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Compact algorithms generating and solving finite-difference approximations of partial differential equations for propagation of waves obtained by new method. Based on concept of discrete dispersion relation. Used in wave propagation to relate frequency to wavelength and is key measure of wave fidelity.
The purpose of a group management scheme is to enable its associated transfer layer protocol to be responsive to user determined reliability requirements for multicasting. Group management (GM) must assist the client process in coordinating multicast group membership, allow the user to express the subset of the multicast group that a particular multicast distribution must reach in order to be successful (reliable), and provide the transfer layer protocol with the group membership information necessary to guarantee delivery to this subset. GM provides services and mechanisms that respond to the need of the client process or process level management protocols to coordinate, modify, and determine attributes of the multicast group, especially membership. XTP GM provides a link between process groups and their multicast groups by maintaining a group membership database that identifies members in a name space understood by the underlying transfer layer protocol. Other attributes of the multicast group useful to both the client process and the data transfer protocol may be stored in the database. Examples include the relative dispersion, most recent update, and default delivery parameters of a group.
The origin of the 100 percent elliptical polarization of Jupiter's decametric radio emission is investigated. The transfer of polarized radiation when coupling of the Stokes parameters is important is studied, and it is found, in agreement with earlier authors, that the density in and near the source region must be so low that the polarization remains fixed along the ray path. The polarization of the cyclotron maser radiation in these circumstances is determined, and it is found that the dispersion relation of the rarefied plasma composed of energetic, anisotropic electrons is like that in the vacuum. It is also found that the growth rate is sufficient to saturate the maser and account for the observed brightness temperature. Possible sources of plasma in and near the source region in Jupiter's inner, polar magnetosphere are considered.
The stability of the planar interface of a structureless solid growing from a depositing component dilute in a carrier fluid is studied when the main solute transport mechanism is thermal (Soret) diffusion. A linear stability analysis, carried out in the limit of low growth Peclet number, leads to a dispersion relation which shows that the planar front is unstable either when the thermal diffusion factor of the condensing component is positive and the latent heat release is small or when the thermal diffusion factor is negative and the solid grows over a thermally-insulating substrate. Furthermore, the influence of interfacial energy effects and constitutional supersaturation in the vicinity of the moving interface is analyzed in the limit of very small Schmidt numbers (small solute Fickian diffusion). The analysis is relevant to physical vapor deposition of very massive species on cold surfaces, as in recent experiments of organic solid film growth under microgravity conditions.
A qualitative analysis is conducted to reconcile the experimental data from critical ionization velocity (CIV) studies with CIV theories. The experimental data are reviewed demonstrating that: (1) the wave frequency is variable and low; (2) the wave polarization is almost isotropic; (3) electron energization is not easily reconciled with the observed wave spectrum; and (4) ambient electron density plays a role in determining CIV triggering conditions. Analytical treatment is given to the dispersion relation of the lower hybrid wave (LWH) instability driven by the streaming of an ion beam generated by the interaction of the neutral cloud with the ambient atmosphere. By incorporating the LWH instabilities of strong turbulence and finite-size effects into theoretical CIV relationships, the observations can be interpreted. The issues raised by the experimental data are understood within the context of a hypothesis of backward propagating nonlinearly collapsing wavepackets.
A normal mode analysis for nonaxisymmetric perturbations in a thin, differentially rotating disk with a vertical structure that is isothermal and convectively unstable is performed. The vertical gravity is assumed to be external and constant. The perturbation scale is assumed to be much shorter than the radius of the disk but comparable to or less than the thickness. The initial value problem is formulated in shearing coordinates. Dispersion relations are obtained for the three limiting cases of zero shear, axisymmetric perturbations, and small radial wavelengths. The full effects of shear are studied by integrating numerically the initial value problem. Nonaxisymmetric local Fourier modes are found to have a radial wavenumber that increases linearly with time in proportion to the shear times the azimuthal wavenumber. While Coriolis forces exert stabilizing effects on the convective modes, reducing their growth rate and the range of unstable wavelengths, shear has destablizing effects inasmuch as it reduces the epicyclic frequency at a given angular velocity. In a Keplerian disk, perturbations with azimuthal wavelengths about 2 times smaller than vertical wavelengths grow exponentially.
