Pc1-2 Discrete regular daytime pulsation bursts at high latitudes     

R.J. Morris  K.D. Cole 《Planetary and Space Science》1985,33(1):53-67

A family of related Pc1-2 (0.2–10 s) discrete daytime geomagnetic pulsations is presented using pulsation data obtained at Davis, Antarctica, a typical polar-cap station. The morphological properties of IPRP and Pclb pulsation regimes, which maximize in amplitude and frequency of occurrence under the projection of the polar cusp, are examined. Furthermore, two other variations of discrete pulsation bursts yet to be named are also presented, viz IPFP (Intervals of Pulsations with Falling Period) and IPAP (Intervals of Pulsations with Alternating Period) which are observed on rare occasions. It is also suggested that the Pc1b (0.2–5 s) should be extended to incorporate Pc2b (5–10 s) which from the results in this paper are physically the same phenomenon and could be collectively classified as IPCP (Intervals of Pulsations with Constant Period).  相似文献   

Numerical simulation of comet nuclei I. Water-ice comets     

G. Herman  M. Podolak 《Icarus》1985,61(2):252-266

A one-dimensional simulation of pure water-ice cometary nuclei is presented, and the effect of the nucleus as a heat reservoir is considered. The phase transition from amorphous to crystalline ice is studied for two cases: (1) where the released latent heat goes entirely into heating adjacent layers and (2) where the released latent heat goes entirely into sublimation. For a Halley-like orbit it was found that for case 1 the phase boundary penetrates about 15 m on the first orbit and does not advance until sublimation brings the surface to some 10 m from the phase boundary. For case 2 the phase boundary penetrates about 1 m below the surface and remains at this depth as the surface sublimates. For an orbit like that of Schwassmann-Wachmann 1 the phase boundary penetrates about 50 m initially for case 1 and about 1 m for case 2. There is no further transformation until the entire comet is heated slowly to near the transition temperature, after which the entire nucleus is converted to crystalline ice. For an Encke-type orbit case 1 gives a nearly continuous transition of the entire nucleus to crystalline ice, while for case 2 the initial penetration is about 8 m and remains at this depth relative to the surface as sublimation decreases the cometary radius. Thus the entire comet is converted to crystalline ice just before it is completely dissipated.  相似文献   

Ice‐cover variability on shallow lakes at high latitudes: model simulations and observations     

Claude R. Duguay  Greg M. Flato  Martin O. Jeffries  Patrick Mnard  Kim Morris  Wayne R. Rouse 《水文研究》2003,17(17):3465-3483

A one‐dimensional thermodynamic model for simulating lake‐ice phenology is presented and evaluated. The model can be driven with observed daily or hourly atmospheric forcing of air temperature, relative humidity, wind speed, cloud amount and snowfall. In addition to computing the energy balance components, key model output includes the temperature profile at an arbitrary number of levels within the ice/snow (or the water temperature if there is no ice) and ice thickness (clear ice and snow‐ice) on a daily basis, as well as freeze‐up and break‐up dates. The lake‐ice model is used to simulate ice‐growth processes on shallow lakes in arctic, sub‐arctic, and high‐boreal forest environments. Model output is compared with field and remote sensing observations gathered over several ice seasons. Simulated ice thickness, including snow‐ice formation, compares favourably with field measurements. Ice‐on and ice‐off dates are also well simulated when compared with field and satellite observations, with a mean absolute difference of 2 days. Model simulations and observations illustrate the key role that snow cover plays on the seasonal evolution of ice thickness and the timing of spring break‐up. It is also shown that lake morphometry, depth in particular, is a determinant of ice‐off dates for shallow lakes at high latitudes. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

Europan surface phenomena     

Aharon Eviatar  Akiva Bar-nun  Morris Podolak 《Icarus》1985,61(2):185-191

The interaction of corotating iogenic plasma with the surface of Europa in light of recent ice sputtering, experimental results, and published Voyager data has been examined. It has been found that the residual atmosphere of Europa is made up of sputtered molecular oxygen and is exospheric from the surface outwards. It was also found that if sputtering, redistribution, and escape are considered and the sulfur dioxide/water mixing ratio is held constant over a UV observing depth, the observed sulfur dioxide density on the trailing hemisphere lends support to the hypothesis that liquid water from the interior of Europa is boiling out and being deposited as a frost layer on the surface at the rate of about 0.04 μm/year.  相似文献   

