The polarization of Pc5 ULF waves around dawn: A possible ionospheric conductivity gradient effect     

G. Chisham  D. Orr  T. K. Yeoman  D. K. Milling  M. Lester  J. A. Davies 《Annales Geophysicae》1995,13(2):159-167

In this study over 100 Pc5 events observed on the SAMNET magnetometer array have been catalogued. The ground horizontal polarization of these waves has been investigated and it has been shown that there are large differences between the polarization azimuth distributions on either side of noon. In the morning the azimuth distributions are spread evenly across all possible orientations whereas in the afternoon they are predominantly in a North-South orientation. The difference appears to be the result of a specific polarization pattern that occurred across the SAMNET array for 31 of the Pc5 events. These 31 Pc5 events all occurred in the morning sector, close to the dawn terminator. It is suggested that this special polarization pattern is a consequence of ionospheric conductivity gradients that occur at around dawn. This theory is reinforced by the comparison of the local times of these polarization variations with the local time of sunrise. By employing ionospheric conductivity data from the EISCAT radar for two of these Pc5 events, this connection has been studied in more detail.  相似文献   

Late Cretaceous genesis of the Kula plate     

Mark T. Woods  Geoffrey F. Davies 《Earth and Planetary Science Letters》1982,58(2):161-166

We propose that the Pacific-Kula ridge began spreading approximately 85 m.y. B.P., during Late Cretaceous time. Extrapolation of the Great Magnetic Bight backwards in time results in an implausible ridge configuration. This implies that plate velocity vectors for the Pacific, Kula, and Farallon plates were not constant during this interval. Evidence for splitting of the Kula plate from the Pacific plate along the Chinook trough is the relationship between the north-striking Amlia and Adak fracture zones, the Chinook and Emperor troughs, and the magnetic lineations south of the Aleutian trench. If this hypothesis is correct, it will require that Mesozoic reconstructions of the Pacific basin and their relation to Cenozoic reconstructions be re-examined. A previously unrecognized Mesozoic plate may be required. We propose calling this the Izanagi plate.  相似文献   

The reflection of wave energy by undulations on the seabed     

A. G. Davies 《Dynamics of Atmospheres and Oceans》1982,6(4):207-232

The problem of the reflection of incident surface water waves by undulations on the seabed is treated on the basis of linear perturbation theory. The flow is assumed to be two-dimensional, uniplanar and nonseparating, and the undulations are assumed to be of limited horizontal extent, enabling results for the reflected and transmitted waves to be obtained by Fourier transform methods. A steady state formulation of the problem is adopted, and this provides general results which are applied to the case of a patch of sinusoidal undulations on the bed. The principal conclusions are that the reflection coefficient is both oscillatory in the ratio of the length of the patch to the surface wavelength and also, as expected, critically dependent on the ratio of the surface to bed wavelengths. In addition, the transmitted wave may experience a small phase shift, even in cases of zero reflection. Examples are presented which indicate that relatively few bottom undulations, with wavenumber equal to approximately twice the surface wavenumber, may give rise to a very substantial reflected wave. A possible consequence of this is a coupling between ripple growth and wave reflection, which may be important in problems of coastal protection.  相似文献   

Superdeep diamonds from the Juina area, Mato Grosso State, Brazil   总被引：4，自引：1，他引：3  

F. Kaminsky  O. Zakharchenko  R. Davies  W. Griffin  G. Khachatryan-Blinova  A. Shiryaev 《Contributions to Mineralogy and Petrology》2001,140(6):734-753

