Showing papers on "Electric field published in 1984"

Electric field assisted dissociation of charge transfer states as a mechanism of photocarrier production

Abstract: The lowest charge‐transfer excited state (CT1) of electron donor–acceptor crystals or polymers is demonstrated to be a plausible precursor of free charge carriers when such materials are photoexcited Rate constants for the dissociation of charge–transfer states are formulated for two approximate descriptions of CT1: classical ion pair and Wannier exciton The electric field dependence of the dissociation rate constant is postulated to be given by Onsager’s 1934 theory (O‐34) of ion pair dissociation This formulation of CT1 dissociation obviates the need to invoke electron–hole ‘‘thermalization’’ lengths of 2 to 3 nm in order to explain free charge carrier formation in donor–acceptor materials

Monitoring fibroblast behavior in tissue culture with an applied electric field

Abstract: Mammalian fibroblasts have been cultured on evaporated gold electrodes subjected to an alternating electric field at 4000 Hz. The system consists of a large (approximately equal to 2 cm2) and a small (approximately equal to 3 X 10(-4) cm2) electrode bathed in tissue culture medium. The applied electric field produces a voltage drop at the boundary between the solution and the small electrode of a few mV at a current density of a few mA/cm2. The small population of cells that attach and spread on this electrode have a marked effect on the measured impedance and also cause it to fluctuate with time. The amplitude of these fluctuations is greatly reduced by cytochalasin B (10 microM), suggesting they are a consequence of cell movement.

Electrostatics of conductors

Abstract: Macroscopic electrodynamics is concerned with the study of electromagnetic fields in space that is occupied by matter. Electrodynamics deals with physical quantities averaged over elements of volume that are physically infinitesimal and ignore the microscopic variations of the quantities that result from the molecular structure of matter. The fundamental equations of the electrodynamics of continuous media are obtained by averaging the equations for the electromagnetic field in a vacuum. The form of the equations of macroscopic electrodynamics and the significance of the quantities appearing in them depend on the physical nature of the medium and on the way in which the field varies with time. Charges present in a conductor must be located on its surface. The presence of charges inside a conductor would cause an electric field in it. These charges can be distributed on its surface, however, in such a way that the fields that they produce in its interior are mutually balanced. The mean field in the vacuum is almost the same as the actual field. The two fields differ only in the immediate neighborhood of the body, where the effect of the irregular molecular fields is noticeable, and this difference does not affect the averaged field equations.

Nonequilibrium steady states of stochastic lattice gas models of fast ionic conductors

Abstract: We investigate theoretically and via computer simulation the stationary nonequilibrium states of a stochastic lattice gas under the influence of a uniform external fieldE. The effect of the field is to bias jumps in the field direction and thus produce a current carrying steady state. Simulations on a periodic 30 × 30 square lattice with attractive nearest-neighbor interactions suggest a nonequilibrium phase transition from a disordered phase to an ordered one, similar to the para-to-ferromagnetic transition in equilibriumE=0. At low temperatures and largeE the system segregates into two phases with an interface oriented parallel to the field. The critical temperature is larger than the equilibrium Onsager value atE=0 and increases with the field. For repulsive interactions the usual equilibrium phase transition (ordering on sublattices) is suppressed. We report on conductivity, bulk diffusivity, structure function, etc. in the steady state over a wide range of temperature and electric field. We also present rigorous proofs of the Kubo formula for bulk diffusivity and electrical conductivity and show the positivity of the entropy production for a general class of stochastic lattice gases in a uniform electric field.

Magnetotelluric responses of three-dimensional bodies in layered earths

Abstract: The electromagnetic fields scattered by a three‐dimensional (3-D) inhomogeneity in the earth are affected strongly by boundary charges. Boundary charges cause normalized electric field magnitudes, and thus tensor magnetotelluric (MT) apparent resistivities, to remain anomalous as frequency approaches zero. However, these E‐field distortions below certain frequencies are essentially in‐phase with the incident electric field. Moreover, normalized secondary magnetic field amplitudes over a body ultimately decline in proportion to the plane‐wave impedance of the layered host. It follows that tipper element magnitudes and all MT function phases become minimally affected at low frequencies by an inhomogeneity. Resistivity structure in nature is a collection of inhomogeneities of various scales, and the small structures in this collection can have MT responses as strong locally as those of the large structures. Hence, any telluric distortion in overlying small‐scale extraneous structure can be superimposed to ar...

