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1993

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Utah State University

Articles 1 - 15 of 15

Full-Text Articles in Physics

Onset Of Convection For Autocatalytic Reaction Fronts In A Vertical Slab, J. Huang, D. A. Vasquez, Boyd F. Edwards, P. Kolodner Dec 1993

Onset Of Convection For Autocatalytic Reaction Fronts In A Vertical Slab, J. Huang, D. A. Vasquez, Boyd F. Edwards, P. Kolodner

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A fully three-dimensional linear stability analysis shows that ascending autocatalytic reaction fronts in vertical slabs are unstable to convection for large-wavelength perturbations at all finite values of the dimensionless driving parameter S=δga3/νDC. This parameter involves a fractional density difference δ between the unreacted and reacted fluids, the acceleration of gravity g, the slab width a, the kinematic viscosity ν, and the catalyst molecular diffusivity DC. Buoyancy dominates over the competing curvature dependence of the front velocity in a band 0c of unstable dimensionless wave numbers, with qc→S/24 as S→0 and qc …

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Convective Turing Patterns, D. A. Vasquez, J. W. Wilder, Boyd F. Edwards Sep 1993

Convective Turing Patterns, D. A. Vasquez, J. W. Wilder, Boyd F. Edwards

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Turing patterns involve regions of different chemical compositions which lead to density gradients that, in liquids, are potentially unstable hydrodynamically. Nonlinear hydrodynamics coupled with a model of Turing pattern formation show that convection modifies and coexists with some Turing patterns and excludes others, and thereby plays a significant role in pattern selection.

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New Singularity In Anisotropic, Time-Dependent Solutions To Maximally Gauss-Bonnet Extended Gravity, T. Kitaura, James Thomas Wheeler Jul 1993

New Singularity In Anisotropic, Time-Dependent Solutions To Maximally Gauss-Bonnet Extended Gravity, T. Kitaura, James Thomas Wheeler

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Among the solutions for anisotropic, time-dependent, maximally Gauss-Bonnet extended gravity, we find a class of curvature singularities for which the metric components remain finite. These new singularities therefore differ in type from the standard Kasner-like divergences expected for this class of theories. We study perturbative solutions near the singularity and show that there exist solutions with timelike paths that reach the singularity in finite proper time. Solving the equation of geodesic deviation in the same approximation, we show that the comoving coordinate system does not break down at the singularity. A brief classification of the corresponding singularity types in Robertson-Walker …

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Ionicity Of Alkali Metal Adsorbates, Reply, D. Mark Riffe, G. K. Wertheim, P. H. Citrin Jul 1993

Ionicity Of Alkali Metal Adsorbates, Reply, D. Mark Riffe, G. K. Wertheim, P. H. Citrin

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A Comment on the Letter by D. M. Riffe et al. Phys. Rev. Lett. 64, 571 (1990).

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Modification Of The Eikonal Relation For Chemical Waves To Include Fluid Flow, J. W. Wilder, D. A. Vasquez, Boyd F. Edwards May 1993

Modification Of The Eikonal Relation For Chemical Waves To Include Fluid Flow, J. W. Wilder, D. A. Vasquez, Boyd F. Edwards

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Propagating wave fronts resulting from autocatalytic chemical reactions have been the focus of much recent research. For the most part, the hydrodynamics resulting from such reactions has been neglected. In this work, a relation is derived for the normal speed of a propagating wave front as a function of the local curvature when fluid motion is allowed. This ‘‘eikonal’’ equation is a generalization of one which was derived in the absence of fluid flow. It is also shown that small variations in the fluid density due to the chemical reaction do not change the form of the relation.

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Evidence Of Preferential Directions For Gravity Wave Propagation Due To Wind Filtering In The Middle Atmosphere, Michael J. Taylor, E. H. Ryan, T. F. Tuan, R. Edwards Apr 1993

Evidence Of Preferential Directions For Gravity Wave Propagation Due To Wind Filtering In The Middle Atmosphere, Michael J. Taylor, E. H. Ryan, T. F. Tuan, R. Edwards

