Magnetic Field Spectrum Research Articles

Electromagnetic anomaly before earthquakes measured by electromagnetic experiments

Three experiments are carried out for earthquake monitoring using electromagnetic (EM) methods in recent years. Some earthquakes occurred in chance of the measurement time period for each experiment and the anomalies were recorded before the shocks. The observation at a site 20 km away from the epicenter of Zhangbei MS6.2 earthquake in 1998 shows that the apparent resistivity decreases in the strike direction before and/or during the earthquake and the resistivity increases in the de- cline direction. This anomalous variation in apparent resistivity accounts for about 20%. The apparent resistivities at a site in the epicentral area decrease in the strike and decline directions before and/or during the earthquake and increase after shocks. The experiments using artificial electromagnetic signals with super low frequency carried out in 1999 show that the resolution and stability of electric and magnetic spectra are improved. The spectra of electric and magnetic fields and apparent resistivity at the Baodi station began to anomalously change two days before the Qian'an earthquake with 120 km distant to the station. The anomalous variation of electric and magnetic spectra is about twice as great as normal variation and the apparent resistivity changes by about 20%. The measurements in active seismic area of Yunnan province in the year 2005 indicate that the electric and magnetic spectra anomalously change by one order before the Taoyuan earthquake about 100 km away from the observa- tories. But the measurements at the sites in Beijing area 2 000 km away from the epicenter do not show any anomaly.

Dynamics and waves near multiple magnetic null points in reconnection diffusion region

Identifying the magnetic structure in the region where the magnetic field lines break and how reconnection happens is crucial to improving our understanding of three‐dimensional reconnection. Here we show the in situ observation of magnetic null structures in the diffusion region, the dynamics, and the associated waves. Possible spiral null pair has been identified near the diffusion region. There is a close relation among the null points, the bipolar signature of the Z component of the magnetic field, and enhancement of the flux of energetic electrons up to 100 keV. Near the null structures, whistler‐mode waves were identified by both the polarity and the power law of the spectrum of electric and magnetic fields. It is found that the angle between the fans of the nulls is quite close to the theoretically estimated maximum value of the group‐velocity cone angle for the whistler wave regime of reconnection.

Coupled longitudinal phonon-plasmon spectrum in quantizing magnetic field

Quantum magnetic field effects on longitudinal phonon propagation are discussed, taking account of dynamic and static electron screening in the phonon spectrum. For the low frequency sound mode branch subject to static screening we discuss: (1) Quantum magnetic field corrections to the Bohm-Staver result, (2) DeHaas-Van Alphen oscillatory corrections to the Kohn anomaly at intermediate field strength, (3) Restructuring of the Kohn anomaly at higher magnetic field including details of the behavior of ω(p) near p ∼ 2pp. Considering higher frequency branches of the phonon spectrum in quantizing magnetic field, we recognize that dynamic electron screening of the phonon spectrum couples it to the many branches of the electron plasmon spectrum in magnetic field. In connection with this, coupled phonon-plasmon modes and resonances (including the Bernstein and “quantum” types of resonances in magnetic field) are discussed in detail.

Driving toroidally asymmetric current through the tokamak scrape-off layer. II. Magnetic field structure and spectrum

The structure of the magnetic field perturbations due to nonaxisymmetric field-aligned currents in the tokamak scrape-off layer (SOL) are analytically calculated near the X-point. Paper I [I. Joseph et al., Phys. Plasmas 16, 052510 (2009)] demonstrated that biasing divertor target plates in a toroidally asymmetric fashion can generate an appreciable toroidally asymmetric parallel current density in the SOL along the separatrix. Here, the magnetic field perturbation caused by a SOL current channel of finite width and stepwise constant amplitude at the target plate is derived. Flux expansion amplifies the magnetic perturbation near the X-point, while phase interference causes the SOL amplitude to be reduced at large toroidal mode number. Far enough from the current channel, the magnetic field can be approximated as arising from a surface current near the separatrix with differing amplitudes in the SOL and the divertor leg. The perturbation spectrum and resonant components of this field are computed analytically asymptotically close to the separatrix in magnetic flux coordinates. The size of the stochastic layer due to the applied perturbation that would result without self-consistent plasma shielding is also estimated. If enough resonant field is generated, control of the edge pressure gradient may allow stabilization of edge localized modes.

