Magnetic Field Oscillations Research Articles

Detection of subnanotesla oscillatory magnetic fields using MRI.

Direct mapping of neuronal currents using MRI would have fundamental impacts on brain functional imaging. Previous reports indicated that the stimulus-induced rotary saturation (SIRS) mechanism had the best potential of direct detection of neural oscillations; however, it lacked the high-sensitivity level needed. In this study, a novel strategy is proposed in an effort to improve the detection sensitivity. In our modified SIRS sequence, an external oscillatory magnetic field is used as the excitation pulse in place of the standard 90-degree excitation pulse. This approach could potentially lead to tens or even hundreds times of enhancement in the detection sensitivity for low field signals. It also helps to lower the physiological noise, allows for shorter pulse repetition time, and is less affected by the blood oxygen level. We demonstrate that a 100-Hz oscillatory magnetic field with magnitude as low as 0.25 nanotesla generated in a current loop can be robustly detected using a 3-Tesla MRI scanner. The modified SIRS sequence offers higher detection sensitivity as well as several additional advantages. These promising results suggest that the direct detection of neural oscillation might be within the grasp of the current MRI technology.

Exchange enhancement of the electron g factor in strained InGaAs/InP heterostructures

The exchange enhancement of the electron g factor in strained InGaAs/InP heterostructures with a two-dimensional electron gas is studied. Analysis of the temperature dependence of the resistance in the minima of the Shubnikov-de Haas oscillations in perpendicular magnetic fields up to 12 T in the vicinity of the odd filling factors of the Landau levels yields the values of the effective electron Lande factor g* from −8.6 to −10.1. The experimental values are compared with the results of theoretical calculations of the g factor of quasiparticles. The calculations are performed using an eight-band k · p Hamiltonian and take into account exchange interaction in the two-dimensional electron gas. It is shown that, under the conditions of a large overlap between the spin-split Landau levels, the maximum value of the quasiparticle g factor can be attained in the vicinity of even filling factors. This is caused by the nonparabolicity of the electron dispersion relation.

Transport characteristics of focused beam deposited nanostructures

AbstractWe review the transport properties of different nanostructures produced by ion- and electron-beam deposition, as prepared as well as after certain treatments. In general, the available literature indicates that the transport properties are determined by conduction processes typical for disordered metallic grains embedded in a carbon-rich matrix, including intergrain tunneling and variable range hopping mechanisms. Special emphasis is given to the superconducting behavior found in certain Tungsten-Carbide nanostructures that, in a certain field and temperature range, is compatible with that of granular superconductivity. This granular superconductivity leads to phenomena like magnetic field oscillations as well as anomalous hysteresis loops in the magnetoresistance.

Electronic and Thermodynamic Properties of Sn and SbI3 Doped Single Crystals p - Bi2Te3

Bi2Te3 - Sb2Te3 solid solutions, the most commonly used thermoelectrics (~300K), are characterized by inhomogeneities and defects of various types, and doping contributes to spatial inhomogeneity. These impurity energy states fall into the band gap with small high-field SdH oscillations. However, it is known that Sn impurity pins the Fermi level and tremendously improves the spatial homogeneity, leading to observation of high amplitude SdH oscillations in lower magnetic field. The iodine (I) impurity was chosen to shift the Fermi level and affect the filling factor of impurity states. Numerous experimental results on magnetoresistivity, specific heat and magnetic susceptibility indicate that the most probable model for one-electron states in Bi2Te3 solid solutions doped with Sn is the presence of two impurity bands one filled and one empty- with the Fermi level pinned in-between. Earlier we reported on SdH oscillations at T = 4.2K and in magnetic Field up to 10T. Here, we report on investigations of the magnetoresistivity (2 < T < 295K) and its oscillations (T < 12K). We deduce the carriers mobility and concentration and their temperature dependence along with that of the dampening of SdH amplitude. We also report on specific heat measurements in the range (2 < T < 295K), with a low T power law of exponent p=3.5 behavior.