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All-sky TV images of wave structure in the near-infrared hydroxyl (OH) nightglow emission were recorded over a 3-month period during May, June, and July 1988 from a high-altitude site at the Mountain Research Station (40.0° N, 105.6° W, 3050 m), near Nederland, Colorado. Well-defined, coherent wave patterns associated with the passage of short period (<1 hour) gravity waves were observed on a total of 22 occasions. The wave motions exhibited similar spatial and temporal properties during each month but a distinct tendency for northward propagation (68% of the wave azimuths within ± 40.0° N), with some eastward motion in May and June, was observed throughout the campaign. Although it is theoretically well known that upward propagating gravity waves can be blocked at a critical layer produced by the interaction of the waves with the horizontal background wind, observational evidence of this phenomenon is rare. To investigate the possibility that the asymmetry in the wave propagation directions was caused by the critical layer, a model based on mean climatological background winds and numerical tidal wave modes valid for any mid-latitude site and time of the year was constructed to show the regions forbidden to upward gravity wave propagation from critical layer theory. These “blocking diagrams” which vary with height and time were constructed for the OH altitude (∼87 km) for the present paper. Comparison of the predicted (i.e., least restricted) and the observed directions of the wave motion show almost complete agreement. This suggests that middle atmospheric winds can play an important role in determining the flux and the azimuthal distribution of short-period waves reaching the upper atmosphere.

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Ta(110) Surface And Subsurface Core-Level Shifts And 4f7/2 Lineshapes, D. Mark Riffe, G. K. Wertheim Mar 1993

Ta(110) Surface And Subsurface Core-Level Shifts And 4f7/2 Lineshapes, D. Mark Riffe, G. K. Wertheim

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High-resolution 4f core-level spectra of the Ta(110) surface region have been obtained at 80 and 300 K with 70- and 100-eV synchrotron radiation. The data show that the subsurface core-level binding-energy shift (compared to deeper-lying atoms) for a close-packed bcc(110) surface can be substantial: 65±15 meV for the first underlayer atoms of Ta(110). The surface core-level shift is 360±12 meV at 80 K and decreases by 13±2 meV at 300 K. Final-state screening in both the bulk and surface layers is well described by a constant singularity index of 0.133±0.012. An enhanced phonon broadening at the surface corresponds to a …

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Parameterized Study Of The Ionospheric Modification Associated With Sun-Aligned Polar Cap Arcs, D. J. Crain, Jan Josef Sojka, Robert W. Schunk, Lie Zhu Jan 1993

Parameterized Study Of The Ionospheric Modification Associated With Sun-Aligned Polar Cap Arcs, D. J. Crain, Jan Josef Sojka, Robert W. Schunk, Lie Zhu

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Sun-aligned (SA) arcs are a prevalent feature of the polar cap ionosphere during northward interplanetary magnetic field conditions. These arcs are, like the auroral arc, a complex electrodynamic system coupling the ionosphere and magnetosphere. The electron precipitation and convection electric field associated with this system modify the polar cap ionospheric plasma distribution. In the past decade, a wealth of observational information has indicated the complexity of these ionospheric modifications, but only a limited number of model studies have been carried out to elucidate the changes associated with SA arcs. In this investigation, an extensive parametric study has been conducted to …

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Femtosecond Thermionic Emission In The Space-Charge Limited Regime, D. Mark Riffe, X. Y. Wang, M. C. Downer, D. L. Fisher, T. Tajima, J. L. Erskine Jan 1993

Femtosecond Thermionic Emission In The Space-Charge Limited Regime, D. Mark Riffe, X. Y. Wang, M. C. Downer, D. L. Fisher, T. Tajima, J. L. Erskine

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We study femtosecond-laser-pulse-induced electron emission from W(100), Al(110), and Ag(lll) in the sub-damage regime (1–44 mJ/cm2 fluence) by simultaneously measuring the incident-light reflectivity, total electron yield, and electron-energy distribution curves of the emitted electrons. The total-yield results are compared with a space-charge-limited extension of the Richardson-Dushman equation for short-time-scale thermionic emission and with particle-in-a-cell computer simulations of femtosecond-pulsed-induced thermionic emission. Quantitative agreement between the experimental results and two calculated temperature-dependent yields is obtained and shows that the yield varies linearly with temperature beginning at a threshold electron temperature of ~0.25 eV The particle-in-a-cell simulations also reproduce the experimental electron-energy …

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Gravitational Observables And Local Symmetries, Charles G. Torre Jan 1993

Gravitational Observables And Local Symmetries, Charles G. Torre

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Using a recent classification of local symmetries of the vacuum Einstein equations, it is shown that there can be no observables for the vacuum gravitational field (in a closed universe) built as spatial integrals of local functions of Cauchy data and their derivatives.

