The viscous overstability, nonlinear wavetrains, and finescale structure in dense planetary rings     

Henrik N. Latter  Gordon I. Ogilvie 《Icarus》2009,202(2):565-583

This paper addresses the fine-scale axisymmetric structure exhibited in Saturn's A and B-rings. We aim to explain both the periodic microstructure on 150-220 m, revealed by the Cassini UVIS and RSS instruments, and the irregular variations in brightness on 1-10 km, reported by the Cassini ISS. We propose that the former structures correspond to the peaks and troughs of the nonlinear wavetrains that form naturally in a viscously overstable disk. The latter variations on longer scales may correspond to modulations and defects in the wavetrains' amplitudes and wavelength. We explore these ideas using a simple hydrodynamical model which captures the correct qualitative behaviour of a disk of inelastically colliding particles, while also permitting us to make progress with analytic and semi-analytic techniques. Specifically, we calculate a family of travelling nonlinear density waves and determine their stability properties. Detailed numerical simulations that confirm our basic results will appear in a following paper.  相似文献   

ESEEM and multi-frequency EPR study on Mn2+ luminescence centres in leucophanite     

Hassane El Mkami  Henrik Friis  Adrian A. Finch  G. M. Smith 《Physics and Chemistry of Minerals》2010,37(8):519-528

The location and coordination of Mn²⁺ paramagnetic luminescent centres in three natural samples of leucophanite (ideally, NaCaBeSi₂O₆F) have been investigated by electron paramagnetic resonance (EPR) and electron spin-echo envelope modulation (ESEEM) spectroscopy. Continuous wave (cw)-EPR was carried out at multiple frequencies (9, 94 and 188 GHz) which allowed the precise determination of the spin Hamiltonian parameters. The simulation of the cw-EPR spectra shows that the signal originates from one manganese located in a single distorted site. This distortion was close to axial symmetry with a slight rhombicity and was consistent with an eight coordinated Mn²⁺ substituting for Ca²⁺. Spectral differences between the samples could be attributed mainly to D-strain effect. Three-pulse ESEEM spectroscopy was also used to investigate the coordination environment of the manganese and showed weak coupling between the manganese and beryllium (⁹Be), sodium (²³Na) and fluorine (¹⁹F) nuclei in the second and subsequent coordination spheres. The Mn ligand distances estimated from the ESEEM data are in good agreement with bond distances estimated from X-ray diffraction data. To our best knowledge, only few ESEEM studies have been reported so far in Earth science and this is believed to be the first detailed ESEEM study of Mn²⁺ in a natural mineral system.  相似文献   

Analysis of limestone micromechanical properties by optical microscopy     

Lucas R.D. Jensen  Henrik Friis  Erling Fundal  Per Møller  Mads Jespersen 《Engineering Geology》2010,110(3-4):43-50

Four dense Scandinavian limestones were analyzed to determine their mechanical properties. The generation of dust (? 10 μm) and fines (? 90 μm) during a closed circuit vertical roller mill comminution process was correlated with the calcite crystal size distributions of each limestone. Thin sections were analyzed and by means of stereology the calcite crystal size distributions for each limestone was measured. The dust generation of limestones is governed by a surface abrasive mechanism (R² = 0.99) and the production of fines is governed by the mechanical strength of limestones (R² = 0.99). The overall limestone degradation mechanism is predominantly controlled by the calcite cleavage planes which reduce the power consumption during the comminution process. This study is the first step in determining the influence of limestone texture on the wear rates in heterogenous raw mixes used in closed circuit comminution equipment.  相似文献   

Bias aware Kalman filters: Comparison and improvements   总被引：1，自引：0，他引：1  

Jean-Philippe Drécourt  Henrik Madsen  Dan Rosbjerg 《Advances in water resources》2006

This paper reviews two different approaches that have been proposed to tackle the problems of model bias with the Kalman filter: the use of a colored noise model and the implementation of a separate bias filter. Both filters are implemented with and without feedback of the bias into the model state. The colored noise filter formulation is extended to correct both time correlated and uncorrelated model error components. A more stable version of the separate filter without feedback is presented. The filters are implemented in an ensemble framework using Latin hypercube sampling. The techniques are illustrated on a simple one-dimensional groundwater problem. The results show that the presented filters outperform the standard Kalman filter and that the implementations with bias feedback work in more general conditions than the implementations without feedback.  相似文献   

Preparation of a Synthetic Titanite Glass Calibration Material for In Situ Microanalysis by Direct Fusion in Graphite Electrodes: A Preliminary Characterisation by EPMA and LA-ICP-MS   总被引：1，自引：2，他引：1  

Magne Ødegård  Øyvind Skår  Henrik Schiellerup  Norman J. Pearson 《Geostandards and Geoanalytical Research》2005,29(2):197-209

