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 Polar cap arcs (2)
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 F region (2)
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Articles 1  19 of 19
FullText Articles in Physics
Onset Of Convection For Autocatalytic Reaction Fronts In A Vertical Slab, J. Huang, D. A. Vasquez, Boyd F. Edwards, P. Kolodner
Onset Of Convection For Autocatalytic Reaction Fronts In A Vertical Slab, J. Huang, D. A. Vasquez, Boyd F. Edwards, P. Kolodner
All Physics Faculty Publications
A fully threedimensional linear stability analysis shows that ascending autocatalytic reaction fronts in vertical slabs are unstable to convection for largewavelength perturbations at all finite values of the dimensionless driving parameter S=δga^{3}/νD_{C}. 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 D_{C}. Buoyancy dominates over the competing curvature dependence of the front velocity in a band 0c of unstable dimensionless wave numbers, with q_{c}→S/24 as S→0 and q ...
Solar Cycle And Seasonal Variationsin F Region Electrodynamics At Millstone Hill, M. J. Buonsanto, M. E. Hagan, J. E. Salah, Bela G. Fejer
Solar Cycle And Seasonal Variationsin F Region Electrodynamics At Millstone Hill, M. J. Buonsanto, M. E. Hagan, J. E. Salah, Bela G. Fejer
Bela G. Fejer
Incoherent scatter radar observations of ion drifts taken at Millstone Hill (42.6°N, 288.5°E) during 73 experiments in the period February 1984 to February 1992 are used to construct, for the first time at this station, average quiettime E×B drift patterns for both solar cycle maximum and minimum, for the summer, winter, and equinox seasons. The daily variation of V⊥N shows a reversal from northward to southward drifts near noon, and a return to northward drifts in the premidnight hours. The weaker southward drift in the afternoon in summer noted by Wand and Evans (1981 ...
Convective Turing Patterns, D. A. Vasquez, J. W. Wilder, Boyd F. Edwards
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.
New Singularity In Anisotropic, TimeDependent Solutions To Maximally GaussBonnet Extended Gravity, T. Kitaura, James Thomas Wheeler
New Singularity In Anisotropic, TimeDependent Solutions To Maximally GaussBonnet Extended Gravity, T. Kitaura, James Thomas Wheeler
All Physics Faculty Publications
Among the solutions for anisotropic, timedependent, maximally GaussBonnet 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 Kasnerlike 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 RobertsonWalker ...
Ionicity Of Alkali Metal Adsorbates, Reply, D. Mark Riffe, G. K. Wertheim, P. H. Citrin
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).
Modification Of The Eikonal Relation For Chemical Waves To Include Fluid Flow, J. W. Wilder, D. A. Vasquez, Boyd F. Edwards
Modification Of The Eikonal Relation For Chemical Waves To Include Fluid Flow, J. W. Wilder, D. A. Vasquez, Boyd F. Edwards
All Physics Faculty Publications
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.
Properties Of Rigid Foams For Application As Materials For Light Weight Structures In Space, Huichen Chi
Properties Of Rigid Foams For Application As Materials For Light Weight Structures In Space, Huichen Chi
All Graduate Theses and Dissertations
The mechanical and mesoscopic structural properties of rigid cellular foam made of polystyrene have been investigated. Basic properties (e.g., density, total and available porosity, permeability, surface area, isotropicity, and cell size and cell wall thickness distributions) were measured. In most cases, alternative methods were used to determine which methods were most appropriate for the type of samples we studied. Standard compression and deflection mechanical tests were performed. The stressstrain curves and related mechanical properties were found to agree with standard cellular structural models of opencell foams.
We investigated the effects of small (~<5 atm) hydrostatic stress applied to foam samples for long periods of time (~one day). We observed large changes (up to a factor of three) in the stressstrain diagrams, Young's modulus, elastic collapse stress, ultimate strength, resilience, Poisson's ratio, permeability, penetration depth, and available porosity. Effects were most pronounced above 2 atm applied pressure differential, but were observed even for 1 atm loads. Shortterm exposure to loads up to ten times as large did not cause comparable changes. These changes were interpreted as resulting from observed changes in the mesoscopic structure occurring near the surface using standard cellular structural models.
This work was originally motivated by applications ...
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
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
All Physics Faculty Publications
Allsky TV images of wave structure in the nearinfrared hydroxyl (OH) nightglow emission were recorded over a 3month period during May, June, and July 1988 from a highaltitude site at the Mountain Research Station (40.0° N, 105.6° W, 3050 m), near Nederland, Colorado. Welldefined, 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 midlatitude 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 shortperiod waves reaching the upper atmosphere.
Ta(110) Surface And Subsurface CoreLevel Shifts And 4f7/2 Lineshapes, D. Mark Riffe, G. K. Wertheim
Ta(110) Surface And Subsurface CoreLevel Shifts And 4f7/2 Lineshapes, D. Mark Riffe, G. K. Wertheim
All Physics Faculty Publications
Highresolution 4f corelevel spectra of the Ta(110) surface region have been obtained at 80 and 300 K with 70 and 100eV synchrotron radiation. The data show that the subsurface corelevel bindingenergy shift (compared to deeperlying atoms) for a closepacked bcc(110) surface can be substantial: 65±15 meV for the first underlayer atoms of Ta(110). The surface corelevel shift is 360±12 meV at 80 K and decreases by 13±2 meV at 300 K. Finalstate screening in both the bulk and surface layers is well described by a constant singularity index of 0.133±0.012. An ...
