Free Space Region Research Articles

Ligand-induced asymmetry as observed through fluorophore rotations and free energy couplings: application to neurophysin.

Changes that occur in subunit neurophysin structure upon ligand binding were explored by two methods. First, the thermal coefficient of the viscosity around the subunit tyrosine was monitored, which yields information on the environmental flexibility and free rotational space of the fluorophore. Initially, it was determined that the environmental flexibility and the free space around each subunit tyrosine are unperturbed upon dimerization. Binding of the tripeptide analogue of oxytocin causes the once homologous environments of the subunit tyrosines to become drastically different such that one moves onto a closely packed environment whereas the other moves into a region of larger free space. Even though the subunits as seen by each tyrosine are very different, the specific binding sites as seen by the ligands are similar. It was also found that ligand binding is stabilized by ring stacking and that energy transfer occurs between the tyrosine of the ligand and the neurophysin subunit tyrosine. Second, changes in subunit structure upon ligation were also followed by the determination of the order of free energy coupling between ligand binding and oligomerization, which tells how each ligand affects the subunit affinity. Since the binding of ligand is cooperative and induces dimer formation, there is second-order coupling between ligand binding and dimerization and the binding of the second ligand is responsible for the increase in subunit affinity.

Phased-array performance degradation due to mirror misfigures, piston errors, jitter, and polarization errors

The purpose of this paper is to develop relationships between the normalized far-field peak intensity (Strehl ratio) and system perturbations, such as mirror misfigure, piston errors, jitter, and polarization errors, for phased-array transmitters. Some of these sources of degradation are unique to phased arrays, whereas others are shared in common with single-aperture transmitters. It was found that for a phased array with many apertures, piston errors must be controlled to within ~λ/20 to ensure a Strehl ratio of 0.9. However, if adaptive optics is used to adjust the path lengths optimally in the free-space regions, then the gain lengths need only be controlled to within λ/(20δn) for a master-oscillator power-amplifier system. The factor δn = (δn/δk)δk is simply the variation of the gain’s index of refraction within one standard deviation in the wave number of the lasing lines. The results found for the effects of jitter are valid for arbitrary-shaped apertures as well as for arbitrary array patterns. For identical apertures and uncorrelated jitter among the apertures, the Strehl ratio is independent of the number of apertures and their arrangement. On the other hand, for correlated jitter, the far-field degradation is sensitive to the array pattern and is least sensitive for a close-packed array. Both the case of isolated perturbations for one element of the array and that of random errors in all the elements of the array are studied.

Theory of generation of ULF pulsations by ionospheric modification experiments

A model characterized by three regions is considered. A thin dynamo region is separated by a region of free space from a perfectly conducting earth below. Above, the dynamo region is bounded by a magnetosphere in which hydromagnetic waves in the fast and in the Alfvén mode can propagate. Two limited extensions of a previous theory that is valid for frequencies well below 1 Hz and that neglects the effect of currents flowing into the magnetosphere, are considered. First, the frequency dependence of the ionospheric current system is determined, neglecting currents flowing into the magnetosphere. Secondly, the effects of currents flowing into the magnetosphere are evaluated in the low frequency limit and the Alfvén wave fields excited in the magnetosphere in the equatorial plane are calculated.

Twist reflector design using E-type and H-type modes

A complete solution of plane wave scattering from a twist reflector of infinite extent for arbitrary incidence is presented. The solution is accomplished through the use of an E -and H -type modal representation of the fields in the twister and free space regions. The equivalent circuits for strip and parallel plate twister gratings are derived by transforming E and H mode results [5] to appropriate type mode parameters. The modal reflection coefficients \Gamma' and \Gamma'' are determined from these equivalent circuit parameters and together with the use of a type mode description of the incident and reflected fields lead to two simple conditions for an optimum design; one geometric, namely \tan \phi = \sec \theta and one electrical, \Gamma/\Gamma' = - 1 . The computed performance of this optimum twist reflector in terms of a cross-polarized suppression ratio versus incidence angle for various planes of incidence represents the best that can be done with any physical structure. A design procedure including design formulas and curves is given from which twist reflector dimensions can be determined. Experimental verification for two specific implementations, parallel plates and parallel strips twist reflectors, is described.

