical harmonic analysis, to separate the measured geomagnetic field into its spectrum, from long to short wavelength fields. They do this for the rate of change of the field as well.

Voorhies, Coerte V.↗

Onset of the Magnetic Explosion in Solar Flares and Coronal Mass Ejections

We present observations of the magnetic field configuration and its transformation in six solar eruptive events that show good agreement with the Hirayama-Shibata model for eruptive flares. The observations arc X-ray images from the Yohkoh Soft X-ray Telescope (SXT) and magnetograms from Kitt Peak National Solar Observatory, interpreted together with the 1-8 Angstrom X-ray flux observed by GOES. The observations show: 1. Each event is a magnetic explosion that occurs in an initially closed magnetic bipole in which the core field is sheared and twisted in the shape of a sigmoid, having an oppositely curved elbow on each end. The arms of the opposite elbows are sheared past each other so that they overlap and are crossed low above the neutral line in the middle of the bipole. 2. Although four of the explosions arc ejective (blow open the bipole) and two are confined (arc arrested within the closed bipole), all six begin the same way. In the SXT iniages, the explosion begins with brightening and expansion of the two elbows together with the appearance of short bright sheared loops low over the neutral line under the crossed arms and, rising up from the crossed arms, long strands connecting the far ends of the elbows. 3. All six events arc single-bipole events in that during the onset and early development of the explosion they show no evidence for reconnection between the exploding bipole and any surrounding magnetic fields. We conclude that in each of our events the magnetic explosion was unleashed by runaway tether-cutting via implosive/explosive rcconnection in the middle of the sigmoid, as in the Hirayama-Shibata model. The similarity of the onsets of the two confined explosions to the onsets of the four ejective explosions and their agreement with the model indicate that runaway reconnection inside a sheared core field can begin whether or not the overlying fields (the envelope field and contiguous fields that press down on the envelope) allow the explosion to be ejective. We consider the implications ofthis for the unleashing of magnetic explosions in field configurations composed of to or more impacted bipoles.

Moore, R. L.↗

Impurity temperature screening in stellarators close to quasisymmetry

Impurity temperature screening is a favourable neoclassical phenomenon involving an outward radial flux of impurity ions from the core of fusion devices. Quasisymmetric magnetic fields lead to intrinsically ambipolar neoclassical fluxes that give rise to temperature screening for low enough $\unicode[STIX]{x1D702}^{-1}\equiv d\ln n/d\ln T$ . In contrast, neoclassical fluxes in generic stellarators will depend on the radial electric field, which is predicted to be inward for ion-root plasmas, potentially leading to impurity accumulation. Here, we examine the impurity particle flux in a number of approximately quasisymmetric stellarator configurations and parameter regimes while varying the amount of symmetry breaking in the magnetic field. For the majority of this work, neoclassical fluxes have been obtained using the SFINCS drift-kinetic equation solver with electrostatic potential $\unicode[STIX]{x1D6F7}=\unicode[STIX]{x1D6F7}(r)$ , where $r$ is a flux-surface label. Results indicate that achieving temperature screening is possible, but unlikely, at low-collisionality reactor-relevant conditions in the core. Thus, the small departures from symmetry in nominally quasisymmetric stellarators are large enough to significantly alter the neoclassical impurity transport. A further look at the magnitude of these fluxes when compared to a gyro-Bohm turbulence estimate suggests that neoclassical fluxes are small in configurations optimized for quasisymmetry when compared to turbulent fluxes.

Physics↗

Inhomogeneous disorder Dirac Fermions: from heavy fermion superconductors to graphene (Final Report)

This is the final report on the award designed to foster a partnership between Louisiana State University and Los Alamos National Laboratory (LANL) in conducting fundamental research in support of energy needs. The general focus of the research effort was on developing a better understanding of materials with new functionalities. We investigated two distinct and very promising classes of new materials, which serve as a testing ground for many of the novel phenomena in condensed matter physics: the heavy fermion 115 series, where the interplay of strong interactions between the electrons leads to a rich variety of competing phases and anomalous properties, and newly discovered pnictide superconductors. The former focus was planned; the latter emerged during the collaborative effort with LANL. Our objective was to determine the origin, and to establish a functional effective theory description of the phases in these systems, and transitions between them. We report on the main accomplishments under the award that serves to clarify the nature of superconductivity in both families of materials. In particular, we collaborated with experimentalists to predict and analyze the magnetic field and temperature dependence of the bulk thermodynamic and transport properties and to determine the gap shape in CeCoIn₅ and in Ba(Fe 1-x Co x )₂As₂, investigated the Kondo temperature in the presence of spin-orbit coupling in the conduction band, and provided theoretical guidance for local probes such as scanning tunneling spectroscopy of vortex cores and impurity resonances, and magnetic force microscopy of the superconducting states.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

