A submillimeter view of protoplanetary dust disks     

Sean M. Andrews  Jonathan P. Williams 《Astrophysics and Space Science》2008,313(1-3):119-122

We present some results from our submillimeter single-dish and aperture synthesis imaging surveys of protoplanetary disks using the JCMT, CSO, and Submillimeter Array (SMA) on Mauna Kea, Hawaii. Employing a simple disk model, we simultaneously fit the spectral energy distributions and spatially resolved submillimeter continuum emission from our SMA survey to constrain disk structure properties, including surface density profiles and sizes. The typical disk structure we infer is consistent with a fiducial accretion disk model with a viscosity parameter α≈0.01. Combined with a large, multiwavelength single-dish survey of similar disks, we show how these observations provide evidence for significant grain growth and rapid evolution in the outer regions of disks, perhaps due to an internal photoevaporation process. In addition, we discuss SMA observations of the disks in the Orion Trapezium (proplyds) in the context of disk evolution in a more extreme environment.  相似文献   

Coupled normal-mode sensitivity to inner-core shear velocity and attenuation     

J. Andrews  A. Deuss  J. Woodhouse 《Geophysical Journal International》2006,167(1):204-212

We have studied the response of normal modes to perturbations in inner-core shear velocity and attenuation, using fully coupled mode synthetics. Our results indicate that (i) mode pairs   _n S _l – _{n ±1} S _l   are strongly coupled by anelasticity, (ii) this coupling causes shear velocity perturbations to strongly affect the Q values of modes through exchange of inner-core characteristics, (iii) there is no evidence for a weakly attenuating inner core in shear, and (iv) the discrepancy between attenuation models returned from normal modes and body waves is small. These results suggest that inversions for inner-core attenuation and shear velocity should be performed simultaneously and should take account of the strong cross-coupling due to attenuation.  相似文献   

The Front-to-Back Asymmetry of Coronal Emission     

Andrews  M.D. 《Solar physics》2002,208(2):317-324

Several recent papers have considered the observation of halo coronal mass ejections (CMEs) using the assumption that coronal emission is symmetric with respect to angular position from the Sun. This paper presents a simple but rigorous treatment of the observation of a single electron in the solar corona. The brightness of an electron as a function of height and angle from the solar limb is presented. The conclusion is reached that there is a front-to-back asymmetry of coronal emission that becomes significant at large angle and/or large height. The observation of halo CMEs is considered. The suggestion is made that a mass cut-off makes it likely that halo CMEs will be more massive, wider, and faster than the typical CMEs. Front-side halos should be more commonly observed than CMEs from the back side.  相似文献   

On the reconstruction of pleistocene ice sheets: A review     

John T. Andrews 《Quaternary Science Reviews》1982,1(1):1-30

Pleistocene ice sheets can be reconstructed through three separate approaches: (1) Evidence based on glacial geological studies, such as erratic trains, till composition, crossing striations and exposures of multiple tills/nonglacial sediments. (2) Reconstructions based on glaciological theory and observations. These can be either two- or three-dimensional models; they can be constrained by ‘known’ ice margins at specific times; or they can be ‘open-ended’ with the history of growth and retreat controlled by parameters resting entirely within the model. (3) Glacial isostatic rebound after deglaciation provides a measure of the distribution of mass (ice) across a region. A ‘best fit’ ice sheet model can be developed that closely approximates a series of relative sea level curves within an area of a former ice sheet; in addition, the model should also provide a reasonable sea level fit to relative sea level curves at sites well removed from glaciation.This paper reviews some of the results of a variety of ice sheet reconstructions and concentrates on the various attempts to reconstruct the ice sheets of the last (Wisconsin, Weischelian, Würm, Devensian) glaciation. Evidence from glacial geology suggests flow patterns at variance with simple, single-domed ice sheets over North America and Europe. In addition, reconstruction of ice sheets from glacial isostatic sea level data suggests that the ice sheets were significantly thinner than estimates based on 18 ka equilibrium ice sheets (cf. Denton and Hughes, 1981). The review indicates it is important to differentiate between ice divides, which control the directions of glacial flow, and areas of maximum ice thickness, which control the glacial isostatic rebound of the crust upon deglaciation. Recent studies from the Laurentide Ice Sheet region indicate that the center of mass was not over Hudson Bay; that a major ice divide lay east of Hudson Bay so that flow across the Hudson Bay and James Bay lowlands was from the northeast; that Hudson Bay was probably open to marine invasions two or three times during the Wisconsin Glaciation; and that the Laurentide Ice Sheet was thinner than an equilibrium reconstruction would suggest.  相似文献   

