A Chandra view of the multiple merger in Abell 2744     

Joshua C. Kempner  Laurence P. David 《Monthly notices of the Royal Astronomical Society》2004,349(2):385-392

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Variability in the power spectrum of solar five-minute oscillations     

Frank Hill  Juri Toomre  Laurence J. November 《Solar physics》1983,82(1-2):411-425

Two-dimensional power spectra of solar five-minute oscillations display prominent ridge structures in (k, ω) space, where k is the horizontal wavenumber and ω is the temporal frequency. The positions of these ridges in k and ω can be used to probe temperature and velocity structures in the subphotosphere. We have been carrying out a continuing program of observations of five-minute oscillations with the diode array instrument on the vacuum tower telescope at Sacramento Peak Observatory (SPO). We have sought to establish whether power spectra taken on separate days show shifts in ridge locations; these may arise from different velocity and temperature patterns having been brought into our sampling region by solar rotation. Power spectra have been obtained for six days of observations of Doppler velocities using the Mgi λ5173 and Fei λ5434 spectral lines. Each data set covers 8 to 11 hr in time and samples a region 256″ × 1024″ in spatial extent, with a spatial resolution of 2″ and temporal sampling of 65 s. We have detected shifts in ridge locations between certain data sets which are statistically significant. The character of these displacements when analyzed in terms of eastward and westward propagating waves implies that changes have occurred in both temperature and horizontal velocity fields underlying our observing window. We estimate the magnitude of the velocity changes to be on the order of 100 m s^-1; we may be detecting the effects of large-scale convection akin to giant cells.  相似文献   

The effect of extracellular polymers (EPS) on the proton adsorption characteristics of the thermophile Bacillus licheniformis S-86     

Janette Tourney  Bryne T. Ngwenya  J.W. Fred Mosselmans  Laurence Tetley  Gregory L. Cowie 《Chemical Geology》2008,247(1-2):1-15

This study investigated proton adsorption to an extracellular polymeric substance (EPS) producing bacterial strain, Bacillus licheniformis S-86, in order to characterise and quantify the contribution made by EPS to cell surface reactivity. Potentiometric titrations were conducted using both untreated cells and cells from which the EPS layer had been extracted. Surface-complexation modelling indicated the presence of four different functional groups in both untreated and EPS-free cells. These sites are assigned to phosphodiester, (pK_a 3.3–3.4), carboxylic (pK_a 5.3–5.4), phosphoryl/ (pK_a 7.4–7.5) and hydroxyl/amine (pK_a 9.9–10.1) type groups. The pK_a values for the four groups were very similar for untreated and EPS-free cells, indicating no qualitative difference in composition, but site concentrations in the untreated cells were statistically found to be significantly higher than those in the EPS-free cells for the pK_a 3.3–3.4 and pK_a 9.9–10.1 sites. Infrared analysis provided supporting evidence that site 2 is carboxylic in nature but did not reveal any difference in IR absorption between the native and EPS-free cells. Dissolved organic carbon (DOC) analysis conducted during this study indicated that DOC release by cells is significant, and that the EPS layer is the major contributor.  相似文献   

Voracious planktonic hydroids: unexpected predatory impact on a coastal marine ecosystem     

Laurence P. Madin  Stephen M. Bollens  Erich Horgan  Mari Butler  Jeffrey Runge  Barbara K. Sullivan  Grace Klein-Macphee  Edward Durbin  Ann G. Durbin  Donna Van Keuren  Stphane Plourde  Ann Bucklin  M. Elizabeth Clarke 《Deep Sea Research Part II: Topical Studies in Oceanography》1996,43(7-8)

Hydroids are typically attached, benthic cnidarians that feed on a variety of small prey. During sampling on Georges Bank in spring 1994, we found huge numbers of hydroids suspended in the plankton. They fed on young stages of copepods that are an important prey for fish, as well as on young fish themselves. Two independent methods were used to estimate feeding rates of the hydroids; both indicate that the hydroids are capable of consuming from 50% to over 100% of the daily production of young copepods. These results suggest that hydroids can have a profound effect on the population dynamics of zooplankton and young fish on Georges Bank.  相似文献   

