Dependence Of Magnetic Field Strength Research Articles

Preload, frequency, vibrational amplitude and magnetic field strength dependence of magnetosensitive rubber

The preload dependence of magnetosensitive rubber is experimentally studied along with the influence of frequency, vibrational amplitude and magnetic field strength. The material studied is an iron particle filled natural rubber with a volume particle fraction of 33%, which is close to the optimal particle concentration. The results of the measurements show that an increased preload decreases the influence of magnetic field strength and they also suggest that an increase in the magnetic field reduces the influence of the preload. Measurements of the magnitude of the dynamic shear modulus also display a preload dependence. These results imply that in a description of these materials, the preload should be taken into account, especially since magnetosensitive elastomers are used in applications where they are often exposed to preloads.

Magnetic field in active regions of the sun at coronal heights

AbstractA method is developed for estimation of the vertical structure of the magnetic field in active regions using multi-wave spectral-polarization measurements of radio waves which gives not only the dependence of magnetic field strength on height but also determines two-dimensional form of a magnetic flux tube, emitted in the microwave range of wavelengths.

On the voxel size and magnetic field strength dependence of spectral resolution in magnetic resonance spectroscopy

While the inherent low sensitivity of in vivo MR spectroscopy motivated a trend towards higher magnetic fields, B(0), it has since become apparent that this increase does not seem to translate into the anticipated improvement in spectral resolution. This is attributed to the decrease of the transverse relaxation time, T(2)*, in vivo due to macro- and mesoscopic tissue susceptibility. Using spectral contrast-to-noise ratio (SCNR) arguments, we show that if in biological systems the linewidth (on the frequency scale) increases linearly with the field, the spectral resolution (in parts per million) improves approximately as the fifth-root of B(0) for chemically shifted lines and decreases as about B(0)(4/5) (in hertz) for a structure of J-coupled multiplets. It is also shown that for any given B(0) there is a unique voxel size that is optimal in spectral resolution, linking the spectral and spatial resolutions. Since in practical applications the spatial resolution may be dictated by the target anatomy, nomograms to determine the B(0) required to achieve the desired spectral resolution at that voxel size are presented. More generally, the scaling of the nomograms to determine the achievable spectral and spatial resolutions at any given field is described.

On the influence of the parameters of a massive ferromagnetic body on the characteristics of its magnetic field

The magnetic field parameters of a ferromagnetic body are investigated. It is shown that, under the conditions of magnetostatics, it is only possible to obtain a magnetic field if a magnetized medium exists; moreover, the magnetic field vector can be determined as the gradient of the scalar magnetic potential. A formula describing the dependence of magnetic field strength in a magnetized body on its geometric parameters and the magnetization value is derived with the help of the Gauss-Ostrogradskii theorem and vector analysis formulas.

Thermal evolution of rotating hybrid stars

As a neutron star spins down, the nuclear matter is continuously converted into quark matter due to the core density increase, and then latent heat is released. We have investigated the thermal evolution of neutron stars undergoing such deconfinement phase transition. We have taken into account the conversion in the frame of the general theory of relativity. The released energy has been estimated as a function of changed rate of deconfinement baryon number. The numerical solutions to the cooling equation are seen to be very different from those without the heating effect. The results show that neutron stars may be heated to higher temperatures which is well matched with pulsar's data despite the onset of fast cooling in neutron stars with quark matter cores. It is also found that the heating effect has a magnetic field strength dependence. This feature could be particularly interesting for high temperatures of low-field millisecond pulsars at a later stage. The high temperature could fit the observed temperature for PSR J0437-4715.

The correlation between magnetic pressure and density in compressible MHD turbulence