Quantitative aspects of magnetospheric physics

Abstract: 1: Introduction.- 2: Charged-Particle Motion in Magnetic and Electric Fields.- 3: Trapping Region and Currents Due to Trapped Particles.- 4: Electric Fields.- 5: Wave-Particle Interactions.

Quantitative Aspects of Magnetospheric Physics

Abstract: 1: Introduction.- 2: Charged-Particle Motion in Magnetic and Electric Fields.- 3: Trapping Region and Currents Due to Trapped Particles.- 4: Electric Fields.- 5: Wave-Particle Interactions.

On the control of magnetospheric convection by the spatial distribution of ionospheric conductivities

Abstract: A self-consistent semianalytical model of magnetospheric convection including the effect of the latitude and local time variations of ionospheric conductivities is presented. The motions of the inner edge of the magnetospheric ring current, and the associated field-aligned currents, produced by the externally imposed dawn-to-dusk potential drop across the magnetospheric cavity are computed by using a linear approximation. The coupling between the different diurnal harmonics in the local time variations of fields and currents produced by the local time dependence of ionospheric conductivities is described by an appropriate matrix formalism. The calculations show that the enhancement of auroral conductivities by electron precipitation in the auroral zone significantly enhances both the typical duration and the absolute amplitude of the penetration of convection electric fields to midlatitudes. Furthermore, the local time variations of the convection electric field generated at midlatitudes by a sudden increase of the dawn-to-dusk potential drop are in good agreement, both at the initial time and after the steady state is reached, with the available statistical models of the disturbance midlatitude electric field. The amplitude of the steady state field seems sufficient to explain these observations, thus confirming that the concept of the shielding of midlatitudes from the convection electric fields is basically correct but was overestimated in earlier analytical calculations. The large subauroral electric fields observed by several satellites are also reproduced in the model either by a decrease of the subauroral conductivities below the midlatitude values or by the consideration of a very narrow latitudinal extent of the auroral zone. The overall consistency between the results of the model and the electric field observations thus supports the idea that a large class of phenomena related to magnetospheric convection in the dipole regions of the magnetosphere can be described in a reasonably realistic manner by a linear theory.

Embryonic fibroblast motility and orientation can be influenced by physiological electric fields.

Abstract: Epithelial layers in developing embryos are known to drive ion currents through themselves that will, in turn, generate small electric fields within the embryo. We hypothesized that the movement of migratory embryonic cells might be guided by such fields, and report here that embryonic quail somite fibroblast motility can be strongly influenced by small DC electric fields. These cells responded to such fields in three ways: (a) The cells migrated towards the cathodal end of the field by extending lamellipodia in that direction. The threshold field strength for this galvanotaxis was between 1 and 10 mV/mm when the cells were cultured in plasma. (b) The cells oriented their long axes perpendicular to the field lines. The threshold field strength for this response for a 90-min interval in the field was 150 mV/mm in F12 medium and between 50 and 100 mV/mm in plasma. (c) The cells elongated under the influence of field strengths of 400 mV/mm and greater. These fibroblasts were therefore able to detect a voltage gradient at least as low as 0.2 mV across their width. Electric fields of at least 10-fold larger in magnitude than this threshold field have been detected in vivo in at least one vertebrate thus far, so we believe that these field effects encompass a physiological range.

Ultrahigh gradient particle acceleration by intense laser-driven plasma density waves

Abstract: Space-charge waves driven by resonantly beating two laser beams in a high-density plasma can produce ultrahigh electric fields that propagate with velocities close to c. By phase-locking particles in such a wave, particles may be accelerated to very high energies within a very short distance.

