Showing papers on "Magnetic field published in 2005"
Abstract: Recent research activities on the linear magnetoelectric (ME) effect?induction of magnetization by an electric field or of polarization by a magnetic field?are reviewed. Beginning with a brief summary of the history of the ME effect since its prediction in 1894, the paper focuses on the present revival of the effect. Two major sources for 'large' ME effects are identified. (i) In composite materials the ME effect is generated as a product property of a magnetostrictive and a piezoelectric compound. A linear ME polarization is induced by a weak ac magnetic field oscillating in the presence of a strong dc bias field. The ME effect is large if the ME coefficient coupling the magnetic and electric fields is large. Experiments on sintered granular composites and on laminated layers of the constituents as well as theories on the interaction between the constituents are described. In the vicinity of electromechanical resonances a ME voltage coefficient of up to 90?V?cm?1?Oe?1 is achieved, which exceeds the ME response of single-phase compounds by 3?5 orders of magnitude. Microwave devices, sensors, transducers and heterogeneous read/write devices are among the suggested technical implementations of the composite ME effect. (ii) In multiferroics the internal magnetic and/or electric fields are enhanced by the presence of multiple long-range ordering. The ME effect is strong enough to trigger magnetic or electrical phase transitions. ME effects in multiferroics are thus 'large' if the corresponding contribution to the free energy is large. Clamped ME switching of electrical and magnetic domains, ferroelectric reorientation induced by applied magnetic fields and induction of ferromagnetic ordering in applied electric fields were observed. Mechanisms favouring multiferroicity are summarized, and multiferroics in reduced dimensions are discussed. In addition to composites and multiferroics, novel and exotic manifestations of ME behaviour are investigated. This includes (i) optical second harmonic generation as a tool to study magnetic, electrical and ME properties in one setup and with access to domain structures; (ii) ME effects in colossal magnetoresistive manganites, superconductors and phosphates of the LiMPO4 type; (iii) the concept of the toroidal moment as manifestation of a ME dipole moment; (iv) pronounced ME effects in photonic crystals with a possibility of electromagnetic unidirectionality. The review concludes with a summary and an outlook to the future development of magnetoelectrics research.
4,315 citations
Book•
01 Dec 2005
TL;DR: In this article, the authors present an overview of the physics-EEG interface, including the physics of electromagnetic fields and EEG, as well as EEG-based recording strategies, reference issues, and dipole localization.
Abstract: 1. The physics-EEG interface 2. Fallacies in EEG 3. An overview of electromagnetic fields 4. Electric fields and currents in biological tissue 5. Current sources in a homogeneous and isotropic medium 6. Current sources in inhomogeneous and isotropic media 7. Recording strategies, reference issues, and dipole localization 8. High-resolution EEG 9. Measures of EEG dynamic properties 10. Spatial-temporal properties of EEG 11. Neocortical dynamics, EEG, and cognition APPENDICES A. Introduction to the calculus of vector fields B. Quasi-static reduction of Maxwell's equations C. Surface magnetic field due to a dipole at an arbitrary location in a volume conductor D. Derivation of the membrane diffusion equation E. Solutions to the membrane diffusion equation F. Point source in a five layered plane medium G. Radial dipole and dipole layer inside the 4-sphere model H. Tangential dipole inside concentric spherical shells I. Spherical harmonics J. The spline Laplacian K. Impressed currents and cross-scale relations in volume conductors L. Outline of neocortical dynamic global theory
2,484 citations
TL;DR: It is shown that a linear chain of colloidal magnetic particles linked by DNA and attached to a red blood cell can act as a flexible artificial flagellum, which induces a beating pattern that propels the structure, and that the external fields can be adjusted to control the velocity and the direction of motion.
Abstract: Microorganisms such as bacteria and many eukaryotic cells propel themselves with hair-like structures known as flagella, which can exhibit a variety of structures and movement patterns. For example, bacterial flagella are helically shaped and driven at their bases by a reversible rotary engine, which rotates the attached flagellum to give a motion similar to that of a corkscrew. In contrast, eukaryotic cells use flagella that resemble elastic rods and exhibit a beating motion: internally generated stresses give rise to a series of bends that propagate towards the tip. In contrast to this variety of swimming strategies encountered in nature, a controlled swimming motion of artificial micrometre-sized structures has not yet been realized. Here we show that a linear chain of colloidal magnetic particles linked by DNA and attached to a red blood cell can act as a flexible artificial flagellum. The filament aligns with an external uniform magnetic field and is readily actuated by oscillating a transverse field. We find that the actuation induces a beating pattern that propels the structure, and that the external fields can be adjusted to control the velocity and the direction of motion.
