Weak Field Region Research Articles

Numerical simulation of drift-resistive ballooning turbulence in the presence of the GA mode in the plasma edge tokamak

Drift-resistive ballooning turbulence is simulated numerically based on a quasi-three-dimensional computer code for solving nonlinear two-fluid MHD equations in the scrape-off layer plasma in a tokamak. It is shown that, when the toroidal geometry of the magnetic field is taken into account, additional (geodesic) flux terms associated with the first poloidal harmonic (∼sinθ) arise in the averaged equations for the momentum, density, and energy. Calculations show that the most important of these terms is the geodesic momentum flux (the Stringer-Windsor effect), which lowers the poloidal rotation velocity. It is also shown that accounting for the toroidal field geometry introduces experimentally observed, special low-frequency MHD harmonics—GA modes—in the Fourier spectra. GA modes are generated by the Reynolds turbulent force and also by the gradient of the poloidally nonuniform turbulent heat flux. Turbulent particle and heat fluxes are obtained as functions of the poloidal coordinate and are found to show that, in a tokamak, there is a “ballooning effect” associated with their maximum in the weak magnetic field region. The dependence of the density, temperature, and pressure on the poloidal coordinate is presented, as well as the dependence of turbulent fluxes on the toroidal magnetic field.

How to deal with measurement errors and lacking data in nonlinear force-free coronal magnetic field modelling?

Context. The measured solar photospheric magnetic field vector is extrapolated into the solar corona under the assumption of a force-free plasma. In the generic case this problem is nonlinear. Aims. We aim to improve an algorithm for computing the nonlinear force-free coronal magnetic field. We are in particular interested to incorporate measurement errors and to handle lacking data in the boundary conditions.Methods. We solve the nonlinear force-free field equations by minimizing a functional. Within this work we extend the functional by an additional term, which allows us to incorporate measurement errors and treat regions with lacking observational data. We test the new code with the help of a well known semi-analytic test case. We compare coronal magnetic field extrapolations from ideal boundary conditions and boundary conditions where the transversal magnetic field information is lacking or has a poor signal-to-noise ratio in weak field regions. Results. For ideal boundary conditions the new code gives the same result as the old code. The advantage of the new approach, which includes an error matrix, is visible only for non-ideal boundary conditions. The force-free and solenoidal conditions are fulfilled significantly better and the solutions agrees somewhat better with the exact solution. The new approach also relaxes the boundary and allows a deviation from the boundary data in poor signal-to-noise ratio areas. Conclusions. The incorporation of measurement errors in the updated extrapolation code significantly improves the quality of nonlinear force-free field extrapolation from imperfect boundary conditions.

Ascension and Port Stanley geomagnetic observatories and monitoring the South Atlantic Anomaly

Our 15-year experience of operating two remote observatories, Ascension and Port Stanley, in the south Atlantic is described. These observatories help monitor the South Atlantic Anomaly (SAA), a region of weak magnetic field which causes considerable problems for spacecraft operators. One-minute and one-second values from these observatories, and other observatories both inside and outside the SAA, are analysed. We investigate whether the SAA, and its growth over time, are having any tangible effect on the observed external field variations. Whilst only able to illustrate the long-term characteristics of the irregular external field related to the solar cycle and not due to any long-term changes in the internal field, we do isolate micropulsation signals at sites inside the SAA which contain more power than at sites outside.

Magnetic field effects on THz radiation from rectangular shape Bi2212 IJJ’s

The effects of magnetic field on terahertz radiation emitted from the rectangular mesa of intrinsic Josephson junctions (IJJ’s) made of Bi2212 single crystals were studied up to 150Oe. Strong suppression of the radiation power was observed when magnetic field was applied to the c-axis of the sample, while in the case of H//ab-plane, the radiation power showed a weak and broad peak in the weak field region (∼50Oe), then it decreased and diminished around 150Oe. These magnetic field effects are interpreted by the strong influence of magnetic field on the I–V curve, which set up the condition for the THz radiation.

Photon emission of single quantum dot driven by continuous external field studied by using the generating function

We studied the properties of the X- and Y- polarized photon emission from the single quantum dot system driven by external cw laser using the generating function approach developed recently. The results demonstrate that the X- and Y- polarized photon show sine or cosine behavior with the field direction in the weak field region. They show, however, stronger nonlinear behavior in the strong field region.

