Magnetic structure of current sheets in magnetic fields with a singular X line

Direct measurements of magnetic fields in a plasma show that current sheets can form in magnetic configurations with an X line in the presence of a longitudinal magnetic field. It is found that, in a plane perpendicular to the X line and to the direction of the main current, the current sheet has two very different dimensions. The tangential and normal components of the magnetic field and current density in the sheet are determined. The influence of the initial conditions (such as the strength of the longitudinal magnetic field, the gradient of the transverse field, and the plasma ion mass) on the current sheet parameters is investigated.     

Interfacial electric polarizability of purple membranes in solution.

Chloroplasts in higher magnetic fields align with their equatorial plane perpendicular to the field. Because of the nonrandom orientation of the chromophores in the membrane the fluorescence radiation will be partially polarized. The chloroplast concentration, magnetic field, and temperature dependence of the fluorescence polarization has been investigated. The results are compared with a simplified model calculation. It is shown that the concentration dependence can be related to the linear dichroism of the fluorescence radiation and self-adsorption. Taking these effects into account results in the calculation of a higher fluorescence polarization (FP) ratio and higher inclination of chlorophyll dipoles to the membrane plane. Analyzing the magnetic field dependence of the FP ratio, we conclude that in a magnetic field not only will be chloroplasts be aligned, but the thylakoid stacks as well. A decrease in the FP ratio was observed around 20 degrees C. It is suggested that this decrease reflects a phase transition in the photosynthetic membrane.     

Magnetic Field Effect on Fresnel Coefficients of the Thin Slab of Graphite Nanocomposite

In this theoretical study, optical properties of a thin slab including graphite nanoparticles doped in a host medium such as silica and silicon have been investigated. A constant magnetic field is used for controlling Fresnel coefficients of the slab. Depending on the direction of the normal vector of the basal plane of the graphite structure with respect to the electric field of the incident electromagnetic wave (perpendicular or parallel), optical behavior of graphite is different. Electric permittivity of an individual magnetized graphite nanoparticle is calculated by a semi-empirical Drude-like model for both kinds of the normal vector of the basal plane orientation. By means of well-known Maxwell-Garnett theory, effective permittivity of the magnetized nanocomposite slab is obtained. It is shown that at the mid-IR frequency range, exerting a magnetic field of few hundred Tesla can lead to the substantial variations of Fresnel coefficients. Effect of the magnetic field on the optical properties of the slab is more evident when the population of the graphite nanoparticles with the parallel orientation of the basal plane is increased.

     

Equatorial sandhoppers use body scans to detect the earth’s magnetic field

Adults of Talorchestia martensii were individually released in a confined environment, with and without the natural magnetic field, under the sun and in a dark room. The sandhoppers scanned the horizontal component of the magnetic field by left and right oscillations of the entire major body axis. The frequency of this behaviour increased in a zeroed magnetic field, as did the frequencies of other behavioural indicators that reflect the difficulty in identifying the ecologically efficient orientation direction (sea–land axis). Therefore, like head scans in birds, body scans seem to be used by equatorial sandhoppers to detect the magnetic symmetry plane.     

Migratory birds use head scans to detect the direction of the earth's magnetic field

Night-migratory songbirds are known to use a magnetic compass , but how do they detect the reference direction provided by the geomagnetic field, and where is the sensory organ located? The most prominent characteristic of geomagnetic sensory input, whether based on visual patterns or magnetite-mediated forces , is the predicted symmetry around the north-south or east-west magnetic axis. Here, we show that caged migratory garden warblers perform head-scanning behavior well suited to detect this magnetic symmetry plane. In the natural geomagnetic field, birds move toward their migratory direction after head scanning. In a zero-magnetic field , where no symmetry plane exists, the birds almost triple their head-scanning frequency, and the movement direction after a head scan becomes random. Thus, the magnetic sensory organ is located in the bird's head, and head scans are used to locate the reference direction provided by the geomagnetic field.     

Orientation of glutaraldehyde-fixed erythrocytes in strong static magnetic fields

In a uniform static magnetic field up to 8 Telsa, glutaraldehyde-fixed erythrocytes showed an orientation in which their disk plane was perpendicular to the magnetic field. The paramagnetism of membrane-bound hemoglobin was thought to contribute significantly to this orientation. The observation of magnetic orientation is directed toward understanding the fundamental microstructural aspects of the erythrocyte. © 1996 Wiley-Liss, Inc.     