Numerical simulations of comet nuclei: II. The effect of the dust mantle     

M. Podolak  G. Herman 《Icarus》1985,61(2):267-277

The insulating effect of an evolving dust mantle is examined. The role of this mantle in determining the surface temperature of the ice core is studied as a function of the mass fraction of the dust in the ice-dust mixture and the thermal conductivity of the nucleus. Using the so-called “looselattice” model of D.A. Mendis and G.D. Brin (1977, Moon17, 359–372) (which was also extended to include cracks and pores in the mantle), it was found that both high dust to ice ratios and high core conductivities inhibit mantle blowoff. Indeed, it is often possible to build an essentially permanent dust mantle around an ice nucleus, so that the nucleus will take on an asteroidal appearance.  相似文献   

Anoxic nitrification: Evidence from Humber Estuary sediments (UK)   总被引：3，自引：0，他引：3  

Rebecca Bartlett  Robert J.G. Mortimer  Katherine Morris 《Chemical Geology》2008,250(1-4):29-39

Conventional understanding of the nitrogen cycle in marine sediments has changed in recent years with the discovery of an alternative pathway for ammonia oxidation via the reduction of manganese oxides (during anoxic nitrification). In anoxic sediments, the potential for manganese oxides to serve as oxidant for nitrification may be considerable yet previous work on manganese-rich sediments has suggested anoxic nitrification may not be significant. In this study, the potential for anoxic nitrification in a range of sediment types was investigated. Laboratory incubation of sediment from three sites on the Humber Estuary, a microbially diverse environment, showed anoxic accumulation of nitrate, nitrite and dinitrogen gas, with and without the addition of synthetic manganese oxides. Incubation experiments confirmed anoxic nitrification as microbially mediated, with heat-killed controls yielding negative results. The anoxic nitrification reaction significantly depleted ammonia concentrations, and occurred simultaneously with manganese-, iron- and sulphate reduction, and methanogenesis. Taken in conjunction with other studies, results suggest anoxic nitrification may not only be dependent on total manganese concentrations but on manganese dynamics. Anoxic nitrification may be explained as a non-steady state reaction, dependent on the recent stability of a sediment system. Physical perturbation of sediments may cause the redistribution and/or introduction of manganese oxides and promote anoxic nitrification. The significance and persistence of anoxic nitrification is likely to depend on the frequency and magnitude of sediment perturbation, which explains why the reaction varies so widely across studied sites, and why it may not occur in some manganese-rich sediment.  相似文献   

Phthalic acid complexation and the dissolution of forsteritic glass studied via in situ FTIR and X-ray scattering     

Peter M. Morris 《Geochimica et cosmochimica acta》2008,72(8):1970-1985

Multiple Internal Reflection Fourier Transform Infra-Red (MIR-FTIR) spectroscopy was developed and used for in situ flow-through experiments designed to study the process of organic acid promoted silicate dissolution. In tandem with the FTIR analysis, ex situ X-ray scattering was used to perform detailed analyses of the changes in the surface structure and chemistry resulting from the dissolution process. Phthalic acid and forsteritic glass that had been Chemically Vapour Deposited (CVD) onto an internal reflection element were used as reactants, and the MIR-FTIR results showed that phthalic acid may promote dissolution by directly binding to exposed Mg metal ion centers on the solid surface. Integrated infrared absorption intensity as a function of time shows that phthalic acid attachment apparently follows a t^1/2 dependence, indicating that attachment is a diffusive process. The diffusion coefficient of phthalic acid was estimated to be approximately 7 × 10⁻⁶ cm² s⁻¹ in the solution near the interface with the glass. Shifts in the infrared absorption structure of the phthalate complexed with the surface compared to the solute species indicate that phthalate forms a seven-membered ring chelate complex. This bidentate complex efficiently depletes Mg from the glass surface, such that after reaction as much as 95% of the Mg may be removed. Surface depletion in Mg causes adsorbate density to fall after an initial attachment stage for the organic ligand. In addition, the infrared analysis shows that silica in the near surface polymerizes after Mg removal, presumably to maintain charge balance. X-ray reflectivity shows that the dissolution rate of forsteritic glass at pH 4 based on Mg removal in such flow-through experiments was equal to 4 × 10⁻¹² mol cm⁻² s⁻¹ (geometric surface area normalized). Reflectivity also shows how the surface mass density decreases during reaction from 2.64 g cm⁻³ to 2.2 g cm⁻³, consistent with preferential loss of Mg from the surface. Auxiliary batch experiments with forsteritic glass films deposited onto soda glass were also completed to add further constraints to the mechanism of reaction. By combining reflectivity with diffuse scatter measurements it is shown that the primary interface changes little in terms of atomic-scale roughness even after removal of several hundred angstroms of material. These measurements unequivocally show how a dicarboxylic acid bonds to and may chelate the dissolution of a magnesium-bearing silicate. At the molecular level the solid surface retreat may best be described by a depinning model where Mg is preferentially removed and residual silica tetrahedra polymerize and act to episodically “pin” the surface.  相似文献   