We study, both analytically and numerically, the behavior of magnetic pressure and density fluctuations in turbulent isothermal magnetohydrodynamic (MHD) flows in a slab geometry. We first consider “simple” MHD waves, which are the nonlinear analogue of the slow, fast and Alfvén linear waves, and show that the dependence of magnetic field strength B on density ρ in a simple wave depends on the mode which is considered: for the slow mode, , while for the fast mode, . We also perform a perturbative analysis about a circularly-polarized plane Alfvén wave to investigate Alfvén wave pressure, recovering the results of McKee and Zweibel that , with at large Ma, at moderate Ma and long wavelengths, and at low Ma. This wide variety of behaviors implies that a single polytropic description of magnetic pressure is not possible in general, but instead depends on which mode dominates the density fluctuation production. This in turn depends on the angle θ between the magnetic field and the direction of wave propagation and on the Alfvénic Mach number Ma. Typically, at small Ma, the slow mode dominates, and B is anticorrelated with ρ . At large Ma, both modes contribute to density fluctuation production, and the magnetic pressure decorrelates from density, exhibiting a large scatter, which however decreases towards higher densities. In this case, magnetic “pressure” does not act as a restoring force, but rather as a random forcing. These results have implications for the probability density function (PDF) of mass density. The non-systematic behavior of the magnetic pressure causes the PDF to maintain the log-normal shape corresponding to non-magnetic isothermal turbulence, except in cases where the slow mode dominates, in which the PDF develops an excess at low densities because the magnetic “random forcing” becomes density-dependent. Our results are consistent with the low values and apparent lack of correlation between the magnetic field strength and density in surveys of the lower-density molecular gas, and also with the recorrelation apparently seen at higher densities, if the Alfvénic Mach number is relatively large there.

Time profile of solar energetic particles fit using a mean free path considering the radial dependence of both magnetic field strength and fluctuations

The radially dependent mean free path of solar energetic particles was calculated by considering the radially dependent magnetic field fluctuation, its correlation length and the variation of magnetic field strength along an Archimedean interplanetary magnetic field. A longer mean free path can be deduced as radial distance approaches the sun. That is, the mean free path at 0.1 AU is increased by 10 times for some typical values of radial dependence of magnetic field fluctuation (e.g., power index of radial dependence = −2), compared to the value at 1 AU. The focused pitch angle transport equation without adiabatic deceleration was solved for the obtained mean free path. The calculated time profiles are compared with observations of solar energetic particles. In some cases, very good matches to observed data for longer periods are obtained with shorter mean free paths than the original authors mentioned.

Radio frequency vortex dynamics in heavy ion irradiated (Bi–Pb) 2Sr 2Ca 2Cu 3O 10 superconducting platelets

We report measurements of vortex dynamics in heavy ion irradiated (Bi–Pb) 2Sr 2Ca 2Cu 3O 10 superconducting tapes in the high-temperature ( T⩾60 K) and low-field ( μ 0 H<0.3 T) regimes. The temperature, magnetic field strength and angular dependences of radio-frequency (RF) penetration depth λ(H, T) are measured and compared with the behavior of unirradiated samples. The pinning force constant, k p, and viscosity coefficient η are extracted from the field dependence of λ(H, T) . The angular dependence of the RF penetration depth in samples irradiated at the field equivalent fluence of 0.5 and 3.0 T is minimum when the external field is brought into alignment with the columnar defects (CDs). While this characteristic feature of anisotropic pinning due to CDs disappear in a temperature regime where J c is high, in the vortex liquid state closer to T c, this feature grows with the increasing field strength. The overall changes in η, k p and the angular dependence of λ(H, T) are discussed in the framework of the Bose glass theory for vortex dynamics in highly layered superconductors.

Effects of Ionic Strength on the Supramolecular Structure in Liquid Crystalline Solutions of Persistent Length DNA Fragments

Cholesteric liquid crystals of persistence length DNA fragments (contour length 55 nm) were studied with small angle neutron scattering. The cholesteric axis was oriented either perpendicular or parallel to the incoming neutron beam with the help of a magnetic field in the range 0.8-1.65 T. A single diffraction peak is observed, which shows in the perpendicular configuration an angular anisotropy in intensity. A moderate magnetic field strength dependence of the anisotropy has been observed, but at the higher field strength the liquid crystal is nearly completely aligned. A decrease in peak intensity as well as anisotropy is observed with increasing ionic strength, but futed DNA concentration. This indicates a decrease in position and orientation order with increased screening of electrostatic interactions. From a comparison of the anisotropic data to the scattering contribution of a single fragment, the standard deviation of the presumed Gaussian orientation distribution could be derived. The results compare favorably with the second virial theory of lyotropic liquid crystals, provided that electrostatic interactions in terms of an ionic strength dependent effective diameter and DNA flexibility are taken into account.

