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At least 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

NUMERICAL SIMULATION OF FAST-MODE MAGNETOSONIC WAVES EXCITED BY PLASMOID EJECTIONS IN THE SOLAR CORONA

The Atmospheric Imaging Assembly instrument on board the Solar Dynamics Observatory has directly imaged the fast-propagating magnetosonic waves (FMWs) successively propagating outward along coronal magnetic funnels. In this study we perform a numerical investigation of the excitation of FMWs in the interchange reconnection scenario, with footpoint shearing flow being used to energize the system and drive the reconnection. The modeling results show that as a result of magnetic reconnection, the plasma in the current sheet is heated up by Joule dissipation to ∼10 MK and is ejected rapidly, developing the hot outflows. Meanwhile, the current sheet is torn into plasmoids, which are shot quickly both upward and downward. When the plasmoids reach the outflow regions, they impact and collide with the ambient magnetic field there, which consecutively launches FMWs. The FMWs propagate outward divergently away from the impact regions, with a phase speed of the Alfvén speed of ∼1000 km s{sup –1}. In the k – ω diagram of the Fourier wave power, the FMWs display a broad frequency distribution with a straight ridge that represents the dispersion relation. With the WKB approximation, at the distance of 15 Mm from the wave source region, we estimate the energy flux of FMWs to be E ∼ 7.0 × 10{sup 6} erg cm{sup –2} s{sup –1}, which is ∼50 times smaller than the energy flux related to the tube-channeled reconnection outflow. These simulation results indicate that energetically and dynamically the outflow is far more important than the waves.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Fast radio bursts as probes of magnetic fields in the intergalactic medium

We examine the proposal that the dispersion measures (DMs) and Faraday rotation measures (RMs) of extragalactic linearly polarized fast radio bursts (FRBs) can be used to probe the intergalactic magnetic field (IGMF) in filaments of galaxies. The DM through the cosmic web is dominated by contributions from the warm-hot intergalactic medium (WHIM) in filaments and from the gas in voids. On the other hand, RM is induced mostly by the hot medium in galaxy clusters, and only a fraction of it is produced in the WHIM. We show that if one excludes FRBs whose sightlines pass through galaxy clusters, the line of sight (LOS) strength of the IGMF in filaments, B{sub ||}, is approximately C(〈1+z〉/f{sub DM})(RM/DM), where C is a known constant. Here, the redshift of the FRB is not required to be known; f {sub DM} is the fraction of total DM due to the WHIM, while 〈1+z〉 is the redshift of interevening gas weighted by the WHIM gas density, both of which can be evaluated for a given cosmology model solely from the DM of an FRB. Using data on structure formation simulations and a model IGMF, we show that C(〈1+z〉/f{sub DM})(RM/DM) closely reproduces the density-weighted LOS strength of the IGMF in filaments of the large-scale structure.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

The JCMT BISTRO Survey: The Magnetic Field Strength in the Orion A Filament

We determine the magnetic field strength in the OMC 1 region of the Orion A filament via a new implementation of the Chandrasekhar–Fermi method using observations performed as part of the James Clerk Maxwell Telescope (JCMT) B-Fields In Star-forming Region Observations (BISTRO) survey with the POL-2 instrument. We combine BISTRO data with archival SCUBA-2 and HARP observations to find a plane-of-sky magnetic field strength in OMC 1 of B{sub pos}=6.6±4.7 mG, where δB{sub pos}=4.7 mG represents a predominantly systematic uncertainty. We develop a new method for measuring angular dispersion, analogous to unsharp masking. We find a magnetic energy density of ∼1.7×10{sup −7} J m{sup −3} in OMC 1, comparable both to the gravitational potential energy density of OMC 1 (∼10{sup −7} J m{sup −3}) and to the energy density in the Orion BN/KL outflow (∼10{sup −7} J m{sup −3}). We find that neither the Alfvén velocity in OMC 1 nor the velocity of the super-Alfvénic outflow ejecta is sufficiently large for the BN/KL outflow to have caused large-scale distortion of the local magnetic field in the ∼500 yr lifetime of the outflow. Hence, we propose that the hourglass field morphology in OMC 1 is caused by the distortion of a primordial cylindrically symmetric magnetic field by the gravitational fragmentation of the filament and/or the gravitational interaction of the BN/KL and S clumps. We find that OMC 1 is currently in or near magnetically supported equilibrium, and that the current large-scale morphology of the BN/KL outflow is regulated by the geometry of the magnetic field in OMC 1, and not vice versa.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Plasmon dispersions in high electron mobility terahertz detectors

