Magnetic Field Spectrum Research Articles

Secondary Structure and Topology of the Transmembrane Domain of LspA, from Mycobacterium Tuberculosis, using Solid State NMR Spectroscopy: Initial Study

Lipoprotein Signal Peptidase A (LspA) belongs to type II signal peptidases and it is an essential enzyme in one of the three distinct steps in lipoprotein maturation. After targeting the prolipoprotein, LspA cleaves the signal peptide to result in the signal peptide cleaved lipoprotein, which is the precursor for the mature lipoprotein.In Mycobacterium tuberculosis (Mtb), LspA is encoded by the gene Rv1539 resulting in a 202 amino acid residue peptide with a molecular weight of 21 kDa that is predicted to possess four transmembrane helices. It has been recognized in vivo that LspA is an essential protein for Mtb virulence and that it may also contribute to antibiotic resistance. As a result LspA has been identified as a potential drug target for the treatment of tuberculosis. Globomycin, a cyclic-peptide is known to inhibit LspA in a non-competitive manner.Solid-state NMR enables studies of membrane proteins in a lipid bilayer environment. Since the native-like environment is important for studying the structure and function of membrane proteins, ssNMR is increasingly useful. In Oriented Sample ssNMR used here, the protein is uniformly aligned with respect to the magnetic field and 15N spectra of isotopically labeled protein have provided initial structural insights.Here, all sample preparation steps and initial spectroscopy will be emphasized. Expression of LspA in E.coli, purification using IMAC, reconstitution of LspA into proteoliposomes via dialysis and the preparation of bicelles as an alignment media with different types of lipids and detergents will be presented. 1D NMR 31P and 15N spectra and 2D separated local field NMR spectra of uniformly and selectively labeled LspA will be shown.

A model for microinstability destabilization and enhanced transport in the presence of shielded 3D magnetic perturbations

A mechanism is presented that suggests shielded 3D magnetic perturbations can destabilize microinstabilities and enhance the associated anomalous transport. Using local 3D equilibrium theory, shaped tokamak equilibria with small 3D deformations are constructed. In the vicinity of rational magnetic surfaces, the infinite-n ideal MHD ballooning stability boundary is strongly perturbed by the 3D modulations of the local magnetic shear associated with the presence of near-resonant Pfirsch–Schlüter currents. These currents are driven by 3D components of the magnetic field spectrum even when there is no resonant radial component. The infinite-n ideal ballooning stability boundary is often used as a proxy for the onset of virulent kinetic ballooning modes and associated stiff transport. These results suggest that the achievable pressure gradient may be lowered in the vicinity of low order rational surfaces when 3D magnetic perturbations are applied. This mechanism may provide an explanation for the observed reduction in the peak pressure gradient at the top of the edge pedestal during experiments where edge localized modes have been completely suppressed by applied 3D magnetic fields.

Type II superconductivity in SrPd2Ge2

Previous investigations have shown that SrPd2Ge2, a compound isostructural with ‘122’ iron pnictides but iron and pnictogen free, is a conventional superconductor with a single s-wave energy gap and a strongly three-dimensional electronic structure. In this work we reveal the Abrikosov vortex lattice formed in SrPd2Ge2 when exposed to magnetic field by means of scanning tunneling microscopy and spectroscopy. Moreover, by examining the differential conductance spectra across a vortex and estimating the upper and lower critical magnetic fields by tunneling spectroscopy and local magnetization measurements, we show that SrPd2Ge2 is a strong type II superconductor with κ ≫ 2−1/2. Also, we compare the differential conductance spectra in various magnetic fields to the pair-breaking model of Maki and de Gennes for a dirty limit type II superconductor in the gapless region. This way we demonstrate that the type II superconductivity is induced by the sample being in the dirty limit, while in the clean limit it would be a type I superconductor with κ ≪ 2−1/2, in concordance with our previous study (Kim et al (2012) Phys. Rev. B 85 014520).