The reliability of analytical data for tributyltin (TBT) in sea water and its implications on water quality criteria     

James W. Readman  Laurence D. Mee 《Marine environmental research》1991,32(1-4)

To investigate the reliability of analytical data for tributyltin (TBT) in sea water, split water samples were distributed to ten laboratories in six countries. The sub-surface samples comprised: (i) an offshore (0·5 km) water sample, (ii) the same sample but spiked with an undisclosed quantity of TBT standard compound (175 ng TBT⁺ liter⁻¹), and (iii) a sample taken from a yacht marina. The seven acceptable data sets were in good agreement for the spiked sample (178 ± 26 ng TBT⁺ liter⁻¹) but showed a greater variation in concentrations reported for the yacht marina sample (366 ± 93 ng TBT⁺ liter⁻¹). Atomic absorption and gas chromatographic-flame photometric detection techniques produced results of similar accuracy and precision. Samples acidified with 1 ml of 10% (v/v) acetic acid appeared stable for more than 2 weeks when stored refrigerated and in darkness. Analyses of the offshore seawater sample revealed TBT contamination (9 ± 7 ng TBT⁺ liter⁻¹) indicating dispersion of the compound to the shelf waters off Monaco. The spread in values reported by the laboratories demonstrates inherent difficulties in obtaining good precision below approximately 20 ng TBT⁺ liter⁻¹. This observation is discussed with respect to the setting and enforcing of water quality standards.  相似文献   

Helium resonance lines in the flare of 15 June, 1973     

Jason G. Porter  Katharine B. Gebbie  Laurence J. November 《Solar physics》1989,120(2):309-341

Time sequences of He i and He ii resonance line intensities at several sites within the flare of 15 June, 1973 are derived from observations obtained with the Naval Research Laboratory's Slitless Spectroheliograph on Skylab. The data are compared with predictions in six model flare atmospheres based on two values for the heating rate and three for the flux of photoionizing coronal X-rays and EUV. A peak ionizing flux more than 10³ times that in the quiet Sun is indicated. For most conditions in flare kernels the He ii L and L lines are found to be formed by collisional excitation, thereby contributing to the local cooling of the plasma at temperatures above 6 × 10⁴ K. Emission in the higher Lyman lines is generally the result of a mixture of collisional excitation at these temperatures and photoionization and recombination at temperatures near 2.5 × 10⁴ K. We discuss implications for the common practice of deriving stellar coronal fluxes from He ii 1640 Å fluxes assuming dominance of the recombination mechanism.Chief, Quantum Physics Division, National Institute of Standards and Technology.Operated jointly by the National Institute of Standards and Technology and the University of Colorado.Operated by the National Optical Astronomy Observatories of the Association of Universities for Research in Astronomy, Inc. under contract with the National Science Foundation.  相似文献   

