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51.
Infrared spectrophotometric measurements of Neptune's satellite Triton obtained between 1980 and 1982 in the spectral range 0.8–2.5 μm show six individual absorption bands attributable to methane. An additional band in the Triton data is not methane. The Triton spectral data conform more closely to a laboratory spectrum of frozen methane than to a synthetic spectrum of methane gas computed for conditions of low temperature expected at the satellite. Additionally, the strength of the bands vary with Triton's orbital position. The data thus suggest that methane in the ice phase is mostly responsible for the bands in Triton's spectrum, and that the ice is distributed nonuniformly around the satellite's surface. 相似文献
52.
We analyze the dynamics of the broadband frequency spectrum of 338 microwave bursts observed in the years 2001?–?2002 with the Owens Valley Solar Array. A subset of 38 strong microwave bursts that show a single spectral maximum are studied in detail. Our main goal is to study changes in spectral peak frequency ν pk with time. We show that, for a majority of these simple bursts, the peak frequency shows a high positive correlation with flux density – it increases on the rise phase in ≈83% of 24 bursts where it could be cleanly measured, and decreases immediately after the peak time in ≈62% of 34 bursts. This behavior is in qualitative agreement with theoretical expectations based on gyrosynchrotron self-absorption. However, for a significant number of events (≈30?–?36%) the peak frequency variation is much smaller than expected from self-absorption, or may be entirely absent. The observed temporal behavior of ν pk is compared with a simple model of gyrosynchrotron radio emission. We show that the anomalous behavior is well accounted for by the effects of Razin suppression, and further show how an analysis of the temporal evolution of ν pk can be used to uniquely determine the relative importance of self-absorption and Razin suppression in a given burst. The analysis technique provides a new, quantitative diagnostic for the gyrosynchrotron component of solar microwave bursts. Applying this analysis technique to our sample of bursts, we find that in most of the bursts (60%) the spectral dynamics of ν pk around the time of peak flux density is caused by self-absorption. On the other hand, for a significant number of events (≈70%), the Razin effect may play the dominant role in defining the spectral peak and dynamics of ν pk, especially on the early rise phase and late decay phase of the bursts. 相似文献
53.
Kalin T. McDannell David A. Schneider Peter K. Zeitler Paul B. O'Sullivan Dale R. Issler 《地学学报》2019,31(3):189-204
We present a multi‐chronometric approach for reconstructing deep‐time thermal histories using southern Baffin Island as a case study. This continuous thermal history begins with the Palaeoproterozoic Trans‐Hudson Orogeny and is derived from inverse and forward models that integrate thermochronometers spanning some 500°C: new apatite U–Pb ages and K‐feldspar 40Ar/39Ar multi‐diffusion domain data, published (U–Th)/He zircon ages and new multi‐kinetic fission‐track results. Integration of data from a wider temperature range reduces ambiguities in thermal‐history modelling and permits us to constrain the timing of geological processes including, extended post‐orogenic cooling, enhanced later Proterozoic cooling, and then episodic burial and exhumation in the Palaeozoic–Mesozoic. 相似文献
54.
Reflection moveout inversion in azimuthally anisotropic media: accuracy, limitations and acquisition 总被引:1,自引:0,他引:1
AbdulFattah Al-Dajani Tariq Alkhalifah & Frank Dale Morgan 《Geophysical Prospecting》1999,47(5):735-756
Parameter estimation from the elliptical variations in the normal-moveout (NMO) velocity in azimuthally anisotropic media is sensitive to the angular separation between the survey lines in 2D, or equivalently, the source-to-receiver azimuth in 3D, and to the set of azimuths used in the inversion procedure. The accuracy in estimating the orientation of an NMO ellipse, in particular the parameter α, is also sensitive to the magnitude of anisotropy. On the other hand, the accuracy in estimating the semi-axes of the NMO-velocity ellipse is about the same for any magnitude of anisotropy. To invert for the NMO ellipse parameters at least three NMO-velocity measurements along distinct azimuth directions are needed. In order to maximize the accuracy and stability in parameter estimation, it is best to have the azimuths for the three source-to-receiver directions 60° apart. Having more than three distinct source-to-receiver azimuths (e.g. full azimuthal coverage) provides a useful data redundancy that enhances the quality of the estimates. In order to maximize quality in the inversion process, it is recommended to design the seismic data acquisition such that it contains small sectors (≤10°) with adequate fold and offset distribution. Using three NMO-velocity measurements, 60° apart, an azimuthally anisotropic layer overlain by an azimuthally isotropic overburden (as might occur for fractured reservoirs) should have a relative thickness (in time) with respect to the total thickness at least equal to the ratio of the error in the NMO (stacking) velocity to the interval anisotropy of the fractured layer. Coverage along more than three azimuths, however, improves this limitation, which is imposed by Dix differentiation, by at most 50%, depending on the number of observations (NMO velocities) that enter the inversion procedure. 相似文献
55.
