[Al ii] in the ultraviolet spectrum of the symbiotic star RR Telescopii     

F. P. Keenan  B. R. Espey  M. Mathioudakis  K. M. Aggarwal  F. L. Crawford  W. A. Feibelman  F. C. McKenna 《Monthly notices of the Royal Astronomical Society》1999,309(1):195-198

An inspection of a GHRS/ HST spectrum of the symbiotic star RR Telescopii reveals the presence of the [Al  ii ] 3s²¹S – 3s3p ³P₂ line at a vacuum wavelength of 2661.06±0.08 Å, 8.89±0.08 Å away from the Al  ii ] 3s²¹S – 3s3p ³P₁ intercombination transition at 2669.95 Å, in good agreement with the theoretical prediction of Δ λ =8.80 Å. We also find that the Al  ii ] line profile is asymmetric, showing a strong low-density component with a weak high-density wing, redshifted by 30 km s⁻¹, in agreement with the findings of Schild & Schmid, which were based on optical observations. Our measurement of the emission-line ratio R I (2661.06 Å)/ I (2669.95 Å)=0.027±0.003 implies log  N _e=5.8±0.2, in good agreement with the densities found from other ions, such as Si  iii . These results provide strong evidence that we have detected the [Al  ii ] line, the first time (to our knowledge) that this feature has been reliably identified in an astrophysical or laboratory spectrum.  相似文献   

Emission lines of [Ne IV] in the optical and ultraviolet spectra of gaseous nebulae     

Keenan  Aller  Bell  Espey  Feibelman  Hyung  McKenna  & Ramsbottom 《Monthly notices of the Royal Astronomical Society》1998,295(3):683-690

  相似文献   

Emergent polarization structure in spectral lines from rotating stars of spectral type B     

Sean J. McKenna 《Astrophysics and Space Science》1984,107(1):61-69

The emergent polarization structure within spectral lines from rapidly rotating stars of spectral type B5V has been computed for the Mgii 4481 Å doublet and H for several angles of inclination and for rotational speeds of 0.5 and 0.9 of the critical rotational speed. It is found in this study that the emergent polarization within the Mgii 4481 Å doublet is currently unobservable with modern polarimeters. However, the polarization within H should be observable.  相似文献   

Coastal issues and conflicts in North West Europe: A comparative analysis     

M.C. O’Connor  J. McKenna  J.A.G. Cooper 《Ocean & Coastal Management》2010,53(12):727-737

This paper identifies and examines current coastal issues and conflicts in the northwest Europe region through the use of questionnaire surveys. The overall management concerns and outstanding problems across the region are highlighted and the results are presented in relation to previously identified management issues in northeast England, east Grampian in Scotland, and France. The survey presents a list of identified issues, each of which has a direct relationship to one of the 3 pillars of sustainable development, (environment, economic and social), and therefore the Lisbon Agenda (economic and social) and the Gothenburg Agenda (environment and environmental risk). The survey reveals that the important coastal management issues across the region are the large-scale concerns traditionally associated with ICZM: land-use planning, estuary management, natural resource and landscape protection, and water quality. Generally, respondents considered that management response to these issues is poor. Dissatisfaction levels were particularly high in France and Ireland, and to a lesser extent, in England. Water quality management escapes general criticism. This can be regarded as a positive finding, because it indicates that pan-European initiatives (the water-related EU directives) can deal effectively with this pan-European problem. The paper considers the links between the main issues across North West Europe and aims to distinguish how current coastal management practices have affected the range of issues arising.  相似文献   

Modeling viscous damping in nonlinear response history analysis of buildings for earthquake excitation          下载免费PDF全文

Anil K. Chopra  Frank McKenna 《地震工程与结构动力学》2016,45(2):193-211

The Rayleigh damping model, which is pervasive in nonlinear response history analysis (RHA) of buildings, is shown to develop ‘spurious’ damping forces and lead to inaccurate response results. We prove that a viscous damping matrix constructed by superposition of modal damping matrices—irrespective of the number of modes included or values assigned to modal damping ratios—completely eliminates the ‘spurious’ damping forces. This is the damping model recommended for nonlinear RHA. Replacing the stiffness‐proportional part of Rayleigh damping by the tangent stiffness matrix is shown to improve response results. However, this model is not recommended because it lacks a physical basis and has conceptual implications that are troubling: hysteresis in damping force–velocity relationship and negative damping at large displacements. Furthermore, the model conflicts with the constant‐damping model that has been the basis for fundamental concepts and accumulated experience about the inelastic response of structures. With a distributed plasticity model, the structural response is not sensitive to the damping model; even the Rayleigh damping model leads to acceptable results. This perspective on damping provides yet another reason to employ the superior distributed plasticity models in nonlinear RHA. OpenSees software has been extended to include a damping matrix defined as the superposition of modal damping matrices. Although this model leads to a full populated damping matrix, the additional computational demands are demonstrated to be minimal. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

Contrasting recovery of shallow and deep water seagrass communities following climate associated losses in tropical north Queensland,Australia     

Michael A. Rasheed  Skye A. McKenna  Alexandra B. Carter  Robert G. Coles 《Marine pollution bulletin》2014

Tropical seagrass decline and recovery from severe storm impacts was assessed via quarterly measurements of seagrass biomass, species composition and experimental investigations of recovery in north Queensland. Shallow and deep seagrass meadows suffered major declines. Significant recovery in the two years following loss only occurred at deeper sites. Halophila spp. in deep water areas had a high capacity for recovery through the availability of seed banks. In contrast, the shallow species did not recover quickly from experimental disturbance, had poor seed reserves and relied on asexual propagation. The potential for shallow species to recover rapidly from widespread losses was limited as seed banks were limited or non-existent. Understanding inter- and intra-specific differences in seagrass recovery and how this interacts with location is critical to predict the consequences of climate events to tropical seagrasses. This is especially important as more frequent severe storms are predicted as a consequence of climate change.  相似文献   

