Differential Drift Research Articles

Small scale plasma and charged aerosol variations and their importance for polar mesosphere summer echoes

Since 1990 a number of campaigns have been conducted in order to study the small scale structure of charged as well as neutral species within Polar Mesosphere Summer Echo (PMSE) layers. Several VHF radars have been used to study this phenomenon from the ground (EISCAT, CUPRI, ALOMAR SOUSY) and sounding rockets have been launched through different types of PMSE layers. The radar observations appear to occur in two classes: (I) One characterized by broad spectral widths and almost no aspect sensitivity, associated with neutral air turbulence; and (II) another by narrow spectral widths and strong aspect sensitivity, which is observed in the absence of neutral air turbulence. Both radar and in situ measurements show a clear dominance of the non-turbulent type II echoes. We will discuss how localised layers of charged aerosols, ions and electrons observed in situ might lead to single and multiple Fresnel type reflections at VHF wavelengths. We will also discuss the possibility that a two-stream type plasma instability may explain the type II echoes. The important parameter in this respect is the electrical field and a horizontal electric field of about 40 mV/m is needed to excite small scale plasma waves. Such a field may have its origin in a differential drift between the heavy aerosols and the much lighter plasma, caused by a varying background wind, windshears and/or waves of different types, that give rise to a polarisation electric field.

Additional critical values and asymptotic representations for seasonal unit root tests

This paper is concerned with tests for seasonal unit roots in a univariate time series process. The paper extends the procedures and tables of critical values due to Hylleberg et al. (1990) and Ghysels et al. (1994) to obtain tests which are similar (exactly and asymptotically) with respect to both the initial values of the process and the possibility of differential seasonal drift under the null hypothesis of a seasonal unit root. Representations are derived for the limiting distributions of the test statistics in this and other cases of interest. These representations provide an explanation for the similarity between critical values for the statistics in different scenarios. The seasonal unit root properties of real seasonally unadjusted UK non-durable consumption expenditure are re-examined.

Suprathermal electrons observed on the TSS‐1R satellite

Particle measurements up to 27,000 eV were made on the TSS‐1R (Tethered Satellite System) satellite. The TSS satellite developed a positive bias due to the Lorentz force. It was the intent that electron measurements on the TSS satellite could be used to track the spacecraft potential and collected current. What was observed was quite different. Accelerated ionospheric electrons were observed to only ∼70 eV even though larger spacecraft potentials were observed by other diagnostics on the TSS satellite. When observed they agreed with these independent measurements of the potential. In addition to the anticipated accelerated thermals, a suprathermal population of electrons was observed to be centered around 200 eV. This population exhibited a 4 orders‐of‐magnitude increase in intensity as the spacecraft potential exceeded the O+ ram energy. The disappearance of the accelerated thermals is explained by the observation that the suprathermal flux becomes larger in magnitude, thus hiding the thermals. However the suprathermals cannot be the dominant current carriers if they are the result of a DC process as their calculated current magnitude exceeds that observed. These results are best explained if one assumes an AC acceleration of the suprathermal electrons whose free energy is derived from the differential drift between electrons and ions.

On-line measurement system based on biosensors

An on-line intelligent measurement system based on H + -ISFET and penicillinase FET is described. The system consists of a sampling system, a diluting system, a liquor store, a monitoring cell, a series of tubing pumps for driving the sample, a testing circuit and a control circuit. The technology of differential measurement, temperature and drift compensation as well as the calibration curve of the piece-wise fitting method are applied to the system. The measurement system can control a complete process of sampling, calibrating, cleaning, testing, and calculating. The process is controlled by a microprocessor. The real-time testing results based on H + -ISFET and penicillinase FET are presented. The testing results of a pH 6.86 buffer solution during 15 h based on H + -ISFET are: the standard deviation = pH 0.008, the coefficient of variation = 0.1%. The correlation coefficient to test a series of buffer pH values is r = 0.9999, the analysis frequency = 60 samples per hour. The results to test a series of penicillin standard solutions based on the penicillinase FET are: correlation coefficient = 0.9999, the test cycle is about 3 min. This system has been experimentally justified in a pharmacy factory.

