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Full-Text Articles in Physics

Poiseuille Advection Of Chemical Reaction Fronts: Eikonal Approximation, R. S. Spangler, Boyd F. Edwards, J. Chem Dec 2002

Poiseuille Advection Of Chemical Reaction Fronts: Eikonal Approximation, R. S. Spangler, Boyd F. Edwards, J. Chem

All Physics Faculty Publications

An eikonal equation including fluid advection is derived from the cubic reaction-diffusion-advection equation, and is used to investigate the speeds and shapes of chemical reaction fronts subject to Poiseuille flow between parallel plates. Although the eikonal equation is usually regarded as valid when the front thickness is small compared to the radius of curvature of the front and to the size of the system, it is also found to be valid when the reaction front is thick with respect to the gap width. This new regime of applicability of the eikonal equation is consistent with its derivation, which requires only ...


Statistical Characteristics Ofsprite Halo Events Using Coincident Photometric And Imaging Data, R. Miyasato, Michael J. Taylor, H. Fukunishi, Y. Takahashi, H. Nielsen Nov 2002

Statistical Characteristics Ofsprite Halo Events Using Coincident Photometric And Imaging Data, R. Miyasato, Michael J. Taylor, H. Fukunishi, Y. Takahashi, H. Nielsen

All Physics Faculty Publications

Sprite halos are brief, diffuse flashes, which occur at the top of a sprite and precede the development of streamer structures at lower altitudes. We have investigated the characteristics of sprite halos in detail using coincident photometric and imaging data obtained during the Sprites'96 and '99 campaign in Colorado and Wyoming, USA. It is found that the average altitude of the centroid of the halo emission and the mean horizontal diameter of the halo events are ∼80 and ∼86 km, respectively, while the average speed of the descending motion of the sprite halos was ∼4.3 × 107 m ...


Influence Of Terrain On Scaling Laws For River Networks, D. A. Vasquez, D. H. Smith, Boyd F. Edwards Nov 2002

Influence Of Terrain On Scaling Laws For River Networks, D. A. Vasquez, D. H. Smith, Boyd F. Edwards

All Physics Faculty Publications

The upper Cheat River network departs from scaling laws describing a large number of river networks in North America. This departure is traced to its corrugated terrain. The more typical random terrain of the lower Cheat River network obeys the standard scaling laws. We modify the random network model of Scheidegger to include the effects of topography, reproducing the behavior observed in the Cheat River basin.


Femtosecond Pump-Probe Reflectivity Study Of Silicon Carrier Dynamics, A. J. Sabbah, D. Mark Riffe Oct 2002

Femtosecond Pump-Probe Reflectivity Study Of Silicon Carrier Dynamics, A. J. Sabbah, D. Mark Riffe

All Physics Faculty Publications

We have studied the ultrafast optical response of native-oxide terminated Si(001) with pump-probe reflectivity using 800 nm, 28 fs pulses at an excitation density of (5.5±0.3)×1018cm-3. Time-dependent reflectivity changes comprise third-order-response coherent-transient variations arising from anisotropic state filling and linear-response variations arising from excited free carriers, state filling, and lattice heating. A time constant of 32±5fs associated with momentum relaxation is extracted from the coherent-transient variations. The state-filling and free-carrier responses are sensitive to carrier temperature, allowing an electron-phonon energy relaxation time of 260±30fs to be measured. The recovery of ...


Poiseuille Advection Of Chemical Reaction Fronts, Boyd F. Edwards Aug 2002

Poiseuille Advection Of Chemical Reaction Fronts, Boyd F. Edwards

All Physics Faculty Publications

Poiseuille flow between parallel plates alters the shapes and velocities of chemical reaction fronts. In the narrow-gap limit, the cubic reaction-diffusion-advection equation predicts a front-velocity correction equal to the gap-averaged fluid velocity ϵ. In the singular wide-gap limit, the correction equals the midgap fluid velocity 3ϵ/2 when the flow is in the direction of propagation of the reaction front, and equals zero for adverse flow of any amplitude for which the front has a midgap cusp. Stationary fronts are possible only for adverse flow and finite gaps. Experiments are suggested.


Longitudinal Ionosphericeffects In The South Atlantic Sector During Solar Maximum, E. R. De Paula, J. R. Souza, Bela G. Fejer, G. J. Bailey, R. A. Heelis Jul 2002

Longitudinal Ionosphericeffects In The South Atlantic Sector During Solar Maximum, E. R. De Paula, J. R. Souza, Bela G. Fejer, G. J. Bailey, R. A. Heelis

Bela G. Fejer

[1] Large-scale horizontal gradients in ion density and vertical drift observed by the Atmospheric Explorer E satellite in the South Atlantic region (latitudes 10°S–20°S, longitudes 50°W–10°E) during the June solstice at solar maximum are presented and analyzed. These features occur during the nighttime period. The observations near 450-km altitude show vertically downward ion drift velocities exceeding 120 m s−1 and depleted regions where the ion density is around 2 × 104 cm−3. It is shown, using values modeled by the Sheffield University Plasmasphere Ionosphere Model (SUPIM) along the satellite trajectory, that the large ...


