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At least 343 records · Page 19

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NASA NTRS1992

The synchrotron bays of the Crab Nebula - A magnetic structure associated with a presupernova circumstellar disk

We investigate the nature of two large conical indentations in the Crab Nebula's synchrotron emission, commonly known as the east and west 'bays'. Optical images spanning nearly nine decades (1899-1988) reveal the bays to be long-lasting remnant features. Proper motions of the bays away from the remnant's center of expansion are consistent with their locations near the SN progenitor site in A.D. 1054. Based on their morphologies and polarization properties, the bays appear to be portions of a torus encircling the remnant's center of expansion. The nebular magnetic field wrapped around this torus blocks the pulsar's relativistic particles, resulting in the observed lack of synchrotron emission. We propose that this torus is the result of a presupernova disklike ejection not less than 0.1 solar mass by the Crab progenitor, similar to the ring around SN 1987 A. A 0.5 solar mass disk of He-rich filaments along the equatorial plane is probably also related to presupernova mass loss, which might explain the wide range of abundances seen among filaments.

Fesen, Robert A.↗

NASA NTRS1989

Buoyancy-limited magnetic viscosity in quasi-stellar object accretion disk models

Alpha models of astrophysical accretion disks assume an ad hoc viscous stress which scales locally as the total pressure. It is shown here that, if turbulent magnetic Maxwell stresses are the source of this viscosity in quasi-stellar object accretion disk models, then the stress cannot follow the assumed alpha-law in the radiation pressure-dominated inner region of the disk.

Sakimoto, Philip J.↗

DOE OSTI2022

A Nonlocal Magneto-curvature Instability in a Differentially Rotating Disk

A global mode is shown to be unstable to nonaxisymmetric perturbations in a differentially rotating Keplerian disk containing either vertical or azimuthal magnetic fields. In an unstratified cylindrical disk model, using both global eigenvalue stability analysis and linear global initial-value simulations, it is demonstrated that this instability dominates at strong magnetic fields where local standard magnetorotational instability (MRI) becomes stable. Unlike the standard MRI mode, which is concentrated in the high flow shear region, these distinct global modes (with low azimuthal mode numbers) are extended in the global domain and are Alfvén-continuum-driven unstable modes. As its mode structure and relative dominance over MRI are inherently determined by the global spatial curvature as well as the flow shear in the presence of a magnetic field, we call it the magneto-curvature (magneto-spatial-curvature) instability. Consistent with the linear analysis, as the field strength is increased in the nonlinear simulations, a transition from MRI-driven turbulence to a state dominated by global nonaxisymmetric modes is obtained. This global instability could therefore be a source of nonlinear transport in accretion disks at a higher magnetic field than predicted by local models.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1993

Magnetic field sources and their threat to magnetic media

Magnetic storage media (tapes, disks, cards, etc.) may be damaged by external magnetic fields. The potential for such damage has been researched, but no objective standard exists for the protection of such media. This paper summarizes a magnetic storage facility standard, Publication 933, that ensures magnetic protection of data storage media.

Jewell, Steve↗

NASA NTRS1992

Intrinsically asymmetric astrophysical jets

Previous work on the origin of self-collimated electromagnetic jets is extended to the general case where there is no reflection symmetry of the magnetic field about the equatorial plane of the disk. The axisymmetric field structure inside the disk is obtained by solving for the magnetic flux function, and the toroidal magnetic field, from the generalized thin-disk induction equation, which is derived. The asymptotic (large-z) magnetic field structure outside the disk is obtained by solving the force-free Grad-Shafranov equation semianalytically. Jet solutions are found in which the power flow is carried mainly by the Poynting flux of the electromagnetic field and the angular momentum outflow from the disk is carried by the magnetic field. The ratio of jet luminosities (top/bottom) depends directly on the degree of asymmetry of the field and can easily be much greater than unity. It is argued that the degree of field asymmetry in the disk is determined by the asymmetry of the weak galactic field fed into the disk at large distances over long periods of time.

Wang, J. C. L.↗

NASA NTRS1989

Particle dynamics and gamma-ray emission in the magnetospheres of neutron stars with accretion disks

Whether the existence of cold accretion disks around isolated, magnetized neutron stars might result in a distinct observational signature due to the Compton drag interaction between the electrostatically accelerated changes and the thermal radiation emitted by these disks. The particle dynamics resulting from the combined effects of electrostatic acceleration and the Compton drag interaction are determined. In the course of their deceleration, the charged particles boost the thermal disk radiation to gamma-ray energies, which may result in a significant spectral component. Cold disks are too underluminous to significantly retard the flow, and if such disks are present in pulsars, only a small fraction of the particle energy is transferred to the radiation field. The results also indicate that steady, unidirectional particle acceleration is unlikely to be the mechanism operating in gamma-ray burst sources.

