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At least 1,585 records · Page 88
All sourcesNASA NTRSDOE OSTI
NASA NTRS1998

Magnetic Field Effect on Convective Fluid Flow in a Vertical Bridgman System

We studied the magnetic field effects on convective fluid flow in a differentially heated HgCdTe Bridgman growth system with linearly stratified vertical HgTe concentration. We followed J. E. Hart's system set-up, boundary conditions, and formulation. We obtained analytical steady parallel flow solutions for fluid flow's velocity and solutal concentration. These solutions depend only on solutal Rayleigh number. Our analytical solutions had different expressions from that of Hart's, but had similar numerical simulation results for the mean fluid velocity and solutal concentration gradient as a function of solutal Rayleigh number. We studied the effects of a transverse magnetic field on the fluid flow of the Bridgman growth systems both on the ground and in the space. The simulation results and comparison with experimental results will be presented.

Wang, J. C.↗
NASA NTRS1990

Hallmarks of the magnetic field in the solar atmosphere - Structure, evolution, heating, and flaring

Recent observations of the solar magnetic field and its effects on the solar atmosphere are discussed, with an emphasis on large-scale active regions and their implications for the fine-scale magnetic structure and for activity in the so-called quiet regions. Sample magnetograms, sunlight images, H-alpha images, X-ray images, and spectroheliograms are presented and characterized in detail, and the form and action of the magnetic field in flares are considered. It is pointed out that simultaneous observations of all levels (from the photosphere to the corona) at 100-km (about 100-marcsec) resolution are needed to see the extent of fields looping into the corona and understand their structure and activity; large space-based observatories would be required.

Moore, Ronald L.↗
NASA NTRS1988

Observed nonpotential magnetic fields and the inferred flow of electric currents at a location of repeated flaring

The vector magnetic field of an active region at a location of repeated flaring is studied in order to explore the nature of the currents flowing in the areas where the flares initiated. The observed transverse component of the magnetic field is used to obtain the component of electric current density crossing the photosphere along the line-of-sight. It is found that currents flow out of an area of positive magnetic polarity and across the magnetic inversion line into two areas of negative polarity. Characteristics of the calculated source field are discussed.

Hagyard, M. J.↗
NASA NTRS1989

Rotation of the photospheric magnetic fields: A north-south asymmetry

During most of solar cycle 21 the large-scale photospheric field rotated more rapidly in the Northern Hemisphere than in the southern. The large-scale northern field rotated with a 26.9 day period (synodic), was centered at 15 degress N, and covered a latitude zone about 24 degrees wide. The large-scale southern field rotated with a periodicity of 28.1 days, was centered at 26 degrees S, and covered a latitude zone about 32 degrees wide. Our analysis showed rotational power at only a few discrete latitudes and frequencies in each hemisphere. The center of each peak lies near the sunspot differential rotation curve. The largest scale field contributes to the configuration of the coronal and interplanetary magnetic field (IMF). The strength of the first harmonic of the northern field suggests that this structure may be related to the 4-sector pattern observed in the IMF polarity. The southern field had much lower power at the first harmonic of the solar rotation rate and so would contribute only to a 2-sector structure in the IMF. These results were discovered in Fourier analysis of photospheric synoptic charts obtained at the Wilcox Solar Observatory from 1976 to 1986 and confirmed in higher resolution maps from the National Solar Observatory. Mt. Wilson magnetic field measurements from solar cycle 20 show a similar north-south asymmetry.

Antonucci, E.↗
NASA NTRS1990

Rotation of the photospheric magnetic fields - A north-south asymmetry

During most of solar cycle 21 the large-scale photospheric field rotated more rapidly in the Northern Hemisphere than in the southern. The large-scale northern field rotated with a 26.9 day period (synodic), was centered at 15 degrees N, and covered a latitude zone about 24 degrees wide. The large-scale southern field rotated with a periodicity of 28.1 days, was centered at 26 degrees S, and covered a latitude zone about 32 degrees wide. Our analysis showed rotational power at only a few discrete latitudes and frequencies in each hemisphere. The center of each peak lies near to the sunspot differential rotation curve. The largest scale field contributes to the configuration of the coronal and interplanetary magnetic field (IMF). The strength of the first harmonic of the northern field suggests that this structure may be related to the 4-sector pattern observed in the IMF polarity. The southern field had much lower power at the first harmonic of the solar rotation rate and so would contribute only to a 2-sector structure in the IMF. These results were discovered in Fourier analysis of photospheric synoptic charts obtained at the Wilcox Solar Observatory from 1976 to 1986 and confirmed in higher resolution maps from the National Solar Observatory. Mt. Wilson magnetic field measurements from solar cycle 20 show a similar north-south asymmetry.