Tracing groundwater flow and sources of organic carbon in sandstone aquifers using fluorescence properties of dissolved organic matter (DOM)   总被引：7，自引：0，他引：7  

D.J. Lapworth  D.C. Gooddy  A.S. Butcher  B.L. Morris 《Applied Geochemistry》2008,23(12):3384-3390

The fluorescence properties of groundwaters from sites in two UK aquifers, the Penrith Sandstone of Cumbria and the Sherwood Sandstone of South Yorkshire, were investigated using excitation–emission matrix (EEM) fluorescence spectroscopy. Both aquifers are regionally important sources of public supply water and are locally impacted by pollution. The Penrith Sandstone site is in a rural setting while the Sherwood Sandstone site is in suburban Doncaster. Fluorescence analysis of samples from discrete sample depths in the Penrith Sandstone shows decreasing fulvic-like intensities with depth and also shows a good correlation with CFC-12, an anthropogenic groundwater tracer. Tryptophan-like fluorescence centres in the depth profile may also provide evidence of rapid routing of relatively recent applications of organic slurry along fractures. Fluorescence analysis of groundwater sampled from multi-level piezometers installed within the Sherwood Sandstone aquifer also shows regions of tryptophan-like and relatively higher fulvic-like signatures. The fluorescence intensity profile in the piezometers shows tryptophan-like peaks at depths in excess of 50 m and mirrors the pattern exhibited by microbial species and CFCs highlighting the deep and rapid penetration of modern recharge due to rapid fracture flow. Fluorescence analysis has allowed the rapid assessment of different types and relative abundances of dissolved organic matter (DOM), and the fingerprinting of different sources of organic C within the groundwater system. The tryptophan:fulvic ratios found in the Penrith Sandstone were found to be between 0.5 and 3.0 and are characteristic of ratios from sheep waste sources. The Sherwood Sandstone has the lowest ratios (0.2–0.4) indicating a different source of DOM, most likely a mixture of terrestrial and microbial sources, although there is little evidence of pollution from leaking urban sewage systems. Results from these two studies suggest that intrinsic fluorescence may be used as a proxy for, or complementary tool to, other groundwater investigation methods in helping provide a conceptual model of groundwater flow and identifying different sources of DOM within the groundwater system.  相似文献   

Acute toxicity of bromochlorinated seawater to selected estuarine species with a comparison to chlorinated seawater toxicity     

Morris H. Roberts  Jr.  Richard A. Gleeson 《Marine environmental research》1978,1(1):19-30

The acute toxicity of bromochlorinated estuarine water (ca. 20%) was determined for several estuarine organisms. The most sensitive species were oysters (Crassostrea virginica, larvae and juveniles) and copepods (Acartia tonsa) with 48-h LC₅₀'s of 0·10 to 0·21 mg BrCl/litre. Palaemonetes pugio was most tolerant with a 96-h LC₅₀ of 0·70 mg BrCl/litre. The fish species tested (Menidia menidia, Brevoortia tyrannus and Leiostomus xanthurus) all had a 96-h LC₅₀ of 0·21–0·23 mg BrCl/litre.The BrCl toxicity data are compared with Cl₂ toxicity data for the same species. When the LC₅₀'s are expressed as equivalents per litre, BrCl is found to be two to four times less toxic than Cl₂. The ranking of species in terms of sensitivity is the same for both disinfectants.Some data are provided concerning the decay rates of BrCl and Cl₂ in estuarine water. BrCl was found to decay more rapidly than Cl₂ at higher ammonia levels (0·25 mg NH₄-N/litre). The question of chemical speciation is discussed with particular reference to the differential toxicities.  相似文献   

Biological characteristics of a resident population of Atlantic cod (Gadus morhua L.) in southern Labrador     

Morris  Corey J.; Green  John M. 《ICES Journal of Marine Science》2002,59(4):666-678

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