Distribution Of Magnetic Field Strength Research Articles

PROTOSTELLAR DISK FORMATION ENABLED BY WEAK, MISALIGNED MAGNETIC FIELDS

The gas from which stars form is magnetized, and strong magnetic fields can efficiently transport angular momentum. Most theoretical models of this phenomenon find that it should prevent formation of large (>100 AU), rotationally-supported disks around most protostars, even when non-ideal magnetohydrodynamic (MHD) effects that allow the field and gas to decouple are taken into account. Using recent observations of magnetic field strengths and orientations in protostellar cores, we show that this conclusion is incorrect. The distribution of magnetic field strengths is very broad, and alignments between fields and angular momentum vectors within protostellar cores are essentially random. By combining the field strength and misalignment data with MHD simulations showing that disk formation is expected for both weak and misaligned fields, we show that these observations imply that we should expect disk fractions of ~10 - 50% even when protostars are still deeply embedded in their parent cores, and even if the gas is governed by ideal MHD.

Experimental validation of a coupled magneto-thermal model for a flat-parallel-plate active magnetic regenerator

In this study, a flat-parallel-plate active magnetic regenerator made of gadolinium is investigated. The coupling of a one-dimensional thermal model with a two-dimensional magnetic model is presented. The thermal model takes into consideration the magnetocaloric effect as a source term and the energy conservation between a solid and a fluid. The magnetic model considers the space distribution of the internal magnetic field strength, and thus it includes the demagnetization effect. Measurements on an experimental magnetic refrigeration test device are performed using distilled water as the working fluid. At cyclic steady states, the temperatures of the fluid on both sides of the regenerator are calculated numerically and compared with the measurement results. The inability to provide a constant internal magnetic field during the heat transfer process with a permanent magnet is demonstrated. A reasonable agreement between simulations and experiments confirms the validity of the proposed model.

AN ARECIBO SURVEY FOR ZEEMAN SPLITTING IN OH MEGAMASER GALAXIES

We present the results of a comprehensive survey using the Arecibo Observatory for Zeeman splitting of OH lines in OH megamasers (OHMs). A total of seventy-seven sources were observed with the Arecibo telescope. Of these, maser emission could not be detected for eight sources, and two sources were only ambiguously detected. Another twenty-seven sources were detected at low signal-to-noise ratios or with interference that prevented placing any useful limits on the presence of magnetic fields. In twenty-six sources, it was possible to place upper limits on the magnitude of magnetic fields, typically between 10-30 mG. For fourteen sources, the Stokes V spectra exhibit features consistent with Zeeman splitting. Eleven of these fourteen are new detections, and the remaining three are re-detections of Stokes V detections in Robishaw et al. (2008). Among confident new detections, we derive magnetic fields associated with maser regions with magnitudes ranging from 6.1-27.6 mG. The distribution of magnetic field strengths suggests the magnetic fields in OH masing clouds in OHMs are larger than those in Galactic OH masers. The results are consistent with magnetic fields playing a dynamically important role in OH masing clouds in OHMs.

Direct Field-Circuit Coupled Analysis and Corresponding Experiments of Electromagnetic Resonant Coupling System

Based on electromagnetic (EM) field resonantly coupling technology, a novel contactless power transfer system is analyzed in this paper. A detailed field-circuit coupling model is proposed taking into account the influences of near field strength and circuit constraints simultaneously. Therefore, the distribution of magnetic field strength under different angular frequencies and spacing distances are calculated through direct field-circuit coupled method. In the experimental aspect, a resonantly coupling system contains planar disc resonators is designed. It is shown by measurement results that property of system is basically consistent with the results derived in article. Moreover, the solution accuracy is improved through this technology to recharge electrical devices or transfer power.