Hanle diagnostics of weak solar magnetic fields:

Context. The quiet Sun magnetism has been intensively investigated in recent years by various observational techniques. But several issues, such as the question of the isotropy and of the energy density spectrum of the mixed polarity turbulent magnetic fields, are still under debate. Aims. Here we present an inversion method that allows us to constrain the depth-dependence of the magnetic field strength. We use the center-to-limb variations of linear scattering polarization measured in molecular lines of C2 and MgH molecules with different sensitivities to the Hanle effect. We consider six C2-triplets and one MgH line in the spectral range between 515.7 nm and 516.1 nm observed with the THEMIS Telescope. Methods. One of the delicate problems with Hanle diagnostics is to disentangle the effects of elastic depolarizing collisions from the depolarization due to the Hanle effect of the magnetic field. By making use of the different sensitivities of the molecular lines in our spectral range to microturbulent magnetic fields and, by using a non-LTE radiative transfer modeling of the line formation, we are able to determine both the depolarizing collision cross-section and the magnetic strength. We use a standard 1D quiet Sun atmospheric model and we invert the full set of center-to-limb polarization rates measured at line centers, with a depth-dependent magnetic field described by three free parameters. The depolarizing collision cross-section is also treated as a free parameter. A downhill simplex method is used to find the best-fitting values for the collisional and magnetic strength parameters. Results. For the elastic depolarizing collisions cross-section for the C2 lines we obtain α (2) = 1.6 ± 0.4 × 10 −9 cm 3 s −1 , which is within an order of magnitude of the value previously obtained for MgH lines from a differential Hanle effect analysis. The observational constraints provided by the MgH and C2 line polarization give access to the altitude range between z = 200 km and z = 400 km above the base of the photosphere. We find that the turbulent magnetic field strength decreases from 95 Gauss at the altitude z = 200 km to 5 Gauss at z = 400 km. Conclusions. The turbulent magnetic field strength that we derive from the Hanle effect shows a strong vertical gradient in the upper photosphere. We point out that this behavior may explain why very different turbulent magnetic field strengths have been inferred from the interpretation of Hanle depolarization when using different lines formed at different altitudes. We notice that the presence of a strong depth gradient is not compatible with the assumption of isotropy of the turbulent field.

Properties of “site error” of lightning direction-finder (DF) and its modeling

Lightning Location Network (LLN) based on time-of-arrival/direction-finder combined technique (TOA/DF) has been used world-widely for lightning warning and protection purpose. As long as the DF technique is involved in a LLN, it suffers from the “site error”. Firstly, based on experimental data, we have examined the properties of the “site error” as a function of the source azimuth. It is found that for a given DF, the “site error” as a function of source azimuth appears as a complicated sinusoidal waveform with a dominant cycle of either 360° or 180°. A different DF has a different “site error” pattern but this pattern is timely invariant unless the DF's site environment is changed. Secondly, with the magnetic field waveforms recorded by a broadband DF, the properties of the “site error” in frequency domain have been studied. It is found that for a given lightning stroke, the source azimuth determined by the ratio of amplitude of the spectra of magnetic fields from the two orthogonal magnetic loops of a DF varies significantly against the frequency. The source azimuth (or the “site error”) as a function of the frequency usually appears as a complicated curve with some mono-polar and bi-polar impulses superposed on a relative flat line. Thirdly, theoretical explanation and modeling of the “site error” have been attempted based on an electromagnetic dipole model. Simulations with the model show that the mono-polar impulsive fluctuations of the “site error” in frequency domain are due to the reflection of lightning incident signals on “electric-dipole-wise” structures nearby the DF, while the bi-polar ones are due to that of “magnetic-dipole-wise” structures. The model can well interpret the properties of the “site error” as a function of the source azimuth too. Furthermore, possible approaches for making “site error” corrections have been discussed.

Numerical simulations to study kinetic Alfvén wave and whistler wave spectra in solar wind plasma

The numerical simulations of the model equation governing the nonlinear evolution of kinetic Alfven wave (KAW) in solar wind plasmas are performed. The nonlinear dynamical equation of KAW satisfies the modified nonlinear Schrodinger MNLS equation when the ponderomotive nonlinearity is incorporated in the KAW dynamics. The effect of Landau damping is taken into account in the KAW dynamics. The coherent (in the absence of Landau damping) and damped (with Landau damping) localized structures of pump KAW as a consequence of ponderomotive nonlinearity have been studied in the solar wind at 1 AU. A weak whistler signal propagating in these localized structures is amplified which leads to the development of its own coherent and damped localized structures. Magnetic field (KAW) and electric field (whistler wave) power spectra and their spectral indices are calculated. Our results reveal the change in spectral index because of the damping effect which has good agreement with the observations. These damped structures and steeper spectra may be one of the reasons responsible for the plasma heating and particles acceleration in solar wind.