The deuterium fractionation timescale in dense cloud cores: A parameter space exploration

The deuterium fraction, [N{sub 2}D{sup +}]/[N{sub 2}H{sup +}], may provide information about the ages of dense, cold gas structures, which are important for comparing dynamical models of cloud core formation and evolution. Here we introduce a complete chemical network with species containing up to three atoms, with the exception of the oxygen chemistry, where reactions involving H{sub 3}O{sup +} and its deuterated forms have been added, significantly improving the consistency with comprehensive chemical networks. Deuterium chemistry and spin states of H{sub 2} and H{sub 3}{sup +} isotopologues are included in this primarily gas-phase chemical model. We investigate the dependence of deuterium chemistry on these model parameters: density (n{sub H}), temperature, cosmic ray ionization rate, and gas-phase depletion factor of heavy elements (f{sub D}). We also explore the effects of time-dependent freeze-out of gas-phase species and the dynamical evolution of density at various rates relative to free-fall collapse. For a broad range of model parameters, the timescales to reach large values of D{sub frac}{sup N{sub 2}H{sup +}}≳0.1, observed in some low- and high-mass starless cores, are relatively long compared to the local free-fall timescale. These conclusions are unaffected by introducing time-dependent freeze-out and considering models with evolving density, unless the initial f{sub D} ≳ 10. For fiducial model parameters, achieving D{sub frac}{sup N{sub 2}H{sup +}}≳0.1 requires collapse to be proceeding at rates at least several times slower than that of free-fall collapse, perhaps indicating a dynamically important role for magnetic fields in supporting starless cores and thus the regulation of star formation.

79 ASTRONOMY AND ASTROPHYSICS↗

Studies of Plasma Flow Past Jupiters Satellite Io

We have investigated the interaction of Io, Jupiter's innermost Galilean satellite, with the Io plasma torus, and the interaction of Ganymede with the corotating Jovian plasma. With the successful insertion of the Galileo spacecraft into orbit around Jupiter, many new observations have been made of the Jovian magnetosphere. Some of the most exciting results thus far have been in regards to Jupiter's satellites, Io and Ganymede. In both cases the large perturbations to the background (Jovian) magnetic field have been consistent with the satellites' possession of an intrinsic magnetic field. The gravity measurements implying a differentiated core at both Io and Ganymede makes internal generation of a magnetic field by dynamo action in these satellites plausible, and, in the case of Ganymede, the identification of an intrinsic field is apparently unambiguous. For Io the situation is less clear, and further analysis is necessary to answer this important question. During the past year, we have used time-dependent three-dimensional magnetohydrodynamic (MHD) simulations to study these plasma-moon interactions. The results from these simulations have been used directly in the analysis of the Galileo magnetometer data. Our primary emphasis has been on the Io interaction, but we recently presented results on the Ganymede interaction as well. In this progress summary we describe our efforts on these problems to date.

Linker, Jon A.↗

Triggering of the Two X-class Flares of 28 and 29 October 2003

From H-alpha movies from Aryabhatta Research Institute of Observational Sciences and from Prairie View Solar Observatory, hard X-ray movies from RHESSI, line-of-sight magnetogram movies from SOHO/MDI, and vector magnetograms from Marshal Space Flight Center, we examine the magnetic structure and evolution of the large delta-sunspot active region NOAA 10486 in relation to the onset and development of the two X-class flares that occurred in this active region on 28 and 29 October 2003. We find evidence that each of these flares was triggered by strongly sheared magnetic field via "tether-cutting" reconnection with adjacent/overlying strongly sheared field. In the first flare, the initial brightening in H-alpha: (1) was partly rooted in emerging sheared magnetic field along the edge of the large positive-polarity flux domain of the delta sunspot, and (2) consisted of four flare kernels, two in negative magnetic flux and two in positive magnetic flux. In the second flare, the brightening started in the core of a Z-shaped sigmoidal sheared magnetic field and the inner two of four H-alpha kernels were visible in 30-50 Kev hard X-ray image from RHESSI. Each flare spread from the initial quadrupolar brightening to develop into a much larger two-ribbon flare straddling a much more extensive swath of strongly sheared field along the edge of the large positive-flux domain of the delta sunspot, the first flare on the leading side and the second flare on the trailing side of this domain. Thus, localized internal reconnection triggered the explosion of these extensive sheared magnetic fields.

