Low-frequency Magnetic Fluctuations Research Articles

Pitch angle scattering of fast particles by low frequency magnetic fluctuations

The adiabatic invariance of the magnetic moment during particle motion is of fundamental importance to the dynamics of magnetized plasma. The related rate of pitch angle scattering is investigated here for fast particles that thermally stream through static magnetic perturbations. For a uniform magnetic field with a localized perturbation, it is found that the curvature parameter κ2=min(Rc/ρL) does not predict the level of pitch angle scattering. Instead, based on numerical integration of particle orbits in prescribed magnetic fields, we derive predictions for the particle scattering rates, which can be characterized by the relative perturbation amplitude A≃δB/B, and the particle Larmor radius normalized by the field-aligned wavelength of the perturbations, ρL/λ||. Particles with ρL/λ||≃1 are subject to strong pitch angle scattering, while the level of scattering vanishes for both the limits of ρL/λ||≪1 and ρL/λ||≫1. The results are summarized in terms of a scattering operator, suitable for including the described scattering in basic kinetic models.

In-SituEvaluation of Low-Frequency Magnetic Field Fluctuation in an Atomic Comagnetometer

This paper investigates a novel method for the in-situ evaluation of the low-frequency magnetic field fluctuation in an atomic comagnetometer (ACM). An in-situ magnetic field measurement model is first established for the hybrid atomic spin ensembles in the ACM based on the simplified Bloch equations. Then, the ACM is transformed into an in-situ magnetometer by employing a large longitudinal magnetic field and a transversal alternating current (AC) magnetic field, respectively. This in-situ magnetometer is sensitive to the magnetic field fluctuation along the detection laser direction according to the established measurement model and can realize the magnetic field measurement sensitivity of 88fT/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> at 0.1Hz. Finally, the low-frequency magnetic field fluctuation is evaluated with this in-situ magnetometer, whose influence on the stability of the ACM is also analyzed by the Allan variance method. The experimental results show that the contribution of the magnetic field fluctuation along the detection laser direction to the stability of the ACM is about 0.0068 deg/h, which means that the magnetic field error still has a non-negligible effect on the stability of the system. This new in-situ magnetic field measurement method has the advantages of simple operation and no need to replace the atomic vapor cell, which provides an effective means of the magnetic field error evaluation for the ACM.

Wavelet analysis of low frequency magnetic field fluctuations in the Jupiter’s magnetotail

Jupiter’s magnetospheric processes are strongly influenced by the planet fast rotation and by its internal plasma sources. As a consequence, the 10-h rotation period signal is dominant on many datasets. In this work, in order to study low frequency magnetic fluctuations in the Jupiter’s magnetotail, the continuous wavelet transform (CWT) analysis is employed to investigate the magnetic field vector data during six orbits of Galileo spacecraft. Besides the near 10-h Jovian rotation period, the major periods found in this study are 5-h, most likely due to transient disturbances in Jupiter’s magnetosphere, and from 1.5 to 3.5 days, possibly related to global reconfiguration in the magnetosphere.These periodicities may also be related to the mass-loading from the moon Io and to possible modulation by the solar wind. Furthermore, long periods have been also found from 5 to 35 days. The method presented in this study could be very useful for application in other datasets of planetary magnetospheres.

Theory of dual fermion superconductivity in hole-doped cuprates

Since the discovery of the cuprate high-temperature superconductivity in 1986, a universal phase diagram has been constructed experimentally and numerous theoretical models have been proposed. However, there remains no consensus on the underlying physics thus far. Here, we theoretically investigate the phase diagram of hole-doped cuprates based on an itinerant-localized dual fermion model, with the charge carriers doped on the oxygen sites and localized holes on the copper $d_{x^{2}-y^{2}}$ orbitals. We analytically demonstrate that the puzzling anomalous normal state or the strange metal could simply stem from a free Fermi gas of carriers bathing in copper antiferromagnetic spin fluctuations. The short-range high-energy spin excitations also act as the `magnetic glue' of carrier Cooper pairs and induce $d$-wave superconductivity from the underdoped to overdoped regime, distinctly diffrent from the conventional low-frequency magnetic fluctuation mechanism. We further sketch out the characteristic dome-shaped critical temperature $T_c$ versus doping level. The emergence of the pseudogap is ascribed to the localization of partial carriers coupled to the local copper moments or a crossover from the strange metal to a nodal Kondo-like insulator. Our work provides a consistent theoretical framework to understand the typical phase diagram of hole-doped cuprates and paves a distinct way to the studies of both non-Fermi liquid and unconventional superconductivity in strongly correlated systems.