Antonucci, E.↗
NASA NTRS1974

Pioneer Solar Plasma and Magnetic Field Measurements in Interplanetary Space During August 2-17, 1972

Solar wind plasma and magnetic field measurements from Pioneers 9 and 10 during August 2-17, 1972, reveal complex and large-amplitude variations on a one-hour time scale and numerous discontinuities. During this time period an approximate radial alignment of the two spacecraft as seen from the Sun occurred with heliocentric distances of 0.8 AU for Pioneer 9 and 2.2 AU for Pioneer 10, both at 45 deg east of the Earth's solar longitude. The peak hourly average solar wind proton bulk velocity measured at Pioneer 9 was 990 km sec (exp -1) during hour 0 UT of August 5. The peak hourly average proton number density was 62 cm (exp -3) during hour 11 UT of August 3. The peak solar wind speeds are generally much reduced at Pioneer 10 compared with those observes at Pioneer 9. The peak 30 minute average magnetic field magnitude was 85 gamma during 1245 - 1315 UT of August 3. The Pioneer 9 data indicate passage of four fast forward interplanetary shocks, and one slow forward interplanetary shock.

Mihalov, J. D.↗
NASA NTRS1990

Giotto magnetic field observations at the outbound quasi-parallel bow shock of Comet Halley

The investigation of the outbound bow shock of Comet Halley using Giotto magnetometer data leads to the following results: the shock is characterized by strong magnetic turbulence associated with an increasing background magnetic field and a change in direction by 60 deg as one goes inward. In HSE-coordinates, the observed normal turned out to be (0.544, - 0.801, 0.249). The thickness of the quasi-parallel shock was 120,000 km. The shock is shown to be a new type of shock transition called a 'draping shock'. In a draping shock with high beta in the transonic transition region, the transonic region is characterized by strong directional variations of the magnetic field. The magnetic turbulence ahead of the shock is characterized by k-vectors parallel or antiparallel to the average field (and, therefore, also to the normal of the quasi-parallel shock) and almost isotropic magnetic turbulence in the shock transition region. A model of the draping shock is proposed which also includes a hypothetical subshock in which the supersonic-subsonic transition is accomplished.

Neubauer, F. M.↗
NASA NTRS2019

A NICER View of PSR J0030+0451: Evidence for a Global-Scale Multipolar Magnetic Field

Recent modeling of Neutron Star Interior Composition Explorer observations of thermal X-ray pulsations from the surface of the isolated millisecond pulsar PSRJ0030+0451 suggests that the hot emitting regions on the pulsar’s surface are far from antipodal, which is at odds with the classical assumption that the magnetic field in the pulsar magnetosphere is predominantly that of a centered dipole. Here, we review these results and examine previous attempts to constrain the magnetospheric configuration of PSRJ0030+0451. To the best of our knowledge, there is in fact no direct observational evidence that PSRJ0030+0451’s magnetic field is a centered dipole. Developing models of physically motivated, non-canonical magnetic field configurations and the currents that they can support poses a challenging task. However, such models may have profound implications for many aspects of pulsar research, including pulsar braking, estimates of birth velocities, and interpretations of multi-wavelength magnetospheric emission.