Polarization Jet Events And Excitation Of Weak Sar Arcs, S. Sazykin, Bela G. Fejer, Y. I. Galperin, S. A. Grigoriev, L. V. Zinin, M. Mendillo Jun 2002

Polarization Jet Events And Excitation Of Weak Sar Arcs, S. Sazykin, Bela G. Fejer, Y. I. Galperin, S. A. Grigoriev, L. V. Zinin, M. Mendillo

Bela G. Fejer

[1] Polarization Jet (PJ), also known as Sub-Auroral Ion Drift (SAID), events are supersonic westward plasma drifts on the equatorward edge of the diffuse aurora in the evening and nighttime sector. Their optical F-region signatures are weak 630.0 nm red arcs colocated with regions of fast convection. These weak arcs resemble Stable Auroral Red (SAR) arcs observed during the recovery phase of magnetic storms, but have lower intensities, shorter lifetimes, and occur without a significant heat flux from the magnetosphere. Previous model studies underestimated the brightness of weak SAR arcs. We present calculations showing that ion-neutral collisional heating and ...


The Role Of Bandgap In The Secondary Electron Emission Of Small Bandgap Semiconductors: Studies Of Graphitic Carbon, Neal E. Nickles May 2002

The Role Of Bandgap In The Secondary Electron Emission Of Small Bandgap Semiconductors: Studies Of Graphitic Carbon, Neal E. Nickles

All Graduate Theses and Dissertations

The question of whether the small bandgaps of semiconductors play a significant role in their secondary electron emission properties is investigated by studying evaporated graphitic amorphous carbon, which has a roughly 0.5 eV bandgap, in comparison with microcrystalline graphite, which has zero bandgap. The graphitic amorphous carbon is found to have a 30% increase in its maximum secondary electron yield over that of two microcrystalline graphite samples with comparable secondary electron yields: highly oriented pyrolytic graphite and colloidal graphite. The potentially confounding influence of the vacuum level has been isolated through the measurement of the photoelectron onset energy of ...


River Meandering Dynamics, Boyd F. Edwards, D. H. Smith Mar 2002

River Meandering Dynamics, Boyd F. Edwards, D. H. Smith

All Physics Faculty Publications

The Ikeda, Parker, and Sawai river meandering model is reexamined using a physical approach employing an explicit equation of motion. For periodic river shapes as seen from above, a cross-stream surface elevation gradient creates a velocity shear that is responsible for the decay of small-wavelength meander bends, whereas secondary currents in the plane perpendicular to the downstream direction are responsible for the growth of large-wavelength bends. A decay length D=H/2Cf involving the river depth H and the friction coefficient Cf sets the scale for meandering, giving the downstream distance required for the fluid velocity profile to ...


Climatology And Storm Time Dependence Ofnighttime Thermospheric Neutral Winds Over Millstone Hill, Bela G. Fejer, J. T. Emmert, D. P. Sipler Jan 2002

Climatology And Storm Time Dependence Ofnighttime Thermospheric Neutral Winds Over Millstone Hill, Bela G. Fejer, J. T. Emmert, D. P. Sipler

Bela G. Fejer

[1] We use 630.0 nm nightglow Fabry-Perot measurements over Millstone Hill from 1989–1999 to study the climatology and storm time dependence of the midlatitude thermospheric winds. Our quiet time wind patterns are consistent with results from earlier studies. We determine the perturbation winds by subtracting from each measurement the corresponding quiet time averages. The climatological zonal disturbance winds are largely independent of season and solar flux and show large early night westward and small late-night eastward winds similar to disturbance ion drifts. The meridional perturbation winds vary strongly with season and solar flux. When the solar flux is ...


Ultra-Dense Phosphorous Delta-Layer Grown Into Silicon From Ph3 Molecular Precursors, T. -C. Shen, J. -Y. Ji, M. A. Zudov, R. -R. Du, J. S. Kline, J. R. Tucker Jan 2002

Ultra-Dense Phosphorous Delta-Layer Grown Into Silicon From Ph3 Molecular Precursors, T. -C. Shen, J. -Y. Ji, M. A. Zudov, R. -R. Du, J. S. Kline, J. R. Tucker

T. -C. Shen

Phosphorous δ-doping layers were fabricated in silicon by PH3 deposition at room temperature, followed by low-temperature Si epitaxy.Scanning tunneling microscope images indicate large H coverage, and regions of c(2×2) structure. Hall data imply full carrier activation with mobility<40 cm2/V s when the surface coverage is ≲0.2 ML. Conductivity measurements show a ln(T) behavior at low temperatures, characteristic of a high-density two-dimensional conductor. Possible future applications to atom-scale electronics and quantum computation are briefly discussed.