Dielectric properties of transformer paper impregnated by mineral oil based magnetic fluid

The influence of combined magnetic and electric field on permittivity of transformer paper used in power transformers was observed. Transformer paper was impregnated by pure transformer oil ITO 100 and magnetic fluids based on transformer oil ITO 100 with different concentrations of magnetite nanoparticles. The measurements were carried out with help of high precision capacitance bridge. The electric intensity between circular planar electrodes was in the region of weak electric field (E > 106 V/m). The increase of electric permittivity of transformer paper impregnated by magnetic fluid opposite pure transformer paper was observed. The experiments showed that permittivity of insulator system consisting of pure transformer paper and impregnated transformer paper naturally depends on number of paper layers. The magnetodielectric effect was found to be dependent on magnetite nanoparticles concentration in magnetic fluids.

The distributions of plasma parameters in the "Magnetor" device with hot cathode discharge

The dipole magnetic configuration as realized in planet magnetospheres is of interest for controlled fusion plasma confinement. This type of magnetic trap is free of convective instability. Distributions of plasma parameters in "Magnetor" device with double dipole magnetic configuration that allows restrict considerable volume of confined plasma are investigated. The comparison of plasma parameters distribution inside the trap for two plasma generation methods: microwave discharge at 2,45GHz and hot cathode discharge in crossed E and B fields are made. It is shown that plasma is localized inside the magnetic separatrix for both types of discharges. To determine possible additional plasma movements caused by the presence of the electric field in the magnetic configuration measurements of plasma flows in discharge with the hot cathode in crossed E and B fields are carried out. Two directions of plasma flows with flux density 1016÷1017cm−2s−1 inside the magnetic trap are determined: the flow along the axis leading particles leaving from a trap through weak magnetic field region and the flow caused by plasma azimuth drift.

Plasma plume dynamics in magnetically assisted pulsed laser deposition

The expansion of a laser produced plasma perpendicular to a magnetic field is studied with a quadruple Langmuir probe and a B-dot probe. In regions where the kinetic beta is less than one, we find plume deceleration and weak displacement of the magnetic field. As the plume expands into regions of weak magnetic field, plume deceleration stops and the displacement of the magnetic field is large. The diffusion time of the magnetic field lines was consistent with anomalously large resistivity driven by the presence of an instability. Electron temperatures are larger than in the field-free case due to Ohmic heating mediated by the anomalously large resistivity.

A new fast reconnection model in a collisionless regime

Based on the first principles [i.e., (i) by balancing the magnetic field advection with the term containing electron pressure tensor nongyrotropic components in the generalized Ohm’s law; (ii) using the conservation of mass; and (iii) assuming that the weak magnetic field region width, where electron meandering motion supports electron pressure tensor off-diagonal (nongyrotropic) components, is of the order of electron Larmor radius] a simple model of magnetic reconnection in a collisionless regime is formulated. The model is general, resembling its collisional Sweet–Parker analog in that it is not specific to any initial configuration, e.g., Harris-type tearing unstable current sheet, X-point collapse or otherwise. In addition to its importance from the fundamental point of view, the collisionless reconnection model offers a much faster reconnection rate [Mc′less=(c∕ωpe)2∕(rL,eL)] than Sweet–Parker’s classical one (Msp=S−1∕2). The width of the diffusion region (current sheet) in the collisionless regime is found to be δc′less=(c∕ωpe)2∕rL,e, which is independent of the global reconnection scale L and is only prescribed by microphysics (electron inertial length, c∕ωpe, and electron Larmor radius, rL,e). Amongst other issues, the fastness of the reconnection rate alleviates, e.g., the problem of interpretation of solar flares by means of reconnection, as for the typical solar coronal parameters the obtained collisionless reconnection time can be a few minutes, as opposed to Sweet–Parker’s equivalent value of less than a day. The new theoretical reconnection rate is compared to the Magnetic Reconnection Experiment device experimental data by Yamada et al. [Phys. Plasmas 13, 052119 (2006)] and Ji et al. [Geophys. Res. Lett. 35, 13106 (2008)], and a good agreement is obtained.