1,700 citations
TL;DR: The magnetocaloric effect (MCE) in paramagnetic materials has been widely used for attaining very low temperatures by applying a magnetic field isothermally and removing it adiabatically as discussed by the authors.
Abstract: The magnetocaloric effect (MCE) in paramagnetic materials has been widely used for attaining very low temperatures by applying a magnetic field isothermally and removing it adiabatically. The effect can also be exploited for room-temperature refrigeration by using giant MCE materials. Here we report on an inverse situation in Ni-Mn-Sn alloys, whereby applying a magnetic field adiabatically, rather than removing it, causes the sample to cool. This has been known to occur in some intermetallic compounds, for which a moderate entropy increase can be induced when a field is applied, thus giving rise to an inverse magnetocaloric effect. However, the entropy change found for some ferromagnetic Ni-Mn-Sn alloys is just as large as that reported for giant MCE materials, but with opposite sign. The giant inverse MCE has its origin in a martensitic phase transformation that modifies the magnetic exchange interactions through the change in the lattice parameters.
1,680 citations
TL;DR: The current understanding of astrophysical magnetic fields is reviewed in this paper, focusing on their generation and maintenance by turbulence, where analytical and numerical results are discussed both for small scale dynamos, which are completely isotropic, and for large scale dynamo, where some form of parity breaking is crucial.
1,548 citations
Patent•
TL;DR: In this paper, a magnetic resonance imaging (MRI) system is presented for highly precisely detecting and compensating body motions within a short processing time during radial scanning, which includes a control unit that applies radiofrequency magnetic fields and magnetic field gradients to a subject lying down in a static magnetic field and detects magnetic resonance signals generated from the subject.
913 citations
TL;DR: In this article, a 3D magnetohydrodynamics simulation code for applications in the solar convection zone and photosphere was developed, including a non-local and non-grey radiative transfer module and taking into account the effects of partial ionization.
Abstract: We have developed a 3D magnetohydrodynamics simulation code for applications in the solar convection zone and photosphere. The code includes a non-local and non-grey radiative transfer module and takes into account the effects of partial ionization. Its parallel design is based on domain decomposition, which makes it suited for use on parallel computers with distributed memory architecture. We give a description of the equations and numerical methods and present the results of the simulation of a solar plage region. Starting with a uniform vertical field of 200 G, the processes of flux expulsion and convective field amplification lead to a dichotomy of strong, mainly vertical fields embedded in the granular downflow network and weak, randomly oriented fields filling the hot granular upflows. The strong fields form a magnetic network with thin, sheet- like structures extending along downflow lanes and micropores with diameters of up to 1000 km which form occasionally at vertices where several downflow lanes merge. At the visible surface around optical depth unity, the strong field concentrations are in pressure balance with their weakly magnetized surroundings and reach field strengths of up to 2 kG, strongly exceeding the values corresponding to equipartition with the kinetic energy density of the convective motions. As a result of the channelling of radiation, small flux concentrations stand out as bright features, while the larger micropores appear dark in brightness maps owing to the suppression of the convective energy transport. The overall shape of the magnetic network changes slowly on a timescale much larger than the convective turnover time, while the magnetic flux is constantly redistributed within the network leading to continuous formation and dissolution of flux concentrations.
793 citations
TL;DR: In this article, the authors used a hybrid technique to construct the high-latitude electric potentials and magnetic field aligned currents (FAC) as a function of the solar wind parameters.
Abstract: Improved techniques have been developed for empirical modeling of the high-latitude electric potentials and magnetic field aligned currents (FAC) as a function of the solar wind parameters. The FAC model is constructed using scalar magnetic Euler potentials, and functions as a twin to the electric potential model. The improved models have more accurate field values as well as more accurate boundary locations. Non-linear saturation effects in the solar wind-magnetosphere coupling are also better reproduced. The models are constructed using a hybrid technique, which has spherical harmonic functions only within a small area at the pole. At lower latitudes the potentials are constructed from multiple Fourier series functions of longitude, at discrete latitudinal steps. It is shown that the two models can be used together in order to calculate the total Poynting flux and Joule heating in the ionosphere. An additional model of the ionospheric conductivity is not required in order to obtain the ionospheric currents and Joule heating, as the conductivity variations as a function of the solar inclination are implicitly contained within the FAC model's data. The models outputs are shown for various input conditions, as well as compared with satellite measurements. The calculations of the total Joule heating are compared with results obtained by the inversion of ground-based magnetometer measurements. Like their predecessors, these empirical models should continue to be a useful research and forecast tools.
581 citations
TL;DR: Experiments using external magnetic fields demonstrate that almost 100% of the nanotubes become magnetic and can be easily manipulated in magnetic field.