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Symmetries Of The Einstein Equations, Charles G. Torre, Ian M. Anderson Jan 1993

Symmetries Of The Einstein Equations, Charles G. Torre, Ian M. Anderson

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We classify all generalized symmetries of the vacuum Einstein equations in four spacetime dimensions. They consist of constant scalings of the metric, and of the infinitesimal action of generalized spacetime diffeomorphisms. Our results rule out a large class of possible ‘‘observables’’ for the gravitational field, and suggest that the vacuum Einstein equations are not integrable.

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A First-Principle Derivation Of The High Latitude Total Electron Content Distribution, D. J. Crain, Jan Josef Sojka, Robert W. Schunk, P. H. Doherty, J. A. Klobucher Jan 1993

A First-Principle Derivation Of The High Latitude Total Electron Content Distribution, D. J. Crain, Jan Josef Sojka, Robert W. Schunk, P. H. Doherty, J. A. Klobucher

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Calculation of the high-latitude distribution of the vertical total electron content (TEC) is possible using a three-dimensional, time-dependent ionospheric model. Global and local comparisons may be made with observations of TEC. We compare the local diurnal variation of TEC calculated by the model with observations of TEC at Goose Bay, Labrador and Hamilton, Massachusetts. Data from the period of March 1–11, 1989, and monthly averaged data for solar maximum and solar minimum periods are examined. We extend the model to predict diurnal variations of TEC in the polar cap and compare these results with the observed TEC at Thule, Greenland, …

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Modeling Polar Cap F-Region Patches Using Time Varying Convection, Jan Josef Sojka, M. D. Bowline, Robert W. Schunk, D. T. Decker, Cesar E. Valladares, R. Sheehand, D. N. Anders, R. A. Heelis Jan 1993

Modeling Polar Cap F-Region Patches Using Time Varying Convection, Jan Josef Sojka, M. D. Bowline, Robert W. Schunk, D. T. Decker, Cesar E. Valladares, R. Sheehand, D. N. Anders, R. A. Heelis

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Creation of polar cap F‐region patches are simulated for the first time using two independent physical models of the high latitude ionosphere. The patch formation is achieved by temporally varying the magnetospheric electric field (ionospheric convection) input to the models. The imposed convection variations are comparable to changes in the convection that result from changes in the By IMF component for southward interplanetary magnetic field (IMF). Solar maximum‐winter simulations show that simple changes in the convection pattern lead to significant changes in the polar cap plasma structuring. Specifically, in winter, as enhanced dayside plasma convects into the polar …

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A Time-Dependent Model Of Polar Cap Arcs, Lie Zhu, Jan Josef Sojka, Robert W. Schunk, D. J. Crain Jan 1993

A Time-Dependent Model Of Polar Cap Arcs, Lie Zhu, Jan Josef Sojka, Robert W. Schunk, D. J. Crain

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A two-dimensional time-dependent model of polar cap arcs has been developed. The electrodynamics of the polar cap arcs are treated self-consistently in the frame of the coupled magnetosphere-ionosphere system. The focus of this paper is to introduce the physics and mathematical formulation of the model and describe the features of the spatial structure and temporal evolution of the polar cap arcs. The modeling results indicate that the time constant for the formation of the polar cap arcs is around 10 min. It is found that an initial single-arc precipitation pattern associated with a polar cap arc tends to split into …

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Influence Of Horizontal Inhomogeneity In The Ionosphere On The Reflection Of Alfvén Waves, Lie Zhu, Jan Josef Sojka, Robert W. Schunk, D. J. Crain Jan 1993

Influence Of Horizontal Inhomogeneity In The Ionosphere On The Reflection Of Alfvén Waves, Lie Zhu, Jan Josef Sojka, Robert W. Schunk, D. J. Crain

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A study of the reflection of Alfvén waves at a horizontally inhomogeneous ionosphere has been carried out. In this study, the Alfvén speed above the ionosphere is assumed to be uniform and the ionosphere is treated as a height‐integrated conducting slab. Analytical and numerical results indicate that the horizontal nonuniformity of the ionospheric conductivity can lead to a rotation of the reflected wave fields and cause field‐aligned currents that originate in the ionosphere. A strong conductivity nonuniformity in the direction perpendicular to the incident wave field, large Hall to Pedersen conductivity ratios, and low conductivity values lead to a large …

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