THE GREAT-CIRCLE CAMERA: A NEW DRIFT-SCANNING INSTRUMENT

ABSTRACT We discuss the design, construction, and use of a new class of scanning camera that eliminates a critical limitation of standard CCD drift-scan observations. A standard scan, which involves no correction for the differential drift rates and curved stellar paths across the field-of-view, suffers from severe image degradation even when one observes at moderate declinations. Not only does this effect limit the area of the sky over which drift scanning is viable, but as detector sizes increase, CCD mosaics become standard, and dome/telescope seeing improves, the area of sky for which scanning is acceptable (image degradation <~ seeing) will be further reduced unless some action is taken. By modifying the scan path (the path on the sky traced by signal accumulated along a single CCD column) to lie along a great circle on the sky rather than along a path of constant declination, image degradation is minimized. In this paper, we discuss the design and implementation of a stage that rotates and translates the CCD during a drift-scan exposure so that the scan path is along a great circle on the sky. Data obtained during the commissioning run of the great Circle Camera at the Las Campanas 1-m telescope are presented.

Grain formation around carbon stars. 1: Stationary outflow models

Asymptotic giant branch (AGB) stars are known to be sites of dust formation and undergo significant mass loss. The outflow is believed to be driven by radiation pressure on grains and momentum coupling between the grains and gas. While the physics of shell dynamics and grain formation are closely coupled, most previous models of circumstellar shells have treated the problem separately. Studies of shell dynamics typically assume the existence of grains needed to drive the outflow, while most grain formation models assume a constant veolcity wind in which grains form. Furthermore, models of grain formation have relied primarily on classical nucleation theory instead of using a more realistic approach based on chemical kinetics. To model grain formation in carbon-rich AGB stars, we have coupled the kinetic equations governing small cluster growth to moment equations which determine the growth of large particles. Phenomenological models assuming stationary outflow are presented to demonstrate the differences between the classical nucleation approach and the kinetic equation method. It is found that classical nucleation theory predicts nucleation at a lower supersaturation ratio than is predicted by the kinetic equations, resulting in significant differences in grain properties. Coagulation of clusters larger than monomers is unimportant for grain formation in high mass-loss models but becomes more important to grain growth in low mass-loss situations. The properties of the dust grains are altered considerably if differential drift velocities are ignored in modeling grain formation. The effect of stellar temperature, stellar luminosity, and different outflow velocities are investigated. The models indicate that changing the stellar temperature while keeping the stellar luminosity constant has little effect on the physical parameters of the dust shell formed. Increasing the stellar luminosity while keeping the stellar temperature constant results in large differences in grain properties. For small outflow velocities, grains form at lower supersaturation ratios and close to the stellar photosphere, resulting in larger but fewer grains. The reverse is true when grains form under high outflow velocities, i.e., they form at higher supersaturation ratios, farther from the star, and are much smaller but at larger quantities.

Rossby vortices as sources of global magnetic structures on the Sun

A numerical simulation of the process of generation of the magnetic field by Rossby vortices, whose horizontal scale is comparable to the solar radius, has been carried out. Long-lived vortices form global magnetic structures that drift together with vortices. Differential rotation in latitude leads to a longer lifetime of cyclones and corresponding magnetic structures. The cyclone and the magnetic structure travel in longitude with the velocity close to a corresponding differential rotation velocity and drift slowly poleward. The interaction of cyclones located in close latitudes makes one of them move to higher latitudes and the poloidal component of the magnetic field to intensify during the interaction. The formation of large-scale vortices was simulated, when the initial condition was specified by a grid of small-scale vortices with a random amplitude distribution. Merging of vortices of the same sign leads to the formation of large-scale vortices whose size is determined by the geometry of the problem and by the differential rotation profile.

Trapping effects in a-Si:H investigated by the SSPG technique

The use of the SSPG method at different electric fields and background light intensities yields values of the differential electron drift mobility. From the dependence of these values on the electron quasi-Fermi level, as determined by the background illumination, information about the shape of the density of states at approximately 0.4 eV below the conduction band edge was obtained. The density of states near that energy falls off much more slowly towards midgap than near the mobility edge.