Melia, Fulvio↗

NASA NTRS2000

Singular Isothermal Disks: Nonaxiymmetric Bifurcations and Equilibria - Paper 2

We review the difficulties of the classical fission and fragmentation hypotheses for the formation of binary and multiple stars. A crucial missing ingredient in previous theoretical studies is the inclusion of dynamically important levels of magnetic fields. As a minimal model for a candidate presursor to the formation of binary and multiple stars, we therefore formulate and solve the problem of the equilibria of isopedically magnetized, singular isothermal disks, without the assumption of axial symmetry. Considerable analytical progress can be made if we restrict our attention to models that are scale-free, i.e., that have surface densities that vary inversely with distance omega from the rotation axis of the system. In agreement with earlier analysis by Syer and Tremaine, we find that lopsided (M = 1) configurations exist at any dimensionless rotation rate, including zero. Multiple-lobed (M = 2, 3, 4, ...) configurations bifurcate from an underlying axisymmetric sequence at progressively higher dimensionless rates of rotation, but such nonaxisymmetric sequences always terminate in shockwaves before they have a chance to fission into M = 2, 3, 4, ... separate bodies. On the basis of our experience in this paper, we advance the hypothesis that binary and multiple star-formation from smooth (i.e., not highly turbulent) starting states that are supercritical but in unstable mechanical balance requires the rapid (i.e., dynamical) loss of magnetic flux at some stage of the ensuing gravitational collapse.

Galli, Danielle↗

NASA NTRS1981

Pulsar disk systems

It is proposed that radio pulsars have the same basic physical features as X-ray pulsars. Specifically, it is suggested that active radio pulsars are rotating neutron stars surrounded by fossil disks left over from the collapse event and that energy is extracted from the rotation of the neutron star by interaction with the disk to produce pulsar luminosity. Attention is given to a model in which the neutron star acts as a unipolar generator (or Faraday disk dynamo) and the disk acts as a load. The self-excitation of the disk/pulsar system is considered along with aspects of disk persistence, pulsed emission, and magnetically field aligned currents to the disk. An investigation is conducted regarding the possibility of a deposition of material in the form of a disk about a pulsar, taking into account questions concerning disk survival. It is found that there are some promising features regarding a disk system.

Michel, F. C.↗

NASA NTRS1992

Initiation of bipolar flows by magnetic field twisting in protostellar nebulae

A model is developed for the time-dependent twisting of an initial poloidal magnetic field threading a conducting protostellar disk. The region outside the disk is assumed to be filled, at least initially, with a low-density, force-free 'coronal' plasma. The differential rotation of the disk acts to twist the B field in the space outside the disk thus generating a toroidal magnetic field. In turn, the toroidal field acts to pinch the plasma and the poloidal field toward the system axis producing a collimated channel. This channel could facilitate the formation of bipolar flows. The magnitude and duration of the field twisting is expected to be limited by magnetohydrodynamic instability.

Newman, William I.↗

NASA NTRS1993

Ultra-high density recording technologies

The Engineering Research Center in Data Storage Systems at Carnegie Mellon University in cooperation with the National Storage Industry Consortium has selected goals of achieving 10 Gbit/sq in. recording density in magnetic and magneto-optic disk recording and 1 terabyte/cubic in. magnetic tape recording technologies. This talk describes the approaches being taken and the status of research leading to these goals.

Kryder, Mark H.↗

NASA NTRS1984

Magnetically suspended flywheel system study

A program to study the application of a graphite/epoxy, magnetically suspended, pierced disk flywheel for the combined function of spacecraft attitude control and energy storage (ACES) is described. Past achievements of the program include design and analysis computer codes for the flywheel rotor, a magnetically suspended flywheel model, and graphite/epoxy rotor rings that were successfully prestressed via interference assembly. All hardware successfully demonstrated operation of the necessary subsystems which form a complete ACES design. Areas of future work include additional rotor design research, system definition and control strategies, prototype development, and design/construction of a UM/GSFC spin test facility. The results of applying design and analysis computer codes to a magnetically suspended interference assembled rotor show specific energy densities of 42 Wh/lb (92.4 Wh/kg) are obtained for a 1.6 kWh system.