James E. Dale Richard Wünsch Anthony Whitworth Jan Palou 《Monthly notices of the Royal Astronomical Society》2009,398(3):1537-1548
We investigate the gravitational fragmentation of expanding shells in the context of the linear thin-shell analysis. We make use of two very different numerical schemes; the flash adaptive mesh refinement code and a version of the Benz smoothed particle hydrodynamics code. We find that the agreement between the two codes is excellent. We use our numerical results to test the thin-shell approximation and we find that the external pressure applied to the shell has a strong effect on the fragmentation process. In cases where shells are not pressure-confined, the shells thicken as they expand and hydrodynamic flows perpendicular to the plane of the shell suppress fragmentation at short wavelengths. If the shells are pressure-confined internally and externally, so that their thickness remains approximately constant during their expansion, the agreement with the analytical solution is better. 相似文献
56.
William D. Robertson James W. Roy Susan J. Brown Dale R. Van Stempvoort Greg Bickerton 《Ground water》2014,52(1):63-70
Monitoring of a well‐defined septic system groundwater plume and groundwater discharging to two urban streams located in southern Ontario, Canada, provided evidence of natural attenuation of background low level (ng/L) perchlorate (ClO4?) under denitrifying conditions in the field. The septic system site at Long Point contains ClO4? from a mix of waste water, atmospheric deposition, and periodic use of fireworks, while the nitrate plume indicates active denitrification. Plume nitrate (NO3?‐N) concentrations of up to 103 mg/L declined with depth and downgradient of the tile bed due to denitrification and anammox activity, and the plume was almost completely denitrified beyond 35 m from the tile bed. The ClO4? natural attenuation occurs at the site only when NO3?‐N concentrations are <0.3 mg/L, after which ClO4? concentrations decline abruptly from 187 ± 202 to 11 ± 15 ng/L. A similar pattern between NO3?‐N and ClO4? was found in groundwater discharging to the two urban streams. These findings suggest that natural attenuation (i.e., biodegradation) of ClO4? may be commonplace in denitrified aquifers with appropriate electron donors present, and thus, should be considered as a remediation option for ClO4? contaminated groundwater. 相似文献
57.
Jakob Walcher Brent Groves Tamás Budavári Daniel Dale 《Astrophysics and Space Science》2011,331(1):1-51
Fitting the spectral energy distributions (SEDs) of galaxies is an almost universally used technique that has matured significantly
in the last decade. Model predictions and fitting procedures have improved significantly over this time, attempting to keep
up with the vastly increased volume and quality of available data. We review here the field of SED fitting, describing the
modelling of ultraviolet to infrared galaxy SEDs, the creation of multiwavelength data sets, and the methods used to fit model
SEDs to observed galaxy data sets. We touch upon the achievements and challenges in the major ingredients of SED fitting,
with a special emphasis on describing the interplay between the quality of the available data, the quality of the available
models, and the best fitting technique to use in order to obtain a realistic measurement as well as realistic uncertainties.
We conclude that SED fitting can be used effectively to derive a range of physical properties of galaxies, such as redshift,
stellar masses, star formation rates, dust masses, and metallicities, with care taken not to over-interpret the available
data. Yet there still exist many issues such as estimating the age of the oldest stars in a galaxy, finer details of dust
properties and dust-star geometry, and the influences of poorly understood, luminous stellar types and phases. The challenge
for the coming years will be to improve both the models and the observational data sets to resolve these uncertainties. The
present review will be made available on an interactive, moderated web page (), where the community can access and change the text. The intention is to expand the text and keep it up to date over the
coming years. 相似文献
58.