Some recent developments in field calculations

The paper surveys several numerical techniques, under development at the University of Manitoba, that yield improved computational efficiency in the solution of difficult magnetic field problems. The paper includes the following: a description of an isoparametric finite element method for continuously inhomogeneous and orthotropic media within curved boundaries; a description of an isoparametric boundary element method (using integral equation or source distribution techniques) for extensive or open-region problems; and coupled partial differential and integral equation techniques (i.e., picture-frame methods) which are of particular use in solving problems that involve inhomogeneous/orthotropic media within localized regions of free space. Mention is made of sparsity techniques, for the solution of large and sparse systems of equations, and of Kron's diakoptics which is a method of tearing a large system into more easily managed subsystems.

Generation of ULF waves by electric or magnetic dipoles

The generation of ULF waves by ground-based magnetic and electric dipoles is studied with a simplified model consisting of three adjoining homogeneous regions representing the groud, the vacuum (free space) region, and the ionosphere. The system is assumed to be immersed in a homogeneous magnetic field with an arbitrary tilt angle. By the use of Fourier techniques and the method of stationary phase, analytic expressions are obtained for the field strength of the compressional Alfven waves in the ionosphere. Expressions are also obtained for the strength of the torsional Alfven wave in the ionosphere and the ULF magnetic field at ground level. Numerical results are obtained for the compressional Alfven-wave field strength in the ionosphere with a nonvertical geomagnetic field and for the ULF magnetic field at ground level for a vertical geomagnetic field.

“Free-space” boundary conditions for the time dependent wave equation

Boundary conditions for the discrete wave equation which act like an infinite region of free space in contact with the computational region can be constructed using projection operators. Propagating and evanescent waves coming from within the computational region generate no reflected waves as they cross the boundary. At the same time arbitrary waves may be launched into the computational region. Well-known projection operators for one-dimensional waves may be used for this purpose in one dimension. Extensions of these operators to higher dimensions along with numerically efficient approximations to them are described for higher-dimensional problems. The separation of waves into ingoing and outgoing waves inherent in these boundary conditions greatly facilitates diagnostics.

Electromagnetic surface waves supported by a half-space of a counterstreaming plasma

The characteristics of electromagnetic surface waves supported by a plane interface separating a semi-infinite region of free space from another semi-infinite region consisting of a plasma counterstreaming parallel to the interface are investigated for the case in which the wave is progressing parallel to the direction of the streams. The behaviour of the surface waves is quite different from the case in which the plasma is stationary. The surface waves exist for all frequencies. The phase velocity of the surface wave is equal to the electromagnetic wave velocity in free space for very low frequencies, decreases monotonically as the frequency is increased and asymptotically approaches the velocity of the stream for very high frequencies. The surface waves are always stable. In contrast to on unbounded counterstreaming plasma which gives rise to instability for the plane longitudinal wave progressing parallel to the streams, a half-space of counterstreaming plasma does not give rise to any instabilities fo...

Wave amplification of electromagnetic surface waves supported by a plasma-beam half-space

The characteristics of electromagnetic surface waves supported by a plane interface separating a semi-infinite region of free space from another semi-infinite region consisting of a background plasma permeated by a neutralized beam streaming parallel to the interface are investigated for the case in which the wave is progressing either in, or opposite to, the direction of the beam. As compared to the case of a stationary plasma, the surface wave has new branches and drastically different characteristics. For the case of waves progressing opposite to the direction of the beam, there are backward-wave regions as well as regions of instability if the beam is sufficiently strong. These regions of instability occur below the plasma frequency. In general, there are two frequency ranges of instability in which the surface wave undergoes traveling wave amplification in the direction of the stream. The wave amplification characteristics are studied and their dependence on the number density of the beam relative to the background plasma is examined.