δf Monte Carlo calculation of neoclassical transport in perturbed tokamaks

Non-axisymmetric magnetic perturbations can fundamentally change neoclassical transport in tokamaks by distorting particle orbits on deformed or broken flux surfaces. This so-called non-ambipolar transport is highly complex, and eventually a numerical simulation is required to achieve its precise description and understanding. A new δf particle orbit code (POCA) has been developed for this purpose using a modified pitch-angle collision operator preserving momentum conservation. POCA was successfully benchmarked for neoclassical transport and momentum conservation in the axisymmetric configuration. Non-ambipolar particle flux is calculated in the non-axisymmetric case, and the results show a clear resonant nature of non-ambipolar transport and magnetic braking. Neoclassical toroidal viscosity (NTV) torque is calculated using anisotropic pressures and magnetic field spectrum, and compared with the combined and 1/ν NTV theory. Calculations indicate a clear δB2 scaling of NTV, and good agreement with the theory on NTV torque profiles and amplitudes depending on collisionality.

Non-Gaussian features of primordial magnetic fields in power-law inflation

We show that a conformal-invariance violating coupling of the inflaton to electromagnetism produces a cross correlation between curvature fluctuations and a spectrum of primordial magnetic fields. According to this model, in the case of power-law inflation, a primordial magnetic field is generated with a nearly flat power spectrum and rms amplitude ranging from nG to pG. We study the cross correlation, a three-point function of the curvature perturbation and two powers of the magnetic field, in real and momentum space. The cross-correlation coefficient, a dimensionless ratio of the three-point function with the curvature perturbation and magnetic field power spectra, can be several orders of magnitude larger than expected as based on the amplitude of scalar metric perturbations from inflation. In momentum space, the cross-correlation peaks for flattened triangle configurations, and is three orders of magnitude larger than the squeezed triangle configuration. These results suggest likely methods for distinguishing the observational signatures of the model.

Possible Ionospheric Electromagnetic Perturbations Induced by the Ms7.1 Yushu Earthquake

The power spectrum density (PSD) of magnetic field in the ELF/VLF band recorded by the experiment IMSC onboard the DEMETER satellite were used to study the Ms7.1 Yushu earthquake taking place on April 13, 2010 in China. The results indicate that possible ionospheric electromagnetic perturbations occurring 4 days before the earthquake in the frequency range [370–897 Hz]. Along the orbit 30880_up, which passed over the epicenter area on 9 April, enhanced PSD value of magnetic field at 410 Hz was detected both over the epicentral area and its conjugate point in the southern hemisphere. While on revisited orbits during other days (before and after the earthquake) the magnetic field spectra remained at a relatively low level. In order to be sure that the electromagnetic disturbances were induced by this Yushu earthquake, relative variations of the magnetic field in the ELF/VLF range [370–897 Hz] were calculated and compared with the normal background during 3 years (2007, 2008 and 2009) in the time interval from Jan. 1st to Apr. 30 and in the area [23°N–43°N, 86°E–106°E]. The results show that the normal level of magnetic field in this area is relatively low (~10−7 nT2/Hz), but a large increase occurred from Apr. 1st to Apr. 14 which exceeds 5σb. After the earthquake, the magnetic field in the ELF/VLF range gradually decreased. In order to exclude the influence of geomagnetic field activity, we selected only the data recorded during magnetically quiet local night-times. In addition, artificial noises were also removed from the dataset used in the present paper. Therefore the observed perturbations were independent of geomagnetic field disturbances, and might be attributed to seismic activity.

Study of Rotor Faults in Induction Motors Using External Magnetic Field Analysis

This paper presents a new signature for detection of rotor faults in induction motors, such as eccentricity and broken rotor bars, that uses the external magnetic field analysis. The proposed method is based on the variations of axial flux density in the presence of these faults. The low frequency part of the magnetic field spectrum is particularly analyzed. The analysis is realized through a machine modeling based on permeance circuit under eccentricity fault and also by machine modeling based on coupled magnetic circuit theory under broken rotor bars fault. Analytical relations which describe the machine operation under broken bars fault highlight the influence of speed variation to modify the low frequency components of the external magnetic field. The theoretical results have been validated by experimental measurements. In particular, an inverse stator cage induction machine have been used to measure the bar currents under healthy and faulty cases.