Magnetic field detection in the bright A0-type supergiant HD 92207

Context. Recent developments in observational techniques and theories revealed the potential significance of magnetic fields for stellar structure, evolution, and circumstellar environment. At present, the distribution of magnetic field strengths in massive stars from the zero-age main sequence to more evolved stages, which would shed light on the origin of the magnetic field, has not been studied. Aims. We searched for the presence of a magnetic field in the visually brightest early A-type supergiant HD 92207. Methods. Observations were obtained using the low-resolution spectropolarimetric mode of FORS 2 (FOcal Reducer low dispersion Spectrograph) mounted on the 8 m Antu telescope of the VLT. For the mean longitudinal magnetic field measurements, we applied a linear regression analysis in two ways: using only the absorption hydrogen Balmer lines or using the entire spectrum including all available absorption lines. Results. A mean longitudinal magnetic field at a significance level of more than 3σ was detected in two out of three observations distributed over about one year. It is one of the rare cases where a field of about a few hundred Gauss is detected in an early A-type supergiant. All line profiles in the spectra of HD 92207 undergo distinct variations in radial velocities and intensities, probably caused by previously detected non-radial pulsations.

Magnetic energy coupling system based on micro-electro-mechanical system coils

In this paper, a high efficiency wireless energy transfer system based on MEMS coils is first developed. The permanent magnetic core used in the transmitting coil can not only enhance the magnetic flux but also applies a strong and uniform magnetic field distribution around the core. Ansoft hfss is then used to analyze the performance of two coupling coils designed to be resonated at the same frequency. The distribution of magnetic field strength and coupling efficiency is modeled and characterized. High-performance bio-compatible MEMS coils were fabricated on a glass wafer by thick glue photolithography and electroplating technique. We measured a peak value of energy transfer at the resonant frequency of 23 MHz, and the coupling efficiency is higher than 10% within the distance of 10–20 cm by sweeping frequencies from 1 MHz to 200 MHz. Experiments also show that the resonant coupling efficiency is not much affected by the relative position of the two coils in a large range.

HINODEOBSERVATIONS SUGGESTING THE PRESENCE OF A LOCAL SMALL-SCALE TURBULENT DYNAMO

Analysis of observations from the Hinode/Solar Optical Telescope spectropolarimeter (SP) yields results that are consistent with the operation of a small-scale turbulent dynamo in the upper solar convection zone. Examination of 45 Hinode data sets obtained in 2007 reveals only a very small correlation of the net polarity imbalance of the regions of the quiet Sun having very weak flux, relative to the polarity imbalance averaged over each data set. Further, there is no correlation of the average net unsigned flux of those regions of weakest flux with the average unsigned flux of each region studied. Positive correlations, especially of the net unsigned flux, should exist if the internetwork fields were to arise from dispersal of flux from active regions, so the absence of significant correlations supports the small-scale dynamo (SSD) scenario. Considering only regions of weakest flux, the net longitudinal flux increases slightly toward the limb, probably as the result of the dominance of horizontal fields higher in the photosphere. Inferred distributions of magnetic field strength as derived from inversions of Stokes profiles indicate that the magnetic energy of the quiet Sun observed at the resolution of the Hinode SP is dominated by the small fraction of field elements having kilo-Gauss strengths. Because these strong-field elements carry most of the imbalance of magnetic flux measured in each region, they likely arise primarily from dispersal of flux from active regions, rather than from an SSD.

FIRST MAGNETIC FIELD MODELS FOR RECENTLY DISCOVERED MAGNETIC β CEPHEI AND SLOWLY PULSATING B STARS

In spite of recent detections of magnetic fields in a number of beta Cephei and slowly pulsating B (SPB) stars, their impact on stellar rotation, pulsations, and element diffusion is not sufficiently studied yet. The reason for this is the lack of knowledge of rotation periods, the magnetic field strength distribution and temporal variability, and the field geometry. New longitudinal field measurements of four beta Cephei and candidate beta Cephei stars, and two SPB stars were acquired with FORS2 at the VLT. These measurements allowed us to carry out a search for rotation periods and to constrain the magnetic field geometry for four stars in our sample.

Superflares from magnetars revealing the GRB central engine

Abstract Long-duration gamma-ray bursts (GRBs) may be powered by the rotational energy of a millisecond magnetar. I argue that the GRB-driving magnetars lie at the high end of the distribution of magnetic field strengths of magnetars. The field of GRB magnetars decays on time-scale of hundreds of years and can power soft gamma-ray repeater (SGR)-like flares up to ∼100 times more powerful than the 2004 event of SGR 1806–20. A few of these flares per year may have been observed by the Burst and Transient Source Experiment (BATSE) and classified as short-duration GRBs. Association of one of these superflares with a nearby dL≲ 250 Mpc galaxy and the discovery of a, coincident in space, 100-yr-old GRB afterglow (observed in the radio) will be the characteristic signature of the magnetar model for GRBs.