Radial plasma drifts deduced from VLF whistler mode signals: A modelling study     

E.M. Poulter  M.K. Andrews  G.J. Bailey  R.J. Moffett 《Planetary and Space Science》1984,32(5):525-533

VLF whistler mode signals have previously been used to infer radial plasma drifts in the equatorial plane of the plasmasphere and the field-aligned ionosphere-protonosphere coupling fluxes. Physical models of the plasmasphere consisting of O⁺ and H⁺ ions along dipole magnetic field lines, and including radial Ez × B drifts, are applied to a mid-latitude flux tube appropriate to whistler mode signals received at Wellington, New Zealand, from the fixed frequency VLF transmitter NLK (18.6 kHz) in Seattle, U.S.A. These models are first shown to provide a good representation of the recorded Doppler shift and group delay data. They are then used to simulate the process of deducing the drifts and fluxes from the recorded data. Provided the initial whistler mode duct latitude and the ionospheric contributions are known, the drifts at the equatorial plane can be estimated to about ± 20 ms^?1 (～10–15%), and the two hemisphere ionosphere-protonosphere coupling fluxes to about ± 10¹² m^?2 s^?1 (～40%).  相似文献   

(Re)thinking the dynamics between healthcare and place: therapeutic geographies in treatment and care practices   总被引：3，自引：0，他引：3  

Gavin J Andrews 《Area》2004,36(3):307-318

Although the concept of therapeutic landscapes has assisted health geographers to explore the intimate connections between well-being and place, arguably, after a decade of applications, the common interpretations and assumptions of commentators should be reviewed. Based on theoretical insights from psychoanalytic geographies and geographies of nursing, this paper makes two observations. First, that landscape has been almost exclusively interpreted in a physical sense with co-presence being a necessary condition. In turn, this assumption has led to the neglect of non-physical (imagined) places. Second, that therapeutic effects have been interpreted as experiences attained outside of clinical practices. Hence, healthcare workers and their direct treatments and care have also been neglected. To explore these omissions together, an interview survey of complementary therapists investigates the many ways in which imagined places are constructed and manipulated in therapy sessions. Arguably, beyond this example, extending the therapeutic landscape concept to both physical and non-physical features of treatments could provide fresh insights into the dynamics between healthcare and place. In disciplinary terms, the benefits are twofold. Whilst it could help develop a critical tradition in health geography, perhaps ironically, it could also provide a stronger disciplinary connection between qualitative health geography, various forms of medicine and their research traditions.  相似文献   

Holocene sediment properties of the East Greenland and Iceland continental shelves bordering Denmark Strait (64–68°N), North Atlantic     

John T. Andrews  R. Kihl  Greta B. Kristjansdottir  L. M. Smith  Gudrun Helgadottir†  Aslaug Geirsdottir‡  & Anne E. Jennings 《Sedimentology》2002,49(1):5-24

