Magnetic ionization fronts – II. Jump conditions for oblique magnetization     

R. J. R. Williams  J. E. Dyson  T. W. Hartquist 《Monthly notices of the Royal Astronomical Society》2000,314(2):315-323

We present the jump conditions for ionization fronts with oblique magnetic fields. The standard nomenclature of R- and D-type fronts can still be applied, but in the case of oblique magnetization there are fronts of each type about each of the fast- and slow-mode speeds. As an ionization front slows, it will drive first a fast- and then a slow-mode shock into the surrounding medium. Even for rather weak upstream magnetic fields, the effect of magnetization on ionization front evolution can be important.  相似文献   

Ashes to ashes, dust to dust     

David A Williams 《Astronomy& Geophysics》2000,41(3):3.08-3.16

An account of the RAS Presidential Address of 11 February 2000, given by David A Williams , Perren Professor of Astronomy, University College London.  相似文献   

Simultaneous interplanetary scintillation and optical measurements of the acceleration of the slow solar wind     

A. R. Breen  S. J. Tappin  C. A. Jordan  P. Thomasson  P. J. Moran  R. A. Fallows  A. Canals  P. J. S. Williams 《Annales Geophysicae》2000,18(9):995-1002

Simultaneous observations of the slow solar wind off the southeast limb of the Sun were made in May 1999 using optical measurements from the C2 and C3 LASCO coronagraphs on board the SOHO spacecraft and radio-scattering measurements from the MERLIN and EISCAT facilities. The observations show the slow solar wind accelerating outwards from 4.5 solar radii (R), reaching a final velocity of 200–300 km s^-1 by 25–30 R. The acceleration profile indicated by these results is more gentle than the average profile seen in earlier LASCO observations of larger scale features, but is within the variation seen in these studies.  相似文献   

Observations of interplanetary scintillation during the 1998 Whole Sun Month: a comparison between EISCAT, ORT and Nagoya data     

P. J. Moran  S. Ananthakrishnan  V. Balasubramanian  A. R. Breen  A. Canals  R. A. Fallows  P. Janardhan  M. Tokumaru  P. J. S. Williams 《Annales Geophysicae》2000,18(9):1003-1008

Observations of interplanetary scintillation (IPS) allow accurate solar wind velocity measurements to be made at all heliographic latitudes and at a range of distances from the Sun. The data may be obtained with either single, double or multiple antennas, each requiring a different method of analysis. IPS data taken during the 1998 whole sun month (30th July–31st August 1998) by EISCAT, the ORT (Ooty Radio Telescope), India, and the Nagoya IPS system, Japan, allow the results of individual methods of analysis to be compared. Good agreement is found between the velocity measurements using each method, and when combined an improved understanding of the structure of the solar wind can be obtained.On leave from the Physical Research Laboratory, Ahmedabad 380 009, India  相似文献   

Simulation of directional waves in a numerical basin by a desingularized integral equation approach     

A. N. Williams  W. W. Crull 《Ocean Engineering》2000,27(6)

A numerical solution is developed to investigate the generation and propagation of small-amplitude water waves in a semi-infinite rectangular wave basin. The three-dimensional wave field is produced by the prescribed “snake-like” motion of an array of segmented wave generators located along the wall at one end of the tank. The solution technique is based on the boundary element approach and uses an appropriate three-dimensional Green function which explicitly satisfies the tank-wall boundary conditions. The Green function and its derivatives which appear in the integral equation formulation can be shown to be slowly convergent when the source and field points are in close proximity. Therefore, when computing the velocity potentials on the wave generators, the source points are chosen outside the fluid domain, thereby ensuring the rapid convergence of these functions and rendering the integral equations non-singular. Numerical results are shown which illustrate the influence of the various wavemaker and basin parameters on the generated wave field. Finally, the complete wave field produced by the diffraction of oblique waves by a vertical circular cylinder in a basin is presented.  相似文献   

Bistability and Temperature Effects in Mixing Layers Between Molecular and Ionized Regions     

T.K. Nguyen  T.W. Hartquist  D.A. Williams 《Astrophysics and Space Science》2000,271(2):171-179

We propose a simple method of assessing the chemical consequences of mixing at interfaces between cool neutral gas with ionized material. It is shown that the temperature within the interface has a significant effect; at low temperatures (∼ 10 K) the chemistry is significantly suppressed, but for temperatures above 100 K the chemistry initiated by proton/O-atom charge exchange gives rise to a rich and abundant chemistry. The chemical state of the cool neutral gas has an important consequence for the interface. A mainly molecular cool gas can give rise to a high ionization state in the interface; this high ionization state appears stable against parameter variations. C₂H is one of the most easily observed molecules having a high abundance in some interfaces. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Do Low CO₂ Concentrations Affect Pollen-Based Reconstructions of LGM Climates? A Response to “Physiological Significance of Low Atmospheric CO₂ for Plant–Climate Interactions” by Cowling and Sykes     

John W. Williams  Thompson Webb  III  Bryan N. Shurman  Patrick J. Bartlein 《Quaternary Research》2000,53(3):402