Correlated low‐frequency electric and magnetic noise along the auroral field lines

Abstract: Dynamics Explorer 1 measurements of intense low-frequency electric and magnetic noise observed at low altitudes over the auroral zone are described. The intensity of both the electric and magnetic fields decreases rapidly with increasing frequency. Most of the energy is at frequencies below the O(+) cyclotron frequency, and some evidence is found for a cutoff or change in spectral slope near that frequency. The magnetic to electric field ratio decreases rapidly with increasing radial distance and also decreases with increasing frequency. The polarization of the electric field in a plane perpendicular to the earth's magnetic field is essentially random. The transverse electric and magnetic fields are closely correlated, with the average Poynting flux directed toward the earth. The total electromagnetic power flow associated with the noise is substantial. Two general models are discussed to interpret these observations, one based on static electric and magnetic fields imbedded in the ionosphere and the other based on Alfven waves propagating along the auroral field lines.

The adiabatic energy change of plasma electrons and the frame dependence of the cross‐shock potential at collisionless magnetosonic shock waves

Abstract: The adiabatic energy gain of electrons in the stationary electric and magnetic field structure of collisionless shock waves was examined analytically in reference to conditions of the earth's bow shock. The study was performed to characterize the behavior of electrons interacting with the cross-shock potential. A normal incidence frame (NIF) was adopted in order to calculate the reversible energy change across a time stationary shock, and comparisons were made with predictions made by the de Hoffman-Teller (HT) model (1950). The electron energy gain, about 20-50 eV, is demonstrated to be consistent with a 200-500 eV potential jump in the bow shock quasi-perpendicular geometry. The electrons lose energy working against the solar wind motional electric field. The reversible energy process is close to that modeled by HT, which predicts that the motional electric field vanishes and the electron energy gain from the electric potential is equated to the ion energy loss to the potential.

High field transport in GaAs, InP and InAs

Abstract: Calculations of the steady state and transient electron drift velocities and impact ionization rate are presented for GaAs, InP and InAs based on a Monte Carlo simulation using a realistic band structure derived from an empirical pseudopotential. The impact ionization results are obtained using collision broadening of the initial state and are found to fit the experimental data well through a wide range of applied fields. In InP the impact ionization rate is much lower than in GaAs and no appreciable anisotropy has been observed. This is due in part to the larger density of states in InP and the corresponding higher electron-phonon scattering rate. The transient drift velocities are calculated under the condition of high energy injection. The results for InP show that higher velocities can be obtained over 1000–1500 A device lengths for a much larger range of launching energies and applied electric fields than in GaAs. For the case of InAs, due to the large impact ionization rate, high drift velocities can be obtained since the ionization acts to limit the transfer of electrons to the satellite minima. In the absence of impact ionization, the electrons show the usual runaway effect and transfer readily occurs, thus lowering the drift velocity substantially.

Auroral zone electric fields from DE-1 and -2 at magnetic conjunctions. Progress Report

Abstract: Nearly simultaneous measurements of auroral zone electric fields are obtained by the Dynamics Explorer spacecraft at altitudes below 900 km and 4,500 km during magnetic conjunctions. The measured electric fields are usually perpendicular to the magnetic field lines. The north-south meridional electric fields are projected to a common altitude by a mapping function which accounts for the convergence of the magnetic field lines. When plotted as a function of invariant latitude, graphs of the projected electric fields are measured by both DE-1 and DE-2 show that the large-scale electric field is the same at both altitudes, as expected. Superimposed on the large-scale fields, however, are small-scale features with wavelengths less than 100 km which are larger in magnitude at the higher altitude. Fourier transforms of the electric fields show that the magnitudes depend on wavelength. Outside of the auroral zone the electric field spectrums are nearly identical. But within the auroral zone the high and low altitude electric fields have a ratio which increases with the reciprocal of the wavelength. The small-scale electric field variations are associated with field-aligned currents. These currents are measured with both a plasma instrument and magnetometer on DE-1.

Electronic Conduction Processes in Dielectric Liquids

Abstract: A review is given on the physical properties of electronic charge carriers (electrons, holes) in non-polar and polar liquids. The results are used to explain or discuss certain electric conduction phenomena, which occur when these liquids are subjected to a high electric field strength. Electronic conduction in ultrapure liquids is finding an increasing number of applications.