Near‐bed shear stress,turbulence production and dissipation in a shallow and narrow tidal channel          下载免费PDF全文

Aline Pieterse  Jack A. Puleo  Thomas E. McKenna  Rebecca A. Aiken 《地球表面变化过程与地形》2015,40(15):2059-2070

Near‐bed, highly resolved velocity profiles were measured in the lower 0.03 m of the water column using acoustic Doppler profiling velocimeters in narrow tidal channels in a salt marsh. The bed shear stress was estimated from the velocity profiles using three methods: the log‐law, Reynolds stress, and shear stress derived from the turbulent kinetic energy (TKE). Bed shear stresses were largest during ebbing tide, while near‐bed velocities were larger during flooding tide. The Reynolds stress and TKE method gave similar results, while the log‐law method resulted in smaller bed shear stress values during ebbing tide. Shear stresses and turbulent kinetic energy followed a similar trend with the largest peaks during ebbing tide. The maximum turbulent kinetic energy was on the order of 1 × 10^{? 2} m²/s². The fluid shear stress during flooding tide was approximately 30% of the fluid shear stress during ebbing tide. The maximum TKE‐derived shear stress was 0.7 N/m² and 2.7 N/m² during flooding and ebbing tide, respectively, and occurred around 0.02 m above the bed. Turbulence dissipation was estimated using the frequency spectrum and structure function methods. Turbulence dissipation estimates from both methods were maximum near the bed (~0.01 m). Both the structure function and the frequency spectrum methods resulted in maximum dissipation estimates on the order of 4 × 10^{? 3} m²/s³. Turbulence production exceeded turbulence dissipation at every phase of the tide, suggesting that advection and vertical diffusion are not negligible. However, turbulence production and dissipation were within a factor of 2 for 77% of the estimates. The turbulence production and dissipation decreased quickly away from the bed, suggesting that measurements higher in the water column cannot be translated directly to turbulence production and dissipation estimates near the bed. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

GHOST – A Novel Airborne Gas Chromatograph for In Situ Measurements of Long-Lived Tracers in the Lower Stratosphere: Method and Applications     

Oliver Bujok  Viceith Tan  Erich Klein  Ralf Nopper  Reimar Bauer  Andreas Engel  Marie-Theres Gerhards  Armin Afchine  Daniel S. McKenna  Ulrich Schmidt  Frank G. Wienhold  Horst Fischer 《Journal of Atmospheric Chemistry》2001,39(1):37-64

A novel fully-automated airborne gas chromatograph for in situmeasurements of long-lived stratospheric tracers hasbeen developed, combining the high selectivity of a megabore PLOTcapillary column with recently developed sampling and separationtechniques. The Gas cHromatograph for theObservation of Stratospheric Tracers (GHOST)has been successfully operated during three STREAM campaigns(Stratosphere TRoposphere Experiment byAirborne Measurement) onboard a Cessna Citation IIaircraft in two different modes: Either N₂O andCF₂Cl₂(CFC-12) or CFC-12 and CFCl₃ (CFC-11) have been measuredsimultaneously, with a time resolution of 2 min for both modes.Under flight conditions the instrument precision (1) forthese species is better than 0.9%, and the accuracy(1) is better than 2.0% of the tropospheric values ofall measured compounds. The detection limits (3) arebelow 28 ppb for N₂O, 14 ppt for CFC-12, and 8 ppt forCFC-11, respectively, i.e., well below 10 % of the troposphericvalues of all measured compounds. Post-mission optimization of thechromatographic separation showed a possible enhancement of thetime resolution by up to a factor of 2, associated with acomparable increase in precision and detection limit. As test ofactual performance of GHOST results from an in-flight N₂Ointercomparison with a tunable diode laser absorptionspectrometer (TDLAS) are presented. They yield an excellentagreement between both instruments. Furthermore, on the basis ofthe hitherto most extensive set of upper tropospheric and lowerstratospheric data, the relative stratospheric N₂2O lifetime isre-assessed. When referenced to the WMO reference CFC-11 lifetimeof 45 ± 7 years an N₂O lifetime of 91 ± 15 yearsis derived, a value substantially smaller than the WMO referencelifetime of 120 years. Moreover, this value implies astratospheric N₂O sink strength of 16.3 ± 2.7 Tg (N)yr^–1 which is 30% larger than previous estimates.  相似文献   

Particle size and sorting characteristics of sand in transport on the stoss slope of a small reversing dune   总被引：2，自引：0，他引：2  

N. Lancaster  W. G. Nickling  C. McKenna Neuman 《Geomorphology》2002,43(3-4)

Studies of the particle size and sorting characteristics of sand on the stoss slope of a 6-m high reversing dune show that the sand in transport is generally finer and better sorted than surface sand at the same position on the slope. The sand in transport becomes coarser and more poorly sorted as wind speed and rates of mass transport increase toward the dune crest. These patterns reflect changes in the competence of the wind, which is capable of transporting larger grains and a wider range of grain sizes as its speed increases in space and time. Our field observations suggest that the particle size and sorting characteristics of surface sand are highly dependent on antecedent wind conditions and are not an invariant property of the dune, as is widely assumed. The wide range of particle sizes on the surface, as well as its change through time, also has important implications for modeling sediment transport on dunes. Transport thresholds may vary by as much as 30% on the stoss slope of the study dune.  相似文献   

Tidal Effects on Ground Water Discharge Through a Sandy Marine Beach   总被引：4，自引：1，他引：4  

Daniel W. Urish  Thomas E. McKenna 《Ground water》2004,42(7):971-982

  相似文献