Amplification of electromagnetic surface waves supported by a half‐space of a double stream of plasma

The characteristics of electromagnetic surface waves supported by a plane interface separating a semi‐infinite region of free space from another semi‐infinite region consisting of a double stream of plasma streaming parallel to the interface are investigated for the case in which the wave is progressing either in or opposite to the direction of the stream. The behavior of the surface wave is quite different from the case in which the plasma is stationary. For the case of waves progressing opposite to the direction of the streams, there are backward‐wave regions as well as regions of instability which occur below the plasma frequency. In the frequency range of instability, the surface wave undergoes traveling‐wave amplification in the direction of the streams. The wave amplification characteristics are examined and the dependence of each on the relative number density of the two streams as well as on the drift velocity is studied. It is found that the maximum amplification rate increases as the drift velocity is increased.

Instability of electromagnetic surface waves supported by a bounded plasma stream

The characteristics of electromagnetic surface waves supported by a plane interface separating a semi-infinite region of free space from another semi-infinite region consisting of a plasma streaming parallel to the interface are investigated for the case in which the wave is progressing either in or opposite to the direction of the stream. The behavior of the surface wave is drastically different from the case in which the plasma is stationary. The surface wave has backward-wave regions. For a certain range of frequencies below the plasma frequency, the surface wave undergoes traveling wave amplification in the direction of the stream. When its wave number is in a certain range, the surface wave progressing in a direction opposite to that of the stream is convectively unstable. The characteristics of this surface wave instability are analyzed.

Comments on "On the reflection coefficient of a plasma profile of exponentially tapered electron density and fixed collision frequency"

Reflection of plane waves from an inhomogeneous medium is considered. It is pointed out that attempts to define a meaningful reflection coefficient within the medium are unsuccessful. On the other hand, if the observer is in the free-space region, there is a unique decomposition into traveling waves with a correspondingly unique definition of a meaningful reflection coefficient.

Electric Dipole Radiation in the Presence of a Moving, Dispersive Dielectric Half‐Space

The problem of a point electric dipole moving over a dispersive dielectric half‐space is studied. The dipole is located in the free space above the dielectric and is assumed to be time harmonic in its rest frame, oriented perpendicular to the interface, and moving parallel to it. Solutions for this problem are obtained by using integral transform techniques. Expressions for the electric and magnetic fields in the free space region are formulated for the reflected radiation field for the case of an arbitrary, dispersive dielectric and for the lateral wave and surface wave fields for the case of a lossless plasma dielectric. In the rest frame of the source, it is found that all three waves exhibit the frequency of the source and that the field patterns are distorted by the relative motion. In the rest frame of the dielectric, it is found that each of the waves exhibits some form of Doppler shift (different from the primary wave) and that their field patterns are also distorted. It is also found that the criterion for existence of the lateral wave is not modified by the relative motion of the source and dielectric but that the criterion for existence of the surface waves is greatly modified by the relative motion. In addition, for velocities greater than some critical velocity, it is found that a new type of surface wave comes into existence.

Lateral Wave on a Symmetrical Epstein Transition

A theoretical investigation is made of a lateral wave on a diffuse interface. Half of a symmetrical Epstein layer is used as the transition between a free half-space and an underdense plasma region. An electric current line source is placed in the free-space region and an exact integral expression is obtained for the reflected field. A high-frequency asymptotic approximation, uniform with respect to transition thickness, is then obtained for the reflected field. This approximation consists of two terms. The first of these terms can be interpreted as a reflected wave for arbitrary transition thickness, while the second term can be interpreted as a lateral wave contribution when the wavelength is much greater than the transition thickness, and as a reflected wave when the wavelength is much smaller than transition thickness.

SURFACE CURRENT EXCITATION ON AN INHOMOGENEOUSLY SHEATHED PLASMA-IMMERSED CYLINDER BY ELECTROMAGNETIC AND ELECTROKINETIC WAVES