Choudhary, Debi Prasad↗

The properties of the extraordinary mode and surface plasmon modes in the three-dimensional magnetized plasma photonic crystals based on the magneto-optical Voigt effects

In this paper, the properties of the extraordinary mode and surface plasmon modes in the three-dimensional (3D) magnetized plasma photonic crystals (MPPCs) with face-centered-cubic lattices that are composed of the core tellurium (Te) spheres with surrounded by the homogeneous magnetized plasma shells inserted in the air, are theoretically investigated in detail by the plane wave expansion method, as the magneto-optical Voigt effects of magnetized plasma are considered (the incidence electromagnetic wave vector is perpendicular to the external magnetic field at any time). The optical switching or wavelength division multiplexer can be realized by the proposed 3D MPPCs. Our analyses demonstrate that the complete photonic band gaps (PBGs) and two flatbands regions for the extraordinary mode can be observed obviously. PBGs can be tuned by the radius of core Te sphere, the plasma density and the external magnetic field. The flatbands regions are determined by the existence of surface plasmon modes. Numerical simulations also show that if the thickness of magnetized plasma shell is larger than a threshold value, the band structures of the extraordinary mode will be similar to those obtained from the same structure containing the pure magnetized plasma spheres. In this case, the band structures also will not be affected by the inserted core spheres. It is also provided that the upper edges of two flatbands regions will not depend on the topology of lattice. However, the frequencies of lower edges of two flatbands regions will be convergent to the different constants for different lattices, as the thickness of magnetized plasma shell is close to zero.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

Fabrication of Fe{sub 3}O{sub 4}@CuO core-shell from MOF based materials and its antibacterial activity

Magnetic Fe{sub 3}O{sub 4}@CuO nanocomposite with a core/shell structure was successfully synthesized via direct calcinations of magnetic Fe{sub 3}O{sub 4}@HKUST-1 in air atmosphere. The morphology, structure, magnetic and porous properties of the as-synthesized nano composites were characterized by using scanning electron microscope (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (PXRD), and vibration sample magnetometer (VSM). The results showed that the nanocomposite material included a Fe{sub 3}O{sub 4} core and a CuO shell. The Fe{sub 3}O{sub 4}@CuO core-shell can be separated easily from the medium by a small magnet. The antibacterial activity of Fe{sub 3}O{sub 4}-CuO core-shell was investigated against gram-positive and gram-negative bacteria. A new mechanism was proposed for inactivation of bacteria over the prepared sample. It was demonstrated that the core-shell exhibit recyclable antibacterial activity, acting as an ideal long-acting antibacterial agent. - Graphical abstract: Fe{sub 3}O{sub 4}@CuO core-shell release of copper ions. These Cu{sup 2+} ions were responsible for the exhibited antibacterial activity. - Highlights: • The Fe{sub 3}O{sub 4}@CuO core-shell was prepared by MOF method. • This is the first study of antibacterial activity of core-shell consist of CuO and Fe{sub 3}O{sub 4}. • The core-shell can be reused effectively. • Core-shell was separated from the reaction solution by external magnetic field.

36 MATERIALS SCIENCE↗

GRM - Observing the terrestrial gravity and magnetic fields in the 1990's

NASA is proposing to launch a new geopotential fields exploration system called the Geopotential Research Mission (GRM). Two spacecraft will be placed in a circular polar orbit at 160 km altitude. Distances between these satellites will vary from 100 to 600 km. Both scalar and vector magnetic fields will be measured by magnetometers mounted on a boom positioned in the forward direction on the lead satellite. Gravity data will be computed from the measured change in distance between the two spacecraft. This quantity, called the range-rate, will be determined from the varying frequency (Doppler shift) between transmitter and receiver on each satellite. Expected accuracies (at the one-sigma level) are: gravity field, 1.0 milliGal, 5 cm geoid height; magnetics, scalar field 2 nT, vector to 20 arcsec, both resolved to less than 100 km. With these more accurate and higher resolution data, it will be possible to investigate the earth's structure from the crust (with the shorter wavelength gravity and magnetic anomalies) through the mantle (from the intermediate wavelength gravity field) and into the core (using the longer wavelength gravity and magnetic fields).

Taylor, P. T.↗

Non-contacting power transfer device

A transformer for coupling AC electrical energy from a stationary element to a rotating element without the use of sliding contacts is described. The transformer is of the rotary type and includes a ferrite core and two primary windings which are stationary with respect to a seconary winding which rotates within an annular cavity adjacent an axial bore in the core. The core is comprised of two cup type core halves. Electrical connection to the secondary winding is made through a split bobbin assembly which couples to a coaxial shaft assembly located in the axial bore. The electrical coupling to the coaxial shaft assembly is made through a continuous transverse channel connecting the axial bore with the annular cavity. The transverse channel forms a single air gap; however, it is not open directly to free space but is shielded by the magnetic permeable material of the core halves.

