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At least 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

THE FORMATION AND ERUPTION OF SOLAR QUIESCENT PROMINENCES

Following the two-stage catastrophic flux rope model presented by Zhang et al., we investigate how magnetic flux emergence affects the formation and evolution of solar quiescent prominences. The magnetic properties of the flux rope are described with its toroidal magnetic flux per radian Φ{sub p} and poloidal flux Φ{sub ψ}, and Φ{sub p} is defined as the emerging strength (ES) of the magnetic flux. After the first catastrophe, the quiescent prominences are supported by the vertical current sheet and located in cavities below the curved transverse current sheet in the inner corona, for which both ES and Φ{sub ψ} are in the certain ranges. We calculate the strength range as 0.25 < ES < 0.50 for the quadrupolar field, and obtain the equation Φ{sub p}Φ{sub ψ} = const., that is, the relationship between Φ{sub p} and Φ{sub ψ} of the emerging flux for which the quiescent prominences are formed in the inner corona. After the second catastrophe, the quiescent prominences would either fall down onto the solar surface or erupt as an important part of coronal mass ejections. During the eruption of the quiescent prominences, most of the magnetic energy in the flux rope is lost, and less than half of the energy loss of the rope is released in the form of Alfvèn waves. We argue that there would be two important conditions required for the formation and eruption of solar quiescent prominences, a complicated source region and emerging toroidal magnetic flux that exceeds a critical strength.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1994

Studies on equatorial shock formation during plasmaspheric refilling

Investigations based on small-scale simulations of microprocesses occurring when a magnetic flux tube refills with a cold plasma are summarized. Results of these investigations are reported in the following attached papers: (1) 'Numerical Simulation of Filling a Magnetic Flux Tube with a Cold Plasma: The Role of Ion Beam-Driven Instabilities'; and (2) 'Numerical Simulation of Filling a Magnetic Flux Tube with a Cold Plasma: Effects of Magnetically Trapped Hot Plasma'. Other papers included are: 'Interaction of Field-Aligned Cold Plasma Flows with an Equatorially-Trapped Hot Plasma: Electrostatic Shock Formation'; and 'Comparison of Hydrodynamic and Semikinetic Treatments for a Plasma Flow along Closed Field Lines'. A proposal for further research is included.

Singh, N.↗
NASA NTRS1986

Response of an emerging flux tube to a current-driven instability

Emerging magnetic flux plays an important role in the development of active regions on the sun and, perhaps, in the subsequent activation of flares. However, the energy input that produces preflare brightenings and flares probably does not come from the flux emergence itself but from one or more associated energy-releasing processes - likely candidates include magnetic reconnection and various current-driven plasma micro-instabilities. Here the interplay between the changing physical characteristics of an emerging magnetic-flux tube and the onset and evolution of a representative 'bump-on-tail' plasma current-driven instability is investigated. The microinstability heats the ambient material, thus changing the macroscopic characteristics of the plasma in which the model and current-driven instability occurs.

Karpen, J. T.↗
NASA NTRS2016

Flux Cancellation Leading to CME Filament Eruptions

Solar filaments are strands of relatively cool, dense plasma magnetically suspended in the lower density hotter solar corona. They trace magnetic polarity inversion lines (PILs) in the photosphere below, and are supported against gravity at heights of up to approx.100 Mm above the chromosphere by the magnetic field in and around them. This field erupts when it is rendered unstable, often by magnetic flux cancellation or emergence at or near the PIL. We have studied the evolution of photospheric magnetic flux leading to ten observed filament eruptions. Specifically, we look for gradual magnetic changes in the neighborhood of the PIL prior to and during eruption. We use Extreme Ultraviolet (EUV) images from the Atmospheric Imaging Assembly (AIA), and magnetograms from the Helioseismic and Magnetic Imager (HMI), both on board the Solar Dynamics Observatory (SDO), to study filament eruptions and their photospheric magnetic fields. We examine whether flux cancellation or/and emergence leads to filament eruptions. We find that continuous flux cancellation was present at the PIL for many hours prior to each eruption. We present two CME-producing eruptions in detail and find the following: (a) the pre-eruption filament-holding core field is highly sheared and appears in the shape of a sigmoid above the PIL; (b) at the start of the eruption the opposite arms of the sigmoid reconnect in the middle above the site of (tether-cutting) flux cancellation at the PIL; (c) the filaments first show a slow-rise, followed by a fast-rise as they erupt. We conclude that these two filament eruptions result from flux cancellation in the middle of the sheared field, and thereafter evolve in agreement with the standard model for a CME/flare filament eruption from a closed bipolar magnetic field [flux cancellation (van Ballegooijen and Martens 1989 and Moore and Roumelrotis 1992) and runaway tether-cutting (Moore et. al 2001)].

