Changes In Magnetic Field Strength Research Articles

Evaluating brain volume segmentation accuracy and reliability of FreeSurfer and Neurophet AQUA at variations in MRI magnetic field strengths.

We aimed to compare the accuracy and reliability of two segmentation tools for magnetic resonance (MR) volumetry (FreeSurfer and Neurophet AQUA) at two magnetic field strengths (1.5T and 3T). We included 101 patients for the 1.5T-3T dataset and 112 for the 3T-3T dataset from three hospitals and five open-source datasets. The mean volume difference and average volume difference percentage with the change in magnetic field strength were compared between the methods. The hippocampus volume was larger with FreeSurfer than the Neurophet AQUA. In most brain regions, the Neurophet AQUA yielded a smaller average volume difference percentage (all < 10%) than FreeSurfer (all > 10%). The Neurophet AQUA exhibited more stable connectivity and regularity of the segmented components. Regarding volume, the Neurophet AQUA had effect sizes and ICCs comparable to those of FreeSurfer across the magnetic field strengths. With FreeSurfer, the original volume difference was small, whereas the average volume difference percentage was small with the Neurophet AQUA. Image segmentation took 1h with FreeSurfer and 5min with the Neurophet AQUA. When choosing an automatic segmentation method, the differences in image processing time and volume variability due to changes in the magnetic field strength of these methods should be considered.

Distributed magnetic field sensor using nickel-acrylate composite coated silica optical fiber

This paper reports on the development and characterization of a distributed magnetic field sensor utilizing magnetostrictive material and the Rayleigh backscattered signal in optical fiber. The sensor consists of a single-mode fiber (SMF) coated with nickel-acrylate (Ni-Acryl) composite, fabricated using the conventional fiber drawing method. The surface morphology and the coating thickness of the Ni-Acryl were analysed using SEM. The sensor is capable of measuring variations in the strain of the fiber as the external magnetic field strength changes, with a spatial resolution of 20 cm. The experimental results revealed a linear relationship between magnetic field strength and Rayleigh frequency shift, with an average sensitivity of 22.85 MHz/mT. The magnetostrictive strain of the Ni-Acryl composite coated fiber is measured to be 36.4 με in response to the 245 mT applied magnetic field. This study highlights the potential of the developed sensor to be utilized in magnetic field sensing applications, particularly for remote sensing applications in a harsh environment.

A numerical simulation method for solving electromagnetic-mechanical coupling field

A new numerical simulation method based on the finite volume method has been proposed to address the challenges associated with the pseudo-oscillation phenomenon that occurs with an increased Péclet number, as well as determining an appropriate time step for electromagnetic-mechanical coupled physical field calculations. This method utilizes the concept of upwind from Computational Fluid Dynamics (CFD) to solve Maxwell's equations, a finite-volume method for discretizing the electromagnetic diffusion equations. Through computing typical electromagnetic field problems such as TEAM problem 9–1 and comparing the results to the literature, it has been confirmed that this method has sufficient computational accuracy to solve electromagnetic field numerical simulation problems containing moving conductors. Additionally, a three-dimensional simulation model of the electromagnetic coil gun has been established to verify the viability of the proposed method for solving magnetic field problems under the electromagnetic launch (EML) system. The study also examined the change in magnetic field strength and current density of the EML device during the launching process. The results lay the foundation for the technical application of this method.

