Wave Magnetic Field Research Articles

Method for direct detection of pitch angle scattering of energetic electrons caused by whistler mode chorus emissions

AbstractThe Wave‐Particle Interaction Analyzer (WPIA), a new instrument proposed by Fukuhara et al. (2009), measures the relative phase angle between the wave magnetic field vector and the velocity vector of each particle and calculates the energy exchange from waves to particles. In this study, we expand its applicability by proposing a method of using the WPIA to directly detect pitch angle scattering of resonant particles by plasma waves by calculating the g values. The g value is defined as the accumulation value of the Lorentz force acting on each particle and indicates the lost momentum of waves. We apply the proposed method to the results of a one‐dimensional electron hybrid simulation reproducing the generation of whistler mode chorus emissions around the magnetic equator. Using the wave and particle data obtained at fixed observation points assumed in the simulation system, we conduct a pseudo‐observation of the simulation result using the WPIA and analyze the g values. Our analysis yielded significant values indicating the strong pitch angle scattering for electrons in the kinetic energy and pitch angle ranges satisfying the cyclotron resonance condition with the reproduced chorus emissions. The results of this study demonstrate that the proposed method enables us to directly and quantitatively identify the location at which pitch angle scattering occurs in the simulation system and that the method can be applied to the results of space‐based observations by the forthcoming Exploration of energization and Radiation in Geospace (ERG) satellite.

Observation of an Alfvén Wave Parametric Instability in a Laboratory Plasma

A shear Alfvén wave parametric instability is observed for the first time in the laboratory. When a single finite ω/Ω_{i} kinetic Alfvén wave (KAW) is launched in the Large Plasma Device above a threshold amplitude, three daughter modes are produced. These daughter modes have frequencies and parallel wave numbers that are consistent with copropagating KAW sidebands and a low frequency nonresonant mode. The observed process is parametric in nature, with the frequency of the daughter modes varying as a function of pump wave amplitude. The daughter modes are spatially localized on a gradient of the pump wave magnetic field amplitude in the plane perpendicular to the background field, suggesting that perpendicular nonlinear forces (and therefore k_{⊥} of the pump wave) play an important role in the instability process. Despite this, modulational instability theory with k_{⊥}=0 has several features in common with the observed nonresonant mode and Alfvén wave sidebands.

Hydromagnetic waves in a compressed-dipole field via field-aligned Klein–Gordon equations

Abstract. Hydromagnetic waves, especially those of frequencies in the range of a few millihertz to a few hertz observed in the Earth's magnetosphere, are categorized as ultra low-frequency (ULF) waves or pulsations. They have been extensively studied due to their importance in the interaction with radiation belt particles and in probing the structures of the magnetosphere. We developed an approach to examining the toroidal standing Aflvén waves in a background magnetic field by recasting the wave equation into a Klein–Gordon (KG) form along individual field lines. The eigenvalue solutions to the system are characteristic of a propagation type when the corresponding eigenfrequency is greater than a critical frequency and a decaying type otherwise. We apply the approach to a compressed-dipole magnetic field model of the inner magnetosphere and obtain the spatial profiles of relevant parameters and the spatial wave forms of harmonic oscillations. We further extend the approach to poloidal-mode standing Alfvén waves along field lines. In particular, we present a quantitative comparison with a recent spacecraft observation of a poloidal standing Alfvén wave in the Earth's magnetosphere. Our analysis based on the KG equation yields consistent results which agree with the spacecraft measurements of the wave period and the amplitude ratio between the magnetic field and electric field perturbations.

Using the cold plasma dispersion relation and whistler mode waves to quantify the antenna sheath impedance of the Van Allen Probes EFW instrument

AbstractCold plasma theory and parallel wave propagation are often assumed when approximating the whistler mode magnetic field wave power from electric field observations. The current study is the first to include the wave normal angle from the Electric and Magnetic Field Instrument Suite and Integrated Science package on board the Van Allen Probes in the conversion factor, thus allowing for the accuracy of these assumptions to be quantified. Results indicate that removing the assumption of parallel propagation does not significantly affect calculated plasmaspheric hiss wave powers. Hence, the assumption of parallel propagation is valid. For chorus waves, inclusion of the wave normal angle in the conversion factor leads to significant alterations in the distribution of wave power ratios (observed/ calculated); the percentage of overestimates decreases, the percentage of underestimates increases, and the spread of values is significantly reduced. Calculated plasmaspheric hiss wave powers are, on average, a good estimate of those observed, whereas calculated chorus wave powers are persistently and systematically underestimated. Investigation of wave power ratios (observed/calculated), as a function of frequency and plasma density, reveals a structure consistent with signal attenuation via the formation of a plasma sheath around the Electric Field and Waves spherical double probes instrument. A simple, density‐dependent model is developed in order to quantify this effect of variable impedance between the electric field antenna and the plasma interface. This sheath impedance model is then demonstrated to be successful in significantly improving agreement between calculated and observed power spectra and wave powers.

