scholarly journals Quantitative Susceptibility Mapping of Magnetic Quadrupole Moments

2019 ◽  
Vol 2019 ◽  
pp. 1-14
Author(s):  
Junghun Cho ◽  
Dong Zhou ◽  
Youngwook Kee ◽  
Pascal Spincemaille ◽  
Yi Wang
Keyword(s):  
Magnetic Field ◽  
Numerical Simulations ◽  
Susceptibility Map ◽  
Tensor Model ◽  
Quadrupole Moments ◽  
Quadrupole Field ◽  
Susceptibility Maps ◽  
Microstructure Effects ◽  
Quadrupole Term ◽  
The Magnetic Field

We modeled the magnetic field up to the quadrupole term to investigate not only the average susceptibility (dipole), but also the susceptibility distribution (quadrupole) contribution. Expanding the magnetic field up to the 2nd order provides the quadrupole (0th: monopole, 1st: dipole). Numerical simulations were performed to investigate the quadrupole contribution with subvoxel nonuniformity. Conventional dipole and our dipole + quadrupole models were compared in the simulation, the phantom and human brain. Furthermore, the quadrupole field was compared with the anisotropic susceptibility field in the dipole tensor model. In a nonuniformity case, numerical simulations showed a nonnegligible quadrupole field contribution. Our study showed a difference between the two methods in the susceptibility map at the edges; both the phantom and human studies showed sharper structural edges with the dipole + quadrupole model. Quadrupole moments showed contrast mainly at the structural boundaries. The quadrupole moment field contribution was smaller but nonnegligible compared to the anisotropic susceptibility contribution. Nonuniform and uniform source distributions can be separately considered by quadrupole expansion, which were mixed together in the dipole model. In the presence of nonuniformity, the susceptibility maps may be different between the two models. For a comprehensive field model, the quadrupole might need to be considered along with susceptibility anisotropy and microstructure effects.

Dynamics of a disc in a nematic liquid crystal

Soft Matter ◽  
2016 ◽  
Vol 12 (4) ◽  
pp. 1279-1294 ◽  
Author(s):  
Alena Antipova ◽  
Colin Denniston
Keyword(s):  
Magnetic Field ◽  
Liquid Crystal ◽  
Numerical Simulations ◽  
Nematic Liquid Crystal ◽  
Weak Magnetic Field ◽  
Angular Speed ◽  
The Magnetic Field

We explain the motion of a micron-sized ferromagnetic disc immersed in a nematic liquid crystal under the action of a weak magnetic field using numerical simulations. We show that the disc's behaviour can be controlled by the angular speed of the magnetic field and its magnitude.

Zum Rotations-Zeeman-Effekt in Dimethylketen / Molecular Zeeman Effect and the Determination of the Molecular g Values, Paramagnetic Susceptibilities, and Molecular Quadrupole Moments in Dimethylketene

1972 ◽  
Vol 27 (4) ◽  
pp. 597-600 ◽  
Author(s):  
D. Sutter ◽  
L. Charpentier ◽  
H. Dreizler
Keyword(s):  
Magnetic Field ◽  
Zeeman Effect ◽  
Microwave Spectrum ◽  
Quadrupole Moments ◽  
Selection Rules ◽  
Rotational Transitions ◽  
The Magnetic Field

Abstract The rotational Zeeman-Effect in the microwave spectrum of dimethylketene was investigated at fieldstrengths close to 22 kG. Only ΔJ= 1 rotational transitions with ΔM = ± 1 selection rules did show appreciable splittings due to the magnetic field. From the splittings the diagonal elements of the molecular gr-tensor were determined to be: gaa = ∓ 0.020(3) ; gbb = ∓ 0.0165(8) ; gcc= + 0.0126(5). (Only the relative signs of the g-values are obtained from the experiment). The susceptibility anisotropics were found to be close to zero.

Mechanisms for magnetic field reversals

2010 ◽  
Vol 368 (1916) ◽  
pp. 1595-1605 ◽  
Author(s):  
F. Pétrélis ◽  
S. Fauve
Keyword(s):  
Magnetic Field ◽  
Turbulent Flow ◽  
Numerical Simulations ◽  
Large Scale ◽  
Conducting Fluid ◽  
Similar Model ◽  
Electrically Conducting ◽  
Electrically Conducting Fluid ◽  
Simple Mechanism ◽  
The Magnetic Field

We present a review of the different models that have been proposed to explain reversals of the magnetic field generated by a turbulent flow of an electrically conducting fluid (fluid dynamos). We then describe a simple mechanism that explains several features observed in palaeomagnetic records of the Earth’s magnetic field, in numerical simulations and in a recent dynamo experiment. A similar model can also be used to understand reversals of large-scale flows that often develop on a turbulent background.

scholarly journals Dynamically dominant magnetic fields in the diffuse interstellar medium

2008 ◽  
Vol 4 (S259) ◽  
pp. 87-88 ◽  
Author(s):  
Andrew Fletcher ◽  
M. Korpi ◽  
A. Shukurov
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Magnetic Flux ◽  
Interstellar Medium ◽  
Numerical Simulations ◽  
Hydrodynamic Model ◽  
Mass Conservation ◽  
Unstable Flow ◽  
Gas Density ◽  
The Magnetic Field

AbstractObservations show that magnetic fields in the interstellar medium (ISM) often do not respond to increases in gas density as would be naively expected for a frozen-in field. This may suggest that the magnetic field in the diffuse gas becomes detached from dense clouds as they form. We have investigated this possibility using theoretical estimates, a simple magneto-hydrodynamic model of a flow without mass conservation and numerical simulations of a thermally unstable flow. Our results show that significant magnetic flux can be shed from dense clouds as they form in the diffuse ISM, leaving behind a magnetically dominated diffuse gas.