filament↗
DOE OSTI2023

Evolution of 1 / f Flux Noise in Superconducting Qubits with Weak Magnetic Fields

The microscopic description of 1/f magnetic flux noise in superconducting circuits has remained an open question for several decades despite extensive experimental and theoretical investigation. Recent progress in superconducting devices for quantum information has highlighted the need to mitigate sources of qubit decoherence, driving a renewed interest in understanding the underlying noise mechanism(s). Though a consensus has emerged attributing flux noise to surface spins, their identity and interaction mechanisms remain unclear, prompting further study. Here, we apply weak in-plane magnetic fields to a capacitively shunted flux qubit (where the Zeeman splitting of surface spins lies below the device temperature) and study the flux-noise-limited qubit dephasing, revealing previously unexplored trends that may shed light on the dynamics behind the emergent 1/f noise. Notably, we observe an enhancement (suppression) of the spin-echo (Ramsey) pure-dephasing time in fields up to B = 100 G. With direct noise spectroscopy, we further observe a transition from a 1/f to approximately Lorentzian frequency dependence below 10 Hz and a reduction of the noise above 1 MHz with increasing magnetic field. Here, we suggest that these trends are qualitatively consistent with an increase of spin cluster sizes with magnetic field. These results should help to inform a complete microscopic theory of 1/f flux noise in superconducting circuits.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Extended-magnetohydrodynamics in under-dense plasmas

Extended-magnetohydrodynamics (MHD) transports magnetic flux and electron energy in high-energy-density experiments, but individual transport effects remain unobserved experimentally. Two factors are responsible in defining the transport: electron temperature and electron current. Here, each electron energy transport term has a direct analog in magnetic flux transport. To measure the thermally driven transport of magnetic flux and electron energy, a simple experimental configuration is explored computationally using a laser-heated pre-magnetized under-dense plasma. Changes to the laser heating profile precipitate clear diagnostic signatures from the Nernst, cross-gradient-Nernst, anisotropic conduction, and Righi-Leduc heat-flow. With a wide operating parameter range, this configuration can be used in both small and large scale facilities to benchmark MHD and kinetic transport in collisional/semi-collisional, local/non-local, and magnetized/unmagnetized regimes.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Collapse and fragmentation of magnetic molecular cloud cores with the Enzo AMR MHD code. II. Prolate and oblate cores

We present the results of a large suite of three-dimensional models of the collapse of magnetic molecular cloud cores using the adaptive mesh refinement code Enzo2.2 in the ideal magnetohydrodynamics approximation. The cloud cores are initially either prolate or oblate, centrally condensed clouds with masses of 1.73 or 2.73 M {sub ☉}, respectively. The radial density profiles are Gaussian, with central densities 20 times higher than boundary densities. A barotropic equation of state is used to represent the transition from low density isothermal phases, to high density optically thick phases. The initial magnetic field strength ranges from 6.3 to 100 μG, corresponding to clouds that are strongly to marginally supercritical, respectively, in terms of the mass to magnetic flux ratio. The magnetic field is initially uniform and aligned with the clouds' rotation axes, with initial ratios of rotational to gravitational energy ranging from 10{sup –4} to 0.1. Two significantly different outcomes for collapse result: (1) formation of single protostars with spiral arms, and (2) fragmentation into multiple protostar systems. The transition between these two outcomes depends primarily on the initial magnetic field strength, with fragmentation occurring for mass to flux ratios greater than about 14 times the critical ratio for prolate clouds. Oblate clouds typically fragment into several times more clumps than prolate clouds. Multiple, rather than binary, system formation is the general rule in either case, suggesting that binary stars are primarily the result of the orbital dissolution of multiple protostar systems.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1971