Timestamp Semantics And Representation., Curtis Dyreson, Richard T. Snodgrass
Timestamp Semantics And Representation., Curtis Dyreson, Richard T. Snodgrass
Curtis Dyreson
Many database Management systems and operating systems provide support for time values. At the physical level time values are known as timestamps. A timestamp has a physical realization and a temporal interpretation. The physical realization is a pattern of bits while the temporal interpretation is the meaning of each bit pattern, that is, the time each pattern represents. All previous proposals defined timestamps in terms of seconds. However, as we show, there are at least seven definitions of this fundamental time unit. We propose a more precise temporal interpretation, the timeline clock, that constructs a timeline by using different welldefined ...
FRegion Plasma Drifts Over Arecibo: Solar Cycle, Seasonal And Magnetic Activityeffects, Bela G. Fejer
FRegion Plasma Drifts Over Arecibo: Solar Cycle, Seasonal And Magnetic Activityeffects, Bela G. Fejer
Bela G. Fejer
We have used Arecibo incoherent scatter measurements from 1981 to 1990 to determine the characteristics of lowlatitude F region plasma drifts. The measurements show large daytoday variability even during magnetically quiet periods. The average poleward/perpendicular plasma drifts do not change significantly with season and solar cycle except in the midnightmorning sector. The zonal drifts show clear solar cycle and seasonal effects. The afternoonnighttime eastward drifts increase with solar flux; the westward drifts in the early morningafternoon sector show a large increase from summer to winter but are independent of solar activity. The two perpendicular velocity components also respond differently ...
A FirstPrinciple Derivation Of The High Latitude Total Electron Content Distribution, D. J. Crain, Jan Josef Sojka, Robert W. Schunk, P. H. Doherty, J. A. Klobucher
A FirstPrinciple Derivation Of The High Latitude Total Electron Content Distribution, D. J. Crain, Jan Josef Sojka, Robert W. Schunk, P. H. Doherty, J. A. Klobucher
All Physics Faculty Publications
Calculation of the highlatitude distribution of the vertical total electron content (TEC) is possible using a threedimensional, timedependent 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 ...
A TimeDependent Model Of Polar Cap Arcs, Lie Zhu, Jan Josef Sojka, Robert W. Schunk, D. J. Crain
A TimeDependent Model Of Polar Cap Arcs, Lie Zhu, Jan Josef Sojka, Robert W. Schunk, D. J. Crain
All Physics Faculty Publications
A twodimensional timedependent model of polar cap arcs has been developed. The electrodynamics of the polar cap arcs are treated selfconsistently in the frame of the coupled magnetosphereionosphere 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 singlearc precipitation pattern associated with a polar cap arc tends to split into ...
Femtosecond Thermionic Emission In The SpaceCharge Limited Regime, D. Mark Riffe, X. Y. Wang, M. C. Downer, D. L. Fisher, T. Tajima, J. L. Erskine
Femtosecond Thermionic Emission In The SpaceCharge Limited Regime, D. Mark Riffe, X. Y. Wang, M. C. Downer, D. L. Fisher, T. Tajima, J. L. Erskine
All Physics Faculty Publications
We study femtosecondlaserpulseinduced electron emission from W(100), Al(110), and Ag(lll) in the subdamage regime (1–44 mJ/cm^{2} fluence) by simultaneously measuring the incidentlight reflectivity, total electron yield, and electronenergy distribution curves of the emitted electrons. The totalyield results are compared with a spacechargelimited extension of the RichardsonDushman equation for shorttimescale thermionic emission and with particleinacell computer simulations of femtosecondpulsedinduced thermionic emission. Quantitative agreement between the experimental results and two calculated temperaturedependent yields is obtained and shows that the yield varies linearly with temperature beginning at a threshold electron temperature of ~0.25 eV The particleinacell ...
Modeling Polar Cap FRegion Patches Using Time Varying Convection, Jan Josef Sojka, M. D. Bowline, Robert W. Schunk, D. T. Decker, C. E. Valladares, R. Sheehand, D. N. Anders, R. A. Heelis
Modeling Polar Cap FRegion Patches Using Time Varying Convection, Jan Josef Sojka, M. D. Bowline, Robert W. Schunk, D. T. Decker, C. E. Valladares, R. Sheehand, D. N. Anders, R. A. Heelis
All Physics Faculty Publications
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 B_{y} 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 ...
Symmetries Of The Einstein Equations, Charles G. Torre, Ian M. Anderson
Symmetries Of The Einstein Equations, Charles G. Torre, Ian M. Anderson
All Physics Faculty Publications
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.
Influence Of Horizontal Inhomogeneity In The Ionosphere On The Reflection Of Alfvén Waves, Lie Zhu, Jan Josef Sojka, Robert W. Schunk, D. J. Crain
Influence Of Horizontal Inhomogeneity In The Ionosphere On The Reflection Of Alfvén Waves, Lie Zhu, Jan Josef Sojka, Robert W. Schunk, D. J. Crain
All Physics Faculty Publications
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 ...
Gravitational Observables And Local Symmetries, Charles G. Torre
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.
Parameterized Study Of The Ionospheric Modification Associated With SunAligned Polar Cap Arcs, D. J. Crain, Jan Josef Sojka, Robert W. Schunk, Lie Zhu
Parameterized Study Of The Ionospheric Modification Associated With SunAligned Polar Cap Arcs, D. J. Crain, Jan Josef Sojka, Robert W. Schunk, Lie Zhu
All Physics Faculty Publications
Sunaligned (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 ...