THE INFLUENCE OF SELECTED PARAMETERS ON MAGNETIC FLUID SEAL TIGHTNESS AND MOTION RESISTANCE

Tribologia ◽  
2017 ◽  
pp. 71-76 ◽  
Author(s):  
Marcin SZCZĘCH ◽  
Wojciech HORAK ◽  
Józef SALWIŃSKI
Keyword(s):  
Magnetic Field ◽  
Numerical Simulations ◽  
Field Distribution ◽  
Magnetic Fluid ◽  
High Speed ◽  
Critical Pressure ◽  
Motion Resistance ◽  
Seal Tightness ◽  
Vacuum Systems ◽  
The Magnetic Field

Magnetic fluid seals belong to the class of non-contact seals. They are used as protective seals for vacuum systems, high speed shafts, precision mechanics, and electromechanical devices. The proper functioning of the magnetic fluid seal is related to creating and maintaining the continuity of the fluid ring on the sealing stage. This is achieved by appropriately shaped magnetic field distribution in the region of the sealing stage. Consequently, one of the main issues with the construction of such seals is to determine the distribution of the magnetic field in this region. This paper presents the results of analytical calculations and numerical simulations, based on which the influence of selected geometric parameters on the critical pressure and motion resistance was determined.

scholarly journals Numerical determination of the cutoff frequency in solar models

2020 ◽  
Vol 640 ◽  
pp. A4 ◽  
Author(s):  
T. Felipe ◽  
C. R. Sangeetha
Keyword(s):  
Magnetic Field ◽  
Wave Propagation ◽  
Magnetic Fields ◽  
Numerical Simulations ◽  
Acoustic Waves ◽  
Cutoff Frequency ◽  
Quiet Sun ◽  
Radiative Losses ◽  
The Magnetic Field

Context. In stratified atmospheres, acoustic waves can only propagate if their frequency is higher than the cutoff value. The determination of the cutoff frequency is fundamental for several topics in solar physics, such as evaluating the contribution of the acoustic waves to the chromospheric heating or the application of seismic techniques. However, different theories provide different cutoff values. Aims. We developed an alternative method to derive the cutoff frequency in several standard solar models, including various quiet-Sun and umbral atmospheres. The effects of magnetic field and radiative losses on the cutoff are examined. Methods. We performed numerical simulations of wave propagation in the solar atmosphere using the code MANCHA. The cutoff frequency is determined from the inspection of phase-difference spectra computed between the velocity signal at two atmospheric heights. The process is performed by choosing pairs of heights across all the layers between the photosphere and the chromosphere to derive the vertical stratification of the cutoff in the solar models. Result. The cutoff frequency predicted by the theoretical calculations departs significantly from the measurements obtained from the numerical simulations. In quiet-Sun atmospheres, the cutoff shows a strong dependence on the magnetic field for adiabatic wave propagation. When radiative losses are taken into account, the cutoff frequency is greatly reduced and the variation of the cutoff with the strength of the magnetic field is lower. The effect of the radiative losses in the cutoff is necessary to understand recent quiet-Sun and sunspot observations. In the presence of inclined magnetic fields, our numerical calculations confirm that the cutoff frequency is reduced as a result of the reduced gravity experienced by waves that propagate along field lines. An additional reduction is also found in regions with significant changes in the temperature, which is due to the lower temperature gradient along the path of field-guided waves. Conclusions. Our results show solid evidence that the cutoff frequency in the solar atmosphere is stratified. The cutoff values are not correctly captured by theoretical estimates. In addition, most of the widely used analytical cutoff formulae neglect the effect of magnetic fields and radiative losses, whose role is critical for determining the evanescent or propagating nature of the waves.

RANDOM JOSEPHSON NETWORKS AND SPIN GLASSES

1987 ◽  
Vol 01 (01n02) ◽  
pp. 27-37 ◽  
Author(s):  
M.V. FEIGEL’MAN ◽  
L.B. IOFFE ◽  
A.I. LARKIN ◽  
V.M. VINOKUR
Keyword(s):  
Magnetic Field ◽  
Phase Transition ◽  
Numerical Simulations ◽  
Spin Glass ◽  
Spin Glasses ◽  
Equations Of State ◽  
Analytical Theory ◽  
Diamagnetic Response ◽  
The Magnetic Field

The phase transition into a spin glass-like state is predicted for the system of superconductive wires connected by Josephson links and placed into the magnetic field. History-dependent equations of state for T<Tc are derived and diamagnetic response to the variation of the magnetic field is predicted. The experiments that can solve the discrepancy between the analytical theory and the numerical simulations on the existence of the phase transition in the vector spin glasses are discussed.

scholarly journals Simulations of Merging Clusters of Galaxies

2002 ◽  
Vol 12 ◽  
pp. 531-533 ◽  
Author(s):  
Sabine Schindler
Keyword(s):  
Magnetic Field ◽  
Numerical Simulations ◽  
Shock Structure ◽  
Clusters Of Galaxies ◽  
Mass Determination ◽  
X Ray ◽  
Dynamical State ◽  
Merging Process ◽  
Cluster Mergers ◽  
The Magnetic Field

AbstractNumerical simulations of cluster mergers reveal many characteristics of the merging process: shock structure and strength, observational signatures of the dynamical state, effects on the mass determination, turbulence and the evolution of the X-ray luminosity and the magnetic field. In this article I review the results obtained from various simulations over the last years.

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