An implicit δf particle-in-cell method with sub-cycling and orbit averaging for Lorentz ions

A second order implicit δf Lorentz ion hybrid model with sub-cycling and orbit averaging has been developed to study low-frequency, quasi-neutral plasmas. Models using the full Lorentz force equations of motion for ions may be useful for verifying gyrokinetic ion simulation models in applications where higher order terms may be important. In the presence of a strong external magnetic field, previous Lorentz ion models are limited to simulating very short time scales due to the small time step required for resolving the ion gyromotion. Here, we use a simplified model for ion Landau damped ion acoustic waves in a uniform magnetic field as a test bed for developing efficient time stepping methods to be used with the Lorentz ion hybrid model. A detailed linear analysis of the model is derived to validate simulations and to examine the significance of ion Bernstein waves in the Lorentz ion model. Linear analysis of a gyrokinetic ion model is also performed, and excellent agreement with the dispersion results from the Lorentz ion model is demonstrated for the ion acoustic wave. The sub-cycling/orbit averaging algorithm is shown to produce accurate finite-Larmor-radius effects using large macro-time steps sizes, and numerical damping of high frequency fluctuations can be achieved by formulating the field model in terms of the perturbed flux density. Furthermore, a CPU–GPU implementation of the sub-cycling/orbit averaging is presented and is shown to achieve a significant speedup over an equivalent serial code.

Electrothermal Characteristics of YBCO Bulk Magnets Deep in LN2: A Preliminary Analysis for Its Use as Excitation System of Low-Speed Synchronous Generators

This paper introduces a preliminary analysis concerning the use of YBCO bulk magnets when operating in the magnetic environment of low-speed synchronous generators. A coupled electrothermal Comsol model of bulk YBCOs is first developed, tested, and experimentally verified. Joule losses in the HTS magnet are then studied when under ac homogeneous standing magnetic field wave with frequencies and amplitude ranges that are usually found on those generators. The YBCO capability in maintaining its magnetization level when subjected to those conditions was also analyzed since the objective is to replace the permanent magnets currently in use to increase up to +30% the air-gap field.

Magnetic linear and circular dichroism of planar sets of hollow silver nanoparticles: engineering the spectra by hollow size

We study the magneto-optical response of planar clusters of hollow silver nanoparticles. We use the coupled-dipole equations to describe the interaction of the electromagnetic wave with particles. The simplest achiral and anisotropic cluster, a dimer, exhibits magnetic-field-induced linear and circular dichroism. For particular directions of the magnetic field and light wave vector, the magnetic circular dichroism of a planar cluster is considerable. We find that the hollow size can be invoked to tailor the overall shape of the spectrum, in particular its peak position, bandwidth, and FWHM.

Magnetoacoustic Sensing of Magnetic Nanoparticles.

The interaction of magnetic nanoparticles and electromagnetic fields can be determined through electrical signal induction in coils due to magnetization. However, the direct measurement of instant electromagnetic energy absorption by magnetic nanoparticles, as it relates to particle characterization or magnetic hyperthermia studies, has not been possible so far. We introduce the theory of magnetoacoustics, predicting the existence of second harmonic pressure waves from magnetic nanoparticles due to energy absorption from continuously modulated alternating magnetic fields. We then describe the first magnetoacoustic system reported, based on a fiber-interferometer pressure detector, necessary for avoiding electric interference. The magnetoacoustic system confirmed the existence of previously unobserved second harmonic magnetoacoustic responses from solids, magnetic nanoparticles, and nanoparticle-loaded cells, exposed to continuous wave magnetic fields at different frequencies. We discuss how magnetoacoustic signals can be employed as a nanoparticle or magnetic field sensor for biomedical and environmental applications.

