Long edge waves over a continental shelf

Summary The existence of long edge waves over a continental shelf is examined for a representative collection of shelf profiles. The frequencies of both trapped and leaky modes are found, when they exist, in terms of the shelf parameters. Certain theoretical trapped waves over a semi-infinite shelf are shown not to exist in the real ocean.A typical shelf profile on the east coast of Australia is modelled in four different ways from the above collection. The amplification of the leaky mode at the shoreline is found in terms of frequency for each model. Comparison of the results illustrates the severe errors introduced by crude approximations to the profile both at the shoreline and at the shelf cut-off region.

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Lange Randwellen über einem Festlandsockel

Zusammenfassung Die Existenz von langen Randwellen über einem Festlandsockel wird untersucht für eine repräsentative Sammlung von Schelfprofilen. Die Frequenzen sowohl der trapped (geführten) als auch der leaky (energieoder impulsabgebenden) Eigenfunktionsformen werden — wenn sie existieren — ermittelt, ausgedrückt in Schelf-Parametern. Es wird gezeigt, daß gewisse, trapped waves (geführte Wellen) über einem semi-unendlichen Schelf im wirklichen Ozean nicht existieren.Mit Hilfe der obigen Sammlung hat man ein typisches Schelfprofil von der Ostküste Australiens in vier verschiedenen Arten modelliert. Die Verstärkung des leaky mode an der Küstenlinie wird für jedes Modell gefunden in Termen der Frequenz. Der Vergleich der Ergebnisse verdeutlicht die schweren Fehler, die durch grobe Näherungen an das Profil entstehen, sowohl an der Küstenlinie als auch in dem Gebiet des Übergangs vom Schelf in die Tiefsee.

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Ondes transversales à longues crêtes au-dessus d'un plateua continental

Résumé On examine l'existence d'ondes transversales à longues crêtes au-dessus d'un plateau continental, afin de constituer une collection représentative des profils du plateau. On a trouvé les fréquences du «trapped mode» (mode captif) et du «leaky mode» (mode de transfert d'énergie et du moment) — quand ils existent — et cela en fonction des paramètres du plateau. On montre qu'il n'y a pas, dans l'océan réel, certaines ondes captées théoriques qui existeraient au-dessus d'un plateau semi-infini.Grâce à la collection dont il est parlé ci-dessus, on a modelé de quatre façons différentes un profil typique de la côte Est de l'Australie. Pour chaque modèle on a trouvé l'amplification du «leaky mode» sur la ligne de côte en fonction de la fréquence. La comparaison des résultats montre les erreurs importantes introduites par des approximations sommaires du profil, à la fois près de la ligne de côte et dans la région de transition du plateau continental en mer abyssale.

     

Chandra observations of neutron stars: an overview

We present an overview of Chandra X-ray Observatory observations of neutron stars. The outstanding spatial and spectral resolution of this great observatory have allowed for observations of unprecedented clarity and accuracy. Many of these observations have provided new insights into neutron star physics. We present an admittedly biased and overly brief review of these observations, highlighting some new discoveries made possible by the Observatory’s unique capabilities. This includes our analysis of recent multiwavelength observations of the putative pulsar and its pulsar-wind nebula in the IC443 SNR.      

The photoevaporation of discs around young stars in massive clusters

We present models in which the photoevaporation of discs around young stars by an external ultraviolet source (as computed by Adams et al.) is coupled with the internal viscous evolution of the discs. These models are applied to the case of the Orion Nebula Cluster (ONC), where the presence of a strong ultraviolet field from the central OB stars, together with a detailed census of circumstellar discs and photoevaporative flows, is well established. In particular we investigate the constraints that are placed on the initial disc properties in the ONC by the twin requirement that most stars possess a disc on a scale of a few astronomical unit (au), but that only a minority (<20 per cent) are resolved by Hubble Space Telescope ( HST ) at a scale of 50 au. We find that these requirements place very weak constraints on the initial radius distribution of circumstellar discs: the resulting size distribution readily forgets the initial radius distribution, owing to the strong positive dependence of the photoevaporation rate on disc radius. Instead, the scarcity of large discs reflects the relative scarcity of initially massive discs (with mass  >0.1 M_⊙  ). The ubiquity of discs on a small scale, on the other hand, mainly constrains the time-span over which the discs have been exposed to the ultraviolet field (<2 Myr). We argue that the discs that are resolved by HST represent a population of discs in which self-gravity was important at the time that the dominant central OB star switched on, but that, according to our models, self-gravity is unlikely to be important in these discs at the present time. We discuss the implications of our results for the so-called proplyd lifetime problem.     