Abstract: We describe a simple and versatile technique to produce magnetic tubes by filling carbon nanotubes (CNTs) with paramagnetic iron oxide particles (∼10 nm diameter). Commercial ferrofluids were used to fill CNTs with an average outer diameter of 300 nm made via chemical vapor deposition into alumina membranes. Transmission electron microscopy study shows a high density of particles inside the CNT. Experiments using external magnetic fields demonstrate that almost 100% of the nanotubes become magnetic and can be easily manipulated in magnetic field. These one-dimensional magnetic nanostructures can find numerous applications in nanotechnology, memory devices, optical transducers for wearable electronics, and in medicine.
422 citations
TL;DR: By applying magnetic fields (B) in a hexaferrite having magnetic order above room temperature (RT), the system undergoes successive metamagnetic transitions, and shows concomitant ferroelectric order in some of the B-induced phases with long-wavelength magnetic structures.
Abstract: We report on the control of electric polarization (P) by using magnetic fields (B) in a hexaferrite having magnetic order above room temperature (RT). The material investigated is hexagonal Ba0.5Sr1.5Zn2Fe12O22, which is a nonferroelectric helimagnetic insulator in the zero-field ground state. By applying B, the system undergoes successive metamagnetic transitions, and shows concomitant ferroelectric order in some of the B-induced phases with long-wavelength magnetic structures. The magnetoelectrically induced P can be rotated 360 degrees by external B. This opens up the potential for not only RT magnetoelectric devices but also devices based on the magnetically controlled electro-optical response.
347 citations
TL;DR: A nuclear spin gyroscope based on an alkali-metal-noble-gas comagnetometer to detect gyroscopic precession and a promising tool in searches for anomalous spin couplings beyond the standard model.
Abstract: We describe a nuclear spin gyroscope based on an alkali-metal-noble-gas comagnetometer. Optically pumped alkali-metal vapor is used to polarize the noble-gas atoms and detect their gyroscopic precession. Spin precession due to magnetic fields as well as their gradients and transients can be cancelled in this arrangement. The sensitivity is enhanced by using a high-density alkali-metal vapor in a spin-exchange relaxation free regime. With a K-3He comagnetometer we demonstrate rotation sensitivity of 5 x 10(-7) rad s(-1) Hz(-1/2), equivalent to a magnetic field sensitivity of 2.5 fT/Hz(1/2). The rotation signal can be increased by a factor of 10 using 21Ne with a smaller magnetic moment. The comagnetometer is also a promising tool in searches for anomalous spin couplings beyond the standard model.
TL;DR: A review of recent research on linear magnetoelectric (ME) effect can be found in this paper, where the authors focus on the present revival of the effect and two major sources for 'large' ME effects are identified.
Abstract: Recent research activities on the linear magnetoelectric (ME) effect?induction of magnetization by an electric field or of polarization by a magnetic field?are reviewed. Beginning with a brief summary of the history of the ME effect since its prediction in 1894, the paper focuses on the present revival of the effect. Two major sources for 'large' ME effects are identified. (i) In composite materials the ME effect is generated as a product property of a magnetostrictive and a piezoelectric compound. A linear ME polarization is induced by a weak ac magnetic field oscillating in the presence of a strong dc bias field. The ME effect is large if the ME coefficient coupling the magnetic and electric fields is large. Experiments on sintered granular composites and on laminated layers of the constituents as well as theories on the interaction between the constituents are described. In the vicinity of electromechanical resonances a ME voltage coefficient of up to 90?V?cm?1?Oe?1 is achieved, which exceeds the ME response of single-phase compounds by 3?5 orders of magnitude. Microwave devices, sensors, transducers and heterogeneous read/write devices are among the suggested technical implementations of the composite ME effect. (ii) In multiferroics the internal magnetic and/or electric fields are enhanced by the presence of multiple long-range ordering. The ME effect is strong enough to trigger magnetic or electrical phase transitions. ME effects in multiferroics are thus 'large' if the corresponding contribution to the free energy is large. Clamped ME switching of electrical and magnetic domains, ferroelectric reorientation induced by applied magnetic fields and induction of ferromagnetic ordering in applied electric fields were observed. Mechanisms favouring multiferroicity are summarized, and multiferroics in reduced dimensions are discussed. In addition to composites and multiferroics, novel and exotic manifestations of ME behaviour are investigated. This includes (i) optical second harmonic generation as a tool to study magnetic, electrical and ME properties in one setup and with access to domain structures; (ii) ME effects in colossal magnetoresistive manganites, superconductors and phosphates of the LiMPO4 type; (iii) the concept of the toroidal moment as manifestation of a ME dipole moment; (iv) pronounced ME effects in photonic crystals with a possibility of electromagnetic unidirectionality. The review concludes with a summary and an outlook to the future development of magnetoelectrics research.