Relative Solar And Auroral Contribution To The Polar F Region: Implications For National Space Weather Program, M. David, Jan Josef Sojka, Robert W. Schunk, R. Heelis Jan 2002

Relative Solar And Auroral Contribution To The Polar F Region: Implications For National Space Weather Program, M. David, Jan Josef Sojka, Robert W. Schunk, R. Heelis

All Physics Faculty Publications

When plasma in the polar cap F region becomes highly structured, patches, irregularities, and scintillations of HF signals may be observed. The topic of this paper is not the mechanism for structuring or distributing the plasma but rather the source of the plasma. By understanding the plasma source we gain insight into the specification and forecasting of ionospheric structures and irregularities as required for space weather applications. The two major sources of polar cap F region plasma are the solar EUV radiation and the auroral precipitation. The region over which solar EUV production occurs is readily modeled. In contrast, the ...


Visual And Lidar Observations Of Noctilucent Clouds Above Logan, Utah, At 41.7°N, Vincent B. Wickwar, M J. Taylor, J P. Herron, B A. Martineau Jan 2002

Visual And Lidar Observations Of Noctilucent Clouds Above Logan, Utah, At 41.7°N, Vincent B. Wickwar, M J. Taylor, J P. Herron, B A. Martineau

All Physics Faculty Publications

Noctilucent clouds (NLCs) were observed from a midlatitude site (Logan, Utah) on the evenings of 22 and 23 June 1999 mountain daylight time. On both nights the clouds were seen for approximately an hour by experienced observers, and they were photographed. The NLC was also observed on the second evening for approximately an hour in the zenith with the Rayleigh-scatter lidar at the Atmospheric Lidar Observatory, which is operated by the Center for Atmospheric and Space Sciences on the campus of Utah State University. These observations enabled several of the properties of the cloud to be determined. They were within ...


The Lisa Optimal Sensitivity, Thomas A. Prince, Massimo Tinto, Shane L. Larson, J. W. Armstrong Jan 2002

The Lisa Optimal Sensitivity, Thomas A. Prince, Massimo Tinto, Shane L. Larson, J. W. Armstrong

All Physics Faculty Publications

The multiple Doppler readouts available on the Laser Interferometer Space Antenna (LISA) permit simultaneous formation of several interferometric observables. All these observables are independent of laser frequency fluctuations and have different couplings to gravitational waves and to the various LISA instrumental noises. Within the functional space of interferometric combinations LISA will be able to synthesize, we have identified a triplet of interferometric combinations that show optimally combined sensitivity. As an application of the method, we computed the sensitivity improvement for sinusoidal sources in the nominal, equal-arm LISA configuration. In the part of the Fourier band where the period of the ...


Unequal Arm Space-Borne Gravitational Wave Detectors, Shane L. Larson, Ronald W. Hellings, William A. Hiscock Jan 2002

Unequal Arm Space-Borne Gravitational Wave Detectors, Shane L. Larson, Ronald W. Hellings, William A. Hiscock

All Physics Faculty Publications

Unlike ground-based interferometric gravitational wave detectors, large space-based systems will not be rigid structures. When the end stations of the laser interferometer are freely flying spacecraft, the armlengths will change due to variations in the spacecraft positions along their orbital trajectories, so the precise equality of the arms that is required in a laboratory interferometer to cancel laser phase noise is not possible. However, using a method discovered by Tinto and Armstrong, a signal can be constructed in which laser phase noise exactly cancels out, even in an unequal arm interferometer. We examine the case where the ratio of the ...


Quantum Dynamics Of The Polarized Gowdy T3 Model, Charles G. Torre Jan 2002

Quantum Dynamics Of The Polarized Gowdy T3 Model, Charles G. Torre

All Physics Faculty Publications

The polarized Gowdy T3 vacuum spacetimes are characterized, modulo gauge, by a “point particle” degree of freedom and a function φ that satisfies a linear field equation and a nonlinear constraint. The quantum Gowdy model has been defined by using a representation for φ on a Fock space F. Using this quantum model, it has recently been shown that the dynamical evolution determined by the linear field equation for φ is not unitarily implemented on F. In this paper, (1) we derive the classical and quantum model using the “covariant phase space” formalism, (2) we show that time evolution is ...