Midterm periods of solar filaments

AbstractOn the basis of the Carte Synoptique catalogue of solar filaments from March 1919 to December 1989, we measure power spectra of detrended full‐disk (FSFNs, latitudinal bands: 0∘–90∘), low‐latitude (LSFNs, latitudinal bands: &lt;50∘), and high‐latitude (HSFNs, latitudinal bands: ≥50∘) solar filament numbers by Maximum Entropy Method (MEM) and Continuous wavelet transform to detect midterm periods. It is found as the following: (1) FSFNs and LSFNs have the same midterm periodicity, while HSFNs show a different midrange periodicity. Some periods frequently mentioned in other solar indices are also detected from the solar filament numbers, such as 2–3 year period (quasi‐biennial oscillation—QBO), ∼1.7 year, ∼1.3 year, ∼1 year, 150–157 day period (Rieger period), and 6.0–6.4 months (Rieger‐type period). These periods are intermittent during considered time span. Some of them are missing in some solar cycles. (2) QBO is detected from total data and most solar cycles of FSFNs, LSFNs, and HSFNs. It may be related to oscillation of magnetic field of solar surface. (3) Approximately 1.3 year period occasionally appears, but ∼1.7 year period is hardly seen. These two periods probably are seasonal effects. (4) Approximately 1 year period is detected from both total data and every solar cycle of FSFNs and LSFNs but hardly detected from HSFNs. It is perhaps connected with sunspot activity. (5) Rieger period of 5.0–5.2 months is detected in total data and even solar cycles of HSFNs. Rieger‐type period of 6.0–6.4 months is found in total data and most solar cycles, except cycle 18 of LSFNs and FSFNs. These periods seem to be subharmonics of ∼11 year period.

SYNTHETIC OBSERVATIONS OF WAVE PROPAGATION IN A SUNSPOT UMBRA.

Spectropolarimetric temporal series from Fe i λ6301.5 Å and Ca ii infrared triplet lines are obtained by applying the Stokes synthesis code NICOLE to a numerical simulation of wave propagation in a sunspot umbra from MANCHA code. The analysis of the phase difference between Doppler velocity and intensity core oscillations of the Fe i λ6301.5 Å line reveals that variations in the intensity are produced by opacity fluctuations rather than intrinsic temperature oscillations, except for frequencies between 5 and 6.5 mHz. On the other hand, the photospheric magnetic field retrieved from the weak field approximation provides the intrinsic magnetic field oscillations associated to wave propagation. Our results suggest that this is due to the low magnetic field gradient of our sunspot model. The Stokes parameters of the chromospheric Ca ii infrared triplet lines show striking variations as shock waves travel through the formation height of the lines, including emission self-reversals in the line core and highly abnormal Stokes V profiles. Magnetic field oscillations inferred from the Ca ii infrared lines using the weak field approximation appear to be related with the magnetic field strength variation between the photosphere and the chromosphere.

Cassini multi‐instrument assessment of Saturn's polar cap boundary

AbstractWe present the first systematic investigation of the polar cap boundary in Saturn's high‐latitude magnetosphere through a multi‐instrument assessment of various Cassini in situ data sets gathered between 2006 and 2009. We identify 48 polar cap crossings where the polar cap boundary can be clearly observed in the step in upper cutoff of auroral hiss emissions from the plasma wave data, a sudden increase in electron density, an anisotropy of energetic electrons along the magnetic field, and an increase in incidence of higher‐energy electrons from the low‐energy electron spectrometer measurements as we move equatorward from the pole. We determine the average level of coincidence of the polar cap boundary identified in the various in situ data sets to be 0.34° ± 0.05° colatitude. The average location of the boundary in the southern (northern) hemisphere is found to be at 15.6° (13.3°) colatitude. In both hemispheres we identify a consistent equatorward offset between the poleward edge of the auroral upward directed field‐aligned current region of ~1.5–1.8° colatitude to the corresponding polar cap boundary. We identify atypical observations in the boundary region, including observations of approximately hourly periodicities in the auroral hiss emissions close to the pole. We suggest that the position of the southern polar cap boundary is somewhat ordered by the southern planetary period oscillation phase but that it cannot account for the boundary's full latitudinal variability. We find no clear evidence of any ordering of the northern polar cap boundary location with the northern planetary period magnetic field oscillation phase.

Thickness-Dependent Oscillations of the Critical Magnetic Field Induced by the Quantum Size Effect in Superconducting Nanofilms

The superconductor to normal metal transition driven by the in-plane magnetic field is investigated for Al nanofilms. Taking the Pauli breaking mechanism (with no orbital effect) into account, it is shown that the critical field oscillates as a function of the nanofilm thickness. This effect is discussed in terms of the quantization of electron energy caused by the confinement of its motion in the direction perpendicular to the film. The analysis of the critical field in the context of Clogston–Chandrasekhar limit is also included.