The gyroresonance of electrons with parallel transverse cold plasma waves is considered, and the Fokker-Planck equation describing the evolution of the electron distribution function in the presence of a spectrum of turbulence is derived. A new resonance which produces a divergence in the Fokker-Planck coefficients is identified; it results when the electron is in gyroresonance with a wave that has a group velocity equal to the velocity of the electron along the magnetic field. Under the assumption of a power-law spectral density, the Fokker-Planck coefficients are calculated numerically, and their complicated momentum and pitch-angle dependence, as well as the influence of various approximations to the dispersion relation, gyroresonance condition, and spectral density are discussed. It is found that there is no resonance gap at any pitch angle as long as the full gyroresonance condition is used and waves propagating on both directions are present.
In this paper it is argued that the time-frequency analysis of structural impulse responses may be used to reveal the wave types carrying significant energy through a structure. Since each wave type is characterized by its own dispersion relation, each wave type may be associated with particular features appearing in the time-frequency domain representation of an impulse response. Here the Wigner Distribution is introduced as a means for obtaining appropriate time-frequency representations of impulse responses. Practical aspects of the calculation of the Wigner Distribution are discussed and examples of its application to the analysis of structural impulse responses are given. These examples will show that the Wigner Distribution may be conveniently used to distinguish between the contributions of various waves types to a total structural response.
The entries of the radioactive components into earth's atmosphere resulting from an accident or inadvertent abort of a space vehicle powered by nuclear-thermal-rockets are investigated. The study is made for a typical piloted Mars mission vehicle incapacitated by an accident or malfunction during the trans-Mars-injection maneuver due to simultaneous multiple failures of its component systems. The three different accident/abort modes considered are the following: (1) a constant-rate angular pitching motion of the vehicle, (2) a constant-acceleration angular pitching motion of the vehicle, and (3) the rocket engine breaks away from the rest of the vehicle with a finite relative (dispersion) velocity. The speeds and angles of the atmospheric entries are calculated for each mode for different values of the time of the accident, pitching rate, acceleration, and dispersion velocity. For the most severe entry speeds and flight-path angles, the stagnation-point pressures, heat transfer rates, thickness, and mass per unit area of the heat shields necessary to protect the radioactive components from disintegrating, deceleration g-loads, and high ground-impact velocities are calculated. The study points out that the high g-loads and high ground-impact velocities are the most serious problems that must be addressed.
Implicit and explicit spatial differencing techniques with fourth order accuracy have been developed. The implicit technique is based on the Pade compact scheme. A Dispersion Relation Preserving concept has been incorporated into both of the numerical schemes. Two dimensional Euler computation of a spatially-developing free shear flow, with and without external excitation, has been performed to demonstrate the capability of numerical schemes developed. Results are in good agreement with theory and experimental observation regarding the growth rate of fluctuating velocity, the convective velocity, and the vortex-pairing process.
During the Spacelab 2 mission the University of Iowa's Plasma Diagnostics Package (PDP) explored the plasma environment around the shuttle. Wideband spectrograms of plasma waves were obtained from the PDP at frequencies of 0-30 kHz and at distances up to 400 m from the shuttle. Strong low-frequency (below 10 kHz) electric field noise was observed in the wideband data during two periods in which an electron beam was ejected from the shuttle. This noise shows clear evidence of interference patterns caused by the finite (3.89 m) antenna length. The low-frequency noise was the most dominant type of noise produced by the ejected electron beam. Analysis of antenna interference patterns generated by these waves permits a determination of the wavelength, the direction of propagation, and the location of the source region. The observed waves have a linear dispersion relation very similar to that of ion acoustic waves. The waves are believed to be oblique ion acoustic or high-order ion cyclotron waves generated by a current of ambient electrons returning to the shuttle in response to the ejected electron beam.
The Alfvenic drift Kelvin-Helmholtz instability of a high-beta plasma in the presence of equilibrium magnetic and electric fields perpendicular to each other are studied. The plasma components are assumed to have 2D sheared velocity in y and z directions. The dispersion relation is derived, and the instability criterion is determined. It is shown that the equilibrium electric field has either stabilizing or destabilizing effect depending on certain conditions discussed in the paper.
The instability of rectangular jets is investigated using a vortex-sheet model. It is shown that such jets support four linearly independent families of instability waves. Within each family there are infinitely many modes. A way to classify these modes according to the characteristics of their mode shapes or eigenfunctions is proposed. It is demonstrated that the boundary element method can be used to calculate the dispersion relations and eigenfunctions of these instability wave modes. The method is robust and efficient. A parametric study of the instability wave characteristics has been carried out. A sample of the numerical results is reported here. It is found that the first and third modes of each instability wave family are corner modes. The pressure fluctuations associated with these instability waves are localized near the corners of the jet. The second mode, however, is a center mode with maximum fluctuations concentrated in the central portion of the jet flow. The center mode has the largest spatial growth rate. It is anticipated that as the instability waves propagate downstream the center mode would emerge as the dominant instability of the jet.
