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1988

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Articles 151 - 162 of 162

Full-Text Articles in Physics

Observations Of Auroral E Region Plasma Waves And Electron Heating Witheiscat And A Vhf Radar Interferometer, J. Providakes, D. T. Farley, B. G. Fejer, J. Sahr, W. E. Swartz, I. Haggstrom, A. Hedberg, J. A. Nordling Jan 1988

Observations Of Auroral E Region Plasma Waves And Electron Heating Witheiscat And A Vhf Radar Interferometer, J. Providakes, D. T. Farley, B. G. Fejer, J. Sahr, W. E. Swartz, I. Haggstrom, A. Hedberg, J. A. Nordling

All Physics Faculty Publications

Two radars were used simultaneously to study naturally occurring electron heating events in the auroral E-region ionosphere. During a joint campaign in March 1986 the Cornell University Portable Radar Interferometer (CUPRI) was positioned to look perpendicular to the magnetic field to observe unstable plasma waves over Tromsø, Norway, while EISCAT measured the ambient conditions in the unstable region. On two nights EISCAT detected intense but short lived (< 1 min) electron heating events during which the temperature suddenly increased by a factor of 2–4 at altitudes near 108 km and the electron densities were less than 7 × 104 cm−3. On the second of these nights CUPRI was operating and detected strong plasma waves with very large phase velocities at precisely the altitudes and times at which the heating was observed. The altitudes, as well as one component of the irregularity drift velocity, were determined by interferometric techniques. From the observations and our analysis, we conclude that the electron temperature increases were caused by plasma wave heating and not by either Joule heating or particle precipitation.

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An Optically Controlled Closing And Opening Semiconductor Switch, K. H. Schoenbach, V. K. Lakdawala, R. Germer, S. T. Ko Jan 1988

An Optically Controlled Closing And Opening Semiconductor Switch, K. H. Schoenbach, V. K. Lakdawala, R. Germer, S. T. Ko

Electrical & Computer Engineering Faculty Publications

A concept for a bulk semiconductor switch is presented, where the conductivity is increased and reduced, respectively, through illumination with light of different wavelengths. The increase in conductivity is accomplished by electron ionization from deep centers and generation of bound holes. The reduction of conductivity is obtained by hole ionization from the excited centers and subsequent recombination of free electrons and holes. The transient behavior of electron and hole density in a high power semiconductor (GaAs:Cu) switch is computed by means of a rate equation model. Changes in conductivity by five orders of magnitude can be obtained.

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A Photochemical Equilibrium Model For Ionospheric Conductivity, C. E. Rasmussen, Robert W. Schunk, Vincent B. Wickwar Jan 1988

A Photochemical Equilibrium Model For Ionospheric Conductivity, C. E. Rasmussen, Robert W. Schunk, Vincent B. Wickwar

All Physics Faculty Publications

A photochemical equilibrium model of the high-latitude ionosphere has been developed. This model provides densities of the ionospheric constituents, N2 +, O2 +, O+, and NO+, from 85 km to approximately 220 km. These densities are then used to calculate Pedersen and Hall conductivities. A comparison of the model results with Arecibo and Chatanika radar observations was made, covering periods of solar minimum and solar maximum. The comparison showed the model to predict ionospheric densities to within 50% and conductivities to within 40% in the illuminated portion of the ionosphere. In regions …

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Comparison Of Simultaneous Chatanika And Millstone Hill Temperature Measurements With Ionospheric Model Predictions, C. E. Rasmussen, Jan Josef Sojka, Robert W. Schunk, Vincent B. Wickwar, O. De La Beaujariere, J. Foster, J. Holt Jan 1988

Comparison Of Simultaneous Chatanika And Millstone Hill Temperature Measurements With Ionospheric Model Predictions, C. E. Rasmussen, Jan Josef Sojka, Robert W. Schunk, Vincent B. Wickwar, O. De La Beaujariere, J. Foster, J. Holt

All Physics Faculty Publications

As part of the MITHRAS program, the Chatanika and Millstone Hill incoherent scatter radars made coordinated observations of the polar ionosphere on June 27 and 28, 1981. The temperature data obtained during these days were compared with predictions made by a high-latitude ionospheric model. The comparison of the temperature measurements and the results of the ionospheric model depend on the assumptions made both in reducing the data and on the inputs that are needed by the model. The deduction of electron temperature from radar measurements depends upon a knowledge of the mean ion mass as a function of altitude. The …

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A Model Study Of How Electric Field Structures Affect The Polar Cap F Region, Jan Josef Sojka, Robert W. Schunk Jan 1988

A Model Study Of How Electric Field Structures Affect The Polar Cap F Region, Jan Josef Sojka, Robert W. Schunk