Effects of pH on Community Dynamics of Chironomidae in a Large River near Sudbury, Ontario

To identify the mechanism(s) by which acidity alters the composition and richness of chironomid communities and to determine the acid tolerance of chironomid taxa inhabiting a large river, pH was manipulated in three stream channels. The chironomid community that developed after a 10-wk exposure to pH 4.5 was poorer in species and lower in abundance than those exposed to pH 5.9 or 7.4 (reference pH). The reduced density of chironomids at pH 4.5 was not a result of differential egg deposition, behavioural drift, or emergence. Acid-enhanced larval mortality appeared to be the primary mechanism accounting for reduced density. Tribelos predomined in the channel acidified to pH 4.5, while Ablabesmyia was predominant in the other channels. Tanytarsus, Microtendipes, and Nilothauma were the most acid-sensitive taxa. When pH was returned to 7.4 from 4.5, the composition and density of the community changed rapidly; acid-sensitive species invaded quickly, and within 6 wk the community resembled that in the other channels. These results suggest that acidity is important in structuring and maintaining lotic communities and that reduction in acidic emissions should lead to rapid biological recovery of acidified lotic systems, provided that acid-sensitive species have survived in nearby refuges.

Kuiper prize lecture: Some problems in the formation of the planets

I consider several problems of the evolution of the solar nebula, the precipitation of dust particles toward its central plane, and the possibility that a gravitational instability developed in the dust subdisk. A solution of the dispersion equation is given for the development of an instability in a two-component (gas-dust) rotating disk under the assumption that there is no instability in the gaseous component. This solution is compared to one found earlier for one-component disks. I emphasize that a monodisperse “initial” state with equal kilometer-sized bodies, as traditionally assumed in many numerical simulations, never existed. I note that turbulence induced by the gradient of the rotation velocity dω/ dz and differential radial drift of particles having different sizes in the gas does not exclude the possibility of a gravitational instability in a thin equatorial layer of the subdisk which contains a small fraction of the total dust mass. A runaway scenario for the accumulation of preplanetary bodies is discussed. I find that a transition to higher relative velocities of bodies and a slower growth of planet embryos began already before the sum of masses of the embryos reached one-tenth the mass of all the other bodies. Mechanisms for removal of almost all the initial mass of solids from the asteroidal zone are considered. A reliable mechanism for the sweeping out of this zone by bodies which penetrate from Jupiter's zone could work efficiently at values of the velocity parameter θ ≲ 30–50, once Jupiter's embryo reached a mass of ∼5–10 M ⊕. Also at such moderate values of θ the outer planets ejected planetesimals into the cometary cloud.

Plasma sheet instability related to the westward traveling surge

The detailed analysis of an isolated dispersionless substorm is performed on the basis of field and particle data collected in situ by the geostationary satellite GEOS 2 and of data from ground‐based instruments installed close to the GEOS 2 magnetic footprint. These data give evidence for (1) quasi‐periodic variations of the magnetic field configuration, which is alternatively taillike and dipolelike, (2) in‐phase oscillations of the flux of energetic electrons, which is high when the configuration is dipolelike and vice versa, (3) a gradient in the flux of energetic ions, which is, on the average, earthward but undergoes large fluctuations around this average direction, and (4) large transient fluctuations of the quasi‐dc electric field, which reverses its direction from eastward to westward. It is shown that these results are consistent with the development of an instability which leads to a westward propagating “wave”. The source of the instability is the differential drift of energetic electrons and ions in a highly stressed magnetic field configuration (in a high β plasma). Evidence is given for a system of localized field‐aligned currents flowing alternately earthward and equatorward at the leading and trailing edges of the westward propagating wave. This current system resulting from the temporal development of the instability produces the so‐called Pi 2 pulsations, at the ionospheric level. The closure of this current system in the equatorial region leads to a current antiparallel to the tail current, and therefore to its reduction or cancellation. This reduction/cancellation of the tail current restores the dipole magnetic field (dipolarization) and generates a large westward directed induced electric field (injection). Hence, dipolarization and injection are the consequences of the instability. Finally, it is suggested that the westward traveling surge observed simultaneously by all‐sky cameras, close to the magnetic field of GEOS 2, is the image of the instability in the equatorial region transmitted to the upper atmosphere by precipitating electrons.

