In Situ Remediation of Contaminated Ground Water: The Funnel-and-Gate System   总被引：6，自引：0，他引：6  

Robert C. Starr  John A. Cherry 《Ground water》1994,32(3):465-476

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A siphon mechanism for supplying prominence mass     

A. I. Poland  John T. Mariska 《Solar physics》1986,104(2):303-312

We examine a siphon-like mechanism for moving mass from the chromosphere to a gravitational well at the top of a magnetic loop to form a prominence. The calculations assume no apriori flow velocity at the loop base. Instead heating in the loop legs drives the flow. The prominence formation process requires two steps. First, the background heating rate must be reduced to on the order of 1 % of the initial heating rate required to maintain the coronal loop. This forms an initial condensation at the top of the loop. Second, the heating must take place only in the loop legs in order to produce a pressure differential which drives mass up into the well at the top of the loop. The heating rate in the loop must be increased once the prominence has begun to form or full prominence densities can not be achieved in a reasonable time. We conclude that this heating driven siphon-like mechanism is feasible for producing and maintaining prominences.  相似文献   

Evolution of cold shock-bounded slabs     

John M. Blondin  Brian S. Marks 《New Astronomy》1996,1(3):235-244

The stability and evolution of cold, shock-bounded slabs is studied using numerical hydrodynamic simulations. We confirm the analysis of Vishniac (1994) [ApJ, 428, 186], who showed that such slabs are unstable if they are perturbed by a displacement larger than their width. The growth rate of this nonlinear thin shell instability (NTSI) is found to increase with decreasing wavelength, in qualitative agreement with Vishniac's analysis. The NTSI saturates when the bending angle becomes large and the growth in the width of the slab pinches off the perturbation. After saturation, the slab remains greatly extended with an average density much less than the original slab density, supported primarily by supersonic turbulence within the slab. Linear perturbations are also found to be unstable in that they can lead to turbulent flow within the slab, although this response to linear perturbations is distinct from, and much less violent than the NTSI.Richard McCray  相似文献   

Coordinate gauge conditions in theN-body equations of motion in the first-order post-Newtonian approximation of general relativity     

Rajat Roy  N. C. Rana 《Astrophysics and Space Science》1990,167(1):125-138

The explicit forms of the metric as well as the equations of motion in the first-order post-Newtonian approximation are worked out under several gauge conditions. It is noted that the so-called EIH (Einstein, Infeld, and Hoffman) equation of motion for an assembly ofN finite mass points mutually interacting via gravitation is identically obtained under three different gauge conditions, namely the harmonic gauge, Chandrasekhar gauge and a composite Chandrasekhar gauge used by Misneret al. (1970), even though the solutions for the metric are found to be all different. In one case the metric has a component apparently diverging, but finally generates regular affine connections so that the equations of motions become free from any singularity. By use of the Chandrasekhar gauge and his formulation, the second-order contribution to the acceleration of planets in the limit of test particle motion around the Sun has been calculated, the inclusion of which in the EIH set of the equations of motion would extend the relative accuracy of computing the total acceleration of any planet to better than one part in 10¹⁷.  相似文献   

measurement of noise power spectrum in photon counting PMT's and the effect of the observed (1/f) noise on astronomical photometry     

S. S. Raouf  A. R. Sadik  N. K. Wafi  K. M. Aboud 《Astrophysics and Space Science》1986,127(2):207-218

In astronomical photometry, the sensitivity of observations is limited by the dark counts of the photomultiplier tube. In the present work, the effect of dark count noise in photon counting systems is investigated by theory and experimental measurements. Dark counts are considered to be originating from two sources, namely: dc fluctuations and random pulses.Experimental measurements were carried out to determine noise effects in different operating regions of noise dominance. The results provide strong evidence that: in normal operating mode, where the effect of random pulses is dominant, dark counts do not follow Poisson statistics. The observed noise shows strong (1/f) power spectrum, where the observed noise power is found to increase with time of observation.The results are important in photon counting systems operating under dark count limited mode. The conclusions drawn can be useful in obtaining more accurate error estimates and in assessing astronomical photometric observations and data reduction techniques.  相似文献   

The discovery of new galaxy members in the NGC 5044 and 1052 groups     

N. P. F. MKay  C. G. Mundell †  S. Brough  Duncan A. Forbes  D. G. Barnes  P. A. James  P. Goudfrooij  V. Kozhurina-Platais  R. Whitaker 《Monthly notices of the Royal Astronomical Society》2004,352(4):1121-1134

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On the stability of particular solutions of the singly averaged Hill problem     

M. A. Vashkov’yak  N. M. Teslenko 《Astronomy Letters》2005,31(12):844-852

We consider the particular solutions of the evolutionary system of equations in elements that correspond to planar and spatial circular orbits of the singly averaged Hill problem. We analyze the stability of planar and spatial circular orbits to inclination and eccentricity, respectively. We construct the instability regions of both particular solutions in the plane of parameters of the problem.  相似文献   

Friedmann cosmology with a cosmological constant in the modified Brans-Dicke theory     

A. Banerjee  N. Banerjee 《Astrophysics and Space Science》1985,114(1):165-174

Homogenous isotropic cosmologies in the presence of a cosmological constant A are studied in the modified Brans-Dicke theory. A class of exact solutions are obtained in Dicke's revised units for empty space as well as for the models filled with dust or radiation. Behaviour of these models near the singularity are discussed.  相似文献   

Going underground: in search of Carboniferous coal forests     

Howard J. Falcon-Lang  William A. DiMichele  Scott Elrick  & W. John Nelson 《Geology Today》2009,25(5):181-184

The development of coal forests during the Carboniferous is one of the best-known episodes in the history of life. Although often reconstructed as steamy tropical rainforests, these ancient ecosystems were a far cry from anything we might encounter in the Amazon today. Bizarre giant club-mosses, horsetails and tree ferns were the dominant plants, not flowering trees as in modern rainforests. At their height, coal forests stretched all the way from Kansas to Kazakhstan, spanning the entire breadth of tropical Pangaea. Most of what we know of their biodiversity and ecology has been quite literally mined out of the ground through two centuries of hard labour. Without coal mining, our knowledge would be greatly impoverished. Over the past few years, we've been exploring underground coal mines in the United States, where entire forested landscapes have been preserved intact over huge areas. Never before have geologists had the opportunity to walk out through mile upon mile of fossilized forest. In this feature article, we describe some of our recent explorations and attempt to shed new light on these old fossils.  相似文献   

Molecular collisions in the interstellar medium     

John Dyson 《Astrophysics and Space Science》1990,174(2):327-327

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