Vortex Lattice Studies in Type-II Superconductors by SANS

When a type-II superconductor is placed in a magnetic field, it is threaded by swirling whirlpools of electric current known as vortices. The vortices behave like massive entities and provide a unique probe into the nature of the superconducting state in the host material. Furthermore, the collective vortex behavior is of crucial importance for practical applications since vortex motion will lead to dissipation. I will discuss two recent vortex lattice (VL) studies using small-angle neutron scattering (SANS) that exemplify the continuous evolution of this technique. In the first example we used SANS to determine the superconducting anisotropy of Sr2RuO4 (SRO) that is among the few superconductors believed to exhibit p-wave pairing [1]. While there is significant experimental support for unconventional pairing, there is a discrepancy between the anisotropies of the upper critical field and the Fermi surface. Taking advantage of the significant transverse VL field component that arises due to the large anisotropy of SRO we measured the superconducting anisotropy ≍ 60, roughly three times greater than the upper critical field anisotropy. This result imposes significant constraints on possible models of triplet pairing in SRO. In the second example we used a stop-motion technique to study VL transition dynamics in MgB2 [2,3]. Here the VL exhibit extensive metastability in connection with a second order rotational phase transition that cannot be understood based on the single VL domain free energy or vortex pinning. Instead, we have proposed that a jamming of VL domains acting as granular entities is responsible for the metastability. We have performed extensive SANS experiments, as the VL is driven from the metastable to the ground state by small-amplitude AC magnetic field oscillations. This shows a dual power-law behavior indicating a two-step process for the transition to the ground state. Supported by the US Department of Energy (Grant No. DE-FG02-10ER46783).

Doxorubicin-loaded magnetic gold nanoshells for a combination therapy of hyperthermia and drug delivery

In the present work, nanohybrid of an anticancer drug, doxorubicin (Dox) loaded gold-coated superparamagnetic iron oxide nanoparticles (SPIONs@Au) were prepared for a combination therapy of cancer by means of both hyperthermia and drug delivery. The Dox molecules were conjugated to SPIONs@Au nanoparticles with the help of cysteamine (Cyst) as a non-covalent space linker and the Dox loading efficiency was investigated to be as high as 0.32mg/mg. Thus synthesized particles were characterized by HRTEM, UV–Vis, FT-IR, SQUID magnetic studies and further tested for heat and drug release at low frequency oscillatory magnetic fields. The hyperthermia studies investigated to be strongly influenced by the applied frequency and the solvents used. The Dox delivery studies indicated that the drug release efficacy is strongly improved by maintaining the acidic pH conditions and the oscillatory magnetic fields, i.e. an enhancement in the Dox release was observed from the oscillation of particles due to the applied frequency, and is not effected by heating of the solution. Finally, the in vitro cell viability and proliferation studies were conducted using two different immortalized cell lines containing a cancerous (MCF-7 breast cancer) and non-cancerous H9c2 cardiac cell type.

Moderate geomagnetic storms of January 22–25, 2012 and their influences on the wave components in ionosphere and upper stratosphere-mesosphere regions