Magnetic field fluctuations during substorms

Before a magnetospheric substorm and during its early phases the magnetic field magnitude in the geomagnetic tail increases and field lines in the nighttime hemisphere assume a more tail-like configuration. Before the substorm onset a minimum amount of magnetic flux is observed to cross the neutral sheet which means that the neutral sheet currents attain their most earthward locations and their greatest current densities. This configuration apparently results from an increased transport of magnetic flux to the tail caused by a southward interplanetary magnetic field. The field begins relaxing toward a more dipolar configuration at the time of a substorm onset with the recovery probably occurring first between 6 and 10 R sub E. This recovery must be associated with magnetospheric convection which restores magnetic flux to the dayside hemisphere. Field aligned currents appear to be required to connect magnetospheric currents to the auroral electrojets, implying that a net current flows in a limited range of longitudes. Space measurements supporting current systems are limited. More evidence exists for the occurrence of double current sheets which do not involve net current at a given longitude.

Fairfield, D. H.↗
NASA NTRS1972

Magnetic field fluctuations during substorms.

Before a magnetospheric substorm and during its early phases, the magnetic field magnitude in the geomagnetic tail increases and field lines in the night-time hemisphere assume a more tail-like configuration. Before the substorm onset, a minimum amount of magnetic flux is observed to cross the neutral sheet which means that the neutral sheet currents attain their most earthward locations and their greatest current densities. This field configuration apparently results from an increased transport of magnetic flux to the tail caused by a southward interplanetary magnetic field. The field begins relaxing toward a more dipolar configuration at the time of a substorm onset with the recovery probably occurring first between 6 and 10 earth radii and later at greater distances. This recovery must be associated with magnetospheric convection which restores magnetic flux to the dayside hemisphere.

Fairfield, D. H.↗
DOE OSTI2016

Experimental evaluation of electrical conductivity imaging of anisotropic brain tissues using a combination of diffusion tensor imaging and magnetic resonance electrical impedance tomography

Anisotropy of biological tissues is a low-frequency phenomenon that is associated with the function and structure of cell membranes. Imaging of anisotropic conductivity has potential for the analysis of interactions between electromagnetic fields and biological systems, such as the prediction of current pathways in electrical stimulation therapy. To improve application to the clinical environment, precise approaches are required to understand the exact responses inside the human body subjected to the stimulated currents. In this study, we experimentally evaluate the anisotropic conductivity tensor distribution of canine brain tissues, using a recently developed diffusion tensor-magnetic resonance electrical impedance tomography method. At low frequency, electrical conductivity of the biological tissues can be expressed as a product of the mobility and concentration of ions in the extracellular space. From diffusion tensor images of the brain, we can obtain directional information on diffusive movements of water molecules, which correspond to the mobility of ions. The position dependent scale factor, which provides information on ion concentration, was successfully calculated from the magnetic flux density, to obtain the equivalent conductivity tensor. By combining the information from both techniques, we can finally reconstruct the anisotropic conductivity tensor images of brain tissues. The reconstructed conductivity images better demonstrate the enhanced signal intensity in strongly anisotropic brain regions, compared with those resulting from previous methods using a global scale factor.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2015