TL;DR: The signal space separation (SSS) method as mentioned in this paper is based on the fact that the magnetic field can be expressed as a combination of two separate and rapidly converging expansions of harmonic functions with one expansion for signals arising from inside of the measurement volume of the sensor array and another expansion for signal arising from outside of this volume.
Abstract: Measurement of external magnetic fields provides information on electric current distribution inside an object. For example, in magnetoencephalography modern measurement devices sample the magnetic field produced by the brain in several hundred distinct locations around the head. The signal space separation (SSS) method creates a fundamental linear basis for all measurable multichannel signal vectors of magnetic origin. The SSS basis is based on the fact that the magnetic field can be expressed as a combination of two separate and rapidly converging expansions of harmonic functions with one expansion for signals arising from inside of the measurement volume of the sensor array and another for signals arising from outside of this volume. The separation is based on the different convergence volumes of the two expansions and on the fact that the sensors are located in a source current-free volume between the interesting and interfering sources. Individual terms of the expansions are shown to contain uncorrelated information of the underlying source distribution. SSS provides a stable decomposition of the measurement into a fundamental device-independent form when used with an accurately calibrated multichannel device. The external interference signals are elegantly suppressed by leaving the interference components out from the reconstruction based on the decomposition. Representation of multichannel data with the SSS basis is shown to provide a large variety of applications for improved analysis of multichannel data.
TL;DR: In this article, the authors developed an analytical model for predicting the eddy-current loss in the rotor magnet of permanent magnet brushless motors, which can account for space-harmonic magnetomotive forces resulting from the winding distribution and timeharmonic MMFs due to nonsinusoidal phase currents, as well as the effect of curvature and circumferential segmentation of the magnets.
Abstract: We develop an analytical model for predicting the eddy-current loss in the rotor magnets of permanent-magnet brushless machines that have a fractional number of slots per pole, when either all the teeth or only alternate teeth are wound, and in which the unwound teeth may be narrower than the wound teeth The model enables the magnetic field distribution in the air gap and magnet regions to be determined, by neglecting the eddy-current redistribution effect and assuming that the eddy currents are resistance limited It can account for space-harmonic magnetomotive forces (MMFs) resulting from the winding distribution and time-harmonic MMFs due to nonsinusoidal phase currents, as well as for the effect of curvature and circumferential segmentation of the magnets We have validated the model by finite-element analysis, and used it to investigate the eddy-current loss in the magnets of three surface-mounted magnet brushless motors that have similar slot and pole numbers, and employ identical rotors but different stators, when they are operated in brushless ac (BLAC) and dc (BLDC) modes We show that the stator winding configuration, as well as the operational mode, significantly influence the resultant eddy-current loss
TL;DR: In this article, the authors described a new class of phenomena based on the interaction between spatially separated, but closely located ferromagnets and superconductors, the so-called ferromagnet-superconductor hybrids (FSH).
Abstract: The new class of phenomena described in this review is based on the interaction between spatially separated, but closely located ferromagnets and superconductors, the so-called ferromagnet–superconductor hybrids (FSH). Typical FSH are: coupled uniform and textured ferromagnetic and superconducting films, magnetic dots over a superconducting film, magnetic nanowires in a superconducting matrix, etc. The interaction is provided by the magnetic field generated by magnetic textures and supercurrents. The magnetic flux from magnetic structures or topological defects can pin vortices or create them, changing the transport properties and transition temperature of the superconductor. On the other hand, the magnetic field from supercurrents (vortices) strongly interacts with the magnetic subsystem, leading to formation of coupled magnetic–superconducting topological defects. Each time the Nambu and spin matrices are written together we mean the direct product. The proximity of ferromagnetic layer dramatically cha...
TL;DR: This review seeks to clarify the most important implications of higher magnetic field strength for clinical examinations of the whole body and demonstrates that susceptibility-dependent imaging, chemical shift selective imaging, and spectroscopic techniques clearly gain from higher field strength.
Abstract: This review seeks to clarify the most important implications of higher magnetic field strength for clinical examinations of the whole body. An overview is provided on the resulting advantages and disadvantages for anatomical, functional and biochemical magnetic resonance examinations in different regions of the body. It is demonstrated that susceptibility-dependent imaging, chemical shift selective (e.g., fat-suppressed) imaging, and spectroscopic techniques clearly gain from higher field strength. Problems due to shorter wavelength and higher radio frequency energy deposition at higher field strength are reported, especially in examinations of the body trunk. Thorax examinations provided sufficient homogeneity of the radio frequency field for common examination techniques in most cases, whereas abdominal and pelvic imaging was often hampered by undesired dielectric effects. Currently available and potential future strategies to overcome related limitations are discussed. Whole-body MRI at higher field strength currently leads to clearly improved image quality using a variety of established sequence types and for examination of many body regions. But some major problems at higher field strength have to be solved before high-field magnetic resonance systems can really replace the well-established and technically developed magnetic resonance systems operating at 1.5 T for each clinical application.