An E-Field solution for a conducting surface small or comparable to the wavelength

Abstract: A new E -field solution is presented for the electric current and electric charge induced on a perfectly conducting surface illuminated by an incident electromagnetic field. This solution is a moment solution to the electric field integral equation on the surface. The expansion functions consist of a set of functions suitable for expanding the magnetostatic current and a set of functions whose surface divergences are suitable for expanding the electrostatic charge. The testing functions are similar to the expansion functions. With these expansion and testing functions, the new E -field solution works well with surfaces whose maximum dimension may be as small as 10^{-15} wavelengths or as large as a few wavelengths. Previous E -field solutions begin to deteriorate when the maximum dimension of the surface falls below a few hundredths of a wavelength. The new E -field solution is applied to a conducting circular disk and a conducting sphere.

Particle and field characteristics of the high-latitude plasma sheet boundary layer

Abstract: Particle and field data obtained by eight ISEE spacecraft experiments are used to define more precisely the characteristics of the high-latitude boundary region of the plasma sheet. A region immediately adjacent to the high-latitude plasma sheet boundary has particle and field characteristics distinctly different from those observed in the lobe and deeper in the central plasma sheet. Electrons over a broad energy interval are 'field-aligned' and bidirectional, whereas in the plasma sheet the distributions are more isotropic. The region supports intense ion flows, large-amplitude electric fields, and enhanced broad-band electrostatic noise.

Perturbation of the direction of neurite growth by pulsed and focal electric fields

Abstract: We have studied the orientation of neurite growth in the culture of embryonic Xenopus neurons in response to three types of extracellular electric fields: spatially uniform pulsed fields, focally applied steady (DC) fields, and focally applied pulsed fields. Under uniform pulsed fields, neurites showed a preferential orientation toward the cathode pole of the field in a manner similar to that previously found for DC fields. The extent of neurite orientation depended upon the duration, amplitude, and frequency of the pulse but appeared to be similar to that produced by a uniform DC field of an equivalent time- averaged field intensity. For square pulses of 5 msec duration, the minimal amplitude and frequency required to produce a detectable orientation of neurite growth over a period of 24 hr were 2.5 V/cm and 10 Hz, which correspond to a time-averaged field intensity of 125 mV/cm. Steady or pulsed focal fields were applied by passing a current through a micropipette placed near the growth cone of the neurite. Fields of negative polarity (current sink) were found to attract the growth cone, whereas fields of positive polarity (current source) were found to deflect the growth cone away from the pipette. The threshold DC current density needed at the growth cone to perturb its direction of growth within 15 min was 0.2 to 2 pA/micron2 (or 3 to 30 mV/cm); and for focal pulsed currents (pulse duration 5 msec), a typical combination of minimal pulse amplitude and frequency was 4 pA/micron2 and 10 Hz. This threshold focal current is similar to that which occurs at the synaptic cleft during active synaptic activity.

Quasistatic electric field measurements with spherical double probes on the GEOS and ISEE satellites

Abstract: Spherical double probes for measurements of electric fields on the GEOS-1, GEOS-2, and ISEE-1 satellites are described. An essential feature of these satellites is their conductive surfaces which eliminate errors due to differential charging and enable meaningful diagnostic experiments to be carried out. The result of these experiments is a good understanding of interactions between the plasma, the satellite and the probes, including photo-electron emission on satellite and probes. Electric field measurements are compared with measurements of plasma drift perpendicular to the magnetic field in the solar wind and the magnetosphere and the error bar for the absolute values of the electric field is found to be in the range ±(0.5–1.0) mV m-1 whereas relative variations can be determined with much better accuracy. A useful by-product from a spherical double probe system is the determination of satellite floating potential which is related to the plasma electron flux. This measurement allows high time resolution studies of boundary crossings. Examples of electric field measurements, which reflect the recent scientific results, are given for different regions of the magnetosphere from the bow shock, the inner magnetosphere and the tail. Several examples of simultaneous GEOS-ISEE observations are described.

Electric fields in the rheology of disperse systems

Abstract: In the present survey, the influence of electric fields on the structure and rheological properties of disperse systems as well as the effect of deformations on their electrical characteristics are discussed. The properties of these systems are considered in terms of the dielectric permittivity and electrification potential. The considerable thickness of the double electric layer around the disperse phase particles, which is characteristic of disperse systems with nonpolar hydrocarbon dispersion media, provides the possibility for strong electric fields to produce an electric nonuniformity on the surface of the disperse phase particles. The formation of hydrate layers on the particles creates the possibility of polarization of the disperse phase. In plastic disperse systems such as greases, a strong orientation effect is observed, which contributes to the creation of frozen flow patterns when the flow is suddenly stopped. The survey is concluded with a consideration of the process of formation of chain structures in the direction of the lines of force of the electric field whose orientation is normal to the direction of flow, which can lead to complete stoppage of the flow.