Demagnetization using a determined estimated magnetic state

A method for demagnetizing comprising positioning a core within the electromagnetic field generated by a first winding until the generated first electrical current is not substantially increasing, thereby determining a saturation current. A second voltage, having the opposite polarity, is then applied across the first winding until the generated second electrical current is approximately equal to the magnitude of the determined saturation current. The maximum magnetic flux within the core is then determined using the voltage across said first winding and the second current. A third voltage, having the opposite polarity, is then applied across the first winding until the core has a magnetic flux equal to approximately half of the determined maximum magnetic flux within the core.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2020

A Comprehensive Review of Permanent Magnet Transverse Flux Machines: Use in Direct-Drive Applications

The use of direct-drive machines in renewable and industrial applications is increasing at a rapid rate. Transverse flux machines (TFMs) are ideally suited for direct-drive applications due to their high torque density. This article presents a comprehensive review of permanent magnet (PM) TFMs for direct-drive applications. It introduces TFMs and their operating principle and reviews the different types of TFMs proposed in the literature. TFMs are categorized by the number of stator sides, types of stator cores, and magnet arrangement in the rotor. The review covers different design topologies, materials used for manufacturing, structural and thermal analyses, modeling, design optimization, and cogging torque minimization in TFMs. The article also examines various applications and comparisons for TFMs presented in the literature.

air gaps↗
DOE OSTI2015

MULTI-PARAMETRIC STUDY OF RISING 3D BUOYANT FLUX TUBES IN AN ADIABATIC STRATIFICATION USING AMR

We study the buoyant rise of magnetic flux tubes embedded in an adiabatic stratification using two-and three-dimensional, magnetohydrodynamic simulations. We analyze the dependence of the tube evolution on the field line twist and on the curvature of the tube axis in different diffusion regimes. To be able to achieve a comparatively high spatial resolution we use the FLASH code, which has a built-in Adaptive Mesh Refinement (AMR) capability. Our 3D experiments reach Reynolds numbers that permit a reasonable comparison of the results with those of previous 2D simulations. When the experiments are run without AMR, hence with a comparatively large diffusivity, the amount of longitudinal magnetic flux retained inside the tube increases with the curvature of the tube axis. However, when a low-diffusion regime is reached by using the AMR algorithms, the magnetic twist is able to prevent the splitting of the magnetic loop into vortex tubes and the loop curvature does not play any significant role. We detect the generation of vorticity in the main body of the tube of opposite sign on the opposite sides of the apex. This is a consequence of the inhomogeneity of the azimuthal component of the field on the flux surfaces. The lift force associated with this global vorticity makes the flanks of the tube move away from their initial vertical plane in an antisymmetric fashion. The trajectories have an oscillatory motion superimposed, due to the shedding of vortex rolls to the wake, which creates a Von Karman street.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1980

X-ray bright points and the solar cycle

Soft X-ray filtergrams show the presence of large numbers of small closed regions of coronal emission. These features, called X-ray bright points, correspond to small short-lived regions of emerging magnetic flux. X-ray data for 1970 to 1978 shows that the number of X-ray bright points appears to be anticorrelated with traditional activity indices such as sunspot number. A comparison of X-ray data with KPNO magnetograms shows that to within a factor of 2 the average total amount of magnetic flux emerging over the full sun is constant through the entire period of observation. The solar cycle therefore appears to be an oscillation in the wavenumber distribution of emerging flux than of the total quantity of the magnetic flux produced.

Golub, L.↗
DOE OSTI2017

Flare Energy Release in the Lower Solar Atmosphere near the Magnetic Field Polarity Inversion Line