A one-dimensional model for wave-induced ice-floe collisions

In this study, the collision of ice floes under the action of a monotonic wave is quantified. The lateral motion of an ice floe caused by the wave is modeled as the sliding of an object under gravity. In this case, the gravity component in the direction of motion varies with time and space as the wave progresses by the floe. Drag and added mass effects are included in the model. Two floes located at different positions are shown to have a net difference in their drift (caused only be repeated wave passages). In most cases, this differential drift eventually causes floe collision. When two floes collide, a spring and dash-pot model is adopted to calculate the contact force. A one-dimensional wave passing through a one-dimensional array of disc-shaped floes is examined. Two phenomena are apparent from the analysis. First, waves have a herding effect that forms bands of floes with the width equal to the wavelength. Secondly, the frequency of collision is sensitive to the elastic properties of the floes and the wave amplitude. With sufficient values of the damping constant, which operates when two floes collide, the floes stay in contact for prolonged periods, indicating the potential to freeze together and form composite floes, as was observed in the field studies.

A one-dimensional model for wave-induced ice-floe collisions

In this study, the collision of ice floes under the action of a monotonic wave is quantified. The lateral motion of an ice floe caused by the wave is modeled as the sliding of an object under gravity. In this case, the gravity component in the direction of motion varies with time and space as the wave progresses by the floe. Drag and added mass effects are included in the model. Two floes located at different positions are shown to have a net difference in their drift (caused only be repeated wave passages). In most cases, this differential drift eventually causes floe collision. When two floes collide, a spring and dash-pot model is adopted to calculate the contact force. A one-dimensional wave passing through a one-dimensional array of disc-shaped floes is examined. Two phenomena are apparent from the analysis. First, waves have a herding effect that forms bands of floes with the width equal to the wavelength. Secondly, the frequency of collision is sensitive to the elastic properties of the floes and the wave amplitude. With sufficient values of the damping constant, which operates when two floes collide, the floes stay in contact for prolonged periods, indicating the potential to freeze together and form composite floes, as was observed in the field studies.

A measurement of the temperature of the cosmic microwave background at a frequency of 7.5 GHz

view Abstract Citations (23) References (30) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Measurement of the Temperature of the Cosmic Microwave Background at a Frequency of 7.5 GHz Kogut, A. ; Bensadoun, M. ; de Amici, G. ; Levin, S. ; Smoot, G. F. ; Witebsky, C. Abstract We have measured the intensity of the cosmic microwave background (CMB) at a frequency of 7.5 GHz (wavelength 4.0 cm) using a ground-based, total power radiometer calibrated at the antenna aperture by an external cryogenic reference target. The radiometer measured the difference in antenna temperature between the reference target and the zenith sky from a dry, high-altitude site. Subtraction of foreground signals (primarily atmospheric and galactic emission) measured with the same instrument leaves the CMB as the residual. The radiometer measured the atmospheric antenna temperature by correlating the signal change with the air mass in the beam during tip scans. The small galactic signal was subtracted based on extrapolation from lower frequencies and was checked by differential drift scans. The limiting uncertainty in the CMB measurement was the effect of ground radiation in the antenna sidelobes during atmospheric measurements. The thermodynamic temperature of the CMB at 7.5 GHz is 2.60 +/- 0.07 K (68% confidence level). Publication: The Astrophysical Journal Pub Date: May 1990 DOI: 10.1086/168745 Bibcode: 1990ApJ...355..102K Keywords: Radiation Measurement; Relic Radiation; Antenna Radiation Patterns; Calibrating; Microwave Radiometers; Astrophysics; COSMIC BACKGROUND RADIATION; EARTH: ATMOSPHERE full text sources ADS |

Stochastic calculus and Schrödinger semigroups admitting a ground state

Presented are some new results concerning distorted Brownian motion, the Ito process associated with the Schrödinger operator with ground state. These results are obtained by means of Malliavin stochastic calculus. Under the condition of continuously differentiable drift coefficient with bounded partial derivatives, it is proved that the image of a bounded function after action of the expectation operator of distorted Brownian motion is twice continuously differentiable. This approach enabled us to prove that distorted Brownian motion has twice continuously differentiable transition densities. With the additional assumption that the drift is bounded, it is proved that the corresponding initial value problem has an infinitely differentiable fundamental solution.

