Electromagnetic Field Variations Research Articles

Spatial distributions of electromagnetic field variations and injection regions during the 20 November 2007 sawtooth event

Abstract. We report multi-spacecraft and ground-based observations of a "sawtooth" event on 20 November 2007. For this event, data from three THEMIS, two GOES, and four LANL spacecraft are available as well as those from extensively distributed ground magnetometers and all-sky imagers. In the present paper we focus on the spatial extents of the electromagnetic and particle signatures of the first "tooth". In this event, auroral images and ground magnetic bays showed two activations: a pseudo onset and a major onset (we use the term pseudo onset since the former auroral brightening did not significantly expand poleward). Ground magnetic bay observations indicate that the substorm current wedge (SCW) developed after the major onset in an azimuthally wide region of ~14–3 h MLT. Similarly, broad magnetic bay distribution was observed also for the pseudo onset prior to the major onset. Furthermore, around the pseudo onset, magnetic dipolarisations were observed from 0.5 to 5 h MLT. These observations illustrate that, during sawtooth events, activities following not only the major onset but also the pseudo onset can extend more widely than those during usual substorms. Remarkable electromagnetic field fluctuations embedded in the dipolarisation trend were seen at 0.5 and 2.5 h MLT. In particular, comprehensive plasma and field data from THEMIS showed the presence of a long-excited weak magnetosonic wave and an impulsive large-amplitude Alfvén wave with an earthward Poynting flux at around the eastward edge of the SCW; the latter was sufficiently strong for powering aurora (140 mW/m2 when mapped to the ionosphere). These two activations of the electromagnetic wave were identified, corresponding to the pseudo onset and the major onset. On the other hand, the dipolarisation at geosynchronous 0 h MLT was observed only after the major onset, despite its closer location to the centre of the auroral activity in terms of the MLT; this indicates that the inner radial limit of the dipolarisation region at the pseudo onset was tailward of geosynchronous altitude at 0 h MLT. The outer radial limit of the electron injection region was also found at ~10 RE by conjunction measurements with THEMIS satellites. These radial distributions are not significantly different to those expected for usual substorms.

Under the Volcano: Unsaturated Flow and Induced Variations of Electromagnetic and Gravitational Fields

A number of electric and magnetic signals have been observed on La Fournaise volcano, which are attributed to electrokinetic effects induced by the heavy rains which fall in this area. A general modeling of the flow in the unsaturated and saturated zones is proposed for an arbitrary topography, arbitrary spatial distributions of physical properties, and arbitrary rains. This flow generates an electrical field and an associated current which itself induces a variable magnetic field. Variations of water masses inside the volcano are also determined to predict gravity variations. Saturation, electric, magnetic, and gravity fields are given and discussed for various rain distributions including real ones.

Results of the integrated geophysical and geodetic investigation of the stress-strain state in Tashtagol Ore Field

The paper describes observations over the gravitation and electromagnetic field variations in a 900 m deep opening in Tashtagol Iron Ore Mine, as well as monitoring of movements in the rock mass and on the earth’s surface. The results provide characterization of the variability of the stress-strain state in the rock mass, and the rock pressure manifestations in the mining area.

Modeling and Understanding MIMO Propagation in Tunnels

This paper first presents an application of the modal theory for interpreting experimental results of the electromagnetic field variation along a tunnel. The transmitting frequency is assumed to be high enough so that the tunnel behaves as an oversized waveguide. Then, for a Multiple-Input Multiple-Output channel, theoretical results of the channel capacity are given. To explain the decrease of the capacity at large distance from the transmitter, even by assuming a constant signal to noise ratio, an approach based on the calculation of the eigenvalues of the transfer matrix in a reference scenario is described.

Separation of a variable electric field into potential and induced components

It is shown that external exciters of a variable electromagnetic field can always be represented in the form of superposition of a potential electric field and an eddy electric current and the variable electric field excited by them in an arbitrary heterogeneous medium can always be separated into potential and induced components, differently reflecting the geoelectric structure. These distinctions can be successfully used for increasing the efficiency of electromagnetic studies of the Earth.

Generation of acoustic waves during nonlinear interaction of hydroacoustic and electromagnetic fields in the marine environment

The feasibility of generating additional acoustic harmonics in the geomagnetic field during the propagation of an acoustic wave in a conducting medium through a region with a variable electromagnetic field is considered. Two possible mechanisms of such generation are analyzed: a parametric mechanism and a mechanism associated with ponderomotive forces (dynamic). Expressions are derived for three acoustic harmonics generated by a magnetic dipole with a variable magnetic moment.