Moderate geomagnetic storms occurred during January 22–25, 2012 period. The geomagnetic storms are characterized by different indices and parameters. The SYM-H value on January 22 increased abruptly to 67nT at sudden storm commencement (SSC), followed by a sharp decrease to −87nT. A second SSC on January 24 followed by a shock on January 25 was also observed. These SSCs before the main storms and the short recovery periods imply the geomagnetic storms are CME-driven. The sudden jump of solar wind dynamic pressure and IMF Bz are also consistent with occurrence of CMEs. This is also reflected in the change in total electron content (TEC) during the storm relative to quiet days globally. The response of the ionospheric to geomagnetic storms can also be detected from wave components that account for the majority of TEC variance during the period. The dominant coherent modes of TEC variability are diurnal and semidiurnal signals which account upto 83% and 30% of the total TEC variance over fairly exclusive ionospheric regions respectively. Comparison of TEC anomalies attributed to diurnal (DW1) and semidiurnal (SW2) tides, as well as stationary planetary waves (SPW1) at 12 UTC shows enhancement in the positive anomalies following the storm. Moreover, the impact of the geomagnetic storms are distinctly marked in the daily time series of amplitudes of DW1, SW2 and SPW1. The abrupt changes in amplitudes of DW1 (5 TECU) and SW2 (2 TECU) are observed within 20°S–20°N latitude band and along 20°N respectively while that of SPW1 is about 3 TECU. Coherent oscillation with a period of 2.4days between interplanetary magnetic field and TEC was detected during the storm. This oscillation is also detected in the amplitudes of DW1 over EIA regions in both hemispheres. Eventhough upward coupling of quasi two day wave (QTDWs) of the same periodicity, known to have caused such oscillation, are detected in both ionosphere and upper stratosphere, this one can likely be attributed to the geomagnetic storm as it happens after the storm commencement. Moreover, further analysis has indicated that QTDWs in the ionosphere are strengthened as a result of coherent oscillation of interplanetary magnetic field with the same frequency as QTDWs. On the otherhand, occurrences of minor SSW and geomagnetic storms in quick succession complicated clear demarcation of attribution of the respective events to variability of QTDWs amplitudes over upper stratosphere.

Soliton–antisoliton interaction in a parametrically driven easy-plane magnetic wire

Abstract In the present work we study the soliton–antisoliton interaction in an anisotropic easy-plane magnetic wire forced by a transverse uniform and oscillatory magnetic field. This system is described in the continuous framework by the Landau–Lifshitz–Gilbert equation. We find numerically that the spatio-temporal magnetization field exhibits both annihilative and repulsive soliton–antisoliton interactions. We also describe this system with the aim of the associated Parametrically Driven and Damped Nonlinear Schrodinger amplitude equation and give an approximate analytical solution that roughly describes the repulsive interaction.

Alfvén and Helicon Wave Resonances Measured with Demodulator Circuit in TCABR

The measurements of the RF driven magnetic field oscillations with the analog demodulator circuitry in the TCABR tokamak are presented. This circuitry works as the synchronous detector that is used to multiply the signal from magnetic probes by the reference signal, and then to filter out the low frequency signal. The output signal carries the information about the amplitude and the phase of the RF driven plasma oscillations, and can be acquired by the TCABR data acquisition system during all RF pulse.The experiments were carried out in the regimes with the low-density plasma n = 1 – 9 × 1016/m3 and magnetic field 0.04T when helicon wave resonances can be excited. The antennae were fed by the RF generators whose frequency was varied in the range 2–5MHz with variable time Δt = 1 – 5ms. The signals acquired with the analog demodulator circuitry were compared with the results obtained after numerical simulations of the data acquired directly with the high frequency oscilloscope. The very good agreement was demonstrated that makes possible to use the demodulator circuit measurements with long RF pulse duration in future experiments.

Experimental Study and Numerical Modelling of Magnetic Domain Walls Drift in Ferrite Garnet Crystals

The results of study of domain walls oscillations in harmonic magnetic field H = H0sin (2πft) oriented perpendicular to ferrite garnet (TbErGd)3(FeAl)5O12 (111) sample plate for amplitudes that include the drift of domain walls are reported. Numerical modelling of domain walls motion was performed for frequencies f~102 Hz, where the drift is observed experimentally. Comparison of results of numerical modelling with experimental results shows their qualitative agreement. It was established that domain walls oscillations amplitude is a linear function of amplitude of oscillating magnetic field.

The origin of spectral resonance structures of the ionospheric Alfvén resonator. Single high‐altitude reflection or resonant cavity excitation?

A common view is that spectral maxima in observed spectral resonance structures (SRS) of ionospheric Alfvén resonator (IAR) at frequencies f&lt;5 Hz are the signature of resonance frequencies of the IAR. We have studied not only spectra but also waveforms of magnetic fluctuations at IAR frequencies registered at Moshiri station (Japan) and have found that there exist two kinds of signals. The dominant type of signal is a pair of pulses which is caused by an initial exciting impulse and accompanied by a single reflection from the top boundary of the IAR. In the absence of reflection from the lower ionosphere, such signals are not resonant and hence are not caused by IAR excitation. The minority of cases are trains of three or more pulses separated by a nearly constant time interval reflected from both IAR boundaries. We have found that different kinds of signals in time domain may correspond to similar comb‐shaped Fourier spectra. So different kinds of signals in time domain practically cannot be distinguished on the basis of their Fourier spectra. We have calculated waveforms and SRS structures of the magnetic field oscillations generated by a model lightning discharge and IAR resonant frequencies. Calculated IAR resonance frequencies can be in disagreement with those of spectral maxima of pulse trains. Then, an analysis of signal waveforms in time domain is highly required to estimate IAR resonance frequencies.