Low temperature, high magnetic field experiments were carried out with monochromatic terahertz (THz) sources to reveal multimode spectra of magnetoplasmons excited in gated and ungated samples processed on a high electron mobility GaAs/AlGaAs heterostructure. We show that playing with the geometry and thickness of the gate one can control both the plasmon dispersion relation and selection rules for plasmon excitation, giving a tool to a better control of plasmon resonances in THz detectors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Determination of the Alfvén Oscillation Location in the TUMAN-3M Tokamak Plasma

The Alfvén oscillations have been studied in ohmically heated deuterium discharges with LH-transition in the TUMAN-3M tokamak in order to clarify their location in a plasma column. The Alfvén oscillation location was determined by comparison of the oscillation frequency measured with magnetic probes and that calculated from local density assuming the typical dispersion relation for Alfvén waves f = (2π){sup −1}k{sub ||}v{sub A}, where v{sub A} is the Alfvén velocity and k{sub ||} is the the parallel wave number in the direction of the magnetic field. It was found that they are localized in central part of plasma column inside r/a < 0.5 region. Candidate sets of mode numbers have been determined.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Effects of magnetic fields on lithium evolution in F and G stars

To study the effects of magnetic fields on lithium depletion and to explain the characteristics of lithium evolution in F- and G-type stars in open clusters, we construct a stellar model that includes a Tayler-Spruit dynamo-type field. Through examining the features of extra-mixing caused by magnetic fields in stellar interiors and the relationship between lithium depletion and the input parameters (i.e., mass, metallicity, and the initial rotational condition), we find that the magnetic model produces extra-mixing processes different from other models. During pre-main sequence, the model exhibits very efficient extra-mixing, resulting in strong lithium depletion. As a star evolves, the efficiency of extra-mixing rapidly decreases and lithium depletion slows down at old ages. By setting up different initial rotating conditions, the model predicts dispersions of lithium abundance in both main sequence and pre-main sequence stages. The dispersion begins when a radiative core forms in the interior. The number sees a rapid growth afterward, reaching 0.1-0.5 dex at zero age main sequence in G and late-F stars. The increase of dispersion continues in the main sequence, which is slight in G and late-F stars while significant in mid-F stars. Finally, a comparison is carried out between the theoretical results and the observed data in four open clusters. Good agreements are obtained.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

MAGNETIC EFFECTS AND OVERSIZED M DWARFS IN THE YOUNG OPEN CLUSTER NGC 2516

By combining rotation periods with spectroscopic determinations of projected rotation velocity, Jackson et al. have found that the mean radii for low-mass M-dwarfs in the young, open cluster NGC 2516 are larger than model predictions at a given absolute I magnitude or I - K color and also larger than measured radii of magnetically inactive M-dwarfs. The relative radius difference is correlated with magnitude, increasing from a few percent at M{sub I} = 7 to greater than 50% for the lowest luminosity stars in their sample at M{sub I} {approx} 9.5. Jackson et al. have suggested that a two-temperature star spot model is capable of explaining the observations, but their model requires spot coverage fractions of at least 50% in rapidly rotating M-dwarfs. Here we examine these results in terms of stellar models that include the inhibiting effects of magnetic fields on convective energy transport, with and without the effects of star spots. We find that a pure spot model is inconsistent with the color-magnitude diagram. The observations of radii versus color and radii versus absolute magnitude in NGC 2516 are consistent with models which include only magnetic inhibition or a combination of magnetic inhibition and spots. At a given mass we find a large dispersion in the strength of the vertical component of the magnetic field in the stellar photosphere but the general trend is that the vertical field increases with decreasing mass from a few hundred Gauss at 0.65 M{sub Sun} to 600-900 G, depending on spot coverage, in the lowest mass stars in the sample at 0.25 M{sub Sun }.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