Determination of the general public exposure around GSM and UMTS base stations

This paper summarises two studies, in which measurement and calculation methods to determine the exposure of the general public around GSM and UMTS base stations have been developed and applied to different scenarios. The electromagnetic field variations around the stations in space and time are accounted for by appropriate maximisation techniques. Measurements show a bandwidth of exposures from 0.01% to more than 10% of field strength exposure limits. The distance to the station is not a main influencing factor, whereas the orientation to the main lobe and the sight conditions greatly influence exposure. Several commercially available numerical simulation tools were tested for their applicability on exposure forecast. In line-of-sight scenarios, all programs are able to predict the exposure accurately, whereas in non-line-of-sight situations, free space models overestimate the real exposure by some orders of magnitude.

7 T body MRI: B1 shimming with simultaneous SAR reduction

The high frequency of the radiofrequency (RF) fields used in high field magnetic resonance imaging (MRI) results in electromagnetic field variations that can cause local regions to have a large specific absorption rate (SAR) and/or a low excitation. In this study, we evaluated the use of a B1 shimming technique which can simultaneously improve the B+1 homogeneity and reduce the SAR for whole body imaging at 7 T. Optimizations for four individual anatomies showed a reduction up to 74% of the peak SAR values with respect to a quadrature excitation and a simultaneous improvement of the B+1 homogeneity varying between 39 and 75% for different optimization parameters. The average SAR was reduced with approximately 50% for all optimizations. The optimized phase and amplitude settings from an elliptical phantom model were applied to four realistic human anatomy models to evaluate whether a generic application without prior knowledge of the detailed human anatomy is possible. This resulted in an average improvement of the B+1 homogeneity of 37% and an average reduction of the maximum and average SAR of 50 and 55%, respectively. It can be concluded that this generic method can be used as a simple method to improve the prospects of 7 T body imaging.

Electromagnetic fields induced by a cuboidal inclusion with uniform spontaneous polarization and magnetization

Exact closed-form solutions are derived for the electromagnetic fields induced by a cuboidal inclusion with uniform spontaneous polarization and magnetization embedded in an infinite uniaxial multiferroic solid. The method of Green’s function and direct integration are employed to obtain the solution, with the results for the induced electric and magnetic potentials as well as the electric and magnetic fields being in terms of elementary functions. It is observed that all the electric and magnetic field components exhibit logarithmic singularities near the eight corners of the cuboid. There also exist logarithmic singularities for the electric and magnetic fields near certain edges of the cuboid. Numerical results are presented for a typical multiferroic composite to demonstrate the variation of electromagnetic fields in and near the cuboidal inclusion.

Advanced use of soft computing and eddy current test to evaluate mechanical integrity of metallic plates

The up-to-date structural designing makes by now widely use of high performance numerical codes, mainly in terms of computational powerful, cost and sizing, only available till some time before to limited groups of users. This allowed the experts to focus their attention on a qualifying aspect of the designing, i.e. an use of the materials very close to their limit behavior. Late innovative approaches in material mechanics gave in addition the opportunity to build models very close to the actual behavior but without introducing heavy computational aspects. In this paper, phenomena which relate mechanical stresses with electromagnetic properties of a defined material have been exploited in order to reconstruct electromagnetic maps starting from mechanical quantities by means of support vector regression machines (SVRMs). Purpose of the proposed study is to reconstruct a stress map in strained metallic plates by using electromagnetic measures. Moreover, an heuristic approach is proposed in order to estimate electromagnetic behavior of a stressed plate starting from easily measurable mechanical quantities. It would be very interesting when electrical or mechanical measurements are very hard to realize. The proposed approach could be very useful in such situations as quality controls of civil buildings, without the necessity of applying expensive and time-consuming destructive or non-destructive testing. In this way, it is possible to have a substantially precise idea of mechanical stresses in metallic materials by estimating the local variation of electromagnetic field into the same material using a SVRM-based interpolator.

Anomalous Light Scattering by Small Particles

Light scattering by a small spherical particle with a low dissipation rate is discussed based upon the Mie theory. It is shown that if close to the plasmon (polariton) resonance frequencies the radiative damping prevails over dissipative losses, sharp giant resonances with very unusual properties may be observed. In particular, the resonance extinction cross section increases with an increase in the order of the resonance (dipole, quadrupole, etc.); the characteristic values of electric and magnetic near fields for the scattered light are singular in the particle size, while energy circulation in the near field is rather complicated, so that the Poynting vector field includes singular points whose number, types, and positions are very sensitive to fine changes in the incident light frequency. The results may provide new opportunities for a giant, controlled, highly frequency-sensitive enhancement and variation of electromagnetic field at nanoscales.