Altitude Dependence Of Middleand Low-Latitude Thermospheric Disturbance Winds Measured By Windii, J. T. Emmert, Bela G. Fejer, G. G. Shepard, B. H. Solheim Jan 2002

Altitude Dependence Of Middleand Low-Latitude Thermospheric Disturbance Winds Measured By Windii, J. T. Emmert, Bela G. Fejer, G. G. Shepard, B. H. Solheim

Bela G. Fejer

[1] Thermospheric neutral winds exhibit strong altitudinal and latitudinal variation during geomagnetically quiet and active times. We use daytime middle and low-latitude neutral winds measured by the Wind Imaging Interferometer (WINDII) instrument on board the Upper Atmosphere Research Satellite (UARS) over the 90–275 km height range to study the altitude and season dependent climatology of disturbance winds (i.e., with quiet time patterns removed) in magnetic coordinates. The daytime perturbations winds are generally equatorward and westward and decrease toward the magnetic equator. Both the zonal and meridional components decrease sharply below 120 km and are essentially insignificant below 100 ...


Temperature Dependence Of Silicon Carrier Effective Masses With Application To Femtosecondreflectivity Measurements, D. Mark Riffe Jan 2002

Temperature Dependence Of Silicon Carrier Effective Masses With Application To Femtosecondreflectivity Measurements, D. Mark Riffe

All Physics Faculty Publications

The conductivity effective masses of electrons and holes in Si are calculated for carrier temperatures from 1 to 3000 K. The temperature dependence of the electron mass is calculated by use of a phenomenological model of conduction-band nonparabolicity that has been fitted to experimental measurements of the dependence of the electron conductivity effective mass on carrier concentration. The hole mass is investigated by tight-binding calculations of the valence bands, which have been adjusted to match experimental values of the valence-band curvature parameters at the top of the valence band. The calculations are in excellent agreement with femtosecond-laser reflectivity measurements of ...


Calculation Of Spectral Degradation Due To Contaminant Films On Infrared And Optical Sensors, L. Gamble, Jr Dennison, B. Wood, J. Herrick, J. S. Dyer Jan 2002

Calculation Of Spectral Degradation Due To Contaminant Films On Infrared And Optical Sensors, L. Gamble, Jr Dennison, B. Wood, J. Herrick, J. S. Dyer

All Physics Faculty Publications

Molecular surface contaminants can cause degradation of optical systems, especially if the contaminants exhibit strong absorption bands in the region of interest. Different strategies for estimation of spectral degradation responses due to uniform films for various types of systems are reviewed. One tool for calculating the effects of contaminant film thickness on signal degradation in the mid IR region is the simulation program CALCRT. The CALCRT database will be reviewed to correlate spectral n and k values associated with specific classes of organic functional groups. Various schemes are also investigated to estimate the spectral degradation in the UV-Vis region. Experimental ...


Calculation Of Spectral Degradation Due To Contaminantfilms On Infrared And Optical Sensors, L. Gamble, Jr Dennison, B. Wood, J. Herrick, J. S. Dyer Jan 2002

Calculation Of Spectral Degradation Due To Contaminantfilms On Infrared And Optical Sensors, L. Gamble, Jr Dennison, B. Wood, J. Herrick, J. S. Dyer

All Physics Faculty Publications

Molecular surface contaminants can cause degradation of optical systems, especially if the contaminants exhibit strong absorption bands in the region of interest. Different strategies for estimation of spectral degradation responses due to uniform films for various types of systems are reviewed. One tool for calculating the effects of contaminant film thickness on signal degradation in the mid IR region is the simulation program CALCRT. The CALCRT database will be reviewed to correlate spectral n and k values associated with specific classes of organic functional groups. Various schemes are also investigated to estimate the spectral degradation in the UV-Vis region. Experimental ...


The Principle Of Symmetric Criticality In General Relativity, Mark E. Fels, Charles G. Torre Jan 2002

The Principle Of Symmetric Criticality In General Relativity, Mark E. Fels, Charles G. Torre

All Physics Faculty Publications

We consider a version of Palais' principle of symmetric criticality (PSC) that is applicable to the Lie symmetry reduction of Lagrangian field theories. Given a group action on a space of fields, PSC asserts that for any group-invariant Lagrangian, the equations obtained by restriction of Euler–Lagrange equations to group-invariant fields are equivalent to the Euler–Lagrange equations of a canonically defined, symmetry-reduced Lagrangian. We investigate the validity of PSC for local gravitational theories built from a metric and show that there are two independent conditions which must be satisfied for PSC to be valid. One of these conditions, obtained ...