Nonlinear Dynamics of the Parker Scenario for Coronal Heating

The Parker or field line tangling model of coronal heating is studied comprehensively via long-time high-resolution simulations of the dynamics of a coronal loop in Cartesian geometry within the framework of reduced magnetohydrodynamics. Slow photospheric motions induce a Poynting flux which saturates by driving an anisotropic turbulent cascade dominated by magnetic energy. In physical space this corresponds to a magnetic topology where magnetic field lines are barely entangled; nevertheless, current sheets (corresponding to the original tangential discontinuities hypothesized by Parker) are continuously formed and dissipated. Current sheets are the result of the nonlinear cascade that transfers energy from the scale of convective motions (~1000 km) down to the dissipative scales, where it is finally converted to heat and/or particle acceleration. Current sheets constitute the dissipative structure of the system, and the associated magnetic reconnection gives rise to impulsive bursty heating events at the small scales. This picture is consistent with the slender loops observed by state-of-the-art (E)UV and X-ray imagers which, although apparently quiescent, shine brightly in these wavelengths with little evidence of entangled features. The different regimes of weak and strong magnetohydrodynamic turbulence that develop and their influence on coronal heating scalings are shown to depend on the loop parameters, and this dependence is quantitatively characterized: weak turbulence regimes and steeper spectra occur in stronger loop fields and lead to larger heating rates than in weak field regions.

Small-Scale Brightenings in the UV Continuum of an M9.1 Solar Flare

We analyze an M9.1 two-ribbon solar flare which occurred on 2004 July 22 using the TRACE white-light and 1700 A images, the RHESSI, and the SOHO/MDI data. We find many small-scale fast-varying brightenings that appeared in the white-light and 1700 A images along the flare ribbons. Some of them underwent rapid motions in weak magnetic field regions. We identify these short-lived brightenings as UV continuum enhancement. Our preliminary result shows that the brightenings are closely related to the HXR emission. They have a lifetime of 30–60 s and a typical size of about 1''–2''. The intensity enhancement is about 150–200 times the mean value of the quiet-Sun. According to previous works, we infer that the 1700 A enhancement may be dominated by the increased emission of 1680 A continuum coming from the temperature minimum region. The impulsive feature in the 1700 A light curves of the small-scale brightenings may be due to the irradiation of the impulsive CIV line intensity caused by the bombardment of non-thermal electron beams.

HFSZEEMAN—A program for computing weak and intermediate field fine and hyperfine structure Zeeman splittings from MCDHF wave functions

Given electronic wave functions generated by the grasp2K relativistic atomic structure package, this program calculates diagonal magnetic dipole A J and electric quadrupole B J hyperfine interaction constants and Landé g J factors. In addition the program computes diagonal and off-diagonal reduced hyperfine and Zeeman matrix elements and constructs the total interaction matrix for an atom in an external magnetic field. By diagonalizing the interaction matrix and plotting eigenvalues as functions of the magnetic field, Zeeman splittings of hyperfine levels are obtained. The method is applicable in the weak and intermediate field regions and yields results that are useful when analyzing spectra from e.g. EBIT sources and magnetic stars. The program can also be used in the field free limit to calculate mixing coefficients that determine rates of hyperfine induced transitions. For atoms with zero nuclear spin I the program computes splittings of the fine-structure levels. Program summary Program title: HFSZEEMAN Catalogue identifier: ADZS_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/ADZS_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 1528 No. of bytes in distributed program, including test data, etc.: 294 664 Distribution format: tar.gz Programming language:Fortran, Matlab Computer: IBM-compatible PC, unix workstation Operating system: Unix, Linux Classification: 11.6 Subprograms used: Cat Id: ADZL_v1_0; Title: grasp2K v1.0; Reference: CPC 177 (2007) 597 Nature of problem: Prediction of weak and intermediate field Zeeman splittings of fine- and hyperfine structure levels using multiconfiguration Dirac–Hartree–Fock wave functions. Solution method: The electronic wave function for a state labeled ΓJM is expanded in terms of jj-coupled configuration state functions | Γ J M 〉 = ∑ γ c γ | γ J M 〉 . In this representation the reduced matrix elements used to construct the interaction matrix can be computed as sums over one-particle radial integrals. By diagonalizing the interaction matrix and plotting eigenvalue as functions of the magnetic field, Zeeman splittings of fine- and hyperfine structure levels are obtained. Restrictions: The complexity of the cases that can be handled is entirely determined by the grasp2K package [P. Jönsson, H. Xe, C. Froese Fischer, I.P. Grant, Comput. Phys. Commun. 177 (2007) 597] used for the generation of the electronic wave functions. Running time: CPU time required to execute test cases: a few seconds.