Inferring the depth extent of the horizontal supergranular flow     

Laurence J. November 《Solar physics》1994,154(1):1-17

The 2D horizontal velocity field determined from local correlation tracking of granulation and its divergence have remarkably different appearances. The 2D horizontal velocity shows the classical 32 Mm supergranular cellular outflow bounded by the chromospheric network, whereas the divergence is dominated by distinct long-lived sources and sinks of about 7 Mm size. The 2D horizontal velocity shows no obvious evidence for 7 Mm cells, and the divergence exhibits little power with the 32 Mm scale. However, by mass continuity for a steady 3D flow in a stratified atmosphere, the divergence of the 2D horizontal component is equal to the vertical velocity divided by a height scale. Thus the 3D steady solar flow field at the bottom of the photosphere has a vertical component consisting primarily of 7 Mm sources and sinks, which define the 2D cellular-like 32 Mm continuous horizontal outflows.Simultaneous Doppler vertical velocity measurements verify the mass-continuity relation, and give a height scale equal to the density scale height in the photosphere within observational error. The observational result is consistent with our theoretical expectation. Any height scale other than the density scale height would indicate a vertical velocity thate-folds on a scale comparable to or smaller than the density scale height, which we argue is unphysical near the top of the convection zone. The continuity relation indicates that vortex-free steady horizontal velocities seen at the solar surface, i.e., the horizontal supergranular flow, must diminish with depth due to the increasing density scale height. We estimate that the horizontal supergranular flow cannot extend much more than onee-fold increase in the density scale height below the visible solar surface, about 2.4 Mm. Therefore the convection below the solar surface should be characterized by the scale of the principal steady vertical velocity component, i.e., by vertical plumes having a dimension of 7 Mm - what we have called mesogranulation - rather than closed 32 Mm cells as is widely believed.Operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with National Science Foundation.  相似文献   

Neoproterozoic crustal evolution in Southern Chad: Pan-African ocean basin closing, arc accretion and late- to post-orogenic granitic intrusion     

A. Pouclet  M. Vidal  J.-C. Doumnang  J.-P. Vicat  R. Tchameni 《Journal of African Earth Sciences》2006,44(4-5):543

In the Lake Léré region, southern Chad, Neoproterozoic terrains are distributed in four lithostructural groups that reveal the geotectonic evolution of a part of the Pan-African orogenic domain. The first group includes basaltic volcanic rocks and fine-grained detrital sedimentary rocks of pre-tectonic basins that were emplaced in an extensional regime, close to a volcanic arc. The second and third groups include calc-alkaline gabbroic intrusions emplaced at an upper crustal level and a midcrustal tonalite, respectively, that are interpreted to be the roots of an active margin volcanic arc. These first three groups experienced WNW to ESE compression, and may belong to a fore-arc basic—volcanic arc—back-arc basin system that was accreted eastward to the Palaeoproterozoic Adamaoua-Yadé Block. The fourth group includes post-tectonic granite plutons invading the older groups. This paper documents the accretion processes in the southern margin of the Saharan Metacraton.  相似文献   

Modification of streaming potential by precipitation of calcite in a sand–water system: laboratory measurements in the pH range from 4 to 12     

Xavier Guichet  Laurence Jouniaux  Nicole Catel 《Geophysical Journal International》2006,166(1):445-460

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A thermodynamic model for Fe–Mg dioctahedral K white micas using data from phase-equilibrium experiments and natural pelitic assemblages     

Teddy Parra  Olivier Vidal  Phillipe Agard 《Contributions to Mineralogy and Petrology》2002,143(6):706-732

The purpose of this study is to derive a solid-solution model for potassic white micas (KWM) encountered in rocks of various bulk compositions, over a wide range of P-T conditions. A compilation of phengite compositions lead us to propose a seven-thermodynamic-component (muscovite, Fe²⁺-Al-celadonite, Mg-Al-celadonite, annite, phlogopite, pyrophyllite and paragonite) ionic solid-solution model which accounts for the Tschermak, Fe-Mg, di/trioctahedral, pyrophyllitic and paragonitic substitutions observed in nature. A four-site mixing model with symmetric Margules parameters to model the Tschermak substitutions, asymmetric Margules parameters to model the other substitutions, and ideal intersite interaction has been adopted. In contrast to previous models, the relevant thermodynamic data and solid-solution properties are calibrated with independent sets of published experiments conducted for the KMASH, KFASH, KFMASH, and KNASH systems, as well as about 200 natural data involving KWM assemblages. The constraints span a wide range of pressure and temperature conditions (150 to 750 °C, 0.5 to 30 kbar), so that our model does not need to be extrapolated outside the calibration range to be used for P-T thermobarometric purposes. The calculated thermodynamic data are interconsistent with the TWQ thermodynamic database and solid-solution models, including that recently published for chlorites.  相似文献