Nematicity and quantum paramagnetism in FeSe

In common with other iron-based high temperature superconductors, FeSe exhibits a transition to a ``nematic'' phase below 90Kelvin in which the crystal rotation symmetry is spontaneously broken. However, the absence of strong low-frequency magnetic fluctuations near or above the transition has been interpreted as implying the primacy of orbital ordering. In contrast, we establish that quantum fluctuations of spin-1 local moments with strongly frustrated exchange interactions can lead to a nematic quantum paramagnetic phase consistent with the observations in FeSe. We show that this phase is a fundamental expression of the existence of a Berry's phase associated with the topological defects of a N\'eel antiferromagnet, in a manner analogous to that which gives rise to valence bond crystal order for spin 1/2 systems. We present an exactly solvable model realizing the nematic quantum paramagnetic phase, discuss its relation with the spin-1 $J_1-J_2$ model, and construct a field theory of the Landau-forbidden transition between the N\'eel state and this nematic quantum paramagnet.

Effect of magnetic field fluctuation on ultra-low field MRI measurements in the unshielded laboratory environment

Magnetic field fluctuations in our unshielded urban laboratory can reach hundreds of nT in the noisy daytime and is only a few nT in the quiet midnight. The field fluctuation causes the Larmor frequency fL to drift randomly for several Hz during the unshielded ultra-low field (ULF) nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) measurements, thus seriously spoiling the averaging effect and causing imaging artifacts. By using an active compensation (AC) technique based on the spatial correlation of the low-frequency magnetic field fluctuation, the field fluctuation can be suppressed to tens of nT, which is a moderate situation between the noisy daytime and the quiet midnight. In this paper, the effect of the field fluctuation on ULF MRI measurements was investigated. The 1D and 2D MRI signals of a water phantom were measured using a second-order low-Tc superconducting quantum interference device (SQUID) in three fluctuation cases: severe fluctuation (noisy daytime), moderate fluctuation (daytime with AC) and minute fluctuation (quiet midnight) when different gradient fields were applied. When the active compensation is applied or when the frequency encoding gradient field Gx reaches a sufficiently strong value in our measurements, the image artifacts become invisible in all three fluctuation cases. Therefore it is feasible to perform ULF-MRI measurements in unshielded urban environment without imaging artifacts originating from magnetic fluctuations by using the active compensation technique and/or strong gradient fields.

TURBULENCE TRANSPORT MODELING OF THE TEMPORAL OUTER HELIOSPHERE

The solar wind can be regarded as a turbulent magnetofluid, evolving in an expanding solar wind and subject to turbulent driving by a variety of in situ sources. Furthermore, the solar wind and the drivers of turbulence are highly time-dependent and change with solar cycle. Turbulence transport models describing low-frequency magnetic and velocity fluctuations in the solar wind have so far neglected solar cycle effects. Here we consider the effects of solar cycle variability on a turbulence transport model developed by Zank et al. This model is appropriate for the solar wind beyond about 1 AU, and extensions have described the steady-state dependence of the magnetic energy density fluctuations, correlation length, and solar wind temperature throughout the outer heliosphere. We find that the temporal solar wind introduces a periodic variability, particularly beyond ~10 AU, in the magnetic energy density fluctuations, correlation length, and solar wind temperature. The variability is insufficient to account for the full observed variability in these quantities, but we find that the time-dependent solutions trace the steady-state solutions quite well, suggesting that the steady-state models are reasonable first approximations.

TURBULENT PITCH-ANGLE SCATTERING AND DIFFUSIVE TRANSPORT OF HARD X-RAY-PRODUCING ELECTRONS IN FLARING CORONAL LOOPS

Recent observations from {\em RHESSI} have revealed that the number of non-thermal electrons in the coronal part of a flaring loop can exceed the number of electrons required to explain the hard X-ray-emitting footpoints of the same flaring loop. Such sources cannot, therefore, be interpreted on the basis of the standard collisional transport model, in which electrons stream along the loop while losing their energy through collisions with the ambient plasma; additional physical processes, to either trap or scatter the energetic electrons, are required. Motivated by this and other observations that suggest that high energy electrons are confined to the coronal region of the source, we consider turbulent pitch angle scattering of fast electrons off low frequency magnetic fluctuations as a confinement mechanism, modeled as a spatial diffusion parallel to the mean magnetic field. In general, turbulent scattering leads to a reduction of the collisional stopping distance of non-thermal electrons along the loop and hence to an enhancement of the coronal HXR source relative to the footpoints. The variation of source size $L$ with electron energy $E$ becomes weaker than the quadratic behavior pertinent to collisional transport, with the slope of $L(E)$ depending directly on the mean free path $\lambda$ again pitch angle scattering. Comparing the predictions of the model with observations, we find that $\lambda \sim$$(10^8-10^9)$ cm for $\sim30$ keV, less than the length of a typical flaring loop and smaller than, or comparable to, the size of the electron acceleration region.