TL;DR: In this article, a mathematical model of biomagnetic fluid dynamics (BFD) suitable for the description of the Newtonian blood flow under the action of an applied magnetic field, is proposed.
Abstract: In the present study a mathematical model of biomagnetic fluid dynamics (BFD), suitable for the description of the Newtonian blood flow under the action of an applied magnetic field, is proposed. This model is consistent with the principles of ferrohydrodynamics and magnetohydrodynamics and takes into account both magnetization and electrical conductivity of blood. As a representative application the laminar, incompressible, three-dimensional, fully developed viscous flow of a Newtonian biomagnetic fluid (blood) in a straight rectangular duct is numerically studied under the action of a uniform or a spatially varying magnetic field. The numerical results are obtained using a finite differences numerical technique based on a pressure-linked pseudotransient method on a collocated grid. The flow is appreciably influenced by the application of the magnetic field and in particularly by the strength and the magnetic field gradient. A comparison of the derived results is also made with those obtained using the existing BFD model indicating the necessity of taking into account the electrical conductivity of blood.
TL;DR: In this paper, the authors measured the Doppler shifts of the interstellar hydrogen resonance glow to show the direction of the neutral hydrogen flow as it entered the inner heliosphere, and found that the hydrogen flow was deflected relative to the helium flow by about 4°.
Abstract: Using an absorption cell, we measured the Doppler shifts of the interstellar hydrogen resonance glow to show the direction of the neutral hydrogen flow as it enters the inner heliosphere. The neutral hydrogen flow is found to be deflected relative to the helium flow by about 4°. The most likely explanation of this deflection is a distortion of the heliosphere under the action of an ambient interstellar magnetic field. In this case, the helium flow vector and the hydrogen flow vector constrain the direction of the magnetic field and act as an interstellar magnetic compass.
TL;DR: In this paper, the authors present a Chandra and XMM-Newton study of X-ray emission from the lobes of 33 double radio galaxies and quasars and find that > 70 percent of the radio lobes are either at equipartition or electron dominated by a small factor.
Abstract: We present a Chandra and XMM-Newton study of X-ray emission from the lobes of 33 classical double radio galaxies and quasars. We report new detections of lobe-related X-ray emission in 11 sources. Together with previous detections we find that X-ray emission is detected from at least one radio lobe in ~75 percent of the sample. For all of the lobe detections, we find that the measured X-ray flux can be attributed to inverse-Compton scattering of the cosmic microwave background radiation, with magnetic field strengths in the lobes between (0.3 - 1.3) B_eq, where the value B_eq corresponds to equipartition between the electrons and magnetic field assuming a filling factor of unity. There is a strong peak in the magnetic field strength distribution at B ~ 0.7 B_eq. We find that > 70 percent of the radio lobes are either at equipartition or electron dominated by a small factor. The distribution of measured magnetic field strengths differs for narrow-line and broad-line objects, in the sense that broad-line radio galaxies and quasars appear to be further from equipartition; however, this is likely to be due to a combination of projection effects and worse systematic uncertainty in the X-ray analysis for those objects. Our results suggest that the lobes of classical double radio sources do not contain an energetically dominant proton population, because this would require the magnetic field energy density to be similar to the electron energy density rather than the overall energy density in relativistic particles.
TL;DR: The first midinfrared (mid-IR) resonant magnetic nanostructures exhibiting a strong magnetic response corresponding to a negative permeability are experimentally demonstrated.