Perpendicular propagation of electromagnetic solitons in magnetized thermal pair plasmas

The properties of perpendicularly propagating large amplitude electromagnetic solitons are investigated in a thermal, magnetized pair plasma. To obtain a tractable description, these solitons are assumed to be charge neutral and have a linearly polarized magnetic field, and thus represent the nonlinear extension of part of the linear extraordinary mode. From a Sagdeev pseudopotential analysis it transpires that these solitons are compressive and characterized by a wave magnetic field parallel to the static field. The existence domain in compositional parameter space shows pressure-dependent maxima for the soliton velocities, densities and total magnetic field. Physically, an increase in pressure yields a decrease in the acceptable maxima. This is also illustrated on typical pseudopotential and soliton profiles.

Characterization of normal lumbar spine in Sudanese population using magnetic resonance images

Introduction: Various medical imaging technique used in radiology to investigate the anatomy and physiology of the body in both health and disease. The main principle of magnetic resonance images (MRI) use strong magnetic field and radio waves to form an image for a certain area in the human body. The technique widely used in the hospital for medical diagnosis. In clinical practice, the MRI is employed for the assessment of lower back in the human body. Knowledge of lumbar the spine character is important for the clinical assessment of spine diseases. Materials and Methods: This study consisted Fifty nine consecutive patients gender both males and females Ages between 18-45 years adult with no known history of lumbar spine diseases underwent lumbar spine MRI sagittal T1, T2-weighted images. MRI machine 1.5 Tesla was used the selected sequences were Scout: axial sagittal, and coronal. Objective: The aims of this study were to establish a normal range of values for lumbar spine length (high) and classify the signal intensities by use of MRI for normal Sudanese adult with no known history of lumbar spine diseases. Result: Of this study revealed Correlation between measurement of the lumbar spine and body indices. The data were expressed as means ± standard deviation (S.D). Lumbar spine length (high) and signal intensity were analyzed as combine group (male + female) as well as the total sample. The data statistical analyses were performed using excel software programmed and statistical analyses were performed using the independent sample t -test, simple correlation (SPSS software version 20 USA) statistical significance was assumed at P . Conclusion: The lumbar spine length (high) for Sudanese adult were found to be in the ranges of (13.8-23.4) cm with mean = 16.9 ± 1.7 S.D. the signal intensities for lumbar spine in MRI fast spin echo image (FSE) were found to be in the ranges (61-163) pixel value (45-110) pixel value for vertebrae and inter vertebral disc respectively.

Short-duration Electron Precipitation Studied by Test Particle Simulation

Energy spectra of electron microbursts from 170 keV to 340 keV have been measured by the solid-state detectors aboard the low-altitude (680 km) polar-orbiting Korean STSAT-1 (Science and Technology SATellite). These measurements have revealed two important characteristics unique to the microbursts: (1) They are produced by a fast-loss cone-filling process in which the interaction time for pitch-angle scattering is less than 50 ms and (2) The e-folding energy of the perpendicular component is larger than that of the parallel component, and the loss cone is not completely filled by electrons. To understand how wave-particle interactions could generate microbursts, we performed a test particle simulation and investigated how the waves scattered electron pitch angles within the timescale required for microburst precipitation. The application of rising-frequency whistler-mode waves to electrons of different energies moving in a dipole magnetic field showed that chorus magnetic wave fields, rather than electric fields, were the main cause of microburst events, which implied that microbursts could be produced by a quasi-adiabatic process. In addition, the simulation results showed that high-energy electrons could resonate with chorus waves at high magnetic latitudes where the loss cone was larger, which might explain the decreased e-folding energy of precipitated microbursts compared to that of trapped electrons.

Positive rotation wave magnetic field's effect and control on thermal quantum correlation of inhomogeneous Heisenberg spin chain

Abstract The thermal entanglement in an anisotropic Heisenberg XXZ spin chain is investigated under a positive rotation wave magnetic field in this paper. Results show that thermal quantum correlation can be effectively controlled under an inhomogeneous magnetic field. Within inhomogeneous magnetic field limits, a relatively stable area, in which a thermal quantum correlate value is almost unconnected with the spin coupling coefficient in z direction, can be formed. By controlling the magnetic field intensity or the spin coupling coefficient in x and y directions, we can regulate this area's corresponding thermal quantum correlation value and efficiently avoid the phenomenon that thermal quantum correlation be zero. The thermal will increase first then decrease in a certain temperature range under a high magnetic with better uniformity.