Low-frequency Magnetic Fluctuation Measurement during Magnetic Reconnection in Counter-helicity Plasma Merging Experiment

Magnetic fluctuations with frequency lower than the local ion cyclotron frequency was observed associated with magnetic reconnection event in TS-3 plasma merging device. Counter helicity merging of spheromaks provides significant ion heating by magnetic reconnection to form high-β field-reversed configuration plasma. The magnitude of the fluctuation observed during and after the counter helicity merging has stronger dependency on ion mass than reconnection speed, indicating that fast ion outflow from X-point would bring about excitation of large amplitude low frequency magnetic fluctuations which may be responsible for direct ion heating.

Low Frequency Magnetic Fluctuations during Magnetic Reconnection in Laboratory Experiment

Large amplitude magnetic fluctuation with ion cyclotron range frequency was observed inside the current sheet region during magnetic reconnection in the plasma merging experiment for the first time. The fluctuation exhibited parallel phase velocity close to the drift velocity in the current sheet and parallel wavelength comparable with the ion gyroradius, suggesting that the reconnection rate enhancement is influenced by the drift kink instability of the current sheet.

Observation of Low-Frequency Beat Waves in GAMMA-10

In the ion cyclotron range of frequency (ICRF) heating experiments on GAMMA 10, wave-wave and wave-particle interactions are investigated. Low-frequency fluctuations of around 100 kHz with beat frequencies among the AIC modes have been observed. These low-frequency modes are also detected in the signal of electrostatic probes in the central cell and in the signal of end-loss high-energy ion detector. Axial transport (velocity space diffusion) of high-energy ions due to beat waves among the AIC modes is clearly indicated. On the other hand, radial transport of high-energy ions due to the drift-type fluctuations has been observed in the central cell. The excitation of low-frequency magnetic fluctuations of which frequencies, fLF, are less than 1 MHz and satisfy the relation of fLF = fICRF - fAIC, where fICRF is the frequency of the heating ICRF wave and fAIC the frequency of the AIC modes. The parametric decay of the heating ICRF waves to the AIC modes and low-frequency waves will be a possible mechanism.

Chaos in the Hanbit Mirror Device

The chaotic phenomena of the low frequency magnetic fluctuations in the HANBIT device have been studied and measured by two magnetic probe arrays in the central cell. The HANA (HANBIT Nonlinear Analysis) code has been developed and used to analyze the magnetic fluctuations to determine whether or not it is chaotic. The autocorrelation function of the magnetic fluctuations is obtained at two different positions in the HANBIT device and was also used to determine chaotic transition. The influence of the magnetic field strength on the magnetic fluctuations has also been studied. When the magnetic field strength is over 0.229 T~ 0.233 T, wave bifurcation is observed and the Primary Lyapunov Exponent (PLE) value is increased. Hence, there is chaotic transition at a specific magnetic field strength in the HANBIT device.

Geometry of low‐frequency solar wind magnetic turbulence: Evidence for radially aligned Alfvénic fluctuations

We employ “omnitape” magnetic field data to determine the geometry of low‐frequency (5‐ to 12‐hour timescales) magnetic fluctuations in the solar wind. We consider three axisymmetric geometries, slab, two‐dimensional (2‐D), and isotropic, as well as binary combinations of them. Both the radial direction and the mean magnetic field direction are considered as candidate symmetry axes. We apply simultaneously three different tests for distinguishing these geometries. Our analysis decisively favors a binary geometry composed of 2‐D turbulence symmetric with respect to the mean field direction and slab turbulence symmetric with respect to the radial direction. The presence of a slab component with radial symmetry provides observational support for a long‐standing theoretical prediction of radially aligned Alfvénic fluctuations at 1 AU. We also find a variation of turbulence properties with solar wind speed, such that magnetic fluctuations in slow wind are more energetic and possess a greater proportion of slab modes than those in fast wind, and the ratio of longitudinal to transverse power in the 2‐D component increases with wind speed.

Incommensurate Magnetic Fluctuations inYBa2Cu3O6.6

We use inelastic neutron scattering to demonstrate that the low-frequency magnetic fluctuations in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.6}$ ( ${T}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}62.7$ K) change from commensurate to incommensurate on cooling with the incommensurability first appearing at temperatures above ${T}_{c}$. For the energies studied, the susceptibility at incommensurate positions increases on cooling below ${T}_{c}$, accompanied by a suppression of the spin fluctuations at the commensurate points. These results suggest that incommensurate spin fluctuations may be a common feature for all cuprate superconductors.