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Temporal and spatial patterns of trace elements in the Patuxent River: A whole watershed approach   总被引：1，自引：0，他引：1  

Gerhardt F. Riedel  Sherry A. Williams  Georgia S. Riedel  Cynthia C. Gilmour  James G. Sanders 《Estuaries and Coasts》2000,23(4):521-535

Trace element distributions, partitioning, and speciation were examined at 15 sites in the Patuxent River watershed from May 1995 through October 1997 to determine possible sources of trace elements to the river and estuary, to examine the relationship of the trace element discharges to freshwater discharges as well as to land use and geographic region, to validate previous estimates of loadings to the river, and to provide baseline data for trace elements in the Patuxent River watershed and estuary. Six freshwater sites were examined, representing different basins and geographic provinces, and nine sites along the estuarine salinity gradient. Subregions within the watershed varied considerably in concentrations and areal yields for some elements. Concentrations of As, Cd, Ni, Pb, and Zn were elevated in the Coastal Plain sites compared to the Piedmont sites, while Cu and Hg were more evenly distributed. Cadmium, Cu, Hg, Ni, Pb, and Zn showed overall positive correlations with river flow while As and methylHg (meHg) showed negative correlations with river flow. Concentrations of trace elements in the estuarine portion of the river were generally low, and consistent with mixing between Patuxent River water with elevated concentrations and the lower concentrations of the Chesapeake Bay. Interesting features included a local Cd maximum in the low salinity region of the estuary, probably caused by desorption from suspended sediments, and a significant input of water containing high As concentrations from the Chesapeake Bay and from As being released from bottom sediments in summer. Comparisons between the estimated annual flux of trace elements and the estimates of suspected source terms (atmospheric deposition, urban runoff, and known point sources) suggest that, except for Hg, direct atmospheric deposition is small compared to fluvial loads. Current estimates of trace element inputs from point sources or from urban runoff are inadequate for comparison with other sources, because of inappropriate techniques and/or unacceptably high detection limits. A complete examination of trace element dynamics in the Patuxent River (and in other coastal systems) will require better data for these potential sources.  相似文献   

Fish assemblages as indicators of water quality in the middle Thames estuary, England (1980–1989)     

Francisco Gerson Araújo  W. Peter Williams  Roland G. Bailey 《Estuaries and Coasts》2000,23(3):305-317

Fish abundance and environmental data collected over ten years (1980–1989) from the middle Thames estuary, England, were analyzed to detect temporal trends in fish populations and relationship with environmental parameters, and to assess water quality. Fish were collected from the cooling water intake screens of West Thurrock power station, situated 35.5 km below London Bridge, in the mid-estuary. Marine species abundance were highly seasonal, with peaks in December–March for herring (Clupea harengus), sprat (Sprattus sprattus), 3-spined-stickleback (Gasterosteus aculeatus), and poor cod (Trisopterus minutus); July–August for flounder (Platichthys flesus); and September–December for sand goby (Pomatoschistus minutus), whiting (Merlangius merlangus), bass (Dicentrarchus labrax), plaice (Pleuronectes platessa), and dab (Limanda limanda). Bimodal seasonal patterns of peaks or unclear seasonality in abundance characterized marine estuarine-dependent sole (Solea solea), Nilsson's pipefish (Syngnathus rostelattus) (April/May and September/October), and pouting (Trisopterus luscus) (May and November/December); the estuarine smelt (Osmerus eperlanus) (October and January) and the catadromous eel (Anguilla anguilla) (June and October). There was substantial variation in the abundance of common species over the period of ten years, with herring, sand goby, flounder, and plaice showing a stable abundance in 1980–1984, increasing sharply in 1985–1986, and then decreasing successively through the remainder of the decade (1987–1989). The first half of the decade was a period of higher abundance for less tolerant species such as smelt, sprat, and poor cod, while the second half showed higher abundances of species tolerant to harsh environmental conditions such as sand goby, flounder, eel, and plaice. A general pattern of stable fish populations with a slight trend of deterioration was found to emerge over the years, related to the number of species and quantities of common species. Multivariate techniques of principal component and canonical correspondence ordinations were used for assessing relationships between fish populations abundance and environmental variables. The most significant environmental variables correlated with fish species were temperature and dissolved oxygen. High abundances of flounder were associated with high temperature, while high abundance of poor cod, sprat, herring, and 3-spined-stickleback were associated with high dissolved oxygen, flow, ammonical nitrogen, and low temperature. Plaice, whiting, sand goby, bass, and dab were preferentially found in high salinity and suspended solids, while smelt and sole were likely to prefer average values or showed no clear preferences.  相似文献   

Latching hook for instrument retrieval     

Albert J. Williams 《Ocean Engineering》1974,2(6):275

A light-weight, latching hook on a tapered-strength, light-weight pole retrieves delicate instruments from the sea. The pole is capable of extending the operator's reach more than 6 m in securing a lifting line to instruments more massive than 500 kg.  相似文献