Solar Tachocline Confinement by the Nonaxisymmetric Modes of a Dynamo Magnetic Field

We recently presented the first 3D numerical simulation of the solar interior for which tachocline confinement was achieved by a dynamo-generated magnetic field. In this follow-up study, we analyze the degree of confinement as the magnetic field strength changes (controlled by varying the magnetic Prandtl number) in a coupled radiative zone (RZ) and convection zone (CZ) system. We broadly find three solution regimes, corresponding to weak, medium, and strong dynamo magnetic field strengths. In the weak-field regime, the large-scale magnetic field is mostly axisymmetric with regular, periodic polarity reversals (reminiscent of the observed solar cycle) but fails to create a confined tachocline. In the strong-field regime, the large-scale field is mostly nonaxisymmetric with irregular, quasi-periodic polarity reversals and creates a confined tachocline. In the medium-field regime, the large-scale field resembles a strong-field dynamo for extended intervals but intermittently weakens to allow temporary epochs of strong differential rotation. In all regimes, the amplitude of poloidal field strength in the RZ is very well explained by skin-depth arguments, wherein the oscillating field that gives rise to the skin depth (in the medium- and strong-field cases) is a nonaxisymmetric field structure at the base of the CZ that rotates with respect to the RZ. These simulations suggest a new picture of solar tachocline confinement by the dynamo, in which nonaxisymmetric, very long-lived (effectively permanent) field structures rotating with respect to the RZ play the primary role, instead of the regularly reversing axisymmetric field associated with the 22 yr cycle.

Thermodynamic analysis on a magnetic refrigeration system

Magnetic refrigeration utilizes the magnetocaloric effect of a magnetic material, whose temperature changes according to the change of magnetic field strength. It is regarded as an eco-friendly refrigeration technology in that it uses magnetic materials as refrigerants instead of CFC, HCFC, and HFC refrigerants used in vapor compression refrigeration. It is also regarded as an energy-efficient refrigeration technology in that it does not use noisy and power-consuming compressors. This paper presents thermodynamic analysis on a magnetic refrigeration system using experimental results obtained from a magnetic refrigeration apparatus. The magnetic refrigeration apparatus was built using two sets of concentric Halbach cylinders consisting of permanent magnet segments. Specifically the coefficient of performance (COP) of the magnetic refrigeration system was calculated using the energy removed from the working fluid across an AMR bed and the work input to run electric motors.

Using Metal Magnetic Memory to Evaluate the Effect of Welding Method and Weld Temperature on Magnetic Field Strength in Structural Steel.

Tests of welds are carried out inter-operatively, during the execution of the steel structure, as well as after the structure is welded, but even before its assembly. Steel structures already in service are also examined to detect potential cracks, delamination, or corrosion loss of thickness having the effect of weakening the mechanical strength of the structure. Such examinations are some the elements that comprise a structural health assessment. In this paper, the metal memory method was used to evaluate the effect of the welding method and weld temperature on the change in magnetic field strength. S235JR structural steel was used for the study, which was subjected to milling and MMA, TIG, and MIG welding. The results of measurement experiments carried out for each welding method are presented as graphs.

Evaluation and comparison of most prevalent artifact reduction methods for EEG acquired simultaneously with fMRI.

Multimodal neuroimaging using EEG and fMRI provides deeper insights into brain function by improving the spatial and temporal resolution of the acquired data. However, simultaneous EEG-fMRI inevitably compromises the quality of the EEG and fMRI signals due to the high degree of interaction between the two systems. Fluctuations in the magnetic flux flowing through the participant and the EEG system, whether due to movement within the magnetic field of the scanner or to changes in magnetic field strength, induce electrical potentials in the EEG recordings that mask the much weaker electrical activity of the neuronal populations. A number of different methods have been proposed to reduce MR artifacts. We present an overview of the most commonly used methods and an evaluation of the methods using three sets of diverse EEG data. We limited the evaluation to open-access and easy-to-use methods and a reference signal regression method using a set of six carbon-wire loops (CWL), which allowed evaluation of their added value. The evaluation was performed by comparing EEG signals recorded outside the MRI scanner with artifact-corrected EEG signals recorded simultaneously with fMRI. To quantify and evaluate the quality of artifact reduction methods in terms of the spectral content of the signal, we analyzed changes in oscillatory activity during a resting-state and a finger tapping motor task. The quality of artifact reduction in the time domain was assessed using data collected during a visual stimulation task. In the study we utilized hierarchical Bayesian probabilistic modeling for statistical inference and observed significant differences between the evaluated methods in the success of artifact reduction and associated signal quality in both the frequency and time domains. In particular, the CWL system proved superior to the other methods evaluated in improving spectral contrast in the alpha and beta bands and in recovering visual evoked responses. Based on the results of the evaluation study, we proposed guidelines for selecting the optimal method for MR artifact reduction.