A V Bilous↗
NASA NTRS1990

Magnetic field line draping in the plasma depletion layer

Simultaneous IMP 8 solar wind and ISEE 1/2 observations for a northern dawn ISEE 1/2 magnetopause crossing on November 6, 1977. During this crossing, ISEE 1/2 observed quasi-periodic pulses of magnetosheathlike plasma on northward magnetic field lines. The ISEE 1/2 observations were originally interpreted as evidence for strong diffusion of magnetosheath plasma across the magnetopause and the Kelvin-Helmholtz instability at the inner edge of the low-latitude boundary layer. An alternate explanation, in terms of magnetic field merging and flux transfer events, has also been advocated. In this paper, a third interpretation is proposed in terms of quasi-periodic magnetopause motion which causes the satellites to repeatedly exit the magnetosphere and observe draped northward magnetosheath magnetic field lines in the plasma depletion layer.

Sibeck, D. G.↗
DOE OSTI2023

Nickel‐Based Single‐Molecule Catalysts with Synergistic Geometric Transition and Magnetic Field‐Assisted Spin Selection Outperform RuO 2 for Oxygen Evolution

Overcoming slow kinetics and high overpotential in electrocatalytic oxygen evolution reaction (OER) requires innovative catalysts and approaches that transcend the scaling relationship between binding energies for intermediates and catalyst surfaces. Inorganic complexes provide unique, customizable geometries, which can help enhance their efficiencies. However, they are unstable and susceptible to chemical reaction under extreme pH conditions. Immobilizing complexes on substrates creates single‐molecule catalysts (SMCs) with functional similarities to single‐atom catalysts (SACs). Here, in this work, an efficient SMC, composed of dichloro(1,3‐bis(diphenylphosphino)propane) nickel [NiCl 2 dppp] anchored to a graphene acid (GA), is presented. This SMC surpasses ruthenium‐based OER benchmarks, exhibiting an ultra‐low onset and overpotential at 10 mAcm −2 when exposed to a static magnetic field. Comprehensive experimental and theoretical analyses imply that an interfacial charge transfer from the Ni center in NiCl 2 dppp to GA enhances the OER activity. Spectroscopic investigations reveal an in situ geometrical transformation of the complex and the formation of a paramagnetic Ni center, which under a magnetic field, enables spin‐selective electron transfer, resulting in enhanced OER performance. The results highlight the significance of in situ geometric transformations in SMCs and underline the potential of an external magnetic field to enhance OER performance at a single‐molecule level.

in situ geometrical changes↗
NASA NTRS2016

On the Electron Diffusion Region in Asymmetric Reconnection with a Guide Magnetic Field

Particle-in-cell simulations in a 2.5-D geometry and analytical theory are employed to study the electron diffusion region in asymmetric reconnection with a guide magnetic field. The analysis presented here demonstrates that similar to the case without guide field, in-plane flow stagnation and null of the in-plane magnetic field are well separated. In addition, it is shown that the electric field at the local magnetic X point is again dominated by inertial effects, whereas it remains dominated by nongyrotropic pressure effects at the in-plane flow stagnation point. A comparison between local electron Larmor radii and the magnetic gradient scale lengths predicts that distribution should become nongyrotropic in a region enveloping both field reversal and flow stagnation points. This prediction is verified by an analysis of modeled electron distributions, which show clear evidence of mixing in the critical region.

reconnection↗
NASA NTRS2020

HAWC plus Far-infrared Observations of the Magnetic Field Geometry in M51 and NGC 891

Stratospheric Observatory for Infrared Astronomy High-resolution Airborne Wideband Camera Plus polarimetry at154μm is reported for the face-on galaxy M51 and the edge-on galaxy NGC 891. For M51, the polarization vectors generally follow the spiral pattern defined by the molecular gas distribution, the far-infrared(FIR) intensity contours, and other tracers of star formation. The fractional polarization is much lower in the FIR-bright central regions than in the outer regions, and we rule out loss of grain alignment and variations in magnetic field strength as causes. When compared with existing synchrotron observations, which sample different regions with different weighting, we find the net position angles are strongly correlated, the fractional polarizations are moderately correlated, but the polarized intensities are uncorrelated. We argue that the low fractional polarization in the central regions must be due to significant numbers of highly turbulent segments across the beam and along lines of sight in the beam in the central 3 kpc of M51. For NGC 891, the FIR polarization vectors within an intensity contour of1500-MJy sr1are oriented very close to the plane of the galaxy. The FIR polarimetry is probably sampling the magnetic field geometry in NGC 891 much deeper into the disk than is possible with NIR polarimetry and radio synchrotron measurements. In some locations in NGC 891, the FIR polarization is very low, suggesting we are preferentially viewing the magnetic field mostly along the line of sight, down the length of embedded spiral arms. There is tentative evidence for a vertical field in the polarized emission off the plane of the disk.