Significance of Dungey-cycle flows in Jupiter's and Saturn's magnetospheres, and their identification on closed equatorial field lines

Abstract. We consider the contribution of the solar wind-driven Dungey-cycle to flux transport in Jupiter's and Saturn's magnetospheres, the associated voltages being based on estimates of the magnetopause reconnection rates recently derived from observations of the interplanetary medium in the vicinity of the corresponding planetary orbits. At Jupiter, the reconnection voltages are estimated to be ~150 kV during several-day weak-field rarefaction regions, increasing to ~1 MV during few-day strong-field compression regions. The corresponding values at Saturn are ~25 kV for rarefaction regions, increasing to ~150 kV for compressions. These values are compared with the voltages associated with the flows driven by planetary rotation. Estimates of the rotational flux transport in the "middle" and "outer" magnetosphere regions are shown to yield voltages of several MV and several hundred kV at Jupiter and Saturn respectively, thus being of the same order as the estimated peak Dungey-cycle voltages. We conclude that under such circumstances the Dungey-cycle "return" flow will make a significant contribution to the flux transport in the outer magnetospheric regions. The "return" Dungey-cycle flows are then expected to form layers which are a few planetary radii wide inside the dawn and morning magnetopause. In the absence of significant cross-field plasma diffusion, these layers will be characterized by the presence of hot light ions originating from either the planetary ionosphere or the solar wind, while the inner layers associated with the Vasyliunas-cycle and middle magnetosphere transport will be dominated by hot heavy ions originating from internal moon/ring plasma sources. The temperature of these ions is estimated to be of the order of a few keV at Saturn and a few tens of keV at Jupiter, in both layers.

Single glass substrate liquid crystal device using electric field-enforced phase separation and photoinduced polymerization

An in-plane switching liquid crystal (LC) cell using a glass substrate and a photoinduced polymer layer is demonstrated. The fabrication process is based on the electrodynamics of dielectric fluids. When the fringing field is present, the LC molecules tend to aggregate in the strong electric field regions while the monomers diffuse to the weak field regions. After photopolymerization, the LC molecules are confined by a thin polymer layer and polymer walls which define the cell gap. This approach enables single-substrate large panel display devices to be fabricated.

Helioseismically Determined Near‐Surface Flows Underlying a Quiescent Filament

The extended filaments seen in Hα images of the solar disk, and the corresponding prominences when viewed at the solar limb, are one of the great hallmarks of solar magnetism. Such arches of magnetic field and the coronal plasma structures they support are both beautiful and enigmatic. Many models of filament formation and maintenance invoke the existence of surface plasma flows, which are used to drive the magnetic reconnection needed to form twisted loops of flux held down by a coronal arcade. These flows are typically composed of a converging flow, which brings flux elements of opposite polarity together, combined with a tangential shear that stresses the coronal arcade. In this paper we present helioseismic measurements of near-surface flows underlying a single quiescent filament lying within a decayed active region. Newly devised high-resolution ring analyses (HRRA) with both 2° and 4° spatial resolution were applied to Doppler imaging data provided by the Michelson Doppler Imager (MDI) instrument on the SOHO spacecraft. A long-lived filament appearing in 2002 May and April was studied. We find that the filament channel is a region of vigorous subphotospheric convection. The largest observed scales of such convection span the region of weak magnetic field separating the active region's two polarities. Thus, the magnetic neutral line that forms the spine of the filament channel tends to lie along the centers of large convection cells. In temporal and spatial averages of the flow field, we do not find a systematic converging flow. However, we do detect a significant shearing flow parallel to the neutral line. This shear takes the form of two oppositely directed jets, one to either side of the neutral line and within 20 Mm of the line. The jets produce a net shear in the flow speed of 30 m s-1 occurring over a distance of 20 Mm.

Weakly perturbed Schwarzschild lens in the strong deflection limit

We investigate the strong deflection limit of gravitational lensing by a Schwarzschild black hole embedded in an external gravitational field. The study of this model, analogous to the Chang & Refsdal lens in the weak deflection limit, is important to evaluate the gravitational perturbations on the relativistic images that appear in proximity of supermassive black holes hosted in galactic centers. By a simple dimensional argument, we prove that the tidal effect on the light ray propagation mainly occurs in the weak field region far away from the black hole and that the external perturbation can be treated as a weak field quadrupole term. We provide a description of relativistic critical curves and caustics and discuss the inversion of the lens mapping. Relativistic caustics are shifted and acquire a finite diamond shape. Sources inside the caustics produce four sequences of relativistic images. On the other hand, retro-lensing caustics are only shifted while remaining point-like to the lowest order.