Studer, P. A.↗

Radio imaging of a type IVM radio burst on the 14th of August 2010

Propagating coronal mass ejections (CMEs) are often accompanied by burst signatures in radio spectrogram data. We present Nançay Radioheliograph observations of a moving source of broadband radio emission, commonly referred to as a type IV radio burst (type IVM), which occurred in association with a CME on the 14th of August 2010. The event was well observed at extreme ultraviolet (EUV) wavelengths by SDO/AIA and PROBA2/SWAP, and by the STEREO SECCHI and SOHO LASCO white light (WL) coronagraphs. The EUV and WL observations show the type IVM source to be cospatial with the CME core. The observed spectra is well fitted by a power law with a negative slope, which is consistent with optically thin gyrosynchrotron emission. The spectrum shows no turn over at the lowest Nançay frequencies. By comparing simulated gyrosynchrotron spectra with Nançay Radioheliograph observations, and performing a rigorous parameter search we are able to constrain several key parameters of the underlying plasma. Simulated spectra found to fit the data suggest a nonthermal electron distribution with a low energy cutoff of several tens to 100 keV, with a nonthermal electron density in the range 10{sup 0}-10{sup 2} cm{sup –3}, in a magnetic field of a few Gauss. The nonthermal energy content of the source is found to contain 0.001%-0.1% of the sources thermal energy content. Furthermore, the energy loss timescale for this distribution equates to several hours, suggesting that the electrons could be accelerated during the CME initiation or early propagation phase and become trapped in the magnetic structure of the CME core without the need to be replenished.

79 ASTRONOMY AND ASTROPHYSICS↗

X-point effects on the ideal MHD modes in tokamaks in the description of dual-poloidal-region safety factor

The flux coordinates with dual-region safety factor (q) in the poloidal direction are developed in this work. The X-point effects on the ideal MHD modes in tokamaks are then analyzed using this coordinate system. Since the X-point effects mainly affect the edge region, the modes localized at the tokamak edge are particularly examined. Two types of modes are studied. The first is related to the conventional peeling or peeling-ballooning modes. The mode existence aligned with the local magnetic field in the poloidally core region as observed experimentally is confirmed. The X points are shown to contribute to a stabilizing effect for the conventionally treated modes with the surface-averaged q and with the tokamak edge portion truncated. The other is the axisymmetric modes localized in the vicinity of X points, which can affect the cross-field-line transport near the X points. The existence of axisymmetric modes points to the possibility of applying a toroidally axisymmetric resonant magnetic perturbation (RMP) in the X-point area for mitigating the edge localized modes, which can be an alternative to the current RMP design. The dual q description also has important implications for the existing non-axisymmetric RMP concept. It helps to understand why the RMP suppression of edge localized modes is difficult to achieve in the double-null tokamak configurations and points to the possibility of further improving the current RMP concept by considering the alignment to the local q.

Fourier analysis↗

Some new results on spherical dynamos

Small-scale motions in the earth's liquid core are likely to be highly anisotropic because of the effects of rotation. Guided by physical considerations, the 'alpha-effect' described by an anisotropic tensor alpha sub ik is formulated and the corresponding boundary value problem for a sphere is solved for a variety of boundary conditions. A converged solution has been obtained only in the case of the Fermi condition of an infinitely conducting exterior of the sphere. Some remarks are made on the hypothetical upper bound on magnetic field strengths in planetary cores originally proposed by Busse (1976).

Busse, F. H.↗

Open-loop correction for an eddy current dominated beam-switching magnet

A beam-switching magnet and the pulsed power supply it requires have been developed for the Japan Proton Accelerator Research Complex. To switch bunched proton beams, the dipole magnetic field must reach its maximum value within 40 ms. In addition, the field flatness should be less than 5 × 10{sup −4} to guide each bunched beam to the designed orbit. From a magnetic field measurement by using a long search coil, it was found that an eddy current in the thick endplates and laminated core disturbs the rise of the magnetic field. The eddy current also deteriorates the field flatness over the required flat-top period. The measured field flatness was 5 × 10{sup −3}. By using a double-exponential equation to approximate the measured magnetic field, a compensation pattern for the eddy current was calculated. The integrated magnetic field was measured while using the newly developed open-loop compensation system. A field flatness of less than 5 × 10{sup −4}, which is an acceptable value, was achieved.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

Correlative cellular ptychography with functionalized nanoparticles at the Fe L-edge