Disk galaxies↗
DOE OSTI2020

HAWC+ Far-infrared Observations of the Magnetic Field Geometry in M51 and NGC 891

Stratospheric Observatory for Infrared Astronomy High-resolution Airborne Wideband Camera Plus polarimetry at 154 μm is reported for the face-on galaxy M51 and the edge-on galaxy NGC 891. For M51, the polarization vectors generally follow the spiral pattern defined by the molecular gas distribution, the far-infrared (FIR) intensity contours, and other tracers of star formation. The fractional polarization is much lower in the FIR-bright central regions than in the outer regions, and we rule out loss of grain alignment and variations in magnetic field strength as causes. When compared with existing synchrotron observations, which sample different regions with different weighting, we find the net position angles are strongly correlated, the fractional polarizations are moderately correlated, but the polarized intensities are uncorrelated. We argue that the low fractional polarization in the central regions must be due to significant numbers of highly turbulent segments across the beam and along lines of sight in the beam in the central 3 kpc of M51. For NGC 891, the FIR polarization vectors within an intensity contour of 1500 MJy sr{sup −1} are oriented very close to the plane of the galaxy. The FIR polarimetry is probably sampling the magnetic field geometry in NGC 891 much deeper into the disk than is possible with NIR polarimetry and radio synchrotron measurements. In some locations in NGC 891, the FIR polarization is very low, suggesting we are preferentially viewing the magnetic field mostly along the line of sight, down the length of embedded spiral arms. There is tentative evidence for a vertical field in the polarized emission off the plane of the disk.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Probing the effect of magnetic field on charge order in the quasi-two-dimensional Mott insulator κ - (ET) 2 Hg(SCN) 2 Cl

Molecular-based quasi-two-dimensional charge-ordered Mott insulators can possess both spin S = 1/2 and an electric dipole degree of freedom residing on each (BEDT - TTF) 2 lattice site. There exists a theoretical proposal for a magnetoelectric effect in such a material, which can manifest itself as a sensitivity of charge distribution on (BEDT - TTF) 2 dimers to the magnetic field. Here in this work, we used Raman scattering spectroscopy in the magnetic field up to 31 T applied perpendicular to the two-dimensional triangular lattice planes to probe a magnetoelectric effect in $\kappa$ - (ET) 2 H g (SCN) 2 Cl. This is a model compound, which shows charge order and antiferromagnetic correlations at temperatures below 30 K. We discuss possible reasons for the absence of an observable magnetoelectric effect in the probed temperature and magnetic field ranges.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Supernova Axions Convert to Gamma Rays in Magnetic Fields of Progenitor Stars

It has long been established that axions could have been produced within the nascent proto-neutron star formed following the type II supernova SN1987A, escaped the star due to their weak interactions, and then converted to gamma rays in the Galactic magnetic fields; the nonobservation of a gamma-ray flash coincident with the neutrino burst leads to strong constraints on the axion-photon coupling for axion masses m a ≲10 –10 eV. In this Letter, we use SN1987A to constrain higher mass axions, all the way to m a ~10 –3 eV, by accounting for axion production from the Primakoff process, nucleon bremsstrahlung, and pion conversion along with axion-photon conversion on the still-intact magnetic fields of the progenitor star. Moreover, we show that gamma-ray observations of the next Galactic supernova, leveraging the magnetic fields of the progenitor star, could detect quantum chromodynamics axions for masses above roughly 50 μ⁢eV, depending on the supernova. We propose a new full-sky gamma-ray satellite constellation that we call the GALactic AXion Instrument for Supernova (GALAXIS) to search for such future signals along with related signals from extragalactic neutron star mergers.

79 ASTRONOMY AND ASTROPHYSICS↗