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451.
Summary. Magnetic variations at high latitudes represent the combination of magnetic perturbations from several different current systems. The relative effects of each of the contributory current systems vary from event to event, resulting in changes in magnetic perturbation patterns in a given local time sector which are a function of several variables, e.g. level of magnetospheric activity, spectrum of precipitating particles responsible for ionospheric conductivity, configuration of interplanetary magnetic field, etc. In this paper we extend the work of Hughes & Rostoker in which the steady state perturbation pattern in each local time sector was established from ground-based meridian line magnetometer data. We shall define the various current systems which contribute to the high-latitude magnetic perturbation pattern and we shall construct the model composite current system which best explains the high-latitude observations in the winter months. We shall discuss changes in this pattern as a function of season and as a function of the configuration of the interplanetary magnetic field. Finally, we shall relate the geometrical configuration of the ionosphere—magnetosphere current flow to the convection of plasma in the magnetosphere. 相似文献
452.
J. L. Hughes 《Astrophysics and Space Science》1977,46(2):L15-L18
It is now possible to test Dirac's Large Numbers hypothesis in the Earth-Mars system using microwave ranging techniques such as those associated with the Viking lander. The consequences of such a test are discussed. 相似文献
453.
Characterisation of mine waste rock with respect to acid generation potential is a necessary part of routine mine operations,
so that environmentally benign waste rock stacks can be constructed for permanent storage. Standard static characterisation
techniques, such as acid neutralisation capacity (ANC), maximum potential acidity, and associated acid–base accounting, require
laboratory tests that can be difficult to obtain rapidly at remote mine sites. We show that a combination of paste pH and
a simple portable carbonate dissolution test, both techniques that can be done in the field in a 15 min time-frame, is useful
for distinguishing rocks that are potentially acid-forming from those that are acid-neutralising. Use of these techniques
could allow characterisation of mine wastes at the metre scale during mine excavation operations. Our application of these
techniques to pyrite-bearing (total S = 1–4 wt%) but variably calcareous coal mine overburden shows that there is a strong
correlation between the portable carbonate dissolution technique and laboratory-determined ANC measurements (range of 0–10 wt%
calcite equivalent). Paste pH measurements on the same rocks are bimodal, with high-sulphur, low-calcite rocks yielding pH
near 3 after 10 min, whereas high-ANC rocks yield paste pH of 7–8. In our coal mine example, the field tests were most effective
when used in conjunction with stratigraphy. However, the same field tests have potential for routine use in any mine in which
distinction of acid-generating rocks from acid-neutralising rocks is required. Calibration of field-based acid–base accounting
characteristics of the rocks with laboratory-based static and/or kinetic tests is still necessary. 相似文献
454.
Recent experiments in macrophyte dominated communities on the relationship between biological diversity and ecosystem functioning
suggest that effects and mechanisms of genetic-genotypic and species diversity are analogous. As previously shown for species
diversity, genotypic diversity enhances ecosystem productivity and recovery from disturbance. These findings generalize ecological
theory, and provide an empirical basis for explicitly considering the maintenance of genetic or genotypic diversity for conservation
strategies. Macrophyte systems such as seagrasses or salt-marshes may be excellent systems to test the interaction between
diversity across several (genetic versus species) levels of biological organization because they are relatively species poor
while simultaneously allowing the manipulation of genotypic diversity by taking advantage of clonal propagation in many species. 相似文献
455.
This study documents two different modes of berm development: (1) vertical growth at spring tides or following significant beach cut due to substantial swash overtopping, and (2) horizontal progradation at neap tides through the formation of a proto-berm located lower and further seaward of the principal berm. Concurrent high-frequency measurements of bed elevation and the associated wave runup distribution reveal the details of each of these berm growth modes. In mode 1 sediment is eroded from the inner surf and lower swash zone where swash interactions are prevalent. The net transport of this sediment is landward only, resulting in accretion onto the upper beach face and over the berm crest. The final outcome is a steepening of the beach face gradient, a change in the profile shape towards concave and rapid vertical and horizontal growth of the berm. In mode 2 sediment is eroded from the lower two-thirds of the active swash zone during the rising tide and is transported both landward and seaward. On the falling tide sediment is eroded from the inner surf and transported landward to backfill the zone eroded on the rising tide. The net result is relatively slow steepening of the beach face, a change of the profile shape towards convex, and horizontal progradation through the formation of a neap berm. The primary factor determining which mode of berm growth occurs is the presence or absence of swash overtopping at the time of sediment accumulation on the beach face. This depends on the current phase of the spring-neap tide cycle, the wave runup height (and indirectly offshore wave conditions) and the height of the pre-existing berm. A conceptual model for berm morphodynamics is presented, based on sediment transport shape functions measured during the two modes of berm growth. 相似文献
456.