Abstract: In 1968, Veselago proposed the concept of a negative refractive index or left-handed material (LHM) [1] with both a negative permittivity" and a negative permeability � . Many interesting properties are associated with negative materials including a negative index of refraction, backward phase propagation, a reversed Doppler effect, and backward emission of Cherenkov radiation. Recently, interest in LHMs has increased substantially with the theoretical prediction that a planar slab of LHM functions as a perfect lens without any diffractive loss of resolution [1,2] and with the first demonstrations of LHMs in the rf and THz electromagnetic regions [3‐6]. While metals provide a negative permittivity at frequencies below the plasma frequency, naturally occurring materials with negative permeability are not available. Composite electromagnetic materials with resonant structures with sizes much less than the wavelength can act as an effective homogeneous media with a negative permeability. Pendry et al. proposed a split ring structure that responds to the magnetic field of incident radiation [7]. The split ring structure (SRS) can be viewed as an equivalent inductor-capacitor (LC) tank circuit. In the presence of a time-varying magnetic field, the magnetic field generated by the current induced in the ring opposes the external magnetic field. At frequencies in the vicinity of the resonance, a negative effective permeability can be realized. While most experimental work has been done at microwave and, more recently, THz frequencies, extending the phenomena to infrared (IR) and visible frequencies will greatly increase the range of applications. Based on the previous modeling work [7] and on the fabrication difficulties associated with scaling the SRS to higher frequency, there has been general pessimism about the prospects of extending these properties to optical frequencies in metallic structures. In this Letter, we describe the fabrication, characterization, and modeling of arrays of a new nanostructure design with resonances in the mid-IR region and properties that demonstrate strong magnetic activity indicative of negative permeability. To our knowledge, this is the first experimental work on negative permeability reported in the IR. Importantly, the scaling of these results to even higher frequency is investigated, and a structure exhibiting negative permeability in the technologically important near-IR range 1:3 � m is proposed. The structure (Fig. 1, top) consists of an array of gold ‘‘staples’’ each with two outwardly splayed footings, separated from a thick continuous gold film by a ZnS dielectric layer. Each staple is a LC circuit with the structure part of the inductor associated with the upper loop of the staple and with two capacitors formed between the gold staple footings—the dielectric layer and the bottom gold layer. As a result of the thick Au film, light can only be reflected from or absorbed within the structure; there is no transmission in the IR. For analysis, it is simpler to view this structure in transmission as the staple and its image mir
Patent•
27 Apr 2005TL;DR: An inductive coil assembly with multiple coils arranged at distinct orientations to provide efficient inductive coupling of power or communications or both to a device when the device is arranged at different orientations with respect to the inductive primary coil is described in this article.
Abstract: An inductive coil assembly having multiple coils arranged at distinct orientations to provide efficient inductive coupling of power or communications or both to a device when the device is arranged at different orientations with respect to the inductive primary coil. In one embodiment, the inductive coil assembly includes three coils, each oriented along one of the x, y and z axes of a standard Cartesian three-dimensional coordinate system. The three separate coils provide effective transfer of power and communication when the device is at essentially any orientation with respect to the primary coil. In an alternative embodiment, the multi-axis inductive coil assembly of the present invention can function as a primary to inductively transmit power or communication or both over a plurality of magnetic fields at distinct orientations.
01 Jan 2005
TL;DR: Magnetic Fields in the Early Universe Magnetic Fields in Galaxy Systems, Clusters and Beyond Magnetic fields in Galaxies as discussed by the authors were derived from Radio Continuum Observations and Faraday Rotation Studies and Mesoscale Magnetic Structures in Spiral Galaxies.
Abstract: Magnetic Fields in the Early Universe.- Magnetic Fields in Galaxy Systems, Clusters and Beyond.- Magnetic Fields in Galaxies.- The Origin of Galactic Magnetic Fields.- Magnetic Fields in the Milky Way, Derived from Radio Continuum Observations and Faraday Rotation Studies.- Mesoscale Magnetic Structures in Spiral Galaxies.- Magnetic Fields in Diffuse HI and Molecular Clouds.- Stellar Magnetic Fields.- Importance of Magnetic Helicity in Dynamos.- Numerical Magnetohydrodynamics in Astrophysics.
TL;DR: In this paper, the authors discuss the magnetic field, column density, and turbulence in the cold neutral medium (CNM) and relate the observed and intrinsic quantities by relating their univariate and bivariate probability distribution functions (pdfs).
Abstract: We discuss observations of the magnetic field, column density, and turbulence in the Cold Neutral Medium (CNM). The observed quantities are only indirectly related to the intrinsic astronomical ones. We relate the observed and intrinsic quantities by relating their univariate and bivariate probability distribution functions (pdfs). We find that observations of the line-of-sight component of magnetic field do not constrain the pdf of the total field very well, but do constrain the median value of the total field.
In the CNM, we find a well-defined median magnetic field 6.0 +/-1.8 microGauss. The CNM magnetic field dominates thermal motions. Turbulence and magnetism are in approximate equipartition. We find the probability distribution of HI column density Nperp in the sheets closely follows Nperp^-1 over a range of two orders of magnitude, 0.026 < Nperp < 2.6 (times 10^20 cm^-2). The bivariate distributions are not well enough determined to constrain structural models of CNM sheets.
TL;DR: An area of interest is whether static magnetic fields cause DNA damage, which can be evaluated by determination of the frequency of micronucleus formation, which has been used to confirm that a static magnetic field alone has no such effect.