Barclay Clemesha Dale Simonich Paulo Batista 《Journal of Atmospheric and Solar》2010,72(9-10):740-744
Mesopause region temperature measurements made with a sodium resonance lidar show two unexpected features: (1) Strong positive temperature gradients are often associated with strong gradients in the sodium concentration and (2) positive temperature gradients are generally much stronger than negative ones. Although the structures we see frequently appear to be associated with gravity waves or tides, the asymmetrical temperature oscillations cannot be explained as the result of simple wave propagation. We suggest that strong positive temperature gradients correspond to regions of high atmospheric stability, where eddy diffusion is inhibited, permitting the build-up of strong gradients in temperature and minor constituent mixing ratio. 相似文献
59.
L. Bohlen A.W. Dale S. Sommer T. Mosch C. Hensen A. Noffke F. Scholz K. Wallmann 《Geochimica et cosmochimica acta》2011,75(20):6094-7276
Benthic nitrogen (N) cycling was investigated at six stations along a transect traversing the Peruvian oxygen minimum zone (OMZ) at 11°S. An extensive dataset including porewater concentration profiles and in situ benthic fluxes of nitrate (NO3−), nitrite (NO2−) and ammonium (NH4+) was used to constrain a 1-D reaction-transport model designed to simulate and interpret the measured data at each station. Simulated rates of nitrification, denitrification, anammox and dissimilatory nitrate reduction to ammonium (DNRA) by filamentous large sulfur bacteria (e.g. Beggiatoa and Thioploca) were highly variable throughout the OMZ yet clear trends were discernible. On the shelf and upper slope (80-260 m water depth) where extensive areas of bacterial mats were present, DNRA dominated total N turnover (?2.9 mmol N m−2 d−1) and accounted for ?65% of NO3− + NO2− uptake by the sediments from the bottom water. Nonetheless, these sediments did not represent a major sink for dissolved inorganic nitrogen (DIN = NO3− + NO2− + NH4+) since DNRA reduces NO3− and, potentially NO2−, to NH4+. Consequently, the shelf and upper slope sediments were recycling sites for DIN due to relatively low rates of denitrification and high rates of ammonium release from DNRA and ammonification of organic matter. This finding contrasts with the current opinion that sediments underlying OMZs are a strong sink for DIN. Only at greater water depths (300-1000 m) did the sediments become a net sink for DIN. Here, denitrification was the major process (?2 mmol N m−2 d−1) and removed 55-73% of NO3− and NO2− taken up by the sediments, with DNRA and anammox accounting for the remaining fraction. Anammox was of minor importance on the shelf and upper slope yet contributed up to 62% to total N2 production at the 1000 m station. The results indicate that the partitioning of oxidized N (NO3−, NO2−) into DNRA or denitrification is a key factor determining the role of marine sediments as DIN sinks or recycling sites. Consequently, high measured benthic uptake rates of oxidized N within OMZs do not necessarily indicate a loss of fixed N from the marine environment. 相似文献
60.
Dale S. Rothman 《Climatic change》2000,44(3):351-376
Measuring the impact of global change depends critically upon our ability to gauge the impacts of changes at the local and individual level. There are strong philosophical and practical concerns in measuring environmental, health, social, and economic impacts even at this level. Aggregating these across commodities, individuals, sectors, regions, and time presents further difficulties. Overcoming these obstacles oftentimes runs afoul of our inability to make interpersonal and intergenerational comparisons. This paper walks through the process of economic valuation from the very local scale of individual choices up to global aggregations across goods and services, individuals, space, and time. Along the way, important assumptions, particularly those related to analyses across different scales and aggregation from lower to higher scales will be emphasized. The effects of making different assumptions will be noted. Fundamental questions about the 'scientific' versus 'political/moral/ethical' nature of valuation will also be highlighted. 相似文献