Development and Application of Geospace Environment Simulator for the Analysis of Spacecraft–Plasma Interactions

Space development has been rapidly increasing, and a strong demand should arise regarding the understanding of the spacecraft-plasma interactions, which is one of the very important issues associated with the human activities in space. To evaluate the spacecraft-plasma interactions including plasma kinetics, transient process, and electromagnetic field variation, the authors have started to develop a numerical plasma chamber called Geospace Environment Simulator (GES) by making the most use of the conventional full particle-in-cell plasma simulations. For the development of a proto model of GES, the authors have used the Earth Simulator, which is one of the fastest supercomputers in the world. GES can be regarded as a numerical chamber in which space experiments can be virtually performed and temporal and spatial evolutions of spacecraft-plasma interactions can be analyzed. In this paper, the authors have briefly introduced GES in terms of its concept, modeling, and research targets. As one of the research topics of GES, the authors have investigated the impedance variation of electric field antenna onboard scientific satellites in the photoelectron environment in space. From the preliminary simulation results, the large change of reactance of the antenna impedance below the characteristic frequency corresponding to the local plasma frequency determined by the photoelectron density could be confirmed

The electromagnetic field characteristics of the high-frequency torch discharge

The axial distribution measurements of the electric field harmonic component amplitudes of the high-frequency torch discharge operating in air, argon and helium were carried out. The truth of the high frequency torch discharge model, considering reflection of the electromagnetic wave at the end of the discharge channel, was examined on basis of this experimental data. The absence of coefficient variation of electromagnetic field damping due to the frequency in case of flare discharge ignition in air or argon was found. The ion and electron plasma frequencies estimation of the high-frequency torch discharge operating in various gases was carried out. The suggestion about parametric interaction of the external electromagnetic field with the own oscillations of the discharge plasma was formed.

ITER cryogenic system

The main ITER cryogenic users are the superconducting magnets together with the 80 K thermal shields. The key requirements that drive the design of the ITER cryogenic system are the intrinsically pulsed heat deposition in the magnets due to electromagnetic field and nuclear heating cyclic variations, and the usage of large flow capacity helium circulating pumps for forced-flow cooling of the magnets. This paper highlights the main features of the ITER cryogenic system, including the results of studies to mitigate pulsed heat loads on the helium refrigerator and minimize the heat load from helium circulating pumps.

Error propagation in electromagnetic transfer functions: what role for the magnetotelluric method in detecting earthquake precursors?

SUMMARY Whether electromagnetic precursors exist is a contentious issue that should be considered in the context of the focal depth and epicentral distance at which earthquakes nucleate, and with due regard to externally driven changes in the electromagnetic fields recorded at the Earth’s surface. If high-frequency electromagnetic signals are generated by earthquakes, then these signals may not be detectable because of attenuation occurring between the focal depth of the earthquake and the Earth’s surface. This is particularly true in seismotectonic regions (e.g. subduction zones) where saline fluids (including seawater), raised heat flow and partial melts often lead to enhanced conductivities. A technique for examining temporal variations in magnetotelluric (MT) spectra is presented, which: (i) considers both non-inductive and inductive parts of the MT transfer function; (ii) allows electromagnetic skin depths and adjustment distances to be calculated; (iii) takes account of error propagation in the transfer functions and; and (iv) acknowledges possible nonearthquake sources of observed variations in transfer functions and their errors. Data processing tends to eliminate non-stationary electromagnetic disturbances of the source field. Hence the method is more suited to detecting hypothesized changes in subsurface conductivity than short-lived electromagnetic bursts. The technique is applied to MT data from seismotectonic regions in New Zealand (Mt. Ruapehu), the Andes (western Cordillera) and the Mediterranean (Montecristo island) and to a culturally quiet, control site situated on a stable craton in central Australia. We demonstrate that the MT method can resolve apparent conductivity changes of the order of a few percent over 48 hr time intervals and give thresholds for other interval lengths. Comparison of the results obtained from seismotectonic regions with those from central Australia, and consideration of external source field effects, highlight how temporal variations of electromagnetic fields might be speciously interpreted as earthquake precursors.