Reconstitution and alignment by a magnetic field of a β-barrel membrane protein in bicelles

A protocol is described for the reconstitution of a transmembrane β-barrel protein domain, tOmpA, into lipid bicelles. tOmpA is the largest protein to be reconstituted in bicelles to date. Its insertion does not prevent bicelles from orienting with their plane either parallel or perpendicular to the magnetic field, depending on the absence or presence of paramagnetic ions. In the latter case, tOmpA is shown to align with the axis of the β-barrel parallel to the magnetic field, i.e. perpendicular to the plane of the bilayer, an orientation conforming to that in natural membranes and favourable to structural studies by solid-state NMR. Reconstitution into bicelles may offer an interesting approach for structural studies of membrane proteins in a medium resembling a biological membrane, using either NMR or other biophysical techniques. Our data suggest that alignment in the magnetic field of membrane proteins included into bicelles may be facilitated if the protein is folded as a β-barrel structure.     

Effects of microscope objectives on magnetic field exposures.

Distortions in magnetic field intensity generated by commonly used microscope objectives (1x to 100x) were characterized within a Helmholtz coil-based exposure system. Objectives from a variety of manufacturers distorted applied field intensities by up to 23% in the image plane. Components that contribute to distortions include (1) nickel-chrome plating of objective housings, (2) the presence of steel springs in objectives with compression collars, and (3) steel screws or studs used to hold together separately manufactured parts. Steel springs and screws produce radially asymmetric profiles, whereas distortions generated by nickel-chrome plating are typically radially symmetric. All components can produce spatial gradients in field intensity if objectives are not perfectly aligned with exposure systems or if placed in the earth's magnetic field. Alterations in the magnitude of magnetic field intensities as well as the production of spatial gradients might have an effect on biological responses. By maintaining optical glass components and replacing metallic components, functional objectives can be reconstructed that produce no measurable effects on magnetic flux densities.     

Effects of non-uniform static magnetic fields on the rate of myosin phosphorylation

The effect on myosin phosphorylation from exposure to a magnetic field generated by an array of four permanent magnets was investigated. Two lateral positions in the non-uniform field over the array were explored, each at four vertical distances over the surface of the device. The rate of myosin phosphorylation was found to depend on the position laterally over the array as well as the distance from the device surface. The square magnet array was comprised of axially magnetized, cylindrical NdFeB permanent magnets arranged with poles of alternating polarity in a plane (MagnaBloc trade mark therapeutic device). Detailed dosimetry of the magnet array was compiled: the magnetic flux density averaged over the exposure volume spanned the range 0.7-86 mT for the eight different exposure positions. The corresponding range for the absolute field gradient was 0.4-20 T/m. Comparing the dosimetry to the experimental outcome, our results imply that magnetic field amplitude alone is not sufficient to describe the influence of the field in this preparation.     

Correlation of nuclear acceptor sites for oestrogen receptors with gene transcription in vitro

1. Membrane particles prepared from ultrasonically-disrupted, aerobically-grown Escherichia coli were centrifuged on to a plastic film that was supported perpendicular to the centrifugal field to yield oriented membrane multilayers. In such preparations, there is a high degree of orientation of the planes of the membranes such that they lie parallel to each other and to the supporting film. 2. When dithionite- or succinate-reduced multilayers are rotated in the magnetic field of an e.p.r. spectrometer, about an axis lying in the membrane plane, angular-dependent signals from an iron-sulphur cluster at g(x)=1.92, g(y)=1.93 and g(z)=2.02 are seen. The g=1.93 signal has maximal amplitude when the plane of the multilayer is perpendicular to the magnetic field. Conversely, the g=2.02 signal is maximal when the plane of the multilayer is parallel with the magnetic field. 3. Computer simulations of the experimental data show that the cluster lies in the cytoplasmic membrane with the g(y) axis perpendicular to the membrane plane and with the g(x) and g(z) axes lying in the membrane plane. 4. In partially-oxidized multilayers, a signal resembling the mitochondrial high-potential iron-sulphur protein (Hipip) is seen whose g(z)=2.02 axis may be deduced as lying perpendicular to the membrane plane. 5. Appropriate choice of sample temperature and receiver gain reveals two further signals in partially-reduced multilayers: a g=2.09 signal arises from a cluster with its g(z) axis in the membrane plane, whereas a g=2.04 signal is from a cluster with the g(z) axis lying along the membrane normal. 6. Membrane particles from a glucose-grown, haem-deficient mutant contain dramatically-lowered levels of cytochromes and exhibit, in addition to the iron-sulphur clusters seen in the parental strain, a major signal at g=1.90. 7. Only the latter may be demonstrated to be oriented in multilayer preparations from the mutant. 8. Comparisons are drawn between the orientations of the iron-sulphur proteins in the cytoplasmic membrane of E. coli and those in mitochondrial membranes. The effects of diminished cytochrome content on the properties of the iron-sulphur proteins are discussed.     