Magnetic field strength and spectral distribution of six parsec-scale active galactic nuclei jets

We use observations of six "blazars" with the Very Long Baseline Array (VLBA), at eight frequencies (4.6, 5.1, 7.9, 8.9, 12.9, 15.4, 22.2, 43.1 GHz), to investigate the frequency-dependent position of their VLBI cores ("core-shift") and their overall jet spectral distribution. By cross-correlating the optically thin jet emission, we are able to accurately align the multi-frequency images of three of the jets (1418+546, 2007+777, 2200+420), whose core-shifts and spectra we find consistent with the equipartition regime of the Blandford & Konigl conical jet model, where the position of the radio core from the base of the jet follows $r_{core}\propto\nu^{-1}$. For the jet of 0954+658, we align the higher frequency images using our lower frequency measurements assuming equipartition in the radio core from 4.6-43 GHz. The jet emission of the other two sources in our sample (1156+295, 1749+096) is too sparse for our alignment technique to work. Using our measured core-shifts, we calculate equipartition magnetic field strengths of the order of 10's to 100's of mG in the radio cores of these four AGN from 4.6-43 GHz. Extrapolating our results back to the accretion disk and black hole jet-launching distances, we find magnetic field strengths consistent with those expected from theoretical models of magnetically powered jets.

The Hanle effect in a random magnetic field

Context. The Hanle effect is used to determine weak turbulent magnetic fields in the solar atmosphere, usually assuming that the angular distribution is isotropic, the magnetic field strength constant, and that micro-turbulence holds, i.e. that the magnetic field correlation length is much less than a photon mean free path. Aims. To examine the sensitivity of turbulent magnetic field measurements to these assumptions, we study the dependence of Hanle effect on the magnetic field correlation length, its angular, and strength distributions. Methods. We introduce a fairly general random magnetic field model characterized by a correlation length and a magnetic field vector distribution. Micro-turbulence is recovered when the correlation length goes to zero and macro-turbulence when it goes to infinity. Radiative transfer equations are established for the calculation of the mean Stokes parameters and they are solved numerically by a polarized approximate lambda iteration method. Results. We show that optically thin spectral lines and optically very thick ones are insensitive to the correlation length of the magnetic field, while spectral lines with intermediate optical depths (around 10–100) show some sensitivity to this parameter. The result is interpreted in terms of the mean number of scattering events needed to create the surface polarization. It is shown that the single-scattering approximation holds good for thin and thick lines but may fail for lines with intermediate thickness. The dependence of the polarization on the magnetic field vector probability density function (PDF) is examined in the micro-turbulent limit. A few PDFs with different angular and strength distributions, but equal mean value of the magnetic field, are considered. It is found that the polarization is in general quite sensitive to the shape of the magnetic field strength PDF and somewhat to the angular distribution. Conclusions. The mean field derived from Hanle effect analysis of polarimetric data strongly depends on the choice of the field strength distribution used in the analysis. It is shown that micro-turbulence is in general a safe approximation.