Effect of The Orientation of The Magnetic Field on The Giant Magnetoresistance of Fe/Cr Superlattices

ABSTRACTWe study the magnetoresistance of [Fe/Cr]30/MgO superlattices grown by molecular beam epitaxy at a various magnetic field directions. The theory of the orientation dependence of the effect is developed. It is shown that the magnetic field strength dependence of magnetoresistance can be calculated for arbitrary orientation of magnetic field if this dependence is known for in-plane and perpendicular-to-plane magnetic fields. It is noted that the magnetization curve can be obtained by making use of the results of the magnetoresistance measurements.

Manganese-deoxyribonucleic acid binding modes. Nuclear magnetic relaxation dispersion results

Ion-DNA interactions are discussed and the applied magnetic field strength dependence of water proton spin-lattice relaxation rates is used to study the Mn(II)-DNA interaction both qualitatively and quantitatively. Associations in which the manganese II (Mn(II)) ion is completely immobilized on the DNA are identified as well as a range of associations in which the ion is only partially reorientationally restricted. Quantitative analysis of the strength of the association in which manganese is immobilized is carried out both with and without a counter-ion condensation correction for electrostatic attraction of the mobile ions. From competition experiments with manganese the relative strengths of the interactions of magnesium and calcium with DNA are found to be identical but less than that of manganese with DNA and the affinity of lithium for DNA is found to be slightly higher than that of sodium. The data demonstrate that the reduced mobility of nonsite-bound ions may have a significant effect on DNA-ion binding analyses performed using magnetic resonance and relaxation methods.

The contribution of cyclotron heating induced spatial modulation of electron magnetization to tokamak current drive

Application to a tokamak of electron-cyclotron resonant heating (ECRH) injected at a given poloidal angle will result in a small but important variation of average electron magnetic moment with poloidal angle. This, coupled with the dependence of magnetic field strength on poloidal angle, will result in the transmission of a net toroidal force to the electrons, the direction and magnitude of which depend on the poloidal angle at which the ECRH is injected and the direction of velocity of the electrons by which it is absorbed. The geometries for which this mechanism adds to the Fisch–Boozer mechanism [Phys. Rev. Lett. 45, 720 (1980)] are described, as are those for which this mechanism is in opposition. Taking this mechanism into account, it is demonstrated that electrons at large minor radii can be driven with significantly greater efficiency if resonance is on the inboard side of the tokamak than if it is on the outboard side, and the ratio of the two efficiencies is calculated for the case of Coulomb collisionality in the limit of large Larmor radius. The case of turbulence-dominated collisionality is considered, and a general prediction of the ratio of the efficiency of this drive mechanism to that of the Fisch–Boozer mechanism is presented, in which the ratio of the efficiencies is given in terms of the dependence of the slowing-down time on velocity. The mechanism described herein provides a significant correction to predictions of current drive efficiency for infinite Larmor radius Coulomb collisionality, and is likely to provide a more significant correction yet for more realistic models of collisionality.

Potassium-39 and sodium-23 NMR studies of cation binding to phosvitin.

The binding of Na+ and K+ to hen egg-yolk phosvitin has been studied by 23Na and 39K NMR. A transition in the binding behaviour of these cations is shown to accompany deprotonation of the phosphate groups on this highly charged protein. At lower pH the data are well described by a purely mass-action binding model, whereas at higher pH significant polyelectrolyte effects are apparent. From single ion as well as competition experiments K+ is shown to bind more strongly to phosvitin than does Na+. The temperature and magnetic field strength dependences of the 23Na and 39K relaxation rates provide a picture of phosvitin as a highly flexible macromolecule. This work demonstrates the potential of 39K NMR as a useful tool in the study of protein-cation interactions.

The relation between magnetic field and gas density in interstellar clouds

view Abstract Citations (8) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The relation between magnetic field and gas density in interstellar clouds Brown, R. L. ; Chang, C. -A. Abstract On the basis of 21 cm H I observations and 327 MHz continuum observations of the galactic background, the dependence of magnetic field strength on gas density in interstellar clouds has been investigated. The results indicate that interstellar clouds do not usually condense via isotropic collapse with a frozen-in field. Rather, they collapse by means of anisotropic flattening which is expected to occur along field lines. This result agrees with that expected from theoretical equilibrium models of interstellar clouds (Mouschovias, 1976). Publication: The Astrophysical Journal Pub Date: January 1983 DOI: 10.1086/160580 Bibcode: 1983ApJ...264..134B Keywords: Gas Density; Hydrogen Clouds; Interstellar Gas; Interstellar Magnetic Fields; Nebulae; Radio Sources (Astronomy); Centimeter Waves; Magnetic Flux; Magnetohydrodynamics; Astrophysics full text sources ADS |

NMR relaxation in DNA. I. The contribution of torsional deformation modes of the elastic filament.