Well posed reconstruction of the solar coronal magnetic field

We present and compare two methods for the reconstruction of the solar coronal magnetic field, assumed to be force-free, from photospheric boundary data. Both methods rely on a well posed mathematical boundary value problem and are of the Grad-Rubin type, i.e., the couple (B, a) is computed iteratively. They do differ from each other on the one hand by the way they address the zero-divergence of B issue, and on the other hand by the scheme they use for computing a at each iteration. The comparison of the two methods is done by numerically computing two examples of nonlinear force-free fields associated to large scale strong electric current distributions, whose exact forms can be otherwise determined semi-analytically. In particular, the second solution has a large nonlinearity even in the weak field region - a feature which is not present in the actual magnetograms, but is interesting to consider as it does allow to push the methods to the limits of their range of validity. The best results obtained with those methods give a relative vector error smaller than 0.01. For the latter extreme case, our results show that higher resolution reconstructions with bounded convergence improve the approximated solution, which may be of some interest for the treatment of the data of future magnetographs.

Sedimentation of Substitutional Solute Atoms in Intermetallic Compound of Bi-Pb System under Ultra-Strong Gravitational Field

Ultra-strong gravitational field can induce sedimentation of even atoms in condensed matter. We had realized sedimentation of substitutional solute atoms in some miscible alloys. In this study, the ultra-centrifuge experiments were performed on an intermetallic compound of Bi-Pb system (Bi3Pb7) by changing time duration of experiment time (experimental conditions; maximum centrifugal force: 1.0x106g level, temperature: 130-150 °C, duration: 30-150h, state: solid). Composition changes were observed in the centrifuged samples. And, it was found that the Bi phase appeared from starting state of Bi3Pb7 around the weak gravitational field region of the sample. These results showed that sedimentation of substitutional solute atoms occurred, and induced the structure change in intermetallic compounds.

Effect of upstream rotational field on the formation of magnetic depressions in a quasi‐perpendicular shock downstream

One‐dimensional hybrid simulations are performed to investigate the interaction between an interplanetary rotational magnetic field (RF) and the terrestrial bow shock (a quasi‐perpendicular and supercritical regime). A magnetic depression structure called a transient density event (TDE) that is anticorrelated to the density peak is built up and is enlarged from the RF region after its entry into the magnetosheath. Contrary to the MHD view, in which such a diamagnetic structure is sandwiched by two slow or time‐dependent intermediate shocks, the TDE generation mechanism is strongly associated with effects of particle kinetics. Within the TDE, the proton temperature parallel to the ambient magnetic field Tp∥ increases to be isotropic, while the ordinary shock downstream consists of strong anisotropic protons (Tp⊥/Tp∥ > 1). This parallel heating is due to enforced conversion of the perpendicular proton motion into a parallel one by the imposed RF. The resultant intense parallel/antiparallel flows generate the field gradient at the leading and trailing edges, which act as a mirror force and reduce the magnetic intensity. In this kinetic sense, slow or Alfvén modes predicted by MHD theories are inadequate for regarding as edges of such a structure. Similar to the normal mirror mode process, particles are concentrated and trapped within the weak field region, leading to a density buildup. The trapped particles lose their energy and undergo cooling as they are bounced at the diverging mirror points. Accordingly, the TDE structure hardly collapses and endures through the magnetosheath. Compared with the mirror instability, isotropization forced by external field fluctuations works more efficiently to produce such a magnetic depression. Thus the presence of the quasi‐perpendicular shock, where large temperature anisotropy is generated, is one of the suitable situations for the TDE when interaction with the rotational field has taken place. This interaction model may also be applicable to the magnetic hole formation mechanism in the solar wind.

The field-trap effect under conditions of boiling of dielectric liquids in nonuniform electric fields

The mechanism of field-trap effect under conditions of boiling of dielectric liquids in a nonuniform electric field is discussed, which was experimentally observed in [1, 2]. According to the data of [1, 2], an external electric field initiates the process of boiling up of liquid, with the growth of vapor bubbles to the departure size occurs mainly in regions with low values of electric field (in field traps). This phenomenon is attributed to the effect made on the vapor bubbles by electric forces directed in opposition to the field square gradient. In view of this, the bubbles arising in the region of strong field move to the region of weak field where they grow to the departure size to provide for an increase in local heat transfer. Such a mechanism is observed in the cited experiments [1, 2] in the boiling of liquid nitrogen at a flat conducting surface with single projections leading to a local nonuniformity of electric field on a heated surface.