Propagation of MHD waves in a plasma in a sheared magnetic field with straight field lines

The propagation of MHD plasma waves in a sheared magnetic field is investigated. The problem is solved using a simplified model: a cold plasma is inhomogeneous in one direction, and the magnetic field lines are straight. The waves are assumed to travel in the plane perpendicular to the radial coordinate (i.e., the coordinate along which the plasma and magnetic field are inhomogeneous). It is shown that the character of the singularity at the resonance surface is the same as that in a homogeneous magnetic field. It is found that the shear gives rise to the transverse dispersion of Alfvén waves, i.e., the dependence of the radial component of the wave vector on the wave frequency. In the presence of shear, Alfvén waves are found to propagate across magnetic surfaces. In this case, the transparent region is bounded by two turning points, at one of which, the radial component of the wave vector approaches infinity and, at the other one, it vanishes. At the turning point for magnetosonic waves, the electric and magnetic fields are finite; however, the radial component of the wave vector approaches infinity, rather than vanishes as in the case with a homogeneous field.     

Excitation of magnetic fields by a circularly polarized laser pulse in a plasma channel

The excitation of quasistatic magnetic fields by a circularly polarized laser pulse in a plasma channel is considered. It is shown that, to second order in the amplitude of the electric field of the laser pulse, circular rotation of the plane of polarization of the laser radiation in a radially nonuniform plasma gives rise to a nonlinear azimuthal current and leads to the excitation of the radial and axial components of the magnetic field. The dependence of the magnetic field distribution over the plasma channel on the spatial dimensions of the pulse and on the channel width is investigated for a moderate-power laser pulse. The structure of the magnetic fields excited by a relativistic laser pulse in a wide plasma channel is analyzed.     

Boundary of the adiabatic motion of a charged particle in a dipole magnetic field

The motion of a charged particle in a dipole magnetic field is considered using a quasi-adiabatic model in which the particle guiding center trajectory is approximated by the central trajectory, i.e., a trajectory that passes through the center of the dipole. A study is made of the breakdown of adiabaticity in the particle motion as the adiabaticity parameter χ (the ratio of the Larmor radius to the radius of the magnetic field line curvature in the equatorial plane) increases. Initially, for χ?0.01, the magnetic moment μ of a charged particle undergoes reversible fluctuations, which can be eliminated by subtracting the particle drift velocity. For χ?0.1, the magnetic moment μ undergoes irreversible fluctuations, which grow exponentially with χ. Numerical integration of the equations of motion shows that, during the motion of a particle from the equatorial plane to the mirror point and back to the equator in a coordinate system related to the central trajectory, the analogue of the magnetic moment μ is conserved. In the equatorial plane, this analogue undergoes a jump. The long-term particle dynamics is described in a discrete manner, by approximating the Poincaré mapping. The existence of the regions of steady and stochastic particle motion is established, and the boundary between these regions is determined. The position of this boundary depends not only on the adiabaticity parameter χ but also on the pitch angle. The calculated boundary is found to agree well with that obtained previously by using the model of a resonant interaction between particle oscillations associated with different degrees of freedom.     

Magnetic-Based Fano Resonance by a Trimer with Y-shaped Gap

We introduce a Y-shaped gap into a silver disk to break the structure symmetry which can be looked as a loop-linked structure. Magnetic resonances are excited by incident light when incident electric field is parallel to the trimer plane. Fano resonance is generated by the coupling between bright electric mode and dark magnetic mode. These resonances can be adjusted by tuning the gap size, the radius of trimer, and the position of Y-shaped gap. The extinction cross section of the structure is calculated with the finite element method (FEM). The maximum figure of merit (FOM) is 37.8. Both the magnetic and electric field are greatly enhanced at the Fano dip and the magnetic resonance peak.     

