A generation mechanism for Pc 5 micropulsations in the morning sector     

Gordon Rostoker  Hing-Lan Lam 《Planetary and Space Science》1978,26(5):493-505

In the companion paper (Lam and Rostoker, 1978) we have shown that Pc 5 micropulsations are intimately related to the behaviour and character of the westward auroral electrojet in the morning sector. In this paper we show that Pc 5 micropulsations can be regarded as LC-oscillations of a three-dimensional current loop involving downward field-aligned current flow near noon, which diverges in part to form the ionospheric westward electrojet and returns back along magnetic field lines into the magnetosphere in the vicinity of the ionosphere conductivity discontinuity at the dawn meridian. The current system is driven through the extraction of energy from the magnetospheric plasma drifting sunwards past the flanks of the magnetosphere in a manner discussed by Rostoker and Boström (1976). The polarization characteristics of the pulsations on the ground can be understood in terms of the effects of displacement currents of significant intensity which flow near the F-region peak in the ionosphere and induced currents which flow in the earth. These currents significantly influence the magnetic perturbation pattern at the Earth's surface. Model current system calculations show that the relative phase of the pulsations along a constant meridian can be explained by the composite effect of oscillations of the borders of the electrojet and variations in the intensity of current flow in the electrojet.  相似文献   

Susceptibility assessment of the precursor stage of a landslide threatening Haivan Railway Station,Vietnam     

Lam Huu Quang  Doan Huy Loi  Kyoji Sassa  Kaoru Takara  Hirotaka Ochiai  Khang Dang  Shinro Abe  Shiho Asano  Do Ngoc Ha 《Landslides》2018,15(2):309-325

Haivan Station is an important station on the North-South railway line in central Vietnam. Field investigation has identified a precursor stage of a landslide that would threaten this railway. Therefore, a landslide susceptibility assessment for Haivan Station was urgently needed to protect passenger safety and the national railway. Conducted investigations included air-photo interpretation, drilling, ground water and inclinometer monitoring, laboratory testing, and landslide simulation. This research applied the undrained dynamic loading ring shear apparatus ICL-2 to drill-core samples from the precursor landslide. Samples for ring shear tests were taken from sandy soil layers found at depths of ~21, ~31, and ~50 m in the cores. Each of these was believed to be a possible sliding surface of a landslide, and all were tested to shear failure in the ICL-2 apparatus. The boundary between highly weathered granitic rock and weathered granitic rock was identified at about 50 m depth. The inclinometer monitoring detected slight movement at this depth. Therefore, the present day risk of a landslide forming at 50 m is higher than for one forming at either 21 or 31 m. The landslide dynamic parameters obtained from the ring shear test of the 50-m-deep sample were used in an integrated numerical simulation model LS-RAPID. The simulation result gave the critical pore-pressure ratio for landslide occurrence, and landslide’s likely maximum speed, total volume, and depth of landslide debris that could cover the railway. These estimates serve to raise awareness of the vulnerability of the Vietnam national railway sector to landslide impact.  相似文献   

Multi-link laser interferometry architecture for interspacecraft displacement metrology     

Samuel P. Francis  Timothy T.-Y. Lam  David E. McClelland  Daniel A. Shaddock 《Journal of Geodesy》2018,92(3):241-251

Targeting a future Gravity Recovery and Climate Experiment (GRACE) mission, we present a new laser interferometry architecture that can be used to recover the displacement between two spacecraft from multiple interspacecraft measurements. We show it is possible to recover the displacement between the spacecraft centers of mass in post-processing by forming linear combinations of multiple, spatially offset, interspacecraft measurements. By canceling measurement error due to angular misalignment of the spacecraft, we remove the need for precise placement or alignment of the interferometer, potentially simplifying spacecraft integration. To realize this multi-link architecture, we propose an all-fiber interferometer, removing the need for any ultrastable optical components such as the GRACE Follow-On mission’s triple mirror assembly. Using digitally enhanced heterodyne interferometry, the number of links is readily scalable, adding redundancy to our measurement. We present the concept, an example multi-link implementation and the signal processing required to recover the center of mass displacement from multiple link measurements. Finally, in a simulation, we analyze the limiting noise sources in a 9 link interferometer and ultimately show we can recover the \(80\;\text {nm}/\sqrt{\text {Hz}}\) displacement sensitivity required by the GRACE Follow-On laser ranging interferometer.  相似文献   

