Localized Inhomogeneities Research Articles

Radio caustics from localized interstellar medium plasma structures

In a study of, 36 extragalactic radio sources observed over 7 years with the Green Bank interferometer at 2.7 GHz and 8.1 GHz, Fiedler et al.1 have detected unusual variations in the light curves of several sources. The most dramatic event, found in the flux record of the quasar 0954 + 658, showed large modulations at both frequencies. At 8.1 GHz, there are four spikes over a period ∼80 days in each of which the intensity increases roughly threefold. At 2.7 CfHz, two broad maxima of width ∼30 days bracket the high-frequency event, with a total duration of ∼120 days. At least two other sources show similar 2.7-GHz variations. Fiedler et al. argue cogently that these variations are unlikely to be intrinsic to the sources and that refractive scintillation in the interstellar medium is probably responsible. They postulate the existence of large-scale (1013–1014 cm) localized plasma density inhomo-geneities in the interstellar medium (ISM) to explain these observations. Independent evidence for strong local fluctuations in the ISM electron density on these scales comes froin the unusually large amplitude of low-frequency quasar variation2, the presence of periodicities in pulsar dynamic scintillation spectra3 and non-gaussian spikes in pulsar light curves4. All three phenomena have been interpreted as multiple imaging and focusing by large-scale refractors. Here we present a more detailed interpretation of the event in 0954 + 658, propose possible sites for the refracting clouds and suggest some future observations.

AN INTRODUCTION TO SUBQUANTUM KINETICS: II. An Open Systems Description of Particles and Fields

Abstract A nonlinear reaction-diffusion system, designated as Model G, is described. This system is similar to the Brusselator with the exception that it includes a third reaction intermediate, variable G. It is shown that a reaction-diffusion substrate of the kind specified by Model G is able to give rise to localized steady state concentration inhomogeneities which are autonomous and self-stabilizing and which exhibit many of the properties of subatomic particles. These “dissipative structures” generate substrate concentration gradients (fields) about themselves which serve as physically realistic analogs of gravitational and electrostatic potential fields. Test particles placed in these 1/r fields are found to experience a 1/r2 accelerating force. Thus action-at-a-distance is elucidated, The same field giving rise to “electrostatic” effects is capable of causing closely spaced particles to undergo nuclear binding. Atom formation with electron orbital quantization is also predicted to occur. The fieid/s...

Low Frequency Scattering by Local Inhomogeneities

An asymptotic expansion which is uniformly valid in space is obtained for the low frequency scattering of a plane wave incident on a localized inhomogeneity. The scattering region, which may be simply or multiply (collection of scatterers) connected, has a characteristic length which is small compared with the wave length of the incident wave. The index of refraction n is unity outside the scattering region and it is arbitrary inside the region. The method of matched asymptotic expansions is used in the analysis. The Born approximation is shown to agree with the uniform expansion in the far and the near fields. the leading term in the uniform expansion is a linear functional of $1 - n^2 $. Thus, statistics of the scattered field are easily evaluated from the statistics of n, when n is a random process. The method is then applied to the low frequency scattering of an acoustic plane wave by localized inhomogeneities in the density and the index of refraction. Finally, the scattering by a plane which is acoustically hard except for a small impedance spot, is analyzed by the same method.

Elongation of electromagnetic wave trajectories on paths with localized refractive index inhomogeneities of air

Elongation of electromagnetic wave trajectories on paths with localized refractive index inhomogeneities of air

Ultraviolet interstellar extinction toward 1367 stars observed by ANS

view Abstract Citations (85) References (35) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Ultraviolet interstellar extinction toward 1367 stars observed by ans. Meyer, D. M. ; Savage, B. D. Abstract Five-band UV photometry is used to investigate the character of interstellar extinction toward 1367 stars of spectral type B3 and earlier, with observations producing a galactic average of 5.11, 4.78, 6.52, 4.10 and 1.95 at 1550, 1800, 2200, 2500, and 3300 A, respectively. Differences in the strengths of the UV extinction features of these stars appear to be independent, since objects with either strong or weak 2200 A extinction can have strong, normal or weak far-UV extinction. Further investigation reveals that some of the deviant stars are embedded in localized regions whose average extinction curves diverge dramatically from the galactic mean. The anomalous extinction of these regions attests to localized inhomogeneities in the UV characteristics of interstellar dust. Because the evaluation of UV extinction toward a particular star would be more accurate if based on the average curve of the region immediately surrounding that star rather than that of the entire Galaxy, it is recommended that the galactic average presented be used with caution to correct astronomical data of individual objects. Publication: The Astrophysical Journal Pub Date: September 1981 DOI: 10.1086/159180 Bibcode: 1981ApJ...248..545M Keywords: Astronomical Netherlands Satellite; Interstellar Extinction; Star Distribution; Stellar Spectra; Ultraviolet Astronomy; B Stars; Error Analysis; Interstellar Matter; Magellanic Clouds; Ultraviolet Photometry; Astrophysics full text sources ADS | data products SIMBAD (60)