This paper describes a technique for the preparation of a titanite (CaTiSiO₅) glass calibration material for use in in situ microanalysis of major, minor, and trace elements in geological materials. The starting composition was a titanite matrix doped with minor and trace elements at ∼ 200 μg g^-1. The elements Sc, Y, REEs, Th and U were added in the form of nitrates in solution, and the elements V, Cr, Mn, Fe, Co, Ni, Zr, Nb, Hf and W were added as solid oxides. The synthetic titanite glass was produced by direct fusion by resistance heating in graphite electrodes at 1600-1700 °C, and quenched in air. Backscattered electron images indicate good homogeneity, with no signs of separate phases or vesicles, and analysis of the major elements Ca, Ti and Si by electron microprobe showed relative standard deviations between 0.5 and 0.7%, based on six independent measurements. Deviations from nominal concentrations for Ca, Si and Ti were measured to -1.2, -3.3 and -0.8%, respectively. The homogeneity of the trace elements in the glass was assessed by LA-ICP-MS analyses, using NIST SRM 610, 612 and 616 as external calibrators, and Ca as the internal standard element. Determinations were made both with a quadrupole mass spectrometer and a sector field instrument, and both raster and spot modes of analysis were used. For the majority of doped elements, precision was better than 10%, and relative deviations from nominal values were, with few exceptions, between 5 and 10%.  相似文献   

Geomorphic evolution of small river–lake‐systems in northeast Germany during the Late Quaternary     

Knut Kaiser  Henrik Rother  Sebastian Lorenz  Peter Gärtner  Ralf Papenroth 《地球表面变化过程与地形》2007,32(10):1516-1532

This study investigates the post‐glacial development of four small river–lake systems in the Weichselian belt of northern central Europe. The valleys investigated are part of an immature drainage system characterized by frequent and abrupt changes in flow direction and the presence of numerous stagnant‐ice depressions in the valley course. The depressions contain thick sedimentary sequences which provide excellent archives for the reconstruction of the post‐glacial valley development. Study results indicate that the valleys reuse segments of former subglacial meltwater channels. During the Late Pleniglacial these channels carried meltwater streams. Stagnant‐ice melting occurred in stages from the Oldest Dryas to the early Holocene and was often followed by the formation of lakes in the valley course. Flow reversals occurred during the Late‐glacial–Holocene transition and were in response to general base‐level lowering caused by stagnant‐ice melting, headwater erosion and lake overspills. Lacustrine deposition typically started during the early Late‐glacial comprising mainly silicate gyttjas, whereas organic gyttjas and peats accumulated during the Allerød. The Younger Dryas is associated with a marked increase in fluvial and aeolian sedimentation, and lake‐level high stands. This was followed by early Holocene lake‐level low stands and a subsequent stabilization phase with decreasing silicate input and increasing organic lacustrine deposition. In general, dramatic changes in Late Pleniglacial to early Holocene sedimentation suggest that small‐scale catastrophic events played a more important role in triggering geomorphic changes then previously recognized. Infilling continued until peat accumulation and terrestrialization of lake basins became widespread during the mid‐ to late Holocene. Beginning in the late Holocene anthropogenic influences become important mainly involving an increase in sediment supply due to forest clearing and land use, followed by mill stowage, river course correction and anthropogenic lake‐level manipulations. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

Cosmoclimatology: a new theory emerges   总被引：1，自引：0，他引：1  

Henrik Svensmark 《Astronomy& Geophysics》2007,48(1):1.18-1.24

Henrik Svensmark draws attention to an overlooked mechanism of climate change: clouds seeded by cosmic rays.  相似文献   

Southwest Scandinavia, 40–15 kyr BP: palaeogeography and environmental change     

Michael Houmark‐Nielsen  Kurt Henrik Kjr 《第四纪科学杂志》2003,18(8):769-786

Twelve palaeogeographical reconstructions illustrate environmental changes at the southwest rim of the Scandinavian Ice Sheet 40–15 kyr BP. Synchronised land, sea and glacier configurations are based on the lithostratigraphy of tills and intertill sediments. Dating is provided by optically stimulated luminescence and calibrated accelerator mass spectrometry radiocarbon. An interstadial sequence ca. 40–30 kyr BP with boreo‐arctic proglacial fjords and subarctic flora and occasional glaciation in the Baltic was succeeded by a Last Glacial Maximum sequence ca. 30–20 kyr BP, with the closure of fjords and subsequent ice streams in glacial lake basins in Kattegat and the Baltic. Steadily flowing ice from Sweden bordered the Norwegian Channel Ice Stream. A deglaciation sequence ca. 20–15 kyr BP indicates the transgression of arctic waters, retreat of the Swedish ice and advance of Baltic ice streams succeeded by a return to interstadial conditions. When ameliorated ice‐free conditions prevailed in maritime regions, glaciers advanced through the Baltic and when interstadial regimes dominated the Baltic, glaciers expanded off the Norwegian coast. The largest glacier extent was reached in the North Sea around 29 kyr BP, about 22 kyr BP in Denmark and ca. 18 kyr BP in the Baltic. Our model provides new data for future numerical and qualitative landform‐based models. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