Development of Rapid Consolidation Equipment for Cohesive Soil   总被引：1，自引：1，他引：0  

Khairul Anuar Kassim  Ahmad Safuan A. Rashid  Ahmad Beng Hong Kueh  Chong Siaw Yah  Lam Chee Siang  Norhazilan Mohd Noor  Hossein Moayedi 《Geotechnical and Geological Engineering》2015,33(1):167-174

  相似文献   

Geographic information science and technology at Louisiana State University     

Michael Leitner  Nina N.S. Lam  Fahui Wang  Lei Wang  Shelley Meng  Joshua Kent 《制图学和地理信息科学》2015,42(4):84-90

  相似文献   

The relationship between the Normalized Difference Vegetation Index and drought indices in the South Central United States     

Bushra  Nazla  Rohli  Robert V.  Lam  Nina S. N.  Zou  Lei  Mostafiz  Rubayet Bin  Mihunov  Volodymyr 《Natural Hazards》2019,96(2):791-808

Natural Hazards - Drought indices are useful for quantifying drought severity and have shown mixed success as an indicator of drought damage and biophysical dryness. While spatial downscaling of...  相似文献   

Modeling shear rigidity of stratified bedrock in site response analysis   总被引：1，自引：0，他引：1  

Hing-Ho Tsang  M. Neaz SheikhNelson T.K. Lam 《Soil Dynamics and Earthquake Engineering》2012,34(1):89-98

Where a distinct soil-rock interface exists, the bedrock medium is commonly treated as elastic half-space and the bedrock surface as the lower boundary of the soil-column model for site response analyses (or the lower boundary of the finite element model for soil-structure interaction analyses). While shear wave velocity in bedrock varies with depth, there has been no consensus amongst scientists and practitioners over the value of “effective depth” into bedrock at which the “half-space” shear wave velocity value should be taken for modeling purposes. This paper reports an interesting and important observation that the effective depth into bedrock is sensitive to the shear wave velocity profile of the overlying soil sediments. A simple and heuristic method, namely Resonant Period Equivalence (RPE) Method, is proposed herein for representing a stratified elastic bedrock of inhomogeneous properties by an equivalent homogeneous elastic half-space medium, which is characterized by a single equivalent shear wave velocity (V_R) value. The proposed calculation method has been verified by extensive comparative analyses involving the use of programs SHAKE and NERA and employing the complete shear wave velocity models of both the soil sediments and the underlying stratified bedrock.  相似文献   

Toxic Effects of Cadmium on Fertilizing Capability of Spermatozoa, Dynamics of the First Cleavage and Pluteus Formation in the Sea Urchin Anthocidaris crassispina (Agassiz)     

M.A Vaschenko  Z.P Zhang  P.K.S Lam  R.S.S Wu 《Marine pollution bulletin》1999,38(12):584-1104

Effects of cadmium on the fertilizing capability of spermatozoa, dynamics of the first cleavage and pluteus formation in the sea urchin, Anthocidaris crassispina, were investigated. Exposure to cadmium concentrations of 0.32, 0.56, 1.0, 1.8, 3.2, 5.6 and 10.0 mg l⁻¹ for 30 min caused a significant adverse effect on the fertilizing capability of spermatozoa (EC₅₀=1.7 mg l⁻¹). For the first cleavage kinetics assay, eggs were pre-exposed to 0, 0.18, 0.32, 0.56, 1.0, 1.8, 3.2 and 5.6 mg l⁻¹ of cadmium for 30 min. Fresh sperms of fixed concentrations were then added to the eggs and the percentage of divided zygotes estimated. Concentration-dependent toxic effects on the dynamics of the first cleavage in A. crassispina were observed between 0.18 and 5.6 mg l⁻¹ cadmium. Calculation of EC₅₀ values by probit analysis at different stages of the first cleavage revealed that EC₅₀ values increased gradually from 0.37 mg l⁻¹ at the beginning to 2.24 mg l⁻¹ at the end of the cleavage process. Different larval classes (normal, abnormal and retarded plutei, preplutei, dead embryos/larvae) of A. crassispina obtained from eggs pre-treated with cadmium for 30 min and then exposed to cadmium throughout embryogenesis were also examined. The percentage of abnormal plutei was significantly higher than that of the control at 0.56 mg l⁻¹ cadmium and higher. The 48 h EC₅₀ calculated for the number of normal plutei was 1.5 mg l⁻¹. A χ²-test revealed significant changes in the distribution of proportions of different larval classes only at cadmium concentrations 1.8 mg l⁻¹. At 5.6 mg l⁻¹ of cadmium, small malformed plutei, and oval embryos with rudimentary skeleton, poorly differentiated intestines and no mouth (pre-pluteus stage) were observed. The relative sensitivity of various end points to cadmium is also compared and discussed.  相似文献   