Specific features of long-period oscillations of sunspots and photosphere according to the MDI (SOHO) data

Long-period oscillations of the magnetic field and the line-of-sight velocity have been detected in sunspots based on the synchronous 90-h time series of magnetograms and dopplergrams obtained with the MDI(SOHO) device. The sunspot eigenmode (12–30 h), the periods of which nonlinearly depend on the magnetic field strength, predominates in the magnetic field oscillation spectrum. The mode, which is probably induced by motions of supergranulation cells (33 h), predominates in the sunspot line-of-sight velocity oscillation spectrum. A strong mode (33 h), which indicates that long-period quasi-oscillations of supergranulation exist, was also detected in the velocity power spectrum for a quiet photosphere, together with the known 5-min mode.

Modeling of the solar activity double cycle using dynamical systems

The generation and evolution of the Sun’s magnetic field and other stars is usually related to the dynamo mechanism. This mechanism is based on the consideration of the joint influence of the α effect and differential rotation. Dynamo sources can be located at different depths of the convection zone and can have different intensities. Based on such a system, the dynamical system in the case of the stellar dynamo in a two-layer medium has been constructed with regard to meridional fluxes in order to model the double cycle that corresponds to the simultaneous presence of 22-year and quasi-biennial magnetic field oscillations. It has been indicated that the regime of mixed oscillations can originate because a dynamo wave moves oppositely to the meridional flows in the upper layer of the convection zone. This results in the deceleration of the toroidal field propagation and in the generation of slow oscillations. In deeper layers, the directions of a dynamo wave and meridional flows coincide with each other, as a result of which fast magnetic fields originate. Therefore, the total contribution of two oscillations with different frequencies corresponds to the appearance of quasi-biennial cycles against 22-year cycles. It has been indicated that the beating regime, which can be related to the secular oscillations of solar magnetic activity, originates in the system when the meridional flows are weak.

Breather soliton solutions in a parametrically driven magnetic wire

In the present work we study the pattern formation in a magnetic wire forced by a transversal uniform and oscillatory magnetic field. This system is described in the continuous framework by the Landau-Lifshitz-Gilbert equation. We find numerically that the spatio-temporal magnetization field exhibits a family of breather soliton states. We characterize different types of breathers as a function of the amplitude and frequency of the driven field.

Magnetic turbulence in the cusp region: 3D spectra and vortex cascades

Cluster satellite measurements that were carried out in the region of the altitudinal cusp in the Earth's magnetosphere and adjacent regions were processed to yield the dynamic pattern of the 3D spectra of magnetic field oscillations in the space of wave vectors, with wave numbers ranging from 0.002 to 0.5 rad/km. The anisotropic properties of space spectra that are functions of the mean plasma parameters are studied. The role of the ion cyclotron resonances and the resonance related to the ion inertial length is shown. The vortex structures detected in the cusp and adjacent regions are described. The issue concerning in what directions turbulent energy is primarily transferred in the physical space.

Quasi-periodic Faraday-rotation fluctuations of the polarization plane during radio sounding of the circumsolar plasma

The statistical characteristics of Faraday-rotation fluctuations (FRFs) of a radio-wave polarization plane are analyzed when the circumsolar plasma is sounded by the signals of a Helios 1 space probe. The time spectra and auto- and crosscorrelation functions of FRFs, which are measured simultaneously at two ground stations separated by large distances, have been determined. The wavelet spectra of FRFs are presented, and the opportunities of the wavelet transform and other techniques used to reveal quasi-periodic FRFs with different periods are investigated. It is demonstrated that the quasi-periodic magnetic field oscillations, which are excited by the trains of Alfven waves propagating through the solar supercorona (their period varies between 2 and 160 min), are observed at the heliocentric distances of 3–12 solar radii.