Alluvial diamonds from the Juina area in Mato Grosso, Brazil, have been characterized in terms of their morphology, syngenetic mineral inclusions, carbon isotopes and nitrogen contents. Morphologically, they are similar to other Brazilian diamonds, showing a strong predominance of rounded dodecahedral crystals. However, other characteristics of the Juina diamonds make them unique. The inclusion parageneses of Juina diamonds are dominated by ultra-high-pressure ("superdeep") phases that differ both from "traditional" syngenetic minerals associated with diamonds and, in detail, from most other superdeep assemblages. Ferropericlase is the dominant inclusion in the Juina diamonds. It coexists with ilmenite, Cr-Ti spinel, a phase with the major-element composition of olivine, and SiO₂. CaSi-perovskite inclusions coexist with titanite (sphene), "olivine" and native Ni. MgSi-perovskite coexists with TAPP (tetragonal almandine-pyrope phase). Majoritic garnet occurs in one diamond, associated with CaTi-perovskite, Mn-ilmenite and an unidentified Si-Mg phase. Neither Cr-pyrope nor Mg-chromite was found as inclusions. The spinel inclusions are low in Cr and Mg, and high in Ti (Cr₂O₃<36.5 wt%, and TiO₂>10 wt%). Most ilmenite inclusions have low MgO contents, and some have very high (up to 11.5 wt%) MnO contents. The rare "olivine" inclusions coexisting with ferropericlase have low Mg# (87-89), and higher Ca, Cr and Zn contents than typical diamond-inclusion olivines. They are interpreted as inverted from spinel-structured (Mg, Fe)₂Si₂O₄. This suite of inclusions is consistent with derivation of most of the diamonds from depths near 670 km, and adds ilmenite and relatively low-Cr, high-Ti spinel to the known phases of the superdeep paragenesis. Diamonds from the Juina area are characterized by a narrow range of carbon isotopic composition ('¹³C=-7.8 to -2.5‰), except for the one majorite-bearing diamond ('¹³C=-11.4‰). There are high proportions of nitrogen-free and low-nitrogen diamonds, and the aggregated B center is predominant in nitrogen-containing diamonds. These observations have practical consequences for diamond exploration: Low-Mg olivine, low-Mg and high-Mn ilmenite, and low-Cr spinel should be included in the list of diamond indicator minerals, and the role of high-Cr, low-Ti spinel as the only spinel associated with diamond, and hence as a criterion of diamond grade in kimberlites, should be reconsidered.  相似文献   

Calibration of the high-pressure cohesive strength meter (CSM)     

S. Vardy  J.E. Saunders  T.J. Tolhurst  P.A. Davies  D.M. Paterson 《Continental Shelf Research》2007

Coastal erosion is an immense economic and social problem that has been receiving increased attention in recent years. A number of devices have been developed to determine the sediment stability in coastal areas: laboratory and field flumes; a range of different erosion devices; shear vanes and fall cone penetrometers. The cohesive strength meter (CSM) erosion device was developed to determine in situ the temporal and spatial variations in the erosion threshold of muddy intertidal sediments. Technological developments have enabled considerable improvements to be made to the original design over the last 15 years.  相似文献   

Large-scale impacts of bottom trawling on shelf primary productivity     

Costas Dounas  Ian Davies  George Triantafyllou  Panayota Koulouri  George Petihakis  Christos Arvanitidis  George Sourlatzis  Anastasios Eleftheriou 《Continental Shelf Research》2007

Disturbance of the seabed resulting from bottom trawling affects ecosystem processes, such as the rate and magnitude of nutrient regeneration. The potential responses of the plankton community arising from such effects can be modelled, provided that reliable data on the effects on nutrient fluxes are available. In a north Cretan outer continental shelf and upper slope fishing ground (Heraklion Bay, Crete, Eastern Mediterranean) we applied a new field instrument which can simulate the passage of trawl groundropes across the sea floor and made direct seasonal measurements of the rate of dissolved and particulate nutrient releases resulting from seabed disturbance. These observational data were then integrated in a 3D ecosystem model. Results revealed that bottom trawling may trigger off considerable productivity pulses, in addition to pulses from the natural seasonal cycle.  相似文献   

On the importance of non-hydrostatic processes in determining tidally induced mixing in sill regions     

Jiuxing Xing  Alan M. Davies 《Continental Shelf Research》2007

The importance of using a non-hydrostatic model to compute tidally induced mixing and flow in the region of a sill is examined using idealized topography representing the sill at the entrance to Loch Etive. This site is chosen since detailed measurements were recently made there. Calculations are performed with and without the inclusion of non-hydrostatic dynamics using a vertical slice model for a range of sill widths corresponding to typical sill regions. Initial non-hydrostatic calculations showed that the model could reproduce the observed flow characteristics in the region. However, when calculations were performed using the model in hydrostatic form, the significant artificial convective mixing that occurred in order to remove density inversions led to excessively high vertical mixing. This influenced the computed temperature field and the intensity of the current jet that separated from the sill on its lee side. In addition it affected the magnitude and spatial characteristics of the lee waves generated on the lee side of the sill. Calculations with a range of sill widths, showed that as the sill width decreased the difference between the solution computed with the non-hydrostatic and hydrostatic model increased.  相似文献   