Abstract: There have been few studies on the effects of static magnetic fields at the cellular level, compared to those of extremely low frequency magnetic fields. Past studies have shown that a static magnetic field alone does not have a lethal effect on the basic properties of cell growth and survival under normal culture conditions, regardless of the magnetic density. Most but not all studies have also suggested that a static magnetic field has no effect on changes in cell growth rate. It has also been shown that cell cycle distribution is not influenced by extremely strong static magnetic fields (up to a maximum of 10 T). A further area of interest is whether static magnetic fields cause DNA damage, which can be evaluated by determination of the frequency of micronucleus formation. The presence or absence of such micronuclei can confirm whether a particular treatment damages cellular DNA. This method has been used to confirm that a static magnetic field alone has no such effect. However, the frequency of micronucleus formation increases significantly when certain treatments (e.g., X-irradiation) are given prior to exposure to a 10 T static magnetic field. It has also been reported that treatment with trace amounts of ferrous ions in the cell culture medium and exposure to a static magnetic field increases DNA damage, which is detected using the comet assay. In addition, many studies have found a strong magnetic field that can induce orientation phenomena in cell culture.
TL;DR: In this article, measurements from the Cluster spacecraft of electric fields, magnetic fields, and ions are used to study the structure and dynamics of the reconnection region in the tail at distances of ∼18 RE near 22.4 MLT on 1 October 2001.
Abstract: [1] Measurements from the Cluster spacecraft of electric fields, magnetic fields, and ions are used to study the structure and dynamics of the reconnection region in the tail at distances of ∼18 RE near 22.4 MLT on 1 October 2001. This paper focuses on measurements of the large amplitude normal component of the electric field observed in the ion decoupling region near the reconnection x-line, the structure of the associated potential drops across the current sheet, and the role of the electrostatic potential structure in the ballistic acceleration of ions across the current sheet. The thinnest current sheet observed during this interval was bifurcated into a pair of current sheets and the measured width of the individual current sheet was 60–100 km (3–5 c/ωpe). Coinciding with the pair of thin current sheets is a large-amplitude (±60 mV/m) bipolar electric field structure directed normal to the current sheets toward the midplane of the plasma sheet. The potential drop between the outer boundary of the thin current sheet and the neutral sheet due to this electric field is 4–6 kV. This electric field structure produces a 4–6 kV electric potential well centered on the separatrix region. Measured H+ velocity space distributions obtained inside the current layers provide evidence that the H+ fluids from the northern and southern tail lobes are accelerated into the potential well, producing a pair of counterstreaming, monoenergetic H+ beams. These beams are directed within 20 degrees of the normal direction with energies of 4–6 keV. The data also suggest there is ballistic acceleration of O+ in a similar larger-scale potential well of 10–30 kV spatially coinciding with the larger scale size (∼1000–3000 km) portions of current sheet surrounding the thin current sheet. Distribution functions show counterstreaming O+ populations with energies of ∼20 keV accelerated along the average normal direction within this large-scale potential structure. The normal component of the electric field in the thin current sheet layer is large enough to drive an E × B drift of the electrons ∼10,000 km/s (0.25 x electron Alfven velocity), which can account for the magnitude of the cross-tail current associated with the thin current sheet.
TL;DR: These findings strongly suggest that excitations in which the spins rotate stimulate superconductivity in the neighborhood of a quantum phase transition under high magnetic field.
Abstract: In several metals, including URhGe, superconductivity has recently been observed to appear and coexist with ferromagnetism at temperatures well below that at which the ferromagnetic state forms. However, the material characteristics leading to such a state of coexistence have not yet been fully elucidated. We report that in URhGe there is a magnetic transition where the direction of the spin axis changes when a magnetic field of 12 tesla is applied parallel to the crystal b axis. We also report that a second pocket of superconductivity occurs at low temperature for a range of fields enveloping this magnetic transition, well above the field of 2 tesla at which superconductivity is first destroyed. Our findings strongly suggest that excitations in which the spins rotate stimulate superconductivity in the neighborhood of a quantum phase transition under high magnetic field.
TL;DR: In this article, the current state of the art in the field of cold and ultracold molecules and demonstrates that chemical reactions, inelastic collisions and dissociation of molecules at sub-kelvin temperatures can be manipulated with external electric or magnetic fields.
Abstract: This article reviews the current state of the art in the field of cold and ultracold molecules and demonstrates that chemical reactions, inelastic collisions and dissociation of molecules at subkelvin temperatures can be manipulated with external electric or magnetic fields. The creation of ultracold molecules may allow for spectroscopy measurements with extremely high precision and tests of fundamental symmetries of nature, quantum computation with molecules as qubits, and controlled chemistry. The probability of chemical reactions and collisional energy transfer can be very large at temperatures near zero kelvin. The collision energy of ultracold atoms and molecules is much smaller than perturbations due to interactions with external electric or magnetic fields available in the laboratory. External fields may therefore be used to induce dissociation of weakly bound molecules, stimulate forbidden electronic transitions, suppress the effect of centrifugal barriers in outgoing reaction channels or tune Fes...