Kirk, J. A.↗

DOE OSTI2017

The Polarization Signature of Photospheric Magnetic Fields in 3D MHD Simulations and Observations at Disk Center

Before using three-dimensional (3D) magnetohydrodynamical (MHD) simulations of the solar photosphere in the determination of elemental abundances, one has to ensure that the correct amount of magnetic flux is present in the simulations. The presence of magnetic flux modifies the thermal structure of the solar photosphere, which affects abundance determinations and the solar spectral irradiance. The amount of magnetic flux in the solar photosphere also constrains any possible heating in the outer solar atmosphere through magnetic reconnection. We compare the polarization signals in disk-center observations of the solar photosphere in quiet-Sun regions with those in Stokes spectra computed on the basis of 3D MHD simulations having average magnetic flux densities of about 20, 56, 112, and 224 G. This approach allows us to find the simulation run that best matches the observations. The observations were taken with the Hinode SpectroPolarimeter (SP), the Tenerife Infrared Polarimeter (TIP), the Polarimetric Littrow Spectrograph (POLIS), and the GREGOR Fabry–Pèrot Interferometer (GFPI), respectively. We determine characteristic quantities of full Stokes profiles in a few photospheric spectral lines in the visible (630 nm) and near-infrared (1083 and 1565 nm). We find that the appearance of abnormal granulation in intensity maps of degraded simulations can be traced back to an initially regular granulation pattern with numerous bright points in the intergranular lanes before the spatial degradation. The linear polarization signals in the simulations are almost exclusively related to canopies of strong magnetic flux concentrations and not to transient events of magnetic flux emergence. We find that the average vertical magnetic flux density in the simulation should be less than 50 G to reproduce the observed polarization signals in the quiet-Sun internetwork. A value of about 35 G gives the best match across the SP, TIP, POLIS, and GFPI observations.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1985

Black hole accretion disks - Electrodynamic coupling of accretion-disk coronae and the partitioning of soft and hard X-ray emission

It is demonstrated that the observed large ratio of hard to soft X-ray emission and the bimodel behavior of black hole accreting X-ray sources such as Cygnus X-1 can be described in terms of a magnetically structured accretion disk corona which is electrodynamically coupled to the disk turbulent motions while the disk is thermodynamically coupled to the corona as described by a feedback parameter delta. The observed ratio of hard to soft X-ray emission is independent of the disk thickness, and weakly dependent of the disk parameter alpha relating the disk viscous stresses to the total pressure. Observed values of the luminosity ratio point towards strong differences of the feedback of the low state compared to the high state, in the sense that low state means small feedback (delta less than 0.2) and high state means strong feedback delta of about 0.5.

Kuperus, M.↗

DOE OSTI2016

Elastic metamaterial beam with remotely tunable stiffness

We demonstrate a dynamically tunable elastic metamaterial, which employs remote magnetic force to adjust its vibration absorption properties. The 1D metamaterial is constructed from a flat aluminum beam milled with a linear array of cylindrical holes. The beam is backed by a thin elastic membrane, on which thin disk-shaped permanent magnets are mounted. When excited by a shaker, the beam motion is tracked by a Laser Doppler Vibrometer, which conducts point by point scanning of the vibrating element. Elastic waves are unable to propagate through the beam when the driving frequency excites the first elastic bending mode in the unit cell. At these frequencies, the effective mass density of the unit cell becomes negative, which induces an exponentially decaying evanescent wave. Due to the non-linear elastic properties of the membrane, the effective stiffness of the unit cell can be tuned with an external magnetic force from nearby solenoids. Measurements of the linear and cubic static stiffness terms of the membrane are in excellent agreement with experimental measurements of the bandgap shift as a function of the applied force. In this implementation, bandgap shifts by as much as 40% can be achieved with ∼30 mN of applied magnetic force. This structure has potential for extension in 2D and 3D, providing a general approach for building dynamically tunable elastic metamaterials for applications in lensing and guiding elastic waves.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2024

Structure Formation Through Magnetohydrodynamical Instabilities in Primordial Disks

The shear flow instabilities under the presence of magnetic fields in the primordial disk can greatly facilitate the formation of density structures that serve as seeds prior to the onset of the gravitational Jeans instability. We evaluate the effects of the Parker, magnetorotational and kinematic dynamo instabilities by comparing the properties of these instabilities. We calculate the mass spectra of coagulated density structures by the above mechanism in the radial direction for an axisymmetric magnetohydrodynamic (MHD) torus equilibrium and power density profile models. Our local three-dimensional MHD simulation indicates that the coupling of the Parker and magnetorotational instability creates spiral arms and gas blobs in an accretion disk, reinforcing the theory and model. Such a mechanism for the early structure formation may be tested in a laboratory. The recent progress in experiments involving shear flows in rotating tokamak, field reversed configuration (FRC) and laser plasmas may become a key element to advance in nonlinear studies.