SOURCE RADIATION AND RESPONSES OF WAVE PROPAGATION   总被引：2，自引：0，他引：2  

CHEN Sheng-zao  Gail M Atkinson 《大地构造与成矿学(英文版)》2001,25(1)

Recordings of seismic waves propagating from earthquake source to a station at the earth's surface are a system response function.The convolution operator in time domain can be simplified as a multiplication operator in frequency domain.We discuss in frequency domain the separation of source,path and site effects for global scaling of earthquake source radiation.Also discussed are source scaling model,faulting mechanism,and the H/V inversion problems with crustal and near surface structures.Gross features of apparent source spectra appear to be not much region-dependent although there may be difference between tectonic styles within a region of tectonic mixture for which we need further study as data accumulate.Vertical spectra may be a better approach to approximate source radiation,as it has less crustal amplification effects than horizontal spectra.The H/V ratio is evidently a comprehensive indicator of amplification effects from near surface to deep structure.This gives it potential as an inversion tool to deduce site crustal structure.  相似文献   

Processing of oxidised nitrogen compounds by passage through winter-time orographic cloud     

R. N. Colvile  T. W. Choularton  J. N. Cape  B. J. Bandy  K. N. Bower  R. A. Burgess  T. J. Davies  G. J. Dollard  M. W. Gallagher  K. J. Hargreaves  B. M. R. Jones  S. A. Penkett  R. L. Storeton-West 《Journal of Atmospheric Chemistry》1996,24(3):211-239

Four case studies are described, from a three-site field experiment in October/November 1991 using the Great Dun Fell flow-through reactor hill cap cloud in rural Northern England. Measurements of total odd-nitrogen nitrogen oxides (NO _y ) made on either side of the hill, before and after the air flowed through the cloud, showed that 10 to 50% of the NO _y , called NO _z , was neither NO nor NO₂. This NO _z failed to exhibit a diurnal variation and was often higher after passage through cloud than before. No evidence of conversion of NO _z to NO₃ ^- in cloud was found. A simple box model of gas-phase chemistry in air before it reached the cloud, including scavenging of NO₃ and N₂O₅ by aerosol of surface area proportional to the NO₂ mixing ratio, shows that NO₃ and N₂O₅ may build up in the boundary layer by night only if stable stratification insulates the air from emissions of NO. This may explain the lack of evidence for N₂O₅ forming NO₃ ^- in cloud under well-mixed conditions in 1991, in contrast with observations under stably stratified conditions during previous experiments when evidence of N₂O₅ was found. Inside the cloud, some variations in the calculated total atmospheric loading of HNO₂ and the cloud liquid water content were related to each other. Also, indications of conversion of NO _x to NO _z were found. To explain these observations, scavenging of NO _x and HNO₂ by cloud droplets and/or aqueous-phase oxidation of NO₂ ^- by nitrate radicals are considered. When cloud acidity was being produced by aqueous-phase oxidation of NO _x or SO₂, NO₃ ^- which had entered the cloud as aerosol particles was liberated as HNO₃ vapour. When no aqueous-phase production of acidity was occurring, the reverse, conversion of scavenged HNO₃ to particulate NO₃ ^-, was observed.  相似文献   

Soil sorption of caesium modelled by the Langmuir and Freundlich isotherm equations   总被引：1，自引：0，他引：1  

Linda S. Campbell  B.E. Davies 《Applied Geochemistry》1995,10(6)

The Langmuir and Freundlich adsorption isotherm equations were used to model Cs sorption in 6 Welsh soils. The pH (in water), of the soils ranged from 3.5 to 6.5, and the estimated organic matter (determined by loss-on-ignition) ranged from 4.8% to 46%. According to the Langmuir adsorption isotherm, highest sorption maxima,b, were displayed by the loams at 102–204 mg kg⁻¹, compared with organic soils at 37–86 mg kg⁻¹. Subsoil horizons (15–30 cm depth) did not always display higher sorption maxima than their topsoil (0–15 cm depth) counterparts. Where an improved fit to the Freundlich model was found over the Langmuir model, it was assumed that either concurrent desorption of other species, or bond-energy heterogeneity in the substrate, were significant for these soils.  相似文献