AbstractThe pertinent correlation function for nmr dipolar relaxation of 31P by its neighboring protons in DNA is derived for a comparatively realistic model of the internal Brownian motions. These motions include the collective torsional deformation modes of the elastic filament, uncoupled local overdamped reorienting motions of the P‐H vectors in harmonic potential wells within the nucleotide unit, and the compartively slow end‐over‐end rotations of the local helix axis. These latter slow axial tumbling motions essentially completely determine T2 but have virtually no effect on T1 or the nuclear Overhauser effect (NOE), which are governed almost exclusively by rapid torsional deformations and local reorientations on the nanosecond time scale. The essential behavior of the relevant correlation function for the collective torsional motions has recently been determined experimentally in this laboratory using the decay of fluorescence polarization anisotropy of bound ethidium dye [J. C. Thomas et al. (1980) Biophys. Chem. 12, 177–180]. By using that result to carry out nmr relaxation calculations for various amplitudes and time constants of the uncoupled local motions and comparing them with the experimental data, it can be demonstrated that (within this model of purely dipolar relaxation), only rather small rms amplitudes of local reorientations (&lt;7°) occur and that their relaxation times are near 1 ns. Contrary to previous conclusions in the literature, the collective torsional deformation modes actually make the dominant contribution to T1 and NOE. At t = 1 ns the total rms azimuthal displacement of the P‐H vector in this model is 19.5°, which results from a superposition of torsional deformations with rms displacement 18.2° and uncoupled local motions with rms displacement 7°. The contribution of pure chemical shift anisotropy (CSA) to the 1/T1 relaxation rate is calculated for the first time for the case when torsional deformation modes predominate, and it is predicted to be 46% of the corresponding dipolar relaxation rate or 31% of the total relaxation rate. Unusual magnetic field strength dependence of the pure CSA and dipolar contributions is predicted to arise as a consequence of the collective torsional deformation modes. This seriously weakens empirical arguments in favor of a small (&lt;10%) CSA contribution. In any case, a detailed interpretation of T1 and NOE incorporating both dipolar and CSA relaxation must await the evaluation of the CSA:dipolar interference term, or crossterm, contribution to the relaxation rate. The contribution of pure CSA to 1/T2 relaxation is likewise calculated for the case when local reorienting motions are negligible; it is found to be ≲16% of the corresponding dipolar relaxation rate for the comparatively short (300–600‐base‐pair) DNA fragments of interest. For high‐molecular‐weight DNAs we predict that the slow Rouse‐Zimm coil‐deformation modes will dominate 1/T2 relaxation and the linewidth. Dynamic light‐scattering and other evidence is presented that the remarkable loss of nmr signal from DNA on addition of ethidium bromide, as reported by Hogan and Jardetzky, is actually a consequence of phase separation in such concentrated solutions. A pronounced decrease in T2, due to greatly hindered axial tumbling in the more concentrated phase, is advanced as a plausible line‐broadening explanation for the apparent loss of nmr signal from DNA in that phase.

Galvanomagnetic Properties of n-Type InSb at Low Temperatures II. Magnetic Field-Induced Metal-Nonmetal Transition

Experimental evidence for the magnetic field-induced metal-nonmetal transition in n–InSb has been obtained, under the magnetic fields up to 13.3 kOe, by systematically investigating the Hall coefficient and the transverse magnetoresistivity for the samples containing 2×10 14 –4×10 15 cm -3 of donors with nearly fixed compensation ratio K ∼0.5 at liquid helium temperatures. The donor concentration dependence of the critical magnetic field strength at the transition satisfactorily agrees with the theoretical prediction on the impurity band. The observed minimum metallic conductivity proves practically insensitive to magnetic field and increases with increasing density of localized centers, in accord with Mott's concept of the metal-nonmetal transition in doped semiconductors.