Noguchi, Koichi↗

NASA NTRS1994

1H 1752 + 081: An eclipsing cataclysmic variable with a small accretion disk

We announce the discovery of an eclipsing nova-like cataclysmic variable (CV) as the optical counterpart to the HEAO 1 X-ray source 1H1752 + 081. This CV has an orbital period of 1.882801 hr, a high equivalent width of H-beta, and an average m(sub v) of 16.4 out of the eclipse. A geometric model is constructed from observations of the eclipse ingress and egress in many optical bandpasses. The broad-band emission originates primarily in two regions; the disk/accretion stream 'hot spot' and a compact central component, which may be a spot on the white dwarf surface, the entire white dwarf surface or the boundary layer between the accretion disk and the white dwarf surface. Based on the durations and offsets of the two eclipses we determined the mass ratio q = 2.5 +/- 0.6 and the angle of inclination i = 77 deg +/- 2 deg. If the central component is the entire white dwarf surface the masses of the stars are M(sub 1) = 0.80 +/- 0.06 solar masses and M(sub 2) = 0.32 +/- 0.06 solar masses. The disk is faint and small (R(sub D) = 0.25 +/- 0.05 r(sub L1), where r(sub L1) is the distance from the primary to the L(sub 1) point), compared to other eclipsing CVs. The small disk may result from the removal of angular momentum from the accretion disk by the magnetic field of the white dwarf; this CV may be a DQ Her type with a slowly rotating white dwarf. The emission-line velocities do not show the 'Z-wave' expected from the eclipse of a Keplerian accretion disk, nor do they have the correct phasing to originate near the white dwarf. The most likely origin of the line emission is the hot spot. The secondary star is visible at wavelengths greater than or equal to 6000 A during eclipse. We estimate a spectral type approximately M6 which, together with the observed m(sub 1) = 16.94 during eclipse, results in a distance estimate of 150 +/- 27 pc.

Silber, Andrew D.↗

NASA NTRS2006

Why Do T Tauri Disks Accrete?

Observations of T Tauri stars and young brown dwarfs suggest that the accretion rates of their disks scales roughly with the square of the central stellar mass. No dependence of accretion rate on stellar mass is predicted by the simplest version of the Gammie layered disk model, in which nonthermal ionization of upper disk layers allows accretion to occur via the magnetorotational instability. We show that a minor modification of Gaminie's model to include heating by irradiation from the central star yields a modest dependence of accretion on the mass of the central star. A purely viscous disk model could provide a strong dependence of accretion rate on stellar mass if the initial disk radius (before much viscous evolution has occurred) has a strong dependence on stellar mass. However, it is far from clear that at least the most massive pre-main-sequence disks can be totally magnetically activated by X-rays or cosmic rays. We suggest that a combination of effects are responsible for the observed dependence, with the lowest mass stars having the lowest mass disks, which can be thoroughly magnetically active, while the higher mass stars have higher mass disks that have layered accret,ion and relatively inactive or "dead" central zones at some radii. In such dead zones, we suggest that gravitational instabilities may play a role in allowing accretion to proceed. In this connection, we emphasize the uncertainty in disk masses derived from dust emission and argue that T Tauri disk masses have been systematically underestimated by conventional analyses. Furtlier study of accretion rates, especially in the lowest mass stars, would help to clarify the mechanisms of accretion in T Tauri stars.

Hartmann, Lee↗

DOE OSTI2015

A new reversal mode in exchange coupled antiferromagnetic/ferromagnetic disks: distorted viscous vortex

Magnetic vortices have generated intense interest in recent years due to their unique reversal mechanisms, fascinating topological properties, and exciting potential applications. In addition, the exchange coupling of magnetic vortices to antiferromagnets has also been shown to lead to a range of novel phenomena and functionalities. Here we report a new magnetization reversal mode of magnetic vortices in exchange coupled Ir 20 Mn 80 /Fe 20 Ni 80 microdots: distorted viscous vortex reversal. In contrast to the previously known or proposed reversal modes, the vortex is distorted close to the interface and viscously dragged due to the uncompensated spins of a thin antiferromagnet, which leads to unexpected asymmetries in the annihilation and nucleation fields. Lastly, these results provide a deeper understanding of the physics of exchange coupled vortices and may also have important implications for applications involving exchange coupled nanostructures.

36 MATERIALS SCIENCE↗