Effects of helical magnetic fields on the cosmic microwave background

A complete numerical calculation of the temperature anisotropies and polarization of the cosmic microwave background in the presence of a stochastic helical magnetic field is presented which includes the contributions due to scalar, vector and tensor modes. The correlation functions of the magnetic field contributions are calculated numerically including a Gaussian window function to effectively cut off the magnetic field spectrum due to damping. Apart from parity-even correlations the helical nature of the magnetic field induces parity-odd correlations between the $E$ and $B$ mode of polarization as well as between temperature ($T$) and the polarization $B$ mode.

Effect of current sheets on the power spectrum of the solar wind magnetic field using a cell model

A puzzling observation of solar wind MHD turbulence is the often seen Kolmogorov scaling of k-5/3, even though the solar wind MHD turbulence is dominated by Alfvénic fluctuations. Recently Li et al. (2011) proposed that the presence of current sheets may be the cause of the Kolmogorov scaling. Here, using a cell model of the solar wind we examine the effect of current sheets on the power spectrum of the solar wind magnetic field. We model the solar wind as multiple cells separated by current sheets. We prescribe the spectra of turbulent magnetic field in individual cells as IK-like and examine the spectra along trajectories that cross multiple boundaries. We find that these spectra become softer and are consistent with the Kolmogorov-scaling. Our finding supports our recent proposal of Li et al. (2011).

Constraint on the primordial vector mode and its magnetic field generation from seven-year Wilkinson Microwave Anisotropy Probe observations

It is often stated that the cosmological vector mode has only a decaying mode and gives negligible effects on cosmological observations. The vector mode, however, has a growing mode if there exists anisotropic stress in energy distributions in the universe. In this paper, we consider a primordial vector mode sustained by free-streaming neutrinos and its associated magnetic field generation. First, we put an observational constraint on the amount of the vector mode from the 7-year WMAP data. The constraint is found as ${r}_{v}\ensuremath{\lesssim}\ensuremath{-}\frac{r}{40}+0.012$, where ${r}_{v}$ and $r$ are the amounts of vector and tensor perturbation amplitudes with respect to the scalar one, respectively. Second, we calculate the spectrum of magnetic fields inevitably created from the primordial vector mode, given the constraint on ${r}_{v}$. It is found that the maximum amount of magnetic fields generated from the vector mode is given by $B\ensuremath{\lesssim}{10}^{\ensuremath{-}22}\text{ }\text{ }\mathrm{G}(\frac{{r}_{v}}{0.012}{)}^{1/2}(\frac{k}{0.002}{)}^{({n}_{v}+1)/2}$, with ${n}_{v}$ being a spectral index of the vector mode. We find a nontrivial cancellation of the magnetic field generation in the radiation-dominated era, which creates a characteristic cutoff in the magnetic field spectrum around $k\ensuremath{\approx}1.0\text{ }\text{ }{\mathrm{Mpc}}^{\ensuremath{-}1}$.

High‐resolution NMR spectroscopy in inhomogeneous fields via Hadamard‐encoded intermolecular double‐quantum coherences

A new pulse sequence based on intermolecular double-quantum coherences was proposed to obtain one-dimensional high-resolution liquid NMR spectra in inhomogeneous magnetic fields via Hadamard encoding. In contrast with the conventional intermolecular multiple-quantum coherences method with a two-dimensional acquisition to obtain one one-dimensional high-resolution spectrum, the new method can provide relatively high-resolution spectra directly through one-dimensional acquisition, and can greatly improve the signal-to-noise ratio of the spectrum within a relatively short acquisition time. Theoretical derivation was performed and analytical expressions of the resulting signals are given. Solution samples in purposely de-shimmed magnetic fields and pig brain tissue samples were tested. The experimental results demonstrate that this sequence can retain useful structural information, even when the field inhomogeneity is sufficiently severe to erase almost all spectral information with conventional one-dimensional single-quantum coherence techniques, and good solvent suppression can be achieved. This method may provide a promising technique for applications in in vivo and in vitro NMR.

Noise properties of HTS-SQUID gradiometer in an elevated magnetic field

Abstract Stability of a high-temperature superconducting quantum interference device (HTS-SQUID) gradiometer with ramp-edge Josephson junctions against application of magnetic fields was investigated. The gradiometer with a SQUID inductor having a 5-μm-wide line and pickup coils of 0.5x1.5 mm for one side was used. Noise spectra were measured for the gradiometer in zero field after cooled in dc magnetic fields up to 300 μT. no increase in noise level was observed until 95 T, being higher than the earth field. Noise spectra in elevated magnetic fields up to 320 μT were also measured for the gradiometer cooled in zero field. No appreciable increase in noise was observed until 150 μT. By decreasing the applied field from 320 μT to zero, the gradiometer recovered the original low noise level. These results indicated that the examined gradiometer had a good durability against magnetic fields and could be used without a magnetic shield for various applications.