Magnetic fields and gas in the cluster-influenced spiral galaxy NGC 4254

Aims. The origin of asymmetric radio polarized emission in the Virgo Cluster spiral NGC 4254 is investigated and the influence of cluster environment on the properties of magnetic fields is explored. Methods. Structures of magnetic fields are analyzed with the concept of “magnetic maps”, presenting distributions of different magnetic field components (total, regular, and random) over the entire galaxy, free of Faraday rotation and projection effects. A number of different physical phenomena influencing the magnetic field are modeled analytically and confronted with the galaxy's depolarization pattern and distribution of magnetic field strength obtained from multifrequency polarimetric radio observations. Results. The study of orientation of intrinsic magnetic field vectors in NGC 4254 indicates that their dramatic variation (from 0° to more than 40°) throughout the galaxy cannot arise from the dynamo process alone, but must be dominated by effects such as density waves and local gas flows. We determine within the galaxy the relation between the strength of total magnetic field and the local star-formation rate (SFR) as a power-law with an index of +0.18 ± 0.01. We find the opposite sense of the relation between magnetic field regularity and SFR (-0.32 ± 0.03), and suggest that it results from efficient production of random field with rising turbulence in the regions with actively-forming stars. The distribution of Faraday rotation measures in NGC 4254 indicates a perturbed axisymmetrical mean-field dynamo mode or a mixture of axisymmetrical and bisymmetrical ones with regular field directed outwards from the disk, which is contrary to most observed galaxies. The galaxy's northern magnetic arm, located on the upstream side of the local density wave, with regular field strength of about 8 μ G and the total one of 17 μ G, much resembles those observed in other galaxies. But the magnetic field within two outer arms (shifted downstream of a density wave) is much stronger, up to 13 μ G in the regular field component and 20 μ G in the total field. Our modeling of cluster influence on different magnetic field components indicates that within the outer magnetic arms the dynamo-induced magnetic fields are modified by stretching and shearing forces rather than by cluster ram pressure. Those forces, which are likely triggered by the galaxy's gravitational interaction, produce an anisotropic component of the regular field and enhance the polarized emission. We also show that the magnetic energy within the large interarm regions and the galaxy's outskirts exceeds the gas thermal and turbulent energy, likely becoming dynamically important.

Internetwork magnetic field distribution from simultaneous 1.56 $\mathsf{\mu}$m and 630 nm observations

Aims.We study the contradictory magnetic field strength distributions retrieved from independent analyses of spectropolarimetric observations in the near-infrared (1.56 μm) and in the visible (630 nm) spectral ranges in internetwork regions.

Relationship between the shape of equilibrium magnetic surfaces and the magnetic field strength

A local analysis of the magnetic field near an equilibrium magnetic surface shows that there is generally no relationship between the magnetic field strength and the shape of the surface. However, the relationship exists under additional requirements such as the absence of the toroidal current, symmetry conservation, and the conservation of the magnetic field strength distribution on the nearest surface. An equilibrium magnetic surface can be calculated by specifying three functions of two angular variables—the magnetic field strength, the periodic component of the magnetic potential, and the mean curvature of the surface.

The Effect of Nonuniform Magnetic Field on Natural Convection in an Enclosure

Numerical two-dimensional study is carried out on the laminar and turbulent natural convection of molten sodium in a square enclosure heated from one vertical wall and cooled from an opposing vertical wall. A nonuniform magnetic field is described by a mathematical equation, which describes the distribution of different values of magnetic induction strength along the magnetic pole faces. The external magnetic field is applied in the x direction, for laminar flow with Ra = 106 and for turbulent flow with Ra = 1010. The magnetic field intensity required for suppressing the turbulent natural convection is more than that required for laminar natural convection. The effect of a nonuniform magnetic field differs from that of a uniform magnetic field. The flow profile changes dramatically with external magnetic strength and direction, but this change in flow region depends on the distribution of magnetic field strength along magnetic pole faces. The flow of liquid metal in an enclosure occurs in the low-magnetic-strength region far from the high-magnetic-strength region. This affects the temperature distribution in the enclosure and the values of the Nusselt number.

Magnetocentrifugal Winds in Three Dimensions: A Nonaxisymmetric Steady State

Outflows can be loaded and accelerated to high speeds along rapidly rotating, open magnetic field lines by centrifugal forces. Whether such magnetocentrifugally driven winds are stable is a longstanding theoretical problem. As a step towards addressing this problem, we perform the first large-scale 3D MHD simulations that extend to a distance {approx} 10{sup 2} times beyond the launching region, starting from steady 2D (axisymmetric) solutions. In an attempt to drive the wind unstable, we increase the mass loading on one half of the launching surface by a factor of {radical}10, and reduce it by the same factor on the other half. The evolution of the perturbed wind is followed numerically. We find no evidence for any rapidly growing instability that could disrupt the wind during the launching and initial phase of propagation, even when the magnetic field of the magnetocentrifugal wind is toroidally dominated all the way to the launching surface. The strongly perturbed wind settles into a new steady state, with a highly asymmetric mass distribution. The distribution of magnetic field strength is, in contrast, much more symmetric. We discuss possible reasons for the apparent stability, including stabilization by an axial poloidal magnetic field, which is required to bendmore » field lines away from the vertical direction and produce a magnetocentrifugal wind in the first place.« less