Yan Zhao Carl D. Sayer Hilary H. Birks Michael Hughes Sylvia M. Peglar 《Journal of Paleolimnology》2006,35(2):335-350
We have explored the contemporary spatial relationship between aquatic vegetation and surficial macrofossil and pollen remains
in a small, shallow, English lake. A detailed point-based (n = 87) underwater vegetation survey was undertaken in the middle of the plant-growing season in July 2000. Then following
plant die-back in November 2000, surface sediment samples (upper 1.5 cm) were collected from 30 of these plant survey points
and analysed for plant macro-remains (all 30 samples), and pollen (4 evenly spaced samples). All data were stored as separate
layers in a geographical information system and spatial relationships between the aquatic vegetation and plant remains were
explored. In contrast to pollen types, plant macrofossils were not evenly dispersed across all parts of the lake and, with
the exception of Chara oospores, higher concentrations of remains (particularly for Potamogeton) were found close to areas of source-plant dominance. The spatial pattern of macrophyte–macrofossil relationships revealed
that vegetative remains (particularly leaf fragments) were probably deposited at source, whereas seeds were recovered close
to the shore suggesting slightly wider dispersal. Overall, however, macro-remains best represented local ‘patch-scale’ vegetation
within 20–30 m of the core site. The macro-remains effectively recorded the dominant plants in the lake with 63% of samples
containing a combination of remains of Chara, Elodea, and Potamogeton. However, relationships between macrophytes and fossils were complex. Some species were significantly over-represented by
macrofossils (e.g., Chara spp., Nitella flexilis agg., and Zannichellia palustris), while others were either under-represented (e.g., Potamogeton spp.), or not represented at all (e.g., Lemna trisulca). Pollen represented macrophyte diversity poorly, but some taxa were found (e.g., Myriophyllum spicatum, Ceratophyllum demersum) that were not recorded by macro-remains. We conclude that macrofossil analysis may be very usefully employed to determine
the dominant taxa in past aquatic plant communities of shallow, productive lakes and that the addition of pollen analysis
provides further information on former species richness. 相似文献
457.
ZHAO Jianhu ZHANG Hongmei John E. Hughes Clarke 《地球空间信息科学学报》2007,10(4):250-254
To overcome the shortcomings of the traditional multibeam survey and data processing, a new method is presented for the precise determination of the instantaneous height at the multibeam transducer by ... 相似文献
458.
Philip. D. Hughes 《地球表面变化过程与地形》2007,32(10):1593-1602
A small cirque glacier is present in a north‐east facing cirque on the mountain of ?ljeme (2455 m a.s.l.) in the Durmitor massif, Montenegro. Sediment ridges in front of this glacier are interpreted as moraines and can be attributed to recent glacier activity over the past 130 years. Lichenometry was applied to determine the relative ages of different moraines using Aspicilia calcarea agg. One set of moraines dates from about 1878 AD, whilst another set of inner moraines dates from about 1904 AD. The most recent moraines appear to have formed within the last 11 years in front of an active glacier margin. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
459.
Most of the step-by-step time integration algorithms for structural dynamics require an initial acceleration vector to be specified, in addition to displacement and velocity vectors. A consistent initial acceleration vector may be calculated by solving the equations of motion at the initial time, while a truncated initial acceleration vector is obtained by setting the acceleration values to zero. Although the truncated starting procedure decreases computational effort, it is shown to affect accuracy adversely. For the structural dynamics algorithms considered herein, the rate at which the numerical solution converges to the exact solution is O(Δt) when the truncated starting procedure is used, compared to O(Δt2) when consistent initial acceleration values are used. 相似文献
460.