Stability of a Perfectly Conducting Viscoelastic Fluid with Continuous Viscosity & density Stratification in Presence of a Horizontal Magnetic Field

Rayleigh-Taylor instability of a viscoelastic fluid in presence of a horizontal magnetic field has been investigated. The Graphical representations of numerical solution of the derived generating equations evolved have been presented with the analysis of results obtained and presentation of numerical data. It has been seen that effect of magnetic field under certain condition acts as a destabilising agent. When horizontal magnetic field varies in vertical direction, the magnetic field affects on two-dimensional disturbances perpendicular to the lines of force. The corresponding stability conditions, dependent on magnetic field and gravity wave, but independent of viscosity and viscoelasticity are derived. Also it is observed that the stability of equilibrium is dependent on sufficiently large magnetic field, viscosity and viscoelasticity for exponential density and viscosity stratification.

Global empirical models of plasmaspheric hiss using Van Allen Probes

AbstractPlasmaspheric hiss is a whistler‐mode emission that permeates the Earth's plasmasphere and is a significant driver of energetic electron losses through cyclotron resonant pitch angle scattering. The Electric and Magnetic Field Instrument Suite and Integrated Science instrument on the Van Allen Probes mission provides vastly improved measurements of the hiss wave environment including continuous measurements of the wave magnetic field cross‐spectral matrix and enhanced low‐frequency coverage. Here, we develop empirical models of hiss wave intensity using two years of Van Allen Probes data. First, we describe the construction of the hiss database. Then, we compare the hiss spectral distribution and integrated wave amplitude obtained from Van Allen Probes to those previously extracted from the Combined Release and Radiation Effects Satellite mission. Next, we develop a cubic regression model of the average hiss magnetic field intensity as a function of Kp, L, magnetic latitude, and magnetic local time. We use the full regression model to explore general trends in the data and use insights from the model to develop a simplified model of wave intensity for straightforward inclusion in quasi‐linear diffusion calculations of electron scattering rates.

Identification of Calcification in MRI Brain Images by k-Means Algorithm

Background/Objective: The role of clustering is significant to analyze different kind of applications of its techniques. Similar data are grouped into one and they formed as a cluster. Dissimilar data are grouped into another form in other cluster. Data clustering is an important and active research applied in many fields including multivariate analysis in statistics and some other areas like pattern recognition and machine learning etc. Methods/Statistical Analysis: Boundary detection and outlier analysis is an important concept for pre-processing the data. The boundary considers only pixels lying on and near edges and use of gradient or Laplacian to preliminary processing of images. To find the outlier in a group of patterns is a well-known problem in Data Mining (DM). An outlier is a pattern which is different with respect to the rest of the patterns in the data. The k-Means is one of the familiar clustering methods used by different researchers to find the well-formed clusters. Magnetic Resonance Imaging (MRI) uses a magnetic field and radio waves to create detailed images of the organs and tissues within human body. The k-Means algorithm is used to find the tumor by applying the boundary detection and outlier techniques in this research work in MRI brain images. Findings: The main goal of this research work is to extract the tumor (Calcification) in an MRI brain image by means of clustering pixels to fortify the quality of clustering algorithm. The results of the MRI brain images are analyzed and identified by the proposed algorithm. The result produced by simple k-Means algorithm is very useful to find the tumor in MRI images perfectly. Application/Improvements: The MRI brain images are analyzed and implemented by other methods like classification and some other techniques in future.

Heartbeat of the Sun from Principal Component Analysis and prediction of solar activity on a millenium timescale.

We derive two principal components (PCs) of temporal magnetic field variations over the solar cycles 21–24 from full disk magnetograms covering about 39% of data variance, with σ = 0.67. These PCs are attributed to two main magnetic waves travelling from the opposite hemispheres with close frequencies and increasing phase shift. Using symbolic regeression analysis we also derive mathematical formulae for these waves and calculate their summary curve which we show is linked to solar activity index. Extrapolation of the PCs backward for 800 years reveals the two 350-year grand cycles superimposed on 22 year-cycles with the features showing a remarkable resemblance to sunspot activity reported in the past including the Maunder and Dalton minimum. The summary curve calculated for the next millennium predicts further three grand cycles with the closest grand minimum occurring in the forthcoming cycles 26–27 with the two magnetic field waves separating into the opposite hemispheres leading to strongly reduced solar activity. These grand cycle variations are probed by α − Ω dynamo model with meridional circulation. Dynamo waves are found generated with close frequencies whose interaction leads to beating effects responsible for the grand cycles (350–400 years) superimposed on a standard 22 year cycle. This approach opens a new era in investigation and confident prediction of solar activity on a millenium timescale.