EXPERIMENTAL DETERMINATION OF WHISTLER WAVE DISPERSION RELATION IN THE SOLAR WIND

The origins and properties of large-amplitude whistler wavepackets in the solar wind are still unclear. In this Letter, we utilize single spacecraft electric and magnetic field waveform measurements from the ARTEMIS mission to calculate the plasma frame frequency and wavevector of individual wavepackets over multiple intervals. This allows direct comparison of experimental measurements with theoretical dispersion relations to identify the observed waves as whistler waves. The whistlers are right-hand circularly polarized, travel anti-sunward, and are aligned with the background magnetic field. Their dispersion is strongly affected by the local electron parallel beta in agreement with linear theory. The properties measured are consistent with the electron heat flux instability acting in the solar wind to generate these waves.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2003

Control of Thermal Convection in Layered Fluids Using Magnetic fields

Immiscible fluid layers are found in a host of applications ranging from materials processing, for example the use of encapsulants in float zone crystal growth technique and a buffer layer in industrial Czochralski growth of crystals to prevent Marangoni convection, to heat transfer phenomena in day-to-day processes like the presence of air pockets in heat exchangers. In the microgravity and space processing realm, the exploration of other planets requires the development of enabling technologies in several fronts. The reduction in the gravity level poses unique challenges for fluid handling and heat transfer applications. The present work investigates the efficacy of controlling thermal convective flow using magnetic fluids and magnetic fields. The setup is a two-layer immiscible liquids system with one of the fluids being a diluted ferrofluid (super paramagnetic nano particles dispersed in carrier fluid). Using an external magnetic field one can essentially dial in a volumetric force - gravity level, on the magnetic fluid and thereby affect the system thermo-fluid behavior. The paper will describe the experimental and numerical modeling approach to the problem and discuss results obtained to date.

Ramachandran, N.↗
DOE OSTI2014

Dynamic control of spin wave spectra using spin-polarized currents

We describe a method of controlling the spin wave spectra dynamically in a uniform nanostripe waveguide through spin-polarized currents. A stable periodic magnetization structure is observed when the current flows vertically through the center of nanostripe waveguide. After being excited, the spin wave is transmitted at the sides of the waveguide. Numerical simulations of spin-wave transmission and dispersion curves reveal a single, pronounced band gap. Moreover, the periodic magnetization structure can be turned on and off by the spin-polarized current. The switching process from full rejection to full transmission takes place within less than 3 ns. Thus, this type magnonic waveguide can be utilized for low-dissipation spin wave based filters.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1984

Electromagnetic ion beam instabilities

The linear theory of electromagnetic instabilities driven by an energetic ion beam streaming parallel to a magnetic field in a homogeneous Vlasov plasma is considered. Numerical solutions of the full dispersion equation are presented. At propagation parallel to the magnetic field, there are four distinct instabilities. A sufficiently energetic beam gives rise to two unstable modes with right-hand polarization, one resonant with the beam, the other nonresonant. A beam with sufficiently large T (perpendicular to B)/T (parallel to B) gives rise to the left-hand ion cyclotron anisotropy instability at relatively small beam velocities, and a sufficiently hot beam drives unstable a left-hand beam resonant mode. The parametric dependences of the growth rates for the three high beam velocity instabilities are presented here. In addition, some properties at oblique propagation are examined. It is demonstrated that, as the beam drift velocity is increased, relative maxima in growth rates can arise at harmonics of the ion cyclotron resonance for both right and left elliptically polarized modes.