The oceanographic Polar Front separates the East Greenland and Iceland margins. Surface water temperatures across Denmark Strait vary by 8–12 °C and represent one of the steepest oceanographic gradients on earth. The East Greenland margin is a polar environment, with extensive sea‐ice cover and calving glacier margins; in contrast, the Iceland shelf is much more temperate, and freshwater run‐off is a key component in land–ocean sediment transfers. Average sediment properties from these two contrasting climate and oceanographic continental shelf environments are compared in the spatial domain at 13 sites; the data represent the last 10 000 radiocarbon years of `normal' marine sedimentation for the two regions. The two regions have similar average rates of sediment accumulation (around 43·5 cm kyr<sup>?1</sup>), so that this key variable is factored out in explaining any differences in sediment properties. Dry sediment density, moisture content, hygroscopic moisture, total organic carbon and carbonate contents, mass magnetic susceptibility and the percentages of sand and silt are compared focusing on: (1) median values for sediment properties; and (2) downcore variability, measured by the coefficient of variation (CV). There are significant differences in all but one (hygroscopic moisture) of the sediment properties between Iceland and East Greenland; in four cases, the sense of the differences was not as predicted. In terms of downcore variation (CV), no difference was found between the two regions, nor between the 13 sites, whereas there are some significant differences between the variables. Carbonate and mass magnetic susceptibility have the largest spreads, and moisture content and dry sediment density are the least variable. Protocols are developed to identify the `type core' in a regional series of sites. The results indicate a need to develop a regional perspective on sediment properties, both as inputs to models of sedimentary processes in different polar/arctic environments, and as an indication of which sediment properties might be best suited for palaeoenvironmental downcore time series.  相似文献   

Lithofacies associations and stable isotopes of palustrine and calcrete carbonates: examples from an Indian Maastrichtian regolith     

S. K. Tandon  & J. E. Andrews 《Sedimentology》2001,48(2):339-355

Detailed information on semi‐arid, palustrine carbonate–calcrete lithofacies associations in a sheetwash‐dominated regolith setting is sparse. This is addressed by studying the Lower Limestone of the Lameta Beds, a well‐exposed Maastrichtian regolith in central India. The general vertical lithofacies assemblage for this unit comprises: (a) basal calcareous siltstones and marls with charophytes, ostracods and gastropods; (b) buff micritic limestones associated in their upper parts with calcretized fissure‐fill sandstones; (c) sheetwash as fissure‐fill diamictites and thin pebbly sheets, locally developed over a few metres; and (d) sandy, nodular, brecciated and pisolitic calcretes at the top. The sequence is ‘regressive’, with upsection filling of topographic lows by increased sheetwash. Lateral lithofacies change is marked, but there are no permanent open‐water lake deposits. In topographic lows close to the water table, marshy palustrine or groundwater calcretes formed, whereas on better drained highs, brecciation and calcretization occurred. Prolonged exposure is implied, suggesting that shrinkage was the main cause of brecciation. Evidence for rhizobrecciation and other biological calcrete fabrics is sparse, contrasting with the emphasis on root‐related brecciation in many studies of palustrine lithofacies. Stable isotope (δ¹⁸O and δ¹³C) values are consistent with the palustrine limestones being fed from meteoric‐derived groundwater with a strong input of soil‐zone carbon. There is overlap of both δ¹⁸O and δ¹³C values from the various palustrine and calcrete fabrics co‐occurring at outcrop. This suggests that, in groundwater‐supported wetlands, conversion from palustrine carbonate to calcrete need not show isotopic expression, as the groundwater source and input of soil‐zone carbon are essentially unchanged. Cretaceous–Tertiary δ¹⁸O and δ¹³C values from palustrine lithofacies and associated calcretes appear to be strongly influenced by the inherited values from lakes and wetlands. Hydrologically closed lakes and marine‐influenced water bodies tend to result in low negative palustrine δ¹⁸O and δ¹³C values. During brecciation and calcretization, the degree of isotopic inheritance depends on whether or not alteration occurs in waters that are different from those of the original water body or wetland. Marked biological activity (e.g. rhizobrecciation or root mat development) during calcretization may lower δ¹³C values where C₃ plants are abundant but, in shrinkage‐dominated systems, δ¹³C values will be largely inherited from the palustrine limestones.  相似文献   

The gulf of Mexico Basin: edited by A. Salvador. The geology of North America, volume J. The Geological Society of America, Boulder, Colorado, 1992, ISBN-0-8137-5216-7, US$77.50, 568 pp. plus folder of six maps     

John T. Andrews 《Quaternary Science Reviews》1993,12(10)

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Salt-Water Intrusion in the Costa de Hermosillo, Mexico: A Numerical Analysis of Water Management Proposals   总被引：1，自引：0，他引：1  

Robert W. Andrews 《Ground water》1981,19(6):635-647

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