Spreading and intermittent structure of the upstream boundary of planetary magnetic foreshocks

The fast particles in the electron foreshocks propagate along the magnetic field lines, so that the effect of low frequency magnetic fluctuations is to create both a broadening and a fine structure of the foreshock upstream boundary. This is studied by means of a 3‐D numerical simulation of turbulent magnetic fields. Applications to Venus, Earth, and Mars magnetic foreshocks are done. It is found that a spacecraft can be magnetically connected to the bow shock even when it is upstream of the nominal magnetic foreshock boundary, and that the mixing of magnetic field lines can give an explanation for the broadband bursty electrostatic noise observed within the electron foreshock.

Cross-field renormalized statistical approach: short-wave low-frequency stationary spectra in a magnetized current-carrying plasma

The low-frequency (frequencies not exceeding the ion gyrofrequency) shortwave (wavelengths smaller than the ion gyroradius) potential, cross-field and magnetic stationary fluctuation spectra in a magnetized current-carrying plasma, are studied by using the canonical distribution method, the new general renormalized statistical approach with cross-field correlations, and the self-similar transformation technique. The cross correlations are found to influence substantially the spectra formation, distorting the stationary plasma state.

Magnetic fluctuations in superconducting La2 − xSrxCuO4

Abstract We present studies of the low-frequency magnetic fluctuations in La2 − xSrxCuO4, with particular emphasis on a sample with x = 0.14 and a superconducting Tc of 35 K. This material exhibits incommensurate peaks in χ″( Q ,ω) . The magnitude of the incommensuration, δ, increases with doping, x, in the composition range probed. At low temperatures the coherence length associated with the lowest-frequency fluctuations is larger than the mean spacing between impurities. The superconducting transition suppresses, but does not eliminate, the fluctuations with h ω ≤ 6 meV . This reduction in χ″ to about 60% of the response present just above Tc is isotropic, a result incompatible with a clean d-wave gap function. It is, however, consistent with gapless superconductivity induced by localized magnetic impurities.

Kolmogorov spectra of short-wave convective and magnetostatic fluctuations in a finite-pressure plasma

The Kolmogorov spectra of low-frequency short-wave potential and magnetic fluctuations are obtained in a plasma with finite ion and electron pressures, βi ∼ βe ∼ 1. Magnetic nonlinearities are found to make the dominant contribution to the renormalized width of the convective spectrum and result in its essential broadening, that gives rise to a reduced anomalous electron heat conductivity.

Magnetic fluctuations in superconducting La2−xSrxCuO4 (invited) (abstract)

Studies of the magnetic dynamics of La2−xSrxCuO4 have been carried out with particular emphasis on a sample with x=0.14 that becomes superconducting at Tc=33 K. This material exhibits incommensurate peaks in the inelastic magnetic neutron scattering with the same geometry as found for a metallic (but not superconducting) sample with x=0.075. In both cases the peak in χ″(Q,ω) is shifted from the commensurate position, (π,π) (in the notation of the square lattice of the CuO2 planes), to (π,π)±δ(π,0) and (π,π)±δ(0,π). The magnitude of the incommensuration, δ, increases with doping, x, as δ≊2x. As the temperature is decreased below 100 K the peaks become sharper in Q space and more intense at low frequencies. This development is interrupted by the superconducting transition which suppresses the fluctuations at low frequencies (below about 6 meV). The loss of low frequency magnetic fluctuations is uniform over the region of reciprocal space where the response is present in the normal state.

The directional dependence of magnetic fluctuations generated by cometary ion pickup

The properties of low frequency magnetic fluctuations generated by cometary ion pickup are examined by means of one‐dimensional hybrid simulations, in which newborn ions are created at a constant rate. The helicity and direction of propagation of magnetic fluctuations are investigated for various cometary ion injection angles, α, relative to the solar wind magnetic field. The parameter η represents the relative contribution of wave energy density propagating in the direction away from the comet, parallel to the beam. For small (quasi‐parallel) injection angles, α ∼ 0° and η is of order unity, while for larger (quasi‐perpendicular) angles, α ∼ 90° and η is about 0.5. At intermediate angles, α ∼ 60°, η can vary between 0 and 1, depending on the wave number. The wave properties are consistent with the instabilities (right‐hand cyclotron resonant modes at small α, left‐hand Alfven/ion cyclotron modes at large a) expected from linear theory. Consequences of these results for particle acceleration are also discussed.