Gary, S. P.↗
DOE OSTI2011

Degenerate mixing of plasma waves on cold, magnetized single-species plasmas

In the cold-fluid dispersion relation {omega}={omega}{sub p}/[1+(k{sub perpendicular}/k{sub z}){sup 2}]{sup 1/2} for Trivelpiece-Gould waves on an infinitely long magnetized plasma cylinder, the transverse and axial wavenumbers appear only in the combination k{sub perpendicular}/k{sub z}. As a result, for any frequency {omega}<{omega}{sub p}, there are infinitely many degenerate waves, all having the same value of k{sub perpendicular}/k{sub z}. On a cold finite-length plasma column, these degenerate waves reflect into one another at the ends; thus, each standing-wave normal mode of the bounded plasma is a mixture of many degenerate waves, not a single standing wave as is often assumed. A striking feature of the many-wave modes is that the short-wavelength waves often add constructively along resonance cones given by dz/dr={+-}({omega}{sub p}{sup 2}/{omega}{sup 2}-1){sup 1/2}. Also, the presence of short wavelengths in the admixture for a predominantly long-wavelength mode enhances the viscous damping beyond what the single-wave approximation would predict. Here, numerical solutions are obtained for modes of a cylindrical plasma column with rounded ends. Exploiting the fact that the modes of a spheroidal plasma are known analytically (the Dubin modes), a perturbation analysis is used to investigate the mixing of low-order, nearly degenerate Dubin modes caused by small deformations of a plasma spheroid.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Ion-beam driven lower hybrid waves in a magnetized dusty plasma

An ion beam drives lower hybrid waves to instability in a magnetized dusty plasma via Cerenkov interaction. A dispersion relation and the growth rate of the lower hybrid waves have been derived taking into account the dust charge fluctuations. The frequency and the growth rate of the unstable wave instability increase with relative density of negatively charged dust grains. The lower hybrid modes with phase velocity comparable to the beam velocity possess a large growth rate. Moreover, the growth rate of the instability increases with beam density and scales as the one-third power of the beam density.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Structural, Microstructural, Magnetic, and Ferroelectric Properties of Ba{sup 2+}-Doped BiFeO{sub 3} Nanocrystalline Multifferroic Material

Multiferroic barium-substituted nanocrystalline samples having the general formula Bi{sub 1−x}Ba{sub x}FeO{sub 3} (where x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25) were prepared by cost-effective sol–gel method using metal nitrates as oxidants. The prepared samples were annealed at 650 {sup ∘}C for 4 h using a muffle furnace. The influence of Ba{sup 2+} doping on the structural, microstructural, magnetic, and ferroelectric properties of bismuth ferrite (BFO) was studied systematically by standard techniques. All the samples were characterized by X-ray diffraction (XRD) technique, and the average crystallite size was calculated using Scherrer’s formula. The surface morphology of the prepared nanocrystalline samples was studied using field emission scanning electron microscopy (FE-SEM) technique. FTIR spectra were obtained to confirm the formation of the perovskite structures. The elemental compositional details obtained were confirmed by energy dispersive spectrum (EDS) analysis. Room temperature magnetic measurements were carried out up to a field of 15 kOe, and the parent sample (BFO) shows weak ferromagnetism. Further on, Ba{sup 2+} substitution in BFO shows an increase in magnetization. P–E loops were obtained to study the ferroelectric properties of all the samples. Obtained enhancement in magnetic and ferroelectric properties at room temperature is of great interest within the research community due to its wide range of applications in memory storage, spintronics, sensors, and multi-functional devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

One-step synthesis of homogeneous BaFe{sub 12}O{sub 19}/Y{sub 3}Fe{sub 5}O{sub 12} composite powders