On the characteristic features of the magnetic field diffusion in an inhomogeneous plasma in the EMHD approximation

The magnetic field transport in a spatially inhomogeneous plasma is studied theoretically in the electron MHD approximation. A model problem with a given periodic plasma density profile is considered in plane geometry. In this case, the magnetic field is transported diffusively and the effective diffusion coefficient is determined by the geometric and time parameters of the perturbed density, as well as by the magnetization parameter. It is found that, under certain relationships between the parameters of the problem, there is a kind of resonant effect—a decrease in the plasma conductivity. The problem under consideration does not refer solely to plasma physics: the results obtained here can also be used to describe transport processes in other branches of physics.     

The Effect of Static Magnetic Field on Electrical Properties of Lens

We investigated the effect of a static magnetic field (0.1 T) on various electrical parameters of goat eye lens using a computer-aided AC impedance system (EG&G 278) at 30°C. Results of measurements on voltage-current characteristics showed that the static magnetic field alters the current flow in the tissue. The complex impedance plane drawn between real (Z') and imaginary (Z) parts in the form of a Cole-Cole plot demonstrates that under magnetic field, the distribution factor a decreases from 0.3 ± 0.006 to 0.24 ± 0.005 and the extracellular resistance (Re) increases from 52 ± 1.3 to 60 ± 1.5 K ohms in the lens tissue. An attempt has been made to explain the interactive behavior of the magnetic field with the electrical changes in the lens.     

Evolution of the electron Langmuir oscillations in an inhomogeneous magnetized plasma

The conditions under which the energy of the electron Langmuir oscillations can escape from the plasma into vacuum are determined in the simplest model of a plane slab of an inhomogeneous cold magnetized plasma in a uniform magnetic field.     

Processes that occur before second cleavage determine third cleavage orientation in Xenopus

As in many organisms, the first three cleavage planes of Xenopus laevis eggs form in a well-described mutually orthogonal geometry. The factors dictating this simple pattern have not been unambiguously identified. Here, we describe experiments, using static magnetic fields as a novel approach to perturb normal cleavage geometry, that provide new insight into these factors. We show that a magnetic field applied during either or both of the first two cell cycles can induce the third cell cycle mitotic apparatus (MA) at metaphase and the third cleavage plane to align nearly perpendicular to their nominal orientations without changing cell shape. These results indicate that processes occurring during the first two cell cycles primarily dictate the third cleavage plane and mitotic apparatus orientation. We discuss how mechanisms that can align the MA after it has formed are likely to be of secondary importance in determining cleavage geometry in this system.     

Variability of morphometric indices of cleaving embryos in two anuran amphibians under modified magnetic conditions

We studied the effects of modified magnetic fields, such as reduced geomagnetic field or only its horizontal component, strengthened vertical component, or periodic alteration of horizontal component polarity, on two-, four-, and eight-cell embryos of Bufo viridis and Rana macrocnemis. Modified magnetic conditions did not affect the first or second cleavage furrow geometry. The strengthened vertical component of geomagnetic field alone decreased the frequency of two-cell embryos with the second furrow, which was not closed on the animal pole. Modified magnetic conditions more distinctly affected the vertical sizes of animal and vegetal blastomeres of eight-cell toad embryos due, apparently, to displacement of the third cleavage plane towards the animal or vegetal pole. The response of frog embryos to modified magnetic conditions was much less pronounced.     

Dynamics of plasma−dust structures formed in a trap created in the narrowing of a current channel in a magnetic field

The geometry and dynamics of plasma?dust structures in a longitudinal magnetic field is studied experimentally. The structures are formed in a glow-discharge trap created in the double electric layer produced as a result of discharge narrowing by means of a dielectric insert introduced in the discharge tube. Studies of structures formed in the new type of glow-discharge trap are of interest from the standpoint of future experiments with complex plasmas in superstrong magnetic fields in which the dust component is magnetized. Different types of dielectric inserts were used: conical and plane ones with symmetric and asymmetric apertures. Conditions for the existence of stable dust structures are determined for dust grains of different density and different dispersity. According to the experimental results, the angular velocity of dust rotation is ≥10 s^–1, which is the fastest type of dust motion for all types of discharges in a magnetic field. The rotation is interpreted by analyzing the dynamics of individual dust grains.