Vibrating orifice droplet generator for precision optical studies

A special purpose vibrating orifice droplet generator is described possessing improved short-term monodispersity (instantaneous diameter fluctuations of 2×10−5 and a differential drift of 10−5/min.). We demonstrate that this is sufficient to allow cw laser excitation of specific morphology-dependent resonances (MDRs). Improved performance results from (1) the liquid sample being direct pressure fed to the vibrating orifice from a closed pressure reservoir rather than by a conventional gear driven syringe pump, and (2) the vibrating orifice is driven by a periodic square wave voltage source having a frequency which is four orders of magnitude more constant (1 part in 108 per day) than sources normally used. A novel variational size spectroscopy is also described that is made possible by programmed frequency ramping of the voltage source driving the vibrating orifice. By monitoring elastic scattering during ramped size changes we have been able to infer precise values of droplet index of refraction (to ±0.001) and size (to ±3 parts in 104) at any orifice operating frequency by comparison of the resulting experimental pattern to Lorentz–Mie calculations. This aerosol source permits the next generation of optical experiments (e.g., double resonance, fluorescence rate inhibition and enhancement, cw nonlinear optical, accurate chemical speciation, etc.) to be performed on droplets with diameters of 10–80 μm.

Rotor noise due to atmospheric turbulence ingestion. I - Fluid mechanics

In the present analytical procedure for the prediction of helicopter rotor noise generation due to the ingestion of atmospheric turbulence, different models for turbulence fluid mechanics and the ingestion process are combined. The mean flow and turbulence statistics associated with the atmospheric boundary layer are modeled with attention to the effects of atmospheric stability length, windspeed, and altitude. The turbulence field can be modeled as isotropic, locally stationary, and homogeneous. For large mean flow contraction ratios, accurate predictions of turbulence vorticity components at the rotor face requires the incorporation of the differential drift of fluid particles on adjacent streamlines.

Electrical stability criterion for a short sample with negative differential conductivity and carrier drift and diffusion

Electrical stability criterion for a short sample with negative differential conductivity and carrier drift and diffusion

Genetics and distribution of black spotting in Enoplognatha ovata (Araneae: Theridiidae), and the role of intermittent drift in population differentiation

Breeding studies have shown that the number of black spots on the dorsal surface of the opisthosoma of E. ouata is genetically determined, with a narrow heritability of about 0.7. There is, in addition, evidence that the presence and absence ofspots might be under the control of a small number of major loci. Frequencies of spotted and non-spotted spiders in Nidderdale, Yorkshire, define the same three regions identified by previously studied variation at a regulatory locus. There are no correlations between sites in spotting and colour-morph frequencies. Variation in all three systems is independent of gross environmental factors. It is argued that the present-day patterns of genetic variation in these populations probably result from stochastic events during major environmental disturbance which is known to have occurred 40 years (generations) ago. Genetic drift, both continuous and intermittent, could he an important influence on morph frequencies in many E. ouata populations. Selection on conspicuous polymorphisms is not necessarily strong merely because the variation is conspicuous.

Item Pool Maintenance in the Presence of Item Parameter Drift

Differential linear drift of item location parameters over a 10 ‐year period is demonstrated in data from the College Board Physics Achievement Test. The relative direction of drift is associated with the content of the items and reflects changing emphasis in the physics curricula of American secondary schools. No evidence of drift of discriminating power parameters was found. Statistical procedures for detecting, estimating, and accounting for item parameter drift in item pools for long‐term testing programs are proposed