Highlights: • A simple one-step sol–gel method was used to synthesize composite magnets. • The composite powders show good homogeneity and exchange coupling. • The M{sub r}/M{sub s} value increases with the BaM concentration. - Abstract: BaFe{sub 12}O{sub 19}/Y{sub 3}Fe{sub 5}O{sub 12} (BaM/YIG) composite powders were synthesized via a simple one-step sol–gel method. The phase composition and morphology of the as-synthesized composite powders were characterized using an X-ray diffractometer and a scanning electron microscopy equipped with energy dispersive X-ray spectroscopy. The magnetic properties of the composite powders were investigated by a vibrating sample magnetometer. Compared with the composite powders prepared by the conventional physical mixing method, the BaM/YIG composite powders show better homogeneity and exchange coupling.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Applying the cold plasma dispersion relation to whistler mode chorus waves: EMFISIS wave measurements from the Van Allen Probes

Most theoretical wave models require the power in the wave magnetic field in order to determine the effect of chorus waves on radiation belt electrons. However, researchers typically use the cold plasma dispersion relation to approximate the magnetic wave power when only electric field data are available. In this study, the validity of using the cold plasma dispersion relation in this context is tested using Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) observations of both the electric and magnetic spectral intensities in the chorus wave band (0.1–0.9 f ce ). Results from this study indicate that the calculated wave intensity is least accurate during periods of enhanced wave activity. For observed wave intensities >10⁻³ nT², using the cold plasma dispersion relation results in an underestimate of the wave intensity by a factor of 2 or greater 56% of the time over the full chorus wave band, 60% of the time for lower band chorus, and 59% of the time for upper band chorus. Hence, during active periods, empirical chorus wave models that are reliant on the cold plasma dispersion relation will underestimate chorus wave intensities to a significant degree, thus causing questionable calculation of wave-particle resonance effects on MeV electrons.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Study of a condition for the mode conversion from purely perpendicular electrostatic waves to electromagnetic waves

One of the mechanisms for generating electromagnetic plasma waves (Z-mode and LO-mode) is mode conversion from electrostatic waves into electromagnetic waves in inhomogeneous plasma. Herein, we study a condition required for mode conversion of electrostatic waves propagating purely perpendicular to the ambient magnetic field, by numerically solving the full dispersion relation. An approximate model is derived describing the coupling between electrostatic waves (hot plasma Bernstein mode) and Z-mode waves at the upper hybrid frequency. The model is used to study conditions required for mode conversion from electrostatic waves (electrostatic electron cyclotron harmonic waves, including Bernstein mode) into electromagnetic plasma waves (LO-mode). It is shown that for mode conversion to occur in inhomogeneous plasma, the angle between the boundary surface and the magnetic field vector should be within a specific range. The range of the angle depends on the norm of the k vector of waves at the site of mode conversion in the inhomogeneous region. The present study reveals that inhomogeneity alone is not a sufficient condition for mode conversion from electrostatic waves to electromagnetic plasma waves and that the angle between the magnetic field and the density gradient plays an important role in the conversion process.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Spin-lattice dynamics simulation of external field effect on magnetic order of ferromagnetic iron

Modeling of field-induced magnetization in ferromagnetic materials has been an active topic in the last dozen years, yet a dynamic treatment of distance-dependent exchange integral has been lacking. In view of that, we employ spin-lattice dynamics (SLD) simulations to study the external field effect on magnetic order of ferromagnetic iron. Our results show that an external field can increase the inflection point of the temperature. Also the model provides a better description of the effect of spin correlation in response to an external field than the mean-field theory. An external field has a more prominent effect on the long range magnetic order than on the short range counterpart. Furthermore, an external field allows the magnon dispersion curves and the uniform precession modes to exhibit magnetic order variation from their temperature dependence.

36 MATERIALS SCIENCE↗