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191.
A grid of seismic reflection lines has been used to image basal topography and infer basal conditions and flow processes beneath ~140 km2 of Rutford Ice Stream, West Antarctica. The subglacial topography in this region consists of two troughs flanking a central high and the bed is composed of water-saturated sediments. The two troughs are filled with deforming sediment, whereas the bed in the central region appears to undergo a transition from largely deforming conditions upstream to basal sliding downstream. The deforming bed is very flat along flow, but undulates across flow. Sliding areas show rougher bed topography. Cross-stream bed topography is characterised by streamlined mounds of deforming sediment aligned in the ice flow direction. These bedforms occur superimposed on the bed in regions of both basal sliding and sediment deformation. In places, they form finger-like mounds of material, which extend into the sliding region further downstream. Mean bedform height is 22 m, mean width is 267 m, and many of them extend for at least 1–2 km along flow. We interpret most of these bedforms as drumlins and one as a mega-scale glacial lineation. The juxtaposition of different basal conditions is consistent with models proposed from terrestrial studies in which the glacier bed is a mosaic of stable and deforming bed areas, variable both spatially and temporally. Any theory of subglacial sediment rheology must also be able to account for our conclusion that, at any given time, pervasive deformation extends at least a few metres into the bed and can persist over a considerable area (many km2). Bedform geometry and basal conditions concur with interpretations of former ice streams, with evidence for increasing elongation ratio with distance downstream. However, those studies also identified bedrock cropping out at the ice-bed interface, for which there is no evidence on Rutford Ice Stream. 相似文献
192.
One of the chief claims of the Vaníček Method of spectrum analysis is its, ability to remove ‘systematic noise’ from a time
series with minimal distortion of the spectrum of the remaining series. In this paper, comparisons are made between the Vaníček
Method and the more usual method consisting of preliminary removal of the systematic noise followed by fourier analysis. Formulas
relating the two methods are developed and a series of comparative plots of simple spectra are presented. Results indicate
that the Vaníček Method does not displace a spectral peak falling close to the systematic noise, that it distorts the amplitude
of the peak less than the conventional method of analysis, and that it tends to intensify the side lobes. 相似文献
193.
The oblateness of the Martian upper atmosphere was determined from analysis of photoelectric observations of the 8 April 1976 occultation of ε Geminorum by Mars at seven stations. The oblatness is 0.0096 ± 0.0023, consistent with a mean equator-to-pole temperature difference in excess of ~ 50°K, vertically averaged from the surface to the occulation altitude of ~70 km. The astrometric solution provides precise determination of the occultation path relative to the Martian shadow, and absolute vertical alignment of upper atmospheric temperature profiles obtained by inversion of occultation light curves. The observations can be compared directly with models of atmospheric tides computed for the conditions at the suboccultation regions on Mars. 相似文献
194.
H.A. Taylor R.E. Hartle H.B. Niemann L.H. Brace R.E. Daniell S.J. Bauer A.J. Kliore 《Icarus》1982,51(2):283-295
Across the nightside of Venus, daily measurements from the PV Orbiter Ion Mass Spectrometer often indicate an ionosphere of relatively abundant concentration, with a composition characteristic of the dayside ionosphere. Such conditions are interspersed by other days on which the ionosphere appears to largely “disappear” down to about 200 km, with ion concentrations at lower heights also much reduced. These characteristics, coupled with observations of strong day to night flows of O+ in the upper ionosphere, support arguments that ion transport from the dayside is important for the maintenance of the nightside ionosphere. Also, U.S. and Soviet observations of nightside energetic electron fluxes have prompted consideration of impact ionization as an additional nightside ion source. The details of the ion and neutral composition at low altitudes on the nightside provide an important input for further analysis of the maintenance process. In the range 140–160 km, strong concentrations of O2+ and NO+ indicate that the ionization peak is at times composed of at least two prominent ion species. Nightside concentrations of O2+ and NO+ as large as 105 and 104/cm3, respectively, appear to require sources in addition to that provided by transport. The most probable sources are considered briefly, and no satisfactory explanation is yet found for the observed NO+ concentrations. Further analysis beyond the scope of this paper is required to resolve this issue. 相似文献
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197.
Robert A. Watson Pedro Carreira Kieran Cleary Rod D. Davies Richard J. Davis Clive Dickinson Keith Grainge † Carlos M. Gutiérrez Michael P. Hobson Michael E. Jones Rüdiger Kneissl Anthony Lasenby Klaus Maisinger Guy G. Pooley Rafael Rebolo José Alberto Rubiño-Martin ‡ Ben Rusholme § Richard D. E. Saunders Richard Savage Paul F. Scott Ane Slosar Pedro J. Sosa Molina Angela C. Taylor David Titterington Elizabeth Waldram Althea Wilkinson 《Monthly notices of the Royal Astronomical Society》2003,341(4):1057-1065
198.
199.
A constitutive model is described for the prediction of dynamic crack formation in geological media. The model includes a simple tensile failure criterion for crack formation and a soil plasticity model for both deviatoric and volumetric plasticity. Numerical results are presented which show that the model reproduces the observed dependence of fracture formation on loding rate for dynamic pressure loading of boreholes. The use of the model as a numerical tool for design is demonstrated through two example parameter studies. 相似文献
200.
The notion of an internal boundary layer (IBL) appeared in studies of local advection within the atmospheric boundary layer when air flows over a change in surface conditions. These include surface roughness, thermal and moisture properties. An ability to predict the height of the IBL interface in the atmosphere under neutral stability, accompanied by certain assumptions on the form of the mean flow parameters, have been a means of obtaining information on the velocity profile after step changes in roughness for more than half a century. A compendium of IBL formulae is presented. The approach based on the diffusion analogy of Miyake receives close attention. The empirical expression of Savelyev and Taylor (2001, Boundary Layer Meteorol. 101, 293–301) suggested that turbulent diffusion is not the only factor that influences IBL growth. An argument is offered that an additional element, mean vertical velocity or streamline displacement, should be taken into account. Vertical velocity is parameterized in terms of horizontal velocity differences employing continuity constraints and scaling. Published data are analyzed from a new point of view, which produces two new neutral stratification formulae. The first implies that the roughness lengths of adjacent surfaces are equally important and a combined length scale can be constructed. In addition new formulae to predict the height of the region of diabatic flow affected by a step change in surface conditions are obtained as an extension of the neutral flow case. 相似文献