A theoretical investigation is described which analyzes the effect of an inhomogeneous sheath on the surface currents and scattering cross section of an infinitely long, perfectly conducting, circular cylinder immersed in an isotropic collisionless, compressible plasma by plane electromagnetic (EM) and electrokinetic (EK) waves incident at an arbitrary angle with respect to the cylinder axis. The linearized fluid equations are used in the analysis, with the inhomogeneous sheath taken to extend a finite distance into the plasma from the cylinder surface, on the order of 10 electron Debye lengths, beyond which the plasma is uniform. The numerical results for the surface currents obtained from the inhomogeneous sheath analysis are compared with those obtained when the sheath is replaced by a free-space region, called the vacuum sheath. It is found that for EM wave incidence, the surface currents are practically independent of the sheath and the finite plasma temperature. For EK wave incidence, the sheath has an attenuating effect on the currents produced, compared with the sheathless case, with the attenuating effect of the vacuum sheath being greater than that of the inhomogeneous sheath. The scattering cross sections for EM wave incidence are, for the scattered fields with the same polarization as the incident wave, also independent of the sheath and finite plasma temperature. The scattered fields with polarization different from that of the incident wave have cross sections generally less than 10−2 of the scattered fields with the incident wave polarization.

Guided Waves Propagating Along the Magnetostatic Field at a Plane Boundary of a Semi-Infinite Magnetoionic Medium

The characteristics of the surface waves supported by a plane boundary of a semi-infiite region of gyrotropic plasma are investigated for the case in which the direction of the magnetostatic field is parallel to both the interface and the propagation direction. Two cases are considered, one for which the plasma is terminated by a perfectly conducting screen, and the other for which it is terminated by a semi-infinite region of free space. Surface waves are found to be propagated for all frequencies below both the plasma and the gyromagnetic frequency in the first case, and below both the plasma and 1/ /spl radic/2 times the upper hybrid resonant frequency in the second case. The characteristics of the surface waves are discussed, and numerical results of the phase velocity and the propagation coefficient of the surface waves along the interface, as well as their attenuation rates normal to the interface, are given.

ELECTROMAGNETIC RADIATION FROM AN ACOUSTIC POINT SOURCE WITHIN A COMPRESSIBLE PLASMA BUBBLE

A solution is presented for the boundary-value problem of an acoustic point-source antenna within a homogeneous compressible plasma region with an assumed rigid boundary. The small signal theory of plasma electrodynamics is employed and all damping mechanisms are neglected. It is shown that a fraction of the original acoustic power is converted into electromagnetic waves in the external free-space region.

Plasma Waveguides as Low Loss Structures

The dispersion relation for propagation of an electromagnetic wave between two semi-infinite slabs of plasma is found under the assumption of infinite magnetic field along the slabs. Provided the plasma frequency is above the applied frequency, a fast wave solution exists, with a sinusoidal field distribution in the free-space region between the plasma slabs, and exponential decay into the slabs. A complex plasma frequency is introduced to account for collision losses in the plasma, and substituting this complex plasma frequency into the dispersion relationship, the attenuation constant of the wave is found. The imaginary part of the plasma frequency is computed from collision theory and found to give damping considerably smaller than that obtained with metal guides. An application to high-field linear accelerators is treated in detail.

Note on surface‐field analysis

In a famour memoir Gauss [see 1 of “References” at end of paper] showed that the surface magnetic field of a sphere could be separated into parts of origin external and internal to the sphere, using the method of spherical harmonic analysis. This analysis also afforded a means of computing the field in adjacent regions of free space from a knowledge of the field over the surface of the sphere.Neumann [2], Schmidt [3], Schuster [4], and others have introduced various improvements in the technique of spherical harmonic analysis and have thereby permitted the achievement of results important in the theory of geomagnetism. However, certain limitations of this valuable method have always been keenly in evidence. In such an analysis an infinite sequence of spherical harmonic terms is usually required to completely represent the field. In cases where the surface‐field is of a type adequately represented by only a small number of spherical harmonic terms, so that those remaining may be neglected, the Gaussian method yields a very convenient and useful analysis. But it is sometimes found that the electric or magnetic surface‐field is complicated and/or of a type expressed adequately only by many spherical harmonic terms. A spherical harmonic analysis may then become so laborious that it becomes futile from a practical point of view, although still possible in theory. This particular limitation of the Gaussian method often arises in geomagnetism, where complicated features appear not only in the Earth's general magnetic field but also in many of its additional surface‐fields varying periodically and aperiodically in time.