Observation of the lower hybrid waves near the three-dimensional null pair

Magnetic reconnection is a fundamental process in plasma, which is thought to play important roles both in laboratory and natural plasmas through affecting magnetic topology, heating and accelerating particles. During an event on Oct. 1st, 2001, the Cluster tetrahedron circled around the magnetic reconnection region several times, and Xiao et al. first identified the null pair and found that the spectrum of the null-point oscillation shows the maximum power near the lower-hybrid frequency. In this paper we report the observation of electromagnetic and electrostatic wave enhancements near lower hybrid frequency associated with the reconnection process near the null pair. The lower hybrid waves (LHWs) with quasi-perpendicular propagation were identified and also confirmed by the power law of the spectrum of electric and magnetic fields.

Radio frequency noise from clinical linear accelerators

There is a great deal of interest in image-guided radiotherapy (IGRT), and to advance the state of IGRT, an integrated linear accelerator–magnetic resonance (linac-MR) system has been proposed. Knowledge of the radiofrequency (RF) emissions near a linac is important for the design of appropriate RF shielding to facilitate the successful integration of these two devices. The frequency spectra of both electric and magnetic fields of RF emission are measured using commercially available measurement probes near the treatment couch in three clinical linac vaults with distinct physical layouts. The magnitude spectrum of the RF power emitted from these three linacs is then estimated. The electric field spectrum was also measured at several distances from the linac modulator in order to assess the effects of variations in spatial location in the treatment vault. A large fraction of RF power is emitted at frequencies below 5 MHz. However, the measured RF power at the Larmor frequency (8.5 MHz) of the proposed 0.2 T MR in the linac-MR (0.4–14.6 µW m−2) is still large enough to cause artifacts in MR images. Magnetron-based linacs generally emit much larger RF power than klystron-based linacs. In the frequency range of 1–50 MHz, only slight variation in the measured electric field is observed as a function of measurement position. This study suggests that the RF emissions are strong enough to cause image artifacts in MRI systems.

Theory of magnetic resonance as an orbital state probe

It is demonstrated that magnetic resonance in a magnetically ordered state is a sensitive indirect method for the investigation of the orbital ground state. This idea is illustrated for two perovskite titanates: ${\text{LaTiO}}_{3}$ and ${\text{YTiO}}_{3}$. In contrast to the spin-wave energy spectra, antiferromagnetic resonance spectra in an external magnetic field reveal clear dependence on the orbital state and it can distinguish the state with strong orbital fluctuations from the static orbital order. Our theoretical analysis is based on the model, which explicitly takes into consideration the strong correlation among lattice, orbital, and magnetic degrees of freedom.

Quasi-periodic patterns coupling the Sun, solar wind and the Earth

The spectrum of velocity and magnetic fields in the solar wind is self-similar (power-law type) in the frequency range greater than >1/day indicating well-mixed turbulence. But it loses self-similarity for lower frequencies indicating the presence of large-scale patterns, which are intermittently generated inside the Sun and propagate from the Sun to the Earth. Here we discuss the spatia–temporal characteristics and origin of the 1.3-year quasi-periodic pattern found inside the Sun by helioseismic methods and detected in the solar wind. To identify and characterize this pattern on the Sun we use time series of solar magnetic Carrington maps generated at the Wilcox Solar Observatory and independent component data analysis. This analysis shows the latitudinal distribution of the pattern, its variable frequency and intermittent appearance.

Attenuation of magnetic field components through an AC machine stator

PurposeNon‐intrusive magnetic measurements in AC machines are possible with small flat coils stuck on the external surface of the housing of a running motor. The aim of the paper consists in determining transmission coefficients able to give a direct relationship between the weak external flux density and the airgap one.Design/methodology/approachAn experimental approach shows that the decoupling principle can be applied. Transmission coefficients are determined separately for the stator yoke, the motor housing and the external air.FindingsFor low frequencies and a housing made of steel, eddy current can be neglected. The transmission coefficient depends strongly of the mode (number of poles) of the rotating field. Conversely, for higher harmonic ranks, the additional attenuation caused by eddy currents in the housing does not practically depend on the mode but is strongly dependant on the frequency.Research limitations/implicationsThe transmission coefficients are determined considering a 2D electromagnetic model and several simplifying hypothesis. Experiments prove the validity of the proposed approach up to 550 Hz.Practical implicationsUp to now, many fault detection systems are based on the presence of additional harmonics in the external magnetic field spectrum. With the knowledge of simple transmission coefficients, an analysis of the variation of the magnitude of critical spectrum lines is now possible for a more precise fault detection in AC machines.Originality/valueTo the authors' knowledge, the only alternative way for the interpretation of external field measurements consist in using a numerical method with a full model of the machine which takes a lot of computation time. The proposed transmission coefficients provide a faster method valid for most of the interesting spectrum lines.

