Individual geopotential coefficients of order 15 and 30, from resonant satellite orbits     

D.G. King-Hele  Doreen M.C. Walker 《Planetary and Space Science》1985,33(2):223-238

The analysis of variations in satellite orbits when they pass through 15th-order resonance (15 revolutions per day) yields values of lumped geopotential harmonics of order 15, and sometimes of order 30. The 15th-order lumped harmonics obtained from 24 such analyses over a wide range of orbital inclinations are used here to determine individual harmonic coefficients of order 15 and degree 15,16,…35; and the 30th-order lumped harmonics (from eight of the analyses) are used to evaluate individual coefficients of order 30 and degree 30,32,…40. The new values should be more accurate than any previously obtained. The accuracy of the 15th-order coefficients of degree 15, 16,…23 is equivalent to 1 cm in geoid height, while the 30th-order coefficients of degree 30, 32 and 34 are determined with an accuracy which is equivalent to better than 2 cm in geoid height. The results are used to assess the accuracy of the Goddard Earth Model 10B.  相似文献   

Variation in Ovii X-ray line-emission from the solar corona     

H. R. Rugge  A. B. C. Walker Jr. 《Solar physics》1970,15(2):372-379

The variation in intensity of the solar X-ray resonance (1s ^{2 1} S ₀ - 1s2p ¹ P ₁), intercombination (1s ^{2 1} S ₀ - 1s2p ³ P ₁), and forbidden (1s^{2 1} S ₀ - 1s2s ³ P ₁) lines of helium-like Ovii with 2800 MHz solar radio flux is presented for three solar rotations. A high correlation (r 0.80) exists between the intensities of all three X-ray lines and the 2800 MHz solar flux. The ratio of the forbidden to the intercombination line intensities is found to be essentially independent of long term solar activity. This ratio is used to determine upper limits on the coronal electron density and to make inferences concerning the change in density with solar activity.  相似文献   

Explosive volcanic eruptions - VI. Ejecta dispersal in plinian eruptions: the control of eruption conditions and atmospheric properties   总被引：5，自引：0，他引：5  

L. Wilson  G. P. L. Walker 《Geophysical Journal International》1987,89(2):657-679

Summary. A simple model is developed to relate the maximum down-wind and cross-wind ranges of pyroclasts forming a plinian airfall deposit to the dynamic processes in the eruption cloud from which they fall and the atmospheric wind conditions in the area. The eruption cloud dynamics are in turn related to the eruptive conditions in the vent (vent radius, exsolved magmatic volatile weight fraction, velocity with which material passes through the vent, and mass eruption rate), some or all of which can be deduced if the appropriate field measurements can be made. Some aspects of the stability of convecting volcanic eruption clouds are investigated, and the effects on eruption cloud height of the local atmospheric temperature profile and the value adopted for the entrainment constant (which relates the horizontal flow speed of atmospheric air entering the column to the vertical rise speed of the column material) are explored. It is confirmed that eruption-cloud rise height and pyroclast dispersal are mainly controlled by the mass eruption rate (per unit length of active fissure in the case of linear vents) and, hence, the heat input rate to the cloud; but a significant subsidiary dependence on the amount of exsolved magma volatiles is also found. The eruption cloud model is validated by application to observed historic eruptions, and its use in the analysis of palaeo-eruptions is discussed.  相似文献   

Sister chromatid exchange induction in cultured peripheral blood leukocytes of a coldwater marine fish     

Helen R. Zakour  Marsha L. Landolt  Richard M. Kocan 《Marine environmental research》1984,14(1-4)

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Multispectral observations complementary to the study of high-energy solar phenomena     

Arthur B. C. Walker Jr. 《Solar physics》1988,118(1-2):209-229

Non-thermal phenomena on the Sun are characterized by the transient acceleration of electrons and ions to energies ranging from several keV to tens of GeV, and the impulsive heating of plasma to temperatures exceeding 5 × 10⁷ K. These energetic processes result in the emission of a broad spectrum of electromagnetic radiation and of high-energy neutrons, as well as the escape of high energy electrons and ions from the acceleration region. The determination of the energy spectrum, polarization, and spatial distribution of these emissions, which contain detailed information on the acceleration and heating process, and the conditions at the sites at which this energy is generated and dissipated, is the principal objective of high-energy solar studies.The study of the evolution of magnetic structures in the solar convection zone and atmosphere which underlie the metastable conditions which precede these energetic processes, of the conditions that trigger the release of energy, and of the impact of the energy released on the solar atmosphere, is most effectively carried out by observations of thermal and quasi-thermal phenomena which precede, coincide with, and follow the impulsive acceleration and heating event itself. Multispectral observations of the phenomena associated with non-thermal events on the Sun are reviewed, and the requirements for visible, ultraviolet, extreme ultraviolet, and soft X-ray observations which are necessary for future advances are briefly described.  相似文献   

A small Soret effect in spreading center gabbros     

David Walker  Stephen E. DeLong 《Contributions to Mineralogy and Petrology》1984,85(2):203-208