Application of the SHETRAN basin‐scale,landslide sediment yield model to the Llobregat basin,Spanish Pyrenees

The first application of the SHETRAN basin‐scale, landslide erosion and sediment yield model is carried out for a major landsliding event in the upper 505 km² of the Llobregat basin, in the eastern Spanish Pyrenees, in November 1982. The model simulates the spatial distribution of shallow landslides and their sediment yield. Acknowledging uncertainty in the model parameter evaluation, the aim of the application was not to reproduce the observed occurrence of landslides as accurately as possible with one simulation, but to bracket the observed pattern with several simulations representing uncertainty in the key input conditions. Bounds on the landslide simulations were thus determined as a function of uncertainty in the vegetation root cohesion (used in the model factor of safety calculations). The resulting upper bound considerably overestimates the observed pattern (17 000 landslides compared with an observation of around 700), but it reproduces several of the principal clusters in the observed pattern. The lower bound contains around 500 landslides. The sediment yield estimates (2670–14 630 t km^?2) are comparable to measurements elsewhere in the Pyrenees for extreme events. The results demonstrate an ability to simulate the basin‐scale landslide response to a rainfall event and the resulting sediment yield. They also highlight the need for further research in setting the uncertainty bounds and in avoiding large overestimates of landslide occurrence arising in part from a current inability to model small‐scale controls for a basin of the given size. Copyright © 2006 John Wiley & Sons, Ltd.     

Potential martian mineral resources: Mechanisms and terrestrial analogues

The future exploration of Mars is likely to utilize resources that can be extracted in situ. An overview of the geology of Mars has been presented and several mechanisms that could result in the formation of ore deposits have been identified. These include deposits caused by hydrothermal fluids resulting from volcanic activity, large igneous province formation and impact craters. Surface enrichment of mineral sand deposits is also discussed. Where appropriate, terrestrial analogues of these mechanisms have been discussed and supporting evidence from observations of Mars undertaken to date presented. Types of deposits that are unlikely to be found on Mars are also listed.     

The geology of Australian Mars analogue sites

Australia has numerous landforms and features, some unique, that provide a useful reference for interpreting the results of spacecraft orbiting Mars and exploring the martian surface. Examples of desert landforms, impact structures, relief inversion, long-term landscape evolution and hydrothermal systems that are relevant to Mars are outlined and the relevant literature reviewed. The Mars analogue value of Australia's acid lakes, hypersaline embayments and mound spring complexes is highlighted along with the Pilbara region, where the oldest convincing evidence of life guides exploration for early life on Mars. The distinctive characteristics of the Arkaroola Mars Analogue Region are also assessed and opportunities for future work in Australia are outlined.     

Responses of benthic macroinvertebrates to thin-layer disposal of dredged material in Mississippi Sound, USA

Benthic community responses to thin-layer disposal of dredged material were assessed at three sites in Mississippi Sound, USA. Community composition differed between disposal and reference sites (ANOSIM) for each disposal event. Oweniid sand worms, Amphinomid fire worms, and brittle stars (Ophiuriodea) contributed the most to community distinctions between disposal and reference sites (SIMPER), with higher abundances at the disposal sites. Total infaunal abundance was similar to pre-disposal and reference conditions within 3-10 months of thin-layer disposal. Distinctions in community composition between disposal and reference sites (nMDS) persisted throughout the 16-month study period, but were less pronounced at the site where sediment composition was unchanged by disposal. Size distributions of some taxa (e.g., gastropod and hemichordate) suggest adults recolonized the newly deposited sediments either through vertical migration or lateral immigration from adjacent areas. Thin-layer disposal offers a viable alternative to conventional open-water disposal practices and warrants further exploration for other areas with deeper bathymetries and different hydrodynamic regimes.     

Getting from here to there – energy technology transformation pathways in the EMF27 scenarios

Based on a large number of energy-economic and integrated assessment models, the Energy Modeling Forum (EMF) 27 study systematically explores the implications of technology cost and availability for feasibility and macroeconomic costs of energy system transformations toward climate stabilization. At the highest level, the technology strategy articulated in all the scenarios in EMF27 includes three elements: decarbonization of energy supply, increasing the use of low-carbon energy carriers in end-use, and reduction of energy use. The way that the scenarios differ is in the degree to which these different elements of strategy are implemented, the timing of those implementations, and the associated macroeconomic costs. The study also discusses the value of individual technologies for achieving climate stabilization. A robust finding is that the unavailability of carbon capture and storage and limited availability of bioenergy have the largest impact on feasibility and macroeconomic costs for stabilizing atmospheric concentrations at low levels, mostly because of their combined ability to remove carbon from the atmosphere. Constraining options in the electric sector such as nuclear power, wind and solar energy in contrast has a much smaller impact on the cost of mitigation.