Fourier synthesis techniques for NMR spectroscopy in inhomogeneous fields

This paper describes a method for synthesizing spin rotations with arbitrary space dependence on a sample of noninteracting spin 12 by using nonselective radio frequency pulses and pulsed field gradients. This method is used to map out spatial distribution of inhomogeneous B(0) field and to engineer a space dependent evolution of spins that cancels the space dependent phase spins acquire when precessing in an inhomogeneous magnetic field. The technique allows one to record high resolution spectra in inhomogeneous magnetic field by using only time varying linear gradients and rf fields and is expected to find applications in ex situ NMR.

Statistical Analysis of the High‐Frequency Spectral Break of the Solar Wind Turbulence at 1 AU

The physical mechanism responsible for the dissipation of the solar wind turbulence and the resulting plasma heating is not completely understood. To be a viable means of dissipation, any mechanism has to reproduce several observational features of the turbulence spectra. One important characteristic of the spectrum is its high-frequency break, where the spectral slope becomes considerably steeper than the Kolmogorov-like scaling law observed in the inertial range. The onset of the spectral steepening can be inferred from the observations fairly accurately, and it is a good benchmark to test various theories of the turbulence dissipation. In this paper, a large database of magnetic field spectra and plasma parameters at 1 AU measured by the ACE spacecraft is used to determine the spectral break. The statistical correlation of the data points calculated according to existing theoretical formulae for the break is analyzed, and the least-squares fits to the data are compared with the theoretically predicted scalings. It is concluded that the position of the spectral break is not determined just by a scale of the turbulent fluctuations, but by a combination of their scale and the amplitude at that scale. This suggests that the dissipation of the solar wind turbulence is an essentially nonlinear process.

Spin relaxation in the impurity band of a semiconductor in the external magnetic field

Spin relaxation in the impurity band of a 2D semiconductor with spin-split spectrum and hyperfine interaction in the external magnetic field is considered. Two contributions to the spin relaxation are shown to be relevant: the one given by optimal impurity configurations with the hop-waiting time inversely proportional to the external magnetic field and another one related to electron motion over large distances. The average spin relaxation rate is calculated.

Suppression of the filamentation instability by a flow-aligned magnetic field: testing the analytic threshold with PIC simulations

The impact of a flow-aligned and spatially homogeneous magnetic field on the filamentation instability (FI) is examined in a system of two equal counterstreaming non-relativistic cool electron beams. Particle-in-cell simulations that represent the plane perpendicular to the flow velocity vector confirm the reduction of the linear growth rate by the initial magnetic field. The FI is, however, not inhibited by a magnetic field with the critical strength, for which the solution of the linear dispersion relation predicts a full suppression. The saturation of the electromagnetic fields in the plasma involves a balance between the magnetic pressure gradient and the electric field resulting from the charge displacement. The simulations demonstrate that the magnetic energy gain and the field structure upon saturation do not depend on the initial magnetic field strength. This can be explained by the qualitative similarity of the spectrum of unstable wavenumbers, at least for subcritical strengths of the background magnetic field, and by the vanishing of the pressure gradient of a spatially homogeneous magnetic field. Magnetic trapping is apparently not the saturation mechanism for the considered plasma parameters. The spatial power spectrum of the saturated magnetic fields in the simulation plane can be approximated by a power-law function and the magnetic and electric spectra are similar at high wavenumbers. The final electron velocity distributions are comparable for all magnetic field strengths.

Global FDTD Maxwell's Equations Modeling of Electromagnetic Propagation From Currents in the Lithosphere

Electromagnetic wave propagation from electric currents within the Earth's crust is investigated using a three-dimensional finite-difference time-domain (FDTD) full-vector Maxwell's equations model of the global Earth-ionosphere cavity. The FDTD model employed extends from km below sea level to an altitude of km, and can account for arbitrary horizontal as well as vertical geometrical and electrical inhomogeneities and anisotropies of the ionosphere, lithosphere, and oceans. Using this model, the surface horizontal magnetic field is calculated for different depths and orientations of an electric current occurring below the epicenter of the 1989 Loma Prieta earthquake. Results show that the alignment and depth of the electric current within the Earth's crust yields significant differences for the calculated surface magnetic field time-waveforms and spectra. Further, it is found that EM wave phenomena measured at the Earth's surface due to electric currents buried in the Earth's crust will only have significant spectra below 1 Hz.

Experimental investigation of barium spectra in crossed electric and magnetic fields

This paper has studied the barium spectra in external magnetic and electric fields on the recently-built system based on a super-conducting solenoid with high stability of better than I Gauss. Firstly, we compared the spectra in a high pure magnetic field of B = 4.00000 T for different polarized laser, (sigma(+) and (sigma(-). We prove that after a proper shift, the (sigma(+) and sigma(-) spectra are completely identical. We also investigated the dependence of the spectral feature in the high magnetic field on an additionally applied crossing electric field. Along with the increase of the electric field, there is an energy shift for every spectral line. Based on a transformative Hamiltonian, we explained this energy shift well.