We study the linear stability of an optically thin uniform radiating plasma subject to nonlocal heat transport. We derive the dispersion relation appropriate to this problem, and the marginal wavenumbers for instability. Our analysis indicates that nonlocal heat transport acts to reduce the stabilizing influence of thermal conduction, and that there are critical values for the electron mean free path such that the plasma is always unstable. Our results may be applied to a number of astrophysical plasmas, one such example being the halos of clusters of galaxies.
The problem of gravitational instability of an infinite homogeneous self-gravitating medium carrying a uniform magnetic field in the presence of Hall effect has been investigated to include the effect due to rotation. The dispersion relation has been obtained. It has been found that the Jeans's criterion for the instability remains unaffected even when the effect due to rotation is considered in the presence of Hall effect carrying a uniform magnetic.
A class of parametric instabilities of finite-amplitude, circularly polarized Alfven waves in a plasma with pressure anisotropy is studied by application of the CGL equations. A linear perturbation analysis is used to find the dispersion relation governing the instabilities, which is a fifth-order polynomial and is solved numerically. A large-amplitude, circularly polarized wave is unstable with respect to decay into three waves: one sound-like wave and two side-band Alfven-like waves. It is found that, in addition to the decay instability, two new instabilities that are absent in the framework of the MHD equations can occur, depending on the plasma parameters.
We examined several publicly available spectral analysis software packages looking for one with enough functionality and versatility to meet the analysis needs of astronomers during the next decade. None of those examined can satisfactorily support the wide variety of panchromatic science programs that are now becoming possible. Furthermore, we concluded that none of these packages can be simply modified to include critical functions because of their original (limited) designs. During the next two years we will write a new spectral analysis package, ASpect, that will incorporate the latest analysis techniques for astronomical spectra in all wavelength domains. The ASpect package has several functional requirements. It must operate on spectra from a wide variety of ground-based and space-based instruments spanning wavelengths from radio to gamma rays. It must accommodate non-linear dispersion relations. It must provide a variety of functions, individually or in combination, with which to fit spectral features and the continuum. It is vitally important that known bad data be masked and that, uncertainties be propagated throughout the calculations in order for astronomers to evaluate the reliability of results. Finally, this new package must provide a powerful, intuitive graphical user interface to handle the burden of data input/output (I/O), on-line 'help,' selection of relevant features for analysis, plotting and graphical interaction, and data base management--all in a comprehensible environment. We anticipate that ASpect will take the form of an external package in IRAF (such as the NOAO and STSDAS packages) and will be layered upon the IRAF virtual Operating System to make it available on as many platforms as possible, while making it resistant to changes in operating systems and compilers. Our choice of IRAF is motivated by its portability, its wide use within the astronomical community, and its rich set of existing user applications.
In this report, we extend to cloud physics the work done for single and multiple scattering of electromagnetic waves. We consider the scattering of light, visible or infrared, by a spherical cloud represented by a statistically homogeneous ensemble of configurations of N identical spherical water droplets whose centers are uniformly distributed in its volume V. The ensemble is specified by the average number rho of scatterers in unit volume and by rho f(R) with f(R) as the distribution function for separations R of pairs. The incident light, vector-phi(sub 0) a plane electromagnetic wave with harmonic time dependence, is from outside the cloud. The propagation parameter kappa(sub 0) and the index of refraction eta(sub 0) determine physically the medium outside the distribution of scatterers. We solve the interior problem separately to obtain the bulk parameters for the scatterer equivalent to the ensemble of spherical droplets. With the interior solution or the equivalent medium approach, the multiple scattering problem is reduced to that of an equivalent single scatterer excited from outside illumination. A dispersion relation which determines the bulk propagation parameter K and the bulk index of refraction eta of the cloud is given in terms of the vector equivalent scattering amplitude vector-G and the dyadic scattering amplitude tilde-g of the single object in isolation. Based on this transfer model we will have the ability to consider clouds composed of inhomogeneous distribution of water and/or ice particles and we will be able to take into account particle size distributions within the cloud. We will also be able to study the effects of cloud composition (i.e., particle shape, size, composition, orientation, location) on the polarization of the single or the multiple scattered waves. Finally, this study will provide a new starting point for studying the problem of lightning radiative transfer.