All Physics Faculty Publications

A three-dimensional time-dependent ionospheric model was used to study how electric field structures affect the polar F region. The electric field structures are represented by elongated Volland two-cell models whose dimensions range from tens to 1000 km. These model structures are intended to represent the polar cap electric field for IMF Bz northward conditions. A statistical method is used to generate a set of these structures. Their electric field strength and polarity are varied in order to study the F region’s dependence on this magnetospheric input. For electric field structures whose size and electric field strengths are consistent with …

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Low-Temperature Ion Beam Mixing In Metals, S J. Kim, M A. Nicolet, R S. Averback, David Peak Jan 1988

Low-Temperature Ion Beam Mixing In Metals, S J. Kim, M A. Nicolet, R S. Averback, David Peak

All Physics Faculty Publications

A systematic study of ion-beam mixing of tracer impurities in thin metal films at low temperatures has been conducted. We have investigated the dependence of ion mixing on two matrix properties: atomic mass and cohesive energy. We have also studied the dependence of ion mixing on tracer impurity properties: its heat of mixing with the matrix and its thermal diffusivity in the matrix. The matrices investigated were thin films of C, Al, Ti, Fe, Ni, Cu, Mo, Ru, Ag, Hf, Ta, W, Pt, and Au. The tracer impurities, Al, Ti, Cr, Mn, Fe, Ni, Cu, Y, Nb, Mo, Ru, Ag, …

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Discharge Suppression System For A Double Focusing, Atmospheric Pressure Ionization Mass Spectrometer, Andrew H. Grange, Robert J. O'Brien, Douglas F. Barofsky Jan 1988

Discharge Suppression System For A Double Focusing, Atmospheric Pressure Ionization Mass Spectrometer, Andrew H. Grange, Robert J. O'Brien, Douglas F. Barofsky

Chemistry Faculty Publications and Presentations

An electrical discharge suppression system for a medium throughput (∼2 l/s) pumping line has been devised that works up to potentials of ±15 kV. This device permits atmospheric pressure ionization sources to be interfaced to high-resolution, magnetic sector mass spectrometers with source potentials of 6-10 kV

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Gauge-Invariance And Quantization, Br Holstein Dec 1987

Gauge-Invariance And Quantization, Br Holstein

Barry R Holstein

Quantizing theories such as quantum electrodynamics that contain a gauge invariance are discussed via a simple pedagogical example. Canonical and path integral quantization methods are compared.

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Semiclassical Treatment Of The Double Well, Br Holstein Dec 1987

Semiclassical Treatment Of The Double Well, Br Holstein

Barry R Holstein

The double well potential V(x)= 1/4 λ(x2-α2)2 is addressed using both semiclassical path integral and instanton techniques. The basic physics of the two-state system is shown to arise, and energy levels calculated via the two methods are compared.

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Comment On "Percolation In Isotropic Elastic Media.", Anthony Day, M. Thorpe Dec 1987

Comment On "Percolation In Isotropic Elastic Media.", Anthony Day, M. Thorpe

Anthony Roy Day

No abstract provided.

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Spectral Dimensionality Of Random Superconducting Networks, Anthony Roy Day, W. Xia, M. F. Thorpe Dec 1987

Spectral Dimensionality Of Random Superconducting Networks, Anthony Roy Day, W. Xia, M. F. Thorpe

Anthony Roy Day

We compute the spectral dimensionality d-tilde of random superconducting-normal networks by directly examining the low-frequency density of states at the percolation threshold. We find that d-tilde=4.1±0.2 and 5.8±0.3 in two and three dimensions, respectively, which confirms the scaling relation d-tilde=2d/(2-s/ nu ), where s is the superconducting exponent and nu the correlation-length exponent for percolation. We also consider the one-dimensional problem where scaling arguments predict, and our numerical simulations confirm, that d-tilde=0. A simple argument provides an expression for the density of states of the localized high-frequency modes in this special case. We comment on the connection between our calculations …

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Stability Of Networks Under Tension And Pressure, Anthony Roy Day, H. Yan, M. F. Thorpe Dec 1987

Stability Of Networks Under Tension And Pressure, Anthony Roy Day, H. Yan, M. F. Thorpe

Anthony Roy Day

The number of zero-frequency modes of an elastic network is an important quantity in determining the stability of the network. We present a constraint-counting method for finding this number in general central-force networks that are under an external tension. The technique involves isolating the backbone and then counting constraints in the same way as for free standing networks. A detailed example of this counting is given for a random two-dimensional network subject to an external tension. The results are shown to agree with the number of zero-frequency modes as determined by a direct matrix diagonalization.

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