A Formal Theory of Internal Wave Scattering with Applications to Ocean Fronts

Abstract A concise theory for scattering of internal waves at localized inhomogeneities (i.e., topographic features, baroclinicity in the density field, variations of the mean sea level, jetlike currents) in the oceanic waveguide is presented within the formal framework of quantum mechanical scattering theory. The equations of motion of the wave system are reduced to a form resembling the Schrodinger equation with an interaction operator describing the effect of the ambient inhomogeneities. By standard Green's function techniques integral equations for the scattered field and its Fourier transform (which relates to the amplitudes of the scattered waves) are derived, both for a scattering region of finite extent (representing a two-dimensional scattering problem) and a “wall-like” scattering region of infinite extent (representing a one-dimensional scattering problem). As an example, the theory is applied to the scattering at a straight geostrophic front. The far-field is described in the Born approximatio...

Magnetic holes in the solar wind

An analysis is presented of high resolution interplanetary magnetic field measurements from the magnetometer on Explorer 43 which showed that low magnetic field intensities in the solar wind at 1 AU occur as distinct depressions or 'holes'. These magnetic holes are new kinetic-scale phenomena, having a characteristic dimension on the order of 20,000 km. They occurred at a rate of 1.5/day in the 18-day time span (March 18 to April 6, 1971) that was analyzed. Most of the magnetic holes are characterized by both a depression in the absolute value of the magnetic field, and a change in the magnetic field direction; some of these are possibly the result of magnetic merging. However, in other cases the magnetic field direction does not change; such holes are not due to magnetic merging, but might be a diamagnetic effect due to localized plasma inhomogeneities.

Appearance of flaws in layers of AlxGa1?xSb solid solutions during epitaxial growth from the liquid phase

Layers of AlxGa1−xSb (x≤0.7) solid solutions were grown epitaxially from the liquid phase on substrates of gallium antimonide. Aluminum was added to the melt during the buildup of an epitaxial layer. In these layers were found inclusions of the solvent as well as regions of localized chemical inhomogeneities. It is shown here that the appearance of such flaws is intimately related to the relief which the GaSb substrate has formed by the time aluminum enters into the solution. A mechanism of this flaw formation is suggested.

Multiple scattering of waves by ensembles of particles and the theory of radiation transport

The problem is outlined of how to approach the description of partially coherent multiple scattering of waves by ensembles of particles in terms of the photometric theory of radiation transport in a scattering medium. The treatment is carried out with the example of a scalar monochromatic wavefield. The apparatus of equations of the Dyson and Bethe-Salpeter types is used. This apparatus is shown to be adequate for the fundamental concepts of transport theory. In particular, correspondence is established between an effective inhomogeneity of an ensemble of correlated particles and the volume element of transport theory. It is shown that neglect of the effect of mutal illumination of correlation groups of particles within a given inhomogeneity leads to spatial localization of the inhomogeneities, while the additional hypothesis that the localized inhomogeneities lie close to one another in the Fraunhofer zone gives the transport equation. The contribution is estimated of the effect of mutual illumination of correlation groups of particles within a given inhomogeneity to the partially coherent scattering of waves by the volume of the medium. The role of the detector of the scattered radiation in making this estimate is discussed.

Absorption of thermal neutrons in media with localized inhomogeneities

Absorption of thermal neutrons in media with localized inhomogeneities

Gaussian Light Beams in Inhomogeneous Media

Vector wave solutions are obtained for the propagation of beams of light in media having slow spatial variations of the gain, loss, or index of refraction. The formalism developed here is applicable to a wide range of problems, and an exa mple considered in detail is the propagation of off-axis beams in lenslike laser materials and optical waveguides. A procedure is also described for the diagnosis of localized dielectric inhomogeneities such as plasmas by means of Gaussian laser beams.