Correlation of inflation-produced magnetic fields with scalar fluctuations

If the conformal invariance of electromagnetism is broken during inflation, then primordial magnetic fields may be produced. If this symmetry breaking is generated by the coupling between electromagnetism and a scalar field---e.g. the inflaton, curvaton, or the Ricci scalar---then these magnetic fields may be correlated with primordial density perturbations, opening a new window to the study of non-gaussianity in cosmology. In order to illustrate, we couple electromagnetism to an auxiliary scalar field in a de Sitter background. We calculate the power spectra for scalar-field perturbations and magnetic fields, showing how a scale-free magnetic field spectrum with rms amplitude of ~nG at Mpc scales may be achieved. We explore the Fourier-space dependence of the cross-correlation between the scalar field and magnetic fields, showing that the dimensionless amplitude, measured in units of the power spectra, can grow as large as ~500 H_I/M, where H_I is the inflationary Hubble constant and M is the effective mass scale of the coupling.

Features of the electron-photon drag effect in a spiral ribbon in the external magnetic field

The features of the photon drag effect in a spiral two-dimensional ribbon, which are associated with an asymmetric electron energy spectrum in a longitudinal magnetic field, have been studied theoretically. The effect of the anisotropic transfer of the photon momentum to the electron system in a spectral dependence of the photon drag current density has been revealed.

Green’s function approach to the low temperature properties of Cs2CuCl4: anisotropy effects

We have studied the effect of both axial and transverse anisotropy on the critical field and thermodynamic properties of the field induced three dimensional antiferromagnetic Heisenberg model on the frustrated hexagonal lattice for Cs2CuCl4 compound. The spin model is mapped to a bosonic one with the hard core repulsion constraint and the Green’s function approach has been implemented to get the low energy spectrum and the corresponding thermodynamic properties. To find the critical field (B c ) we have looked for the Bose-Einstein condensation of quasi-particles (magnons) which takes place when the magnon spectrum vanishes at the ordering spiral wave vector. We have also obtained the dispersion of magnon spectrum in the critical magnetic field for each anisotropy parameter to find the spiral wave vector where the spectrum gets its minimum. The magnon energies show a linear dispersion relation close to the quantum critical point. The effect of hard core boson interaction on the single particle excitation energies leads to a temperature dependence of the magnon spectrum versus magnetic field. We have also studied the behavior of specific heat and static structure factor versus temperature and magnetic field.

SPECTRAL SCALING LAWS IN MAGNETOHYDRODYNAMIC TURBULENCE SIMULATIONS AND IN THE SOLAR WIND

The question is addressed to what extent incompressible magnetohydrodynamics (MHD) can describe random magnetic and velocity fluctuations measured in the solar wind. It is demonstrated that distributions of spectral indices for the velocity, magnetic field, and total energy obtained from high resolution numerical simulations are qualitatively and quantitatively similar to solar wind observations at 1 AU. Both simulations and observations show that in the inertial range the magnetic field spectrum E_b is steeper than the velocity spectrum E_v with E_b >~ E_v and that the residual energy E_R = E_b-E_v decreases nearly following a k_perp^-2 scaling.

Turbulence associated with corotating interaction regions at 1 AU: Inertial and dissipation range magnetic field spectra

[1] We examine five Corotating Interaction Regions (CIRs) observed during the recent solar minimum by the Advanced Composition Explorer spacecraft at 1 AU. We apply a familiar series of spectral examinations and compare quantities such as the variance anisotropy, spectral index, cross- and magnetic helicities, and the infered geometry of the wave vectors against established correlations seen in 1 AU turbulence apart from CIR regions. We find variance anisotropies that scale with plasma beta and spectral amplitude, spectral indices in the same familiar ranges, indices within the dissipation range scaling with the apparent rate of energy cascade within the inertial range, and the same linear relationship between cross- and magnetic helicities. As the results agree with earlier established correlations, we conclude that the rate of energy injection by large-scale shear is sufficiently slow as to allow for significant reorganization of the injected energy by the turbulent cascade. This does not mean that there is no evidence of newly injected energy as a result of that large-scale shear. Our companion paper argues that there is significant energy injection within the CIRs. This analysis only argues that energy injection is too slow to fully dominate the interplanetary spectrum.