Collisionless dissipation in quasi‐perpendicular shocks

Abstract: Microscopic dissipation processes in quasi-perpendicular shocks are studied by two-dimensional plasma simulations in which electrons and ions are treated as particles moving in self-consistent electric and magnetic fields. Cross-field currents induce substantial turbulence at the shock front reducing the reflected ion fraction, increasing the bulk ion temperature behind the shock, doubling the average magnetic ramp thickness, and enhancing the upstream field aligned electron heat flow. The short scale length magnetic fluctuations observed in the bow shock are probably associated with this turbulence.

An Electromagnetic Near-Field Sensor for Simultaneous Electric and Magnetic-Field Measurements

Abstract: This paper describes the theory of a single sensor to perform simultaneous electric and magnetic near-field measurements. The theory indicates that it is possible to measure the magnetic-loop and electricdipole currents using a loop antenna terminated with identical loads at two diametrically opposite points. The theory also indicates that it is possible to choose an ideal load impedance for achieving equal electric and magnetic-field responses of the loop. Preliminary experiments have been performed using a plane-wave field to verify these results.

Characteristics of high-frequency and direct-current argon discharges at low pressures: a comparative analysis

Abstract: A theoretical analysis of the characteristics of high-frequency and direct-current argon discharges at low pressures is carried out and a comparison of the basic properties of these discharges is presented. This analysis uses recent calculations of electron energy distributions and collision rate coefficients in argon under the action of uniform AC and DC electric fields together with a steady-state discharge model expressing the balance between collisional ionisation of the gas and the loss of electrons to the wall. The electric field strength, E, required for the steady-state operation of planar and cylindrical discharges is calculated as a function of the gas density N, the angular frequency omega , and the diffusion length for the discharge tube Lambda , assuming predominant direct ionisation.

Two-wave mixing with an applied field and a moving grating

Abstract: The theory for two-wave mixing in a photorefractive material with an external electric field and a moving grating is developed. Large gain enhancements are predicted for grating periods of 10–100 μm and compared with recent observations in Bi12SiO20. Predictions are made for enhancements in GaAs, which has recently been found to be photorefractive in the near infrared.

The production of ion conics by oblique double layers

Abstract: Magnetized test ions are subjected to acceleration through a numerically simulated oblique double layer in order to determine whether they emerge with velocity vectors aligned with or oblique to the ambient magnetic field. A criterion for oblique alignment, depending on the double-layer parameters and on the external magnetization, is obtained. When it is applied to observed and theoretical auroral double layers, this criterion predicts that accelerated heavy ions will be substantially less magnetic field aligned than will accelerated hydrogen ions, thus suggesting auroral double layers as a source of high-energy ion conics. Test particle simulations are also used to investigate the perpendicular heating of ions at low altitudes by the electric fields associated with moving auroral arcs. The rapid motion of small-scale structures in the arcs is suggested as a source of low-energy conical ion distributions, and the slow drifts of the entire arc forms are inferred to heat ionospheric ions.

Modelling of Impurity Flow in the Tokamak Scrape-off Layer

Abstract: The impurity flow parallel to the magnetic field lines in a collisional tokamak scrape-off layer is numerically investigated. The rate equations are solved treating each ionization state as a test fluid which interacts with the given hydrogen background plasma via collisions and the ambipolar electric field. Results for typical impurities (O, Fe, He, etc.) show that collisional friction usually forces the impurities to flow nearly at hydrogen speed. Thermal forces, however, can become dominant locally for small Mach number and large temperature gradient (e.g. strong target recycling), causing impurity flow reversal and subsequent accumulation outside the recycling region. The criterion for flow reversal is roughly M <λi/λT where λi is the hydrogen ion mean free path and λT is the temperature gradient length. Self-sputtering at the target plates is calculated, showing the importance of fractional impurity acceleration in addition to the charge-statedependent electrostatic energy gain.