Magnetically controllable generation of ferrofluid droplets

This paper reports the manipulation of ferrofluid droplets by using a microfluidic flow-focusing device equipped with a magnetic tweezer. Besides the traditional flow rate controlling method, the magnetic field also can be applied to control the size of the droplets. Two major effects in magnetic manipulation process: magnetoviscous effect and magnetic drag effect, were studied. Under a fixed flow rate (CP = 1 mL/h, DP = 0.2 mL/h), the average sizes of ferrofluid droplets were tunable from 135 to 95 μm by varying the magnetic field from 0 to 60 mT. Moreover, square wave magnetic field can be used to periodically generate droplets with different sizes. These results are helpful to understand the generation mechanism of the ferrofluid droplet and supply a novel method for manipulating droplets with a predetermined size and distribution.

The handedness and classification of materials

According to direction relationship among the triplet of vectors of electric field intensity, magnetic field intensity and wave vector of electromagnetic wave, materials have usually been classified as right- and left-handed material. Here, we shall strictly and systematically demonstrate that, in a linear isotropic homogeneous medium, the direction relationship among the triplet of vectors of electric field intensity, magnetic field intensity and wave vector of electromagnetic wave may generally alter between right-handedness and left-handedness with change of time except for the special cases. Thus the further classification of materials according to handedness and corresponding criterion are proposed.

Helicon waves in uniform plasmas. II. High m numbers

Helicons are whistler modes with azimuthal wave numbers. They have been studied in solids and plasmas where boundaries play a role. The present work shows that very similar modes exist in unbounded gaseous plasmas. Instead of boundaries, the antenna properties determine the topology of the wave packets. The simplest antenna is a magnetic loop which excites m = 0 or m = 1 helicons depending on whether the dipole moment is aligned parallel or perpendicular to the ambient background magnetic field B0. While these low order helicons have been described by J. M. Urrutia and R. L. Stenzel [“Helicon modes in uniform plasmas. I. Low m modes,” Phys. Plasmas 22, 092111 (2015)], the present work focuses on high order modes up to m = 8. These are excited by antenna arrays forming magnetic multipoles. Their wave magnetic field has been measured in space and time in a large and uniform laboratory plasma free of boundary effects. The observed wave topology exhibits m pairs of unique field line spirals which may have inspired the name “helicon” to this mode. All field lines converge into these nested spirals which propagate like corkscrews along B0. The field lines near the axis of helicons are perpendicular to B0 and circularly polarized as in parallel whistlers. Helical antennas couple to these transverse fields but not to the spiral fields of helicons. Using a circular antenna array of phased m = 0 loops, right or left rotating or non-rotating multipole antenna fields are generated. They excite m < 0 and m > 0 modes, showing that the plasma supports both modes equally well. The poor excitation of m < 0 modes is a characteristic of loops with dipole moment across B0. The radiation efficiency of multipole antennas has been found to decrease with m.

Helicon modes in uniform plasmas. I. Low m modes

Helicons are whistler modes with azimuthal wave numbers. They arise in bounded gaseous and solid state plasmas, but the present work shows that very similar modes also exist in unbounded uniform plasmas. The antenna properties determine the mode structure. A simple antenna is a magnetic loop with dipole moment aligned either along or across the ambient background magnetic field B0. For such configurations, the wave magnetic field has been measured in space and time in a large and uniform laboratory plasma. The observed wave topology for a dipole along B0 is similar to that of an m = 0 helicon mode. It consists of a sequence of alternating whistler vortices. For a dipole across B0, an m = 1 mode is excited which can be considered as a transverse vortex which rotates around B0. In m = 0 modes, the field lines are confined to each half-wavelength vortex while for m = 1 modes they pass through the entire wave train. A subset of m = 1 field lines forms two nested helices which rotate in space and time like corkscrews. Depending on the type of the antenna, both m=+1 and m = −1 modes can be excited. Helicons in unbounded plasmas also propagate transverse to B0. The transverse and parallel wave numbers are about equal and form oblique phase fronts as in whistler Gendrin modes. By superimposing small amplitude fields of several loop antennas, various antenna combinations have been created. These include rotating field antennas, helical antennas, and directional antennas. The radiation efficiency is quantified by the radiation resistance. Since helicons exist in unbounded laboratory plasmas, they can also arise in space plasmas.