Signal processing method of bubble detection in sodium flow based on inverse Fourier transform to calculate energy ratio

Electromagnetic vortex flowmeter is a new type of instrument for detecting leakage of steam generator, and the signal processing method based on the envelope to calculate energy ratio can effectively detect bubbles in sodium flow. The signal processing method is not affected by changes in the amplitude of the sensor output signal, which is caused by changes in magnetic field strength and other factors. However, the detection sensitivity of the electromagnetic vortex flowmeter is reduced. To this end, a signal processing method based on inverse Fourier transform to calculate energy ratio is proposed. According to the difference between the frequency band of the bubble noise signal and the flow signal, only the amplitude in the frequency band of the flow signal is retained in the frequency domain, and then the flow signal is obtained by the inverse Fourier transform method, thereby calculating the energy ratio. Using this method to process the experimental data, the results show that it can detect 0.1 g/s leak rate of water in the steam generator, and its performance is significantly better than that of the signal processing method based on the envelope to calculate energy ratio.

Application of the mixing of the field of low amplitude AC bias with a strong field of linear slow varying to investigate the magnetic properties of materials

The paper presents a new experimental method of the magnetic hysteresis loop investigation and the constructed measurement instrument. The principle of operation is to mix two magnetic fields: the first of these fields has a very low amplitude, a shape of sine wave (AC bias) and the frequency range order of 10–104 Hz. The second field has a very low frequency (order 10−2 Hz), a high amplitude and forms approximately a triangle wave. The presented, dedicated system is different in comparison to commonly used hystographs and it consists of coreless coils only. The measurement method gives the opportunity of obtaining more detailed measurements of the initial cycles in the material magnetizing process. In a special case, the magnetic field strength changes in the B(H) curve can be stopped at any operating point and then it is possible to investigate local properties using AC bias field for various frequencies. The results revealed a successive displacement of the hysteresis loop and approaching to the steady state position.

Polarization control of THz emission using spin-reorientation transition in spintronic heterostructure

Polarization of electromagnetic waves plays an extremely important role in interaction of radiation with matter. In particular, interaction of polarized waves with ordered matter strongly depends on orientation and symmetry of vibrations of chemical bonds in crystals. In quantum technologies, the polarization of photons is considered as a “degree of freedom”, which is one of the main parameters that ensure efficient quantum computing. However, even for visible light, polarization control is in most cases separated from light emission. In this paper, we report on a new type of polarization control, implemented directly in a spintronic terahertz emitter. The principle of control, realized by a weak magnetic field at room temperature, is based on a spin-reorientation transition (SRT) in an intermetallic heterostructure TbCo2/FeCo with uniaxial in-plane magnetic anisotropy. SRT is implemented under magnetic field of variable strength but of a fixed direction, orthogonal to the easy magnetization axis. Variation of the magnetic field strength in the angular (canted) phase of the SRT causes magnetization rotation without changing its magnitude. The charge current excited by the spin-to-charge conversion is orthogonal to the magnetization. As a result, THz polarization rotates synchronously with magnetization when magnetic field strength changes. Importantly, the radiation intensity does not change in this case. Control of polarization by SRT is applicable regardless of the spintronic mechanism of the THz emission, provided that the polarization direction is determined by the magnetic moment orientation. The results obtained open the prospect for the development of the SRT approach for THz emission control.