We study flare processes in the solar atmosphere using observational data for an M1-class flare of 2014 June 12, obtained by the New Solar Telescope (NST/BBSO) and Helioseismic Magnetic Imager (HMI/SDO). The main goal is to understand triggers and manifestations of the flare energy release in the photosphere and chromosphere using high-resolution optical observations and magnetic field measurements. We analyze optical images, HMI Dopplergrams, and vector magnetograms, and use nonlinear force-free field (NLFFF) extrapolations for reconstruction of the magnetic topology and electric currents. The NLFFF modeling reveals the interaction of two magnetic flux ropes with oppositely directed magnetic fields in the polarity inversion line (PIL). These flux ropes are observed as a compact sheared arcade along the PIL in the high-resolution broadband continuum images from NST. In the vicinity of the PIL, the NST Hα observations reveal the formation of a thin three-ribbon structure corresponding to a small-scale photospheric magnetic arcade. The observational results are evidence in favor of the primary energy release site located in the chromospheric plasma with strong electric currents concentrated near the PIL. In this case, magnetic reconnection is triggered by the interacting magnetic flux ropes forming a current sheet elongated along the PIL.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1972

Photospheric magnetic fields

Knowledge on the nature of magnetic fields on the solar surface is reviewed. At least a large part of the magnetic flux in the solar surface is confined to small bundles of lines of force within which the field strength is of the order of 500 gauss. Magnetic fields are closely associated with all types of solar activity. Magnetic flux appears at the surface at the clearly defined birth or regeneration of activity of an active region. As the region ages, the magnetic flux migrates to form large-scale patterns and the polar fields. Some manifestations of the large-scale distribution are discussed.

Howard, R.↗
NASA NTRS1980

X-ray bright points and the solar cycle

The shape of the Sun's activity spectrum is such that the majority of all magnetic flux emerging at the surface comes in the form of bright points, i.e., regions living less than two days. Examination of soft X-ray data obtained from 1970 to 1978 shows that the number of bright points appears to be anticorrelated with traditional activity indices, such as sunspot number; the anticorrelation persists after corrections are made for obscuration by active regions. Comparison of X-ray data with KPNO magnetograms shows that to within a factor of two, the average total amount of magnetic flux emerging over the full Sun is constant through the entire period of observation. The Solar cycle therefore appears to be more an oscillation in the wavenumber distribution of emerging flux than of the total quantity of magnetic flux produced.

Golub, L.↗
DOE OSTI2013

A FLUX ROPE NETWORK AND PARTICLE ACCELERATION IN THREE-DIMENSIONAL RELATIVISTIC MAGNETIC RECONNECTION

We investigate magnetic reconnection and particle acceleration in relativistic pair plasmas with three-dimensional particle-in-cell simulations of a kinetic-scale current sheet in a periodic geometry. We include a guide field that introduces an inclination between the reconnecting field lines and explore outside-of-the-current sheet magnetizations that are significantly below those considered by other authors carrying out similar calculations. Thus, our simulations probe the transitional regime in which the magnetic and plasma pressures are of the same order of magnitude. The tearing instability is the dominant mode in the current sheet for all guide field strengths, while the linear kink mode is less important even without the guide field, except in the lower magnetization case. Oblique modes seem to be suppressed entirely. In its nonlinear evolution, the reconnection layer develops a network of interconnected and interacting magnetic flux ropes. As smaller flux ropes merge into larger ones, the reconnection layer evolves toward a three-dimensional, disordered state in which the resulting flux rope segments contain magnetic substructure on plasma skin depth scales. Embedded in the flux ropes, we detect spatially and temporally intermittent sites of dissipation reflected in peaks in the parallel electric field. Magnetic dissipation and particle acceleration persist until the end of the simulations, with simulations with higher magnetization and lower guide field strength exhibiting greater and faster energy conversion and particle energization. At the end of our largest simulation, the particle energy spectrum attains a tail extending to high Lorentz factors that is best modeled with a combination of two additional thermal components. We confirm that the primary energization mechanism is acceleration by the electric field in the X-line region. The highest-energy positrons (electrons) are moderately beamed with median angles {approx}30 Degree-Sign -40 Degree-Sign relative to (the opposite of) the direction of the initial current density, but we speculate that reconnection in more highly magnetized plasmas would give rise to stronger beaming. Finally, we discuss the implications of our results for macroscopic reconnection sites, and which of our results may be expected to hold in systems with higher magnetizations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