Geometric dynamics on quantum Nano-surfaces and low-energy spectrum in a homogeneous magnetic field

Low-energy dynamics and spectrum of the Schrodinger operator on curved surfaces in a magnetic field are considered. Nano-and micro-algebraic structures generate a quantization of the surface. On this quantum space, the dynamics has a “surfing” behavior described via curvature invariants. Two types of surface deformations correspond to trapping and transistor types of dynamics.

GENERATION OF LARGE-SCALE MAGNETIC FIELDS FROM PRIMORDIAL DENSITY FLUCTUATIONS

We investigate a generation of magnetic fields from cosmological density perturbations. In the primordial plasma before cosmological recombination, all of the materials except dark matter in the universe exist in the form of photons, electrons, and protons (and a small number of light elements). Due to the different scattering nature of photons off electrons and protons, electric currents and electric fields are inevitably induced, and thus magnetic fields are generated. We numerically obtain the power spectrum of magnetic fields over a wide range of scales, from k ~ 10−5 Mpc −1 to k ~ 109 Mpc −1. Implications of these cosmologically generated magnetic fields are discussed.

Control of non-inductive current in Heliotron J**This manuscript is based on paper EX/P6-14 presented at the 21st IAEA Fusion Energy Conf. (Chengdu, China).

Non-inductive currents of electron cyclotron heated plasmas have been examined in the helical-axis heliotron device, Heliotron J. The bootstrap and EC currents were separated by comparing experiments with positive and negative magnetic field. The estimated bootstrap current was found to be affected by the magnetic field configuration. It increases with an increase in the bumpy component of the magnetic field spectrum, which agrees well with a neoclassical prediction calculated using the SPBSC code. The EC current driven by oblique launch with respect to the magnetic field strongly depends on the field configuration and the location of the EC power deposition. The EC current is enhanced when the EC power is deposited on the magnetic axis. The maximum EC current is IEC = −4.6 kA and the current drive efficiency is η = neRIp/PEC = 8.4 × 1016 A W−1 m−2. The flow direction of the EC current depends on the magnetic field ripple structure where the EC power is deposited.

Particle and field characteristics of broadband electrons observed by the FAST satellite during a geomagnetic storm

Broadband electrons (BBEs) are remarkable flux enhancements (>1013 eV cm−2 s−1) of precipitating electrons over a broad energy range (0.03–30 keV) near the equatorward edge of the auroral oval during geomagnetic storms. We show characteristics of particles (energy spectra and pitch angle distribution) and fields (electric field, magnetic field, and wave spectra) during a BBE event observed by the Fast Auroral SnapshoT (FAST) satellite. The BBEs were observed at an altitude of ∼2000 km at 59°–61° invariant latitudes (ILATs) and 21 h magnetic local time (MLT). The event was observed at ∼7 min after the onset of a substorm during the main phase of the Bastille Day geomagnetic storm (minimum Dst = −301 nT) on 15 July 2000. The precipitation region of the BBEs corresponded to a localized intensification of auroral emission, lasting ∼14 min, observed by the Polar UVI images at 50°–60° geomagnetic latitudes (MLATs) and 20–21 MLTs. These results suggest that rapid particle acceleration was occurring in the inner magnetosphere associated with a storm‐time substorm. The pitch angle distribution of BBEs was isotropic except for a loss cone feature around the field‐aligned upward direction at a higher energy range above ∼1 keV, while field‐aligned electron fluxes were larger than the perpendicular fluxes below ∼1 keV. These results imply that a higher energy part of the BBEs originated from higher altitudes in the inner magnetosphere and that a lower energy part was accelerated parallel to the local magnetic field at lower altitudes near the satellite. Intense fluctuations of electric and magnetic fields were observed during this BBE event. From these results, we discuss possible acceleration of the lower energy part of BBEs through wave–particle interaction.

Property of the spectrum of large-scale magnetic fields from inflation

The property of the spectrum of large-scale magnetic fields generated due to the breaking of the conformal invariance of the Maxwell theory through some mechanism in inflationary cosmology is studied. It is shown that the spectrum of the generated magnetic fields should not be perfectly scale-invariant but be slightly red so that the amplitude of large-scale magnetic fields can be stronger than {approx}10{sup -12} G at the present time. This analysis is performed by assuming the absence of amplification due to the late-time action of some dynamo (or similar) mechanism.