Evolving solar wind flow properties of magnetic inversions observed by Helios

ABSTRACT In its first encounter at solar distances as close as r = 0.16 au, Parker Solar Probe observed numerous local reversals, or inversions, in the heliospheric magnetic field (HMF), which were accompanied by large spikes in solar wind speed. Both solar and in situ mechanisms have been suggested to explain the existence of HMF inversions in general. Previous work using Helios 1, covering 0.3–1 au, observed inverted HMF to become more common with increasing r, suggesting that some heliospheric driving process creates or amplifies inversions. This study expands upon these findings, by analysing inversion-associated changes in plasma properties for the same large data set, facilitated by observations of ‘strahl’ electrons to identify the unperturbed magnetic polarity. We find that many inversions exhibit anticorrelated field and velocity perturbations, and are thus characteristically Alfvénic, but many also depart strongly from this relationship over an apparent continuum of properties. Inversions depart further from the ‘ideal’ Alfvénic case with increasing r, as more energy is partitioned in the field, rather than the plasma, component of the perturbation. This departure is greatest for inversions with larger density and magnetic field strength changes, and characteristic slow solar wind properties. We find no evidence that inversions that stray further from ‘ideal’ Alfvénicity have different generation processes from those which are more Alfvénic. Instead, different inversion properties could be imprinted based on transport or formation within different solar wind streams.

Studying the Effect of Magnetic Strength and Electrical Conductivity on the Velocity Profile of an MHD Flow in a Rectangular Duct

This document presents the results of an investigation of the impact of variations of electrical conductivity and magnetic field strength on the velocity profile inside a rectangular duct, assuming equilibrium flow. It also presents the physics and the complete theory associated with the MHD, then derive the equations to be utilized for computational analysis of the magnetic field and the velocity profiles.The computational work in this document is based on the assumption that the working fluid is air and it is fully developed in the duct. The finite difference method is used to perform the analysis for 2 distinct cases: the first case is when the magnetic field strength changes, while holding the electrical conductivity constant. The second case is when the electrical conductivity is changing while holding the magnetic field strength constant.It has been found that increasing the magnetic field strength has resulted in decreasing the axial velocity profile, which demonstrated that the magnetic field was pulling the flow to its direction, reducing the flow momentum in the axial direction. It has also been found that the electrical conductivity reduced the velocity profile in the axial direction when that conductivity was increasing, demonstrating that the electric field was also pulling the flow in its direction, reducing the flow momentum in the axial direction. Keywords: MHD, Magnetohydrodynamics, Flow in Rectangular Duct, Magnetic Field, Electrical Conductivity, MHD Flow, MHD Flow Velocity Profile DOI: 10.7176/JETP/10-4-03 Publication date: August 31 st 2020

Global‐Scale Observations and Modeling of Far‐Ultraviolet Airglow During Twilight

AbstractThe NASA Global‐scale Observations of the Limb and Disk ultraviolet imaging spectrograph performs observations of upper atmosphere airglow from the sunlit disk and limb of the Earth, in order to infer quantities such as the composition and temperature of the thermosphere. To interpret the measurements, the observational and solar illumination geometry must be considered. We use forward models of upper atmosphere density and composition, photoelectron impact, airglow emissions, radiative transfer, and line‐of‐sight integration, to describe the expected observations, and here test those calculations against observations near the terminator, and near the limb. On the nightside of the terminator, broad regions of faint airglow are seen, particularly near the winter solstice. These are caused by photoelectrons generated in magnetically conjugate areas in the other hemisphere that are still illuminated, transported along field lines, and then precipitated back into the atmosphere. Model calculations demonstrate that this process is the source of the emission, and obtain good agreement with its morphology and intensity. In some regions, the observed emissions are not as bright as the model simulations. Some of the reduction in electron flux is explained by changes in magnetic field strength; in other cases, particularly at high magnetic latitude, the cause is unknown, but must occur along extended field lines that reach into the plasma sheet.