A mechanistic study on the photodegradation of Irgarol-1051 in natural seawater     

Lam KH  Lei NY  Tsang VW  Cai Z  Leung KM  Lam MH 《Marine pollution bulletin》2009,58(2):272-279

The kinetics of the photoinduced degradation and transformation of the antifouling booster biocide, Irgarol-1051, in natural coastal seawater was studied. The measured first-order rate constant for the degradation of Irgarol-1051 was 4.02 ± 0.1 × 10⁻⁴ h⁻¹, while the rate constant for the formation of 2-methylthio-4-tert-butylamino-6-amino-s-triazine (M1), the most dominant degradation product of Irgarol-1051, was 4.6 ± 0.1 × 10⁻⁵ h⁻¹. This considerably slower rate suggested that the transformation of Irgarol-1051 to M1 may not be the predominant pathway of the photodegradation process. During the photodegradation study, a new s-triazine species was observed in the degradation mixtures which, together with M1, appeared immediately upon photolysis and continued to accumulate in the degradation mixture throughout the entire study duration. This is in contrast to the behaviour of the recently identified degradation product of Irgarol-1051, 3-[4-tert-butylamino-6-methylthiol-s-triazin-2-ylamino]- propionaldehyde (M2), which was only detected in the degradation mixture after a long induction period. High-resolution tandem mass spectrometric analysis hinted that the new degradation product (M4) may possess a terminal alcohol and is likely to be an N-allylic alcohol derivative of M1. This suggests that M4 may, indeed, be a precursor of M2 via redox transformation at its N-allylic alcohol functionality.  相似文献   

Measuring and monitoring persistent organic pollutants in the context of risk assessment   总被引：3，自引：0，他引：3  

Wu RS  Chan AK  Richardson BJ  Au DW  Fang JK  Lam PK  Giesy JP 《Marine pollution bulletin》2008,57(6-12):236-244

Due to growing concerns regarding persistent organic pollutants (POPs) in the environment, extensive studies and monitoring programs have been carried out in the last two decades to determine their concentrations in water, sediment, and more recently, in biota. An extensive review and analysis of the existing literature shows that whilst the vast majority of these efforts either attempt to compare (a) spatial changes (to identify "hot spots"), or (b) temporal changes to detect deterioration/improvement occurring in the environment, most studies could not provide sufficient statistical power to estimate concentrations of POPs in the environment and detect spatial and temporal changes. Despite various national POPs standards having been established, there has been a surprising paucity of emphasis in establishing accurate threshold concentrations that indicate potential significant threats to ecosystems and public health. Although most monitoring programs attempt to check compliance through reference to certain "environmental quality objectives", it should be pointed out that many of these established standards are typically associated with a large degree of uncertainty and rely on a large number of assumptions, some of which may be arbitrary. Non-compliance should trigger concern, so that the problem can be tracked down and rectified, but non-compliance must not be interpreted in a simplistic and mechanical way. Contaminants occurring in the physical environment may not necessarily be biologically available, and even when they are bioavailable, they may not necessarily elicit adverse biological effects at the individual or population levels. As such, we here argue that routine monitoring and reporting of abiotic and biotic POPs concentrations could be of limited use, unless such data can be related directly to the assessment of public health and ecological risks. Risk can be inferred from the ratio of predicted environmental concentration (PEC) and the predicted no effect concentration (PNEC). Currently, the paucity of data does not allow accurate estimation of PNEC, and future endeavors should therefore, be devoted to determine the threshold concentrations of POPs that can cause undesirable biological effects on sensitive receivers and important biological components in the receiving environment (e.g. keystone species, populations with high energy flow values, etc.), to enable derivation of PNECs based on solid scientific evidence and reduce uncertainty. Using the threshold body burden of POPs required to elicit damages of lysosomal integrity in the green mussel (Perna virvidis) as an example, we illustrate how measurement of POPs in body tissue could be used in predicting environmental risk in a meaningful way.  相似文献