High-pressure granulite-facies metagabbros in the Ubendian Belt of SW Tanzania: preliminary petrography and P–T estimates     

S. Muhongo  P. Tuisku  S. Mnali  E. Temu  P. Appel  H. Stendal 《Journal of African Earth Sciences》2002,34(3-4)

The Ubende terrane is one of the eight structural blocks constituting the Palaeoproterozoic Ubendian Belt of southwest Tanzania. The Ubende terrane is made up of high-pressure granulite-facies rocks, which occur in association with amphibolites and hornblende gneisses. Preliminary mineralogical studies of the granulite-facies metagabbros indicate that they were derived from rocks, which originally consisted of clinopyroxene and intergranular plagioclase. The mineralogy of these metagabbros consists principally of sodian augite and garnets (50% almandine, 30% pyrope and 20% grossular). Plagioclase, hornblende, quartz, rutile and haematite occur as accessory components. The sodian augite is slightly zoned with Mg content increasing towards the margin and Fe, Al and Na concentrations decrease towards its rim. Thermobarometric calculations indicate that the metagabbros were formed at metamorphic conditions of about 840–900 °C and pressures of 12.2–13.7 kbar. Presence of supersiliceous clinopyroxene (quartz lamellae in clinopyroxene) and decompositional textures suggest that these granulite-facies metagabbros might be retrogressed eclogites.  相似文献   

The architecture of a sandy braided-meandering river system: an example from the lower triassic Soiling Formation (M. Buntsandstein) in W-Germany     

Henrik Olsen 《International Journal of Earth Sciences》1988,77(3):797-814

The dominantly sandy fluvial deposits of the Soiling Formation in the Hessian Depression in W-Germany are excellently exposed in abandoned sandstone quarries N of Kassel near Bad Karlshafen und Stadtoldendorf.Detailed mapping of the quarry walls indicates the presence of five architectural elements: (1) lateral accretion deposits; (2) minor cross-cutting channel-fills; (3) major broad channel deposits; (4) laminated sand sheets; and (5) muddy deposits. The vertical and lateral relationships of these elements suggest deposition in a braided-meandering river system. Sinuous parts of the river channels were dominated by point bar deposition at river high stage (element 1), whereas low-stage flow was characterized by a braided stream pattern (element 2). Less sinuous parts of the river channels were characterized by moderate braiding indices (element 3). Proximal overbank areas were dominated by sandy sheet flows (element 4) whereas distal overbank areas received muddy sediments (element 5).

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Zusammenfassung Die überwiegend sandigen fluviatilen Ablagerungen der Solling-Folge in der Hessischen Senke sind sehr gut in ehemaligen Sandsteinbrüchen nördlich von Kassel in der Nähe von Bad Karlshafen und Stadtoldendorf aufgeschlossen.Detaillierte Kartierung der Bruchwände zeigt fünf architektonische Elemente: (1) Ablagerungen mit lateralem Zuwachs; (2) kleine durchschneidende Rinnenfüllungen; (3) Absätze in großen und breiten Kanälen; (4) laminierte Sandschichten und (5) tonige Ablagerungen. Die vertikalen und lateralen Beziehungen dieser Elemente zueinander deuten auf Sedimentation in einem verflochtenen bis mäandrierenden Flussnetz hin. Die stark gekrümmten Teile der Flussrinnen sind bei Hochwasser von Point Bar-Absätzen dominiert (Element 1), während sie bei Niedrigwasser Ströme mit verflochtenem Muster zeigen (Element 2). Die weniger gekrümmten Teile der Flussrinnen sind durch moderate Vernetzungsindizien charakterisiert (Element 3). Proximale Überflutungsbereiche sind von sandigen Schichtflutsedimenten dominiert (Element 4), während auf den distalen Überflutungsebenen tonige Sedimente abgelagert wurden (Element 5).

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Résumé Les dépôts fluviatiles à dominance sableuse de la Soiling Formation (dépression de Hesse, en Allemagne) affleurent de manière exceptionnelle dans des carrières abandonnées près de Bad-Karlshafen et Stadtoldendorf. Une cartographie détaillée des parois des carrières révèle cinq éléments architecturaux: 1) dépôts à croissance latérale; 2) remplissage entrecroisé de chenaux mineurs; 3) dépôts de chenaux majeurs; 4) couches de sable à structure laminée; 5) dépôts argileux. Les relations verticales et latérales de ces éléments suggèrent une sédimentation dans une rivière anastomosée et à méandres. Les parties sinueuses des chenaux étaient le siège d'une sédimentation de »point bar« lors des périodes de hautes eaux (élément 1), tandis que les périodes de basses eaux se traduisent par un type de cours d'eau anastomosé (élément 2). Les parties moins sinueuses des chenaux étaient caractérisées par des indices d'anastomose plus modérés (élément 3). Les domaines proximaux de crue étaient dominés par des influx sableux (élément 4), tandis que les domaines de crue distaux recevaient des sédiments argileux (élément 5).

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