MAVEN and MEX Multi‐instrument Study of the Dayside of the Martian Induced Magnetospheric Structure Revealed by Pressure Analyses

AbstractA combination of statistical studies and 18 case studies have been used to investigate the structure of the induced Martian magnetosphere. The different plasma and magnetic pressure forces on the dayside of the induced magnetosphere of Mars have been studied using 3.5 years of Mars Atmosphere and Volatile Evolution (MAVEN) and Mars Express (MEX) observations. We present estimates of typical values for the dominant pressure terms, that is, the thermal pressures of the ionosphere and the magnetosheath, the magnetic pressure of the magnetic pile‐up region, and the solar wind dynamic pressure. For 18 typical orbits the altitudes and relative distances of the pressure balance boundaries, the photoelectron boundary, the ion composition boundary, and the induced magnetosphere boundary are estimated. The magnetic pile‐up boundary is discussed but not further studied since earlier characterizations of the magnetic pile‐up boundary do not agree with our results. This study focuses on the transition region between the ionosphere and the magnetosheath on the dayside of Mars. We show that earlier definitions of the photoelectron boundary, ion composition boundary, and induced magnetosphere boundary do not characterize the transition region well, mainly because each boundary is based on measurements from only one or two instruments. In order to characterize the transition region correctly, changes in magnetic field strength and fluctuations, dominant ion species, electron and ion densities and energy distributions need to be considered. This article confirms a complex interaction between Mars and the solar wind and can explain why previous studies have had difficulties to describe the force balance.

The very slow rotation of the magnetic O9.7 V star HD 54879

ABSTRACT The first FOcal Reducer low dispersion Spectrograph (FORS 2) spectropolarimetric observation of the longitudinal magnetic field of HD 54879 of the order of −600 G with a lower limit of the dipole strength of ∼2 kG dates back to 2014. Since then, observations showed a gradual decrease of the absolute value of the mean longitudinal magnetic field. In the course of the most recent monitoring of HD 54879 using FORS 2 spectropolarimetric observations from 2017 October to 2018 February, a longitudinal magnetic field strength change from about −300 G down to about −90 G was reported. A sudden increase of the absolute value of the mean longitudinal magnetic field and an accompanying spectral variability was detected on 2018 February 17. New FORS 2 spectropolarimetric data obtained from 2018 December to 2019 February confirm the very slow magnetic field variability, with the field decreasing from about 150 G to −100 G over 2 months. Such a slow magnetic field variability, related to the extremely slow rotation of HD 54879, is also confirmed using high-resolution High Accuracy Radial velocity Planet Searcher in polarimetric mode and Echelle SpectroPolarimetric Device for the Observation of Stars spectropolarimetry. The re-analysis of the FORS 2 polarimetric spectra from 2018 February indicates that the previously reported field increase and the change of the spectral appearance was caused by improper spectra extraction and wavelength calibration using observations obtained at an insufficient signal-to-noise ratio. The presented properties of HD 54879 are discussed in the context of the Of?p spectral classification.

Comparison between Magnetic Field Distribution Analysis and Metal Magnetic Memory (MMM) Testing Results Around Artificial Cracks under Loads

In this research, the behavior of the metal magnetic memory (MMM) signals near the artificial cracks located the back side of A572 specimens are tried. The materials are used as power transmission tower material. The variations of the MMM signals with the applied tensile loads are studied. It is found that both the tangential component, Hp(x), and the normal component, Hp(y), are effectively showed the stress concentration zone (SCZ) caused by discontinuities. The results of modeling and MMM technique showed that the loads create residual stress and lead to the change of magnetic field strength around artificial cracks in the specimen and that is relatively different from the crack free area. This research is useful for expending MMM method as a tool of nondestructive testing method for detecting subsurface flaws.