HIRF Penetration Into Simplified Fuselage Using a Reverberation Chamber Approach

In this paper, high-intensity radiated fields (HIRF) penetration into aircraft fuselage is studied using a reverberation chamber approach. A mode-stirrer is constructed and placed inside a simplified scale fuselage model. Measurements of the penetrating field are performed for different mode-stirrer positions. The collected sets of data are statistically processed and their probability distributions are investigated. Goodness-of-fit tests are performed between data empirical cumulative density functions and existing theoretical probability models which govern the electromagnetic field variations inside reverberation chambers.

A higher order FDTD method in Integral formulation

We present a fourth-order (4, 4) finite-difference time-domain (FDTD)-like algorithm based on the integral form of Maxwell's equations. The algorithm, which is called the integro-difference time-domain (IDTD) method, achieves its fourth-order accuracy in space and time by taking into account the spatial and temporal variations of electromagnetic fields within each computational cell. In the algorithm, the electromagnetic fields within each cell are represented by space and time integrals (or integral averages) of the fields, i.e., the electric and magnetic fluxes (D,B) are represented by the surface-integral average, and the electric and magnetic fields (E,H) by the line and time integral average. In order to relate the integral average fields in the staggered update equations, we have obtained constitutive relations for these fields. It is shown that the IDTD update equations combined with the constitutive relations are fourth-order accurate both in space and time. The fourth-order correction terms are represented by the modified coefficients in the update equations; the numerical structure remains the same as the conventional second-order update equations and more importantly does not require the storage of field variables at the previous time steps to obtain the fourth-order accuracy in time. Furthermore, the Courant-Friedrichs-Lewy (CFL) stability criteria of this fourth-order algorithm turns out to be identical to the stability limits of conventional second-order FDTD scheme based on differential formulation.

SECOND HARMONIC GENERATION IN TWO-DIMENSIONAL PHOTONIC CRYSTAL CONSISTING OF CENTRO-SYMMETRIC DIELECTRIC

We report optical second harmonic generation (OSHG) in a two dimension photonic crystal of centro-symmetric dielectric based on finite difference time domain (FDTD) algorithm. The electromagnetic field distribution in the structure and the intensity of second harmonic (SH) from electric quadrupole polarization along the waveguide are analyzed. The results show that the acute spatial variation of electromagnetic field results in the radiation of SH, and the intensity is proportional to the square of waveguide length. When the beam intensity of the pumping wave is 1.3 MW/mm2, which has wavelength of 10.6 μm, the conversion efficiency of power is 0.307% for a photonic crystal of Silicon with a length of 40 μm.

Effects of 50 HZ Magnetic Field on Some Factors of Immune System in the Male Guinea Pigs

This study investigated the bio-effects of 50 Hz MF exposure on some Immunological factors (ACTH, cortisol and glucose levels) in male Guinea pigs. The capacity of 50 Hz Electromagnetic Fields (EMF) to interfere in the weakness of endocrine system has been a relevant fact into the scientific community, since we are more susceptible for this kind of exposure, in modern days. The 60-day-old Guinea pigs, which average of weight was 350-400 g, the Guinea pigs were divided in two groups, one of them was the control group while the other one suffered interference of a variable EMF of 50 Hz, intensity 0.207μT, 4 hours a day, for a 5 consecutive he results were collected by the weight comparison before and after of the exposure period, as well as the cortisol and glucose levels. These facts are strongly linked with the immunological system. After 5 days, the animals were anaesthetized and had their blood withdrew from their hearts. The samples produced serum where ACTH (quimiluminescent method), cortisol (radioimunoassay method) and glucose (oxidative method) levels were evaluated. The results showed that 50 Hz EMF acted on rats without any significant weight change, but affecting ACTH,cortisol and glucose levels in a significant way, 33.06% (p=0.033); 27.00% (p=0.004) and 56.23% (p=0.014) respectively when were compared with the control group. For this reason, EMF is able to change the serum levels of ACTH, cortisol and glucose, possible changing the endocrinological regulations.

Laser-induced photoconductivity of ferromagnetic semiconductors

The influence of a strong electromagnetic wave on the kinetic phenomena in ferromagnetic semiconductors (FMSC) is considered. To sequentialy consider this influence, we obtained quantum kinetic equations for electrons and magnons involved into the process of electron-magnon interaction at presence of intense high-frequency electromagnetic field. The new mechanism of a photoconduction not bound with a change of the number of carriers in the conduction band of FMSC and hot conduction electrons is offered. The density of electric current in the FMSC provided by strong variable electromagnetic field and feeble constant electric field has been calculated. The valuation of FMSC electric conductivity in the infrared spectral range is made. It is shown that magnification of the amplitude of variable electromagnetic filed results in increasing the conductivity of FMSC at some fixed tempera- ture.