Precise localization of nanoparticles within a cell is crucial to the understanding of cell-particle interactions and has broad applications in nanomedicine. Here in this paper, we report a proof-of-principle experiment for imaging individual functionalized nanoparticles within a mammalian cell by correlative microscopy. Using a chemically-fixed HeLa cell labeled with fluorescent core-shell nanoparticles as a model system, we implemented a graphene-oxide layer as a substrate to significantly reduce background scattering. We identified cellular features of interest by fluorescence microscopy, followed by scanning transmission X-ray tomography to localize the particles in 3D, and ptychographic coherent diffractive imaging of the fine features in the region at high resolution. By tuning the X-ray energy to the Fe L-edge, we demonstrated sensitive detection of nanoparticles composed of a 22 nm magnetic Fe 3 O 4 core encased by a 25-nm-thick fluorescent silica (SiO 2 ) shell. These fluorescent core-shell nanoparticles act as landmarks and offer clarity in a cellular context. Our correlative microscopy results confirmed a subset of particles to be fully internalized, and high-contrast ptychographic images showed two oxidation states of individual nanoparticles with a resolution of ~16.5 nm. The ability to precisely localize individual fluorescent nanoparticles within mammalian cells will expand our understanding of the structure/function relationships for functionalized nanoparticles.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Magnetic reversals of Jupiter and Saturn

The possibility that the gas-giant planets Jupiter and Saturn undergo solar-type magnetic reversals is examined using dynamo theory and radiotelescope data on decametriic emissions from Jupiter. Possible values are found for the effects of the fluctuating velocity field, the magnetic diffusivity, and change in the rotation rate of a dynamo over a characteristic length. The radio emissions from Jupiter decreased in intensity from 1961-72 and rose steadily to the end of 1978, which could have been caused by a change in the Jovian magnetic field. Since Jupiter may have a small rocky core embedded in metallic hydrogen which comprises 75 percent of the radius of the planet, the planetary magnetic field may extend into the cores of its satellites. The dynamo characteristics, like those of Saturn, would be chaotic, although quasi-periodic reversals could occur over intervals on the order of centuries instead of decades such as with the sun and much longer periods such as with the earth.

Hathaway, D. H.↗

Progress in extending high poloidal beta scenarios on DIII-D towards a steady-state fusion reactor and impact of energetic particles

To prepare for steady-state operation of future fusion reactors (e.g. the International Thermonuclear Experimental Reactor and China Fusion Engineering Test Reactor (CFETR)), experiments on DIII-D have extended the high poloidal beta ( β P ) scenario to reactor-relevant edge safety factor q 95 ~ 6.0, while maintaining a large-radius internal transport barrier (ITB) using negative magnetic shear. Excellent energy confinement quality ( H 98y2 > 1.5) is sustained at high normalized beta ( β N ~ 3.5). This high-performance ITB state with Greenwald density fraction near 100% and q min ≥ 3 is achieved with toroidal plasma rotation V tor ~ 0 at ρ ≥ 0.6. This is a key result for reactors expected to have low V tor . At high β P (≥1.9), large Shafranov shift can stabilize turbulence leading to a high confinement state with a low pedestal and an ITB. At lower β P (<1.9), negative magnetic shear in the plasma core contributes to turbulence suppression and can compensate for reduced Shafranov shift to continue to access a large-radius ITB and excellent confinement with low V tor , consistent with the results of gyrofluid transport simulations. These high- β P cases are characterized by weak/no Alfvén eigenmodes (a.e.) and classical fast-ion transport. At high density, the fast-ion deceleration time decreases and Δ β fast is lower; these reduce a.e. drive. The reverse-shear Alfvén eigenmodes are weaker or stable because the negative magnetic shear region is located at higher radius, away from the peaked fast-ion profile. Resistive wall modes can be a limitation at simultaneous high β N , low internal inductance, and low rotation. Analysis suggests that additional off-axis external current drive could provide a more stable path at reduced q 95 . Based on a DIII-D high- β P plasma with large-radius ITB, two scenarios are proposed for CFETR Q = 5 steady-state operation with ~1 GW fusion power: a lower-$l_i$($l_i$ ~ 0.66) and a higher-$l_i$($l_i$ ~ 0.75) case. Using a Landau closure model, multiple energetic particle (EP) effects on the a.e. stability are analyzed modifying the growth rate of the a.e.s triggered by the neutral-beam-injection EPs and alpha particles, although the stabilizing/destabilizing effect is weak for the cases analyzed. The stabilizing effects of the combined EP species β , energy, and density profile in CFETR need further investigation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