Transmission characteristics of linearly polarized light in reflection-type one-dimensional magnetophotonic crystals

The propagation properties of linearly polarized light in reflection-type one-dimensional magnetophotonic crystals are studied by using the 4 × 4 transmission matrix method. The structure models of reflection-type one-dimensional magnetophotonic crystals are designed, the magnetic field direction control characteristics of reflection spectrum and Kerr rotation angle are discussed, and the effect of applied magnetic field direction and strength on reflection spectrum and Kerr rotation angle are analyzed. The results show that the non-diagonal elements in the dielectric constant of magneto optical materials change when the angle φ between applied magnetic field and optical path changes, the reflectivity and Kerr rotation angle decrease when the angle φ increases; when the applied magnetic field strength changes, the reflectivity and Kerr rotation angle increase when the applied magnetic field strength increases; by adjusting the angle φ and strength of the applied magnetic field, the rotation angle of Kerr can be adjusted to 45°, and a more flat reflection spectrum can be obtained by designing the appropriate structure.

Suppression Method of Jump Points for Multipole Magnetic Encoder at Low Temperature Based on Single-Pole Angle Value Fitting

To eliminate the jump points of multipole angle values after subdivision at low temperature, the magnetic field and temperature field characteristics of a multipole magnetic encoder are analyzed in this study, and the effect of changes in magnetic field strength and temperature field on the precision of angle values is studied. To eliminate the jump point of multipole angle values caused by changes in the temperature field, the suppression method based on single-pole angle value fitting is proposed. The error between the single-pole and multipole angle values is tabulated by the oversampling linear interpolation method, and the precision of fitting single-pole to multipole angle values is effectively improved. The error of the angle value caused by changes in the temperature field is studied and analyzed, and the relationship between the jump angle values and the pole number of the multipole magnetic encoder is obtained. Furthermore, the jump point is compensated for by the jump range of the multipole angle values. Finally, the angle accuracy of the multipole magnetic encoder in a cryogenic chamber is experimentally verified. The experimental results show that the low-temperature jump point compensation method proposed for the multipole magnetic encoder in this paper can effectively suppress the jump of the angle values.

Magnetic field analysis in a diamond anvil cell for Meissner effect measurement by using the diamond NV– center**Project supported by the National Key R&D Program of China (Grant No. 2018YFA0305900) and the National Natural Science Foundation of China (Grant Nos. 11774126, 11674404, and 51772125).

Diamond negatively charged nitrogen-vacancy (NV–) centers provide an opportunity for the measurement of the Meissner effect on extremely small samples in a diamond anvil cell (DAC) due to their high sensitivity in detecting the tiny change of magnetic field. We report on the variation of magnetic field distribution in a DAC as a sample transforms from normal to superconducting state by using finite element analysis. The results show that the magnetic flux density has the largest change on the sidewall of the sample, where NV– centers can detect the strongest signal variation of the magnetic field. In addition, we study the effect of magnetic coil placement on the magnetic field variation. It is found that the optimal position for the coil to generate the greatest change in magnetic field strength is at the place as close to the sample as possible.

Magneto-immutable turbulence in weakly collisional plasmas.

We propose that pressure anisotropy causes weakly collisional turbulent plasmas to self-organize so as to resist changes in magnetic-field strength. We term this effect "magneto-immutability" by analogy with incompressibility (resistance to changes in pressure). The effect is important when the pressure anisotropy becomes comparable to the magnetic pressure, suggesting that in collisionless, weakly magnetized (high-β) plasmas its dynamical relevance is similar to that of incompressibility. Simulations of magnetized turbulence using the weakly collisional Braginskii model show that magneto-immutable turbulence is surprisingly similar, in most statistical measures, to critically balanced MHD turbulence. However, in order to minimize magnetic-field variation, the flow direction becomes more constrained than in MHD, and the turbulence is more strongly dominated by magnetic energy (a nonzero "residual energy"). These effects represent key differences between pressure-anisotropic and fluid turbulence, and should be observable in the β ≳ 1 turbulent solar wind.