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Man's impact on shorelines and nearshore environments: a geomorphological perspective     

H.Jesse Walker 《Geoforum》1984,15(3):395-417

Man evolved late in terms of geologic history, yet he has become a major geomorphologic agent despite his short tenure on Earth. His ability to alter the materials, processes and forms along the world's shorelines are manifested in a variety of ways. Beginning with the indirect changes that resulted from early agriculture and animal husbandry, he progressed to direct modification through reclamation and harbor construction. Today, in the name of shoreline stabilization, thousands of kilometers of natural shoreline have been modified by the addition of artificial structures.  相似文献   

Effects of turbidity caused by nutrient loading on benthic dynamics     

Helen T. Yap  Jrg-Andreas von Oertzen  Ulrich Schiewer 《Continental Shelf Research》1987,7(11-12)

The benthic response to nutrient loading of the water column was determined in the transition from early to mid-summer in a semi-enclosed, shallow estuarine ecosystem of the Baltic Sea. The procedure involved the inoculation of an in situ mesocosm with nitrogen and phosphorus to induce hypertrophic conditions, and comparison of this treated compartment with an untreated control. Sediment activity was assessed as oxygen turn-over, and measured in the laboratory with a simulated in situ polarographic flow-through method. Nutrient addition induced the development of pelagic organisms, particularly phytoplankton and protozoa, which resulted in turbidity within the water column. The latter appeared to depress benthic primary productivity. On the other hand, the greater organic matter load, upon sedimenting, caused increased biological respiration of the sediment. Chemical oxygen demand was unaffected by the increased organic matter input.  相似文献   

Anatomy of the middle ordovician sevier shale basin,eastern Tennessee     

Ganapathy Shanmugam  Kenneth R. Walker 《Sedimentary Geology》1983,34(4):315-337

The Sevier Shale basin in eastern Tennessee comprises one of the thickest clastic sequences (nearly 2500 m) of Middle Ordovician age in North America. The lower one-half of the sequence is composed of Lenoir, Whitesburg, Blockhouse and Sevier Formations, in ascending order. The sequence ranges in age from Whiterockian to lower Wilderness in North American stages.The Middle Ordovician sequence exhibits tidal flat (Mosheim Member of Lenoir Fm.), subtidal (main body of Lenoir Fm.), slope (Whitesburg Fm.), anoxic basin (Blockhouse Fm), turbidite and contourite (Sevier Fm.) facies. The Sevier basin evolved in five stages: First, a widespread marine transgression initiated carbonate-shelf deposition in the study area. Second, a major tectonic downwarping event caused the stable shelf to break and subside rapidly at a rate of 60–65 cm 1000 yrs^?1, and areas of shelf facies became areas of slope and basin facies. Third, global transgressions maintained the deep anoxic conditions for nearly 10 Ma. Fourth, turbidites began to fill the basin from a westward-prograding submarine fan system. Fifth, contour currents reworked the turbidites and progressively ventilated the Sevier basin. The basin-filling process terminated with shallow-water/subaerial clastics at the end of Middle Ordovician.  相似文献   

Evolution of the oceanic plankton     

Helen Tappan  Alfred R. Loeblich 《Earth》1973,9(3):207-240

In its evolution through geologic time, the oceanic plankton has reflected many changes in the physical environment while both causing and being affected by evolutionary change in the associated biota. Temporal variations in the vertical and latitudinal habitat partitioning also affected geochemical balances in the oceans and accumulation rates of biogenic sediments, and perhaps the atmospheric oxygen pressure. Advantages of the oceanic plankton habitat that led to its occupation by a varied sequence of organisms include its extensive geographic area, the possibility for rapid dispersal by currents, the availability of resources (ranging from light, carbon dioxide, and dissolved nutrients for the autotrophic organisms to the various lower trophic levels required by the heterotrophs), and the better oxygenation of the upper water layers.Convergent morphology of the plankton represents adaptations for suspension in the water column, for movement within this water mass to increase nutrient uptake, and for protection against grazing or predation. Necessary attributes for cosmopolitan species include eurythermy and euryhalinity. The patchiness of plankton diversity results from small-scale variations in water masses, the low level of interorganism contact or competition, and the possibility of rapid exploitation of favorable conditions by species present in the local area.Parallel evolution, radiation, and extinction of the various components of the oceanic plankton are similar in pattern to an ecologic succession — early evolution and conditions following major periods of extinction being homologous with the physically controlled or immature modern ecosystem associated with high stress levels and characterized by high net but low gross productivity, low diversity but great intraspecific variability, and small biomass. Evolutionary radiation is associated with more stable physical conditions and, like the seral progression, of an ecosystem, results in high diversity but less intraspecific variability. Increased biological interaction leads to higher gross but lower net productivity, hence more efficient energy and resource utilization. The drop in gross oceanic productivity at times of major extinction is associated with low diversity and decreased rate of accumulation of biogenic ooze, hence the apparent worldwide disconformities, as at the Cretaceous-Tertiary boundary, even in the deep sea.  相似文献