Radio Observations of a Large Sample of Late M, L, and T Dwarfs: The Distribution of Magnetic Field Strengths

I present radio observations of 90 dwarf stars and brown dwarfs of spectral type M5-T8. Three sources exhibit radio activity, in addition to the six objects previously detected in quiescence and outburst, leading to an overall detection rate of ~10% for objects later than M7. The inferred magnetic field strengths are ~102 G in quiescence and nearly 1 kG during flares, while the majority of the nondetected objects have B 50 G. Depending on the configuration and size of the magnetic loops, the surface fields may approach 1 kG even in quiescence, at most a factor of a few smaller than in early M dwarfs. With the larger sample of sources I find continued evidence for (1) a sharp transition around spectral type of M7 from a ratio of radio to X-ray luminosity of log(LR/LX) ~ -15.5 to -12, (2) increased radio activity (LR/Lbol) with later spectral type, in contrast to H? and X-ray observations, and (3) an overall drop in the fraction of active sources from ~30% for M dwarfs to ~5% for L dwarfs, consistent with H? and X-ray observations. Taken together, these trends suggest that some late M and L dwarfs are capable of generating 0.1-1 kG magnetic fields, but the overall drop in the fraction of such objects likely reflects changes in the structure of the chromospheres and coronae, possibly due to increasingly neutral atmospheres and/or a transition to a turbulent dynamo. These possibilities can best be tested through simultaneous observations, which can trace the effect of magnetic dissipation in a direct, rather than a statistical, manner. Still, a more extended radio survey currently holds the best promise for measuring the magnetic field properties of a large number of dwarf stars.

Viscoelasticity of mono- and polydisperse inverse ferrofluids

We report on measurements of a magnetorheological model fluid created by dispersing nonmagnetic microparticles of polystyrene in a commercial ferrofluid. The linear viscoelastic properties as a function of magnetic field strength, particle size, and particle size distribution are studied by oscillatory measurements. We compare the results with a magnetostatic theory proposed by De Gans et al. [Phys. Rev. E 60, 4518 (1999)] for the case of gap spanning chains of particles. We observe these chain structures via a long distance microscope. For monodisperse particles we find good agreement of the measured storage modulus with theory, even for an extended range, where the linear magnetization law is no longer strictly valid. Moreover we compare for the first time results for mono- and polydisperse particles. For the latter, we observe an enhanced storage modulus in the linear regime of the magnetization.

Tsallis Statistics of the Magnetic Field in the Heliosheath

The spacecraft Voyager 1 crossed the termination shock on 2004 December 16 at a distance of 94 AU from the Sun, and it has been moving through the heliosheath toward the interstellar medium since then. The distributions of magnetic field strength B observed in the heliosheath are Gaussian over a wide range of scales, yet the measured profile appears to be filamentary with occasional large jumps in the magnetic field strength B(t). All of the probability distributions of changes in B, dBn ≡ B(t + τ) - B(t) on scales τ from 1 to 128 days, can be fit with the symmetric Tsallis distribution of nonextensive statistical mechanics. At scales ≥32 days, the distributions are Gaussian, but on scales from 1 through 16 days the probability distribution functions have non-Gaussian tails, suggesting that the inner heliosheath is not in statistical equilibrium on scales from 1 to 16 days.

On the Sr I $\mathsf{\lambda}$4607 Å Hanle depolarization signals in the quiet Sun

The Hanle depolarization signals of Sr i λ4607 Å have been used to estimate the unsigned magnetic flux and magnetic energy existing in the quiet Sun photosphere. However, the Sr i λ4607 Å Hanle signals are not sensitive to the unsigned flux and energy. They only bear information on the fraction of photosphere occupied by magnetic field strengths smaller than the Hanle saturation, which do not contribute to the unsigned flux and energy. We deduce an approximate expression for the relationship between magnetic fill factor and Hanle signal. When applied to existing Hanle depolarization measurements, it indicates that only 40% of the quiet Sun is filled by magnetic fields with a strength smaller than 60 G. The remaining 60% of the surface has field strengths above this limit. Such constraint will be needed to determine the distribution of magnetic field strengths existing in the quiet Sun.