TL;DR: In this paper, the authors investigated the correlation anisotropy of solar wind magnetohydrodynamic-scale fluctuations and showed that the nature of the correlation differs in fast and slow (<400 km s-1) streams.
Abstract: Using 5 years of spacecraft data from near Earth orbit, we investigate the correlation anisotropy of solar wind magnetohydrodynamic-scale fluctuations and show that the nature of the anisotropy differs in fast (>500 km s-1) and slow (<400 km s-1) streams. In particular, fast streams are relatively more dominated by fluctuations with wavevectors quasi-parallel to the local magnetic field, while slow streams, which appear to be more fully evolved turbulence, are more dominated by quasi-perpendicular fluctuation wavevectors.
TL;DR: In this article, the authors considered the polarization properties of synchrotron radiation emitted by relativistically moving electron-positron jets carrying large-scale helical magnetic fields.
Abstract: We consider the polarization properties of optically thin synchrotron radiation emitted by relativistically moving electron-positron jets carrying large-scale helical magnetic fields. In our model, the jet is cylindrical, and the emitting plasma moves parallel to the jet axis with a characteristic Lorentz factor {Lambda}. We draw attention to the strong influence that the bulk relativistic motion of the emitting relativistic particles has on the observed polarization. Our computations predict and explain the following behavior. (1) For jets unresolved in the direction perpendicular to their direction of propagation, the position angle of the electric vector of the linear polarization has a bimodal distribution, being oriented either parallel or perpendicular to the jet. (2) If an ultra-relativistic jet with {Lambda} >> 1 whose axis makes a small angle to the line of sight, {theta} {approx} 1/{Lambda}, experiences a relatively small change in the direction of propagation, velocity or pitch angle of the magnetic fields, the polarization is likely to remain parallel or perpendicular; on the other hand, in some cases, the degree of polarization can exhibit large variations and the polarization position angle can experience abrupt 90{sup o} changes. This change is more likely to occur in jets with flatter spectra. (3)more » In order for the jet polarization to be oriented along the jet axis, the intrinsic toroidal magnetic field (in the frame of the jet) should be of the order of or stronger than the intrinsic poloidal field; in this case, the highly relativistic motion of the jet implies that, in the observer's frame, the jet is strongly dominated by the toroidal magnetic field B{sub {phi}}/B{sub z} {ge} {Lambda}. (4) The emission-weighted average pitch angle of the intrinsic helical field in the jet must not be too small to produce polarization along the jet axis. In force-free jets with a smooth distribution of emissivities, the emission should be generated in a limited range of radii not too close to the jet core. (5) For mildly relativistic jets, when a counter jet can be seen, the polarization of the counter jet is preferentially orthogonal to the axis, unless the jet is strongly dominated by the toroidal magnetic field in its rest frame. (6) For resolved jets, the polarization pattern is not symmetric with respect to jet axis. Under certain conditions, this can be used to deduce the direction of the spin of the central object (black hole or disk), whether it is aligned or anti-aligned with the jet axis. (7) In resolved ''cylindrical shell'' type jets, the central parts of the jet are polarized along the axis, while the outer parts are polarized orthogonal to it, in accordance with observations. We conclude that large-scale magnetic fields can explain the salient polarization properties of parsec-scale AGN jets. Since the typical degrees of polarization are {le} 15%, the emitting parts of the jets must have comparable rest-frame toroidal and poloidal fields. In this case, most relativistic jets are strongly dominated by the toroidal magnetic field component in the observer's frame, B{sub {phi}}/B{sub z} {approx} {Lambda}. We also discuss the possibility that relativistic AGN jets may be electromagnetically (Poynting flux) dominated. In this case, dissipation of the toroidal magnetic field (and not fluid shocks) may be responsible for particle acceleration.« less
TL;DR: In this paper, a Bayesian maximum likelihood analysis of Faraday rotation measure (RM) maps of extended radio sources is presented to determine magnetic field power spectra in clusters of galaxies.
Abstract: We present a Bayesian maximum likelihood analysis of Faraday rotation measure (RM) maps of extended radio sources to determine magnetic field power spectra in clusters of galaxies. Using this approach, it is possible to determine the uncertainties in the measurements. We apply this approach to the RM map of Hydra A and derive the power spectrum of the cluster magnetic field. For Hydra A, we measure a spectral index of −5/3 over at least one order of magnitude implying Kolmogorov type turbulence. We find a dominant scale ∼3 kpc on which the magnetic power is concentrated, since the magnetic autocorrelation length is λB = 3 ± 0.5 kpc. Furthermore, we investigate the influences of the assumption about the sampling volume (described by a window function) on the magnetic power spectrum. The central magnetic field strength was determined to be ∼7 ± 2 µG for the most likely geometries.