A Magnetotelluric Traverse in Southern Australia

Summary Telluric potentials were recorded at nine sites stretching 1000 km from east of Wilcannia (N.S.W.) to west of Coober Pedy (S.A.) in south central Australia. These were combined with data obtained from a simultaneous magnetic variometer study, and analysed to yield magnetotelluric impedance tensors. The estimation of probable errors for the impedance tensors was an important part of the calculations. Numerical calculations for two-dimensional models showed that the observed data could be explained by a conductive-resistive-conductive layering. The top conductive (10 ohm-m) layer is approximately 5 km deep and the ensuing resistive section (1000ohm-m) continues to a depth of about 230km. This is underlain by an indeterminably thick conductive layer. Embedded in this general structure is an extremely good conductor (0.1 ohm-m) which strikes north-south. The detection of this good conductor is the main result of this work. The good conductor lies just east of the Adelaide Geosyncline at a depth of a few kilometres, and is of minimum thickness 10 km. It is thought to be related to the accretion of the eastern portion of the Australian continent on to the stable western Precambrian platform by tectonic processes. The strong anisotropy present in the magnetotelluric data at all sites could not be explained by induction in conductive sediments, and is thought to be caused by localized conductivity inhomogeneities distorting current flow.

Martensitic pretransformation phenomena in the Au-47.5 at.% Cd and the Fe-29.7 at.% Ni alloys

Premartensitic transformation characteristics have been investigated in the Au-47.5 at.% Cd and the Fe-29.7 at.% Ni alloys by measuring electrical resistivity and magnetoresistance. Preliminary electron diffraction studies have also been made for the Au–Cd alloy. An anomalous magnetoresistance peak was observed for both of these alloys prior to the martensitic transformation. In the Au–Cd alloy this peak was about 2–3°C above the Ms, and for the Fe–Ni alloy it was about 5–6°C above the Ms. Furthermore, when the Ms was lowered by isothermal stabilization, this peak moved in conjunction with the new Ms temperature. This indicates that the observed peak is truly associated with the premartensitic instability of the parent phase. The electron diffraction pattern of the parent phase of the Au–Cd alloy shows extensive 〈110〉 and 〈112〉 streaking in the temperature range where anomalous magnetoresistance was observed. These experimental results could tentatively be interpreted in terms of anisotropic scattering centers resulting from localized strain inhomogeneities. Presumably these inhomogeneities are associated with the atomic shuffling prior to the actual phase transformation and could be identified with the so-called ``strain embryos''.

The effect of media inhomogeneities upon intracranial electrical fields.

A three-dimensional finite difference computer simulation has been used to model entire intracranial fields from a known dipole generator in both a cat brain and an insulated cube. The model permits pointwise variation of conductivity coincident with intracranial inhomogeneities due to gray and white matter, cerebrospinal fluid, and bone. Adequacy of the model was verified by agreement between the computed results and those obtained in the cube with both homogeneous and inhomogeneous media and in the cat brain along the dipole axis. Variation of brain model parameters showed that neither increasing nor abolishing intracranial inhomogeneities would give differences great enough to be detected experimentally. On the other hand, the position of the boundary could have significant effects on the fields. The model is general enough to be applicable to a variety of multipole generators and generator locations, as well as permitting consideration of the influence of localized inhomogeneities.

Application of the ?straight-lines? method to solving the one-dimensional equation of heat conduction with localized inhomogeneities

The method of product functions is used for obtaining an exact solution to the infinite system of first-order equations which represents an analog of the one-dimensional heat conduction equation with a locally variable thermal conductivity.

Computer Study of Bulk GaAs Devices with Random One-Dimensional Doping Fluctuations

Computer calculations have been performed in order to investigate the influence of one-dimensional random doping fluctuations and localized inhomogeneities on the performance of bulk GaAs oscillators. For the doping fluctuations assumed it was found that accumulation layers dominate in subcritically doped samples (n0×L<1012 cm−2) while dipole layers always form in samples doped above the critical level. Five different modes of oscillation are described and defined with the help of a mode chart. Special attention is paid to the ``quenched multiple-dipole'' mode, which is difficult to obtain if localized inhomogeneities such as dislocations or doping gradients are incorporated in addition to the random fluctuations. It is concluded that a series-connection of accumulation-mode devices, which are not sensitive to doping fluctuations, provides an attractive approach to microwave or millimeter-wave oscillator design.