Statistical lensing by galactic discs     

A. W. Blain  Ole Möller  Ariyeh H. Maller 《Monthly notices of the Royal Astronomical Society》1999,303(2):423-432

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Concerning the generation of geomagnetic giant pulsations by drift-bounce resonance ring current instabilities     

K.-H. Glassmeier  S. Buchert  U. Motschmann  A. Korth  A. Pedersen 《Annales Geophysicae》1999,17(3):338-350

Giant pulsations are nearly monochromatic ULF-pulsations of the Earth’s magnetic field with periods of about 100 s and amplitudes of up to 40 nT. For one such event ground-magnetic observations as well as simultaneous GEOS-2 magnetic and electric field data and proton flux measurements made in the geostationary orbit have been analysed. The observations of the electromagnetic field indicate the excitation of an odd-mode type fundamental field line oscillation. A clear correlation between variations of the proton flux in the energy range 30–90 keV with the giant pulsation event observed at the ground is found. Furthermore, the proton phase space density exhibits a bump-on-the-tail signature at about 60 keV. Assuming a drift-bounce resonance instability as a possible generation mechanism, the azimuthal wave number of the pulsation wave field may be determined using a generalized resonance condition. The value determined in this way, m = −21±4, is in accord with the value m = −27±6 determined from ground-magnetic measurements. A more detailed examination of the observed ring current plasma distribution function f shows that odd-mode type eigenoscillations are expected for the case ∂f/∂W ≥ 0, much as observed. This result is different from previous theoretical studies as we not only consider local gradients of the distribution function in real space, but also in velocity space. It is therefore concluded that the observed giant pulsation is the result of a drift-bounce resonance instability of the ring current plasma coupling to an odd-mode fundamental standing wave. The generation of the bump-on-the-tail distribution causing ≥f/≥W ≥ 0 can be explained due to velocity dispersion of protons injected into the ring current. Both this velocity dispersion and the necessary substorm activity causing the injection of protons into the nightside magnetosphere are observed.  相似文献   

Tracking herring schools with a high resolution sonar. Variations in horizontal area and relative echo intensity   总被引：3，自引：2，他引：1  

Misund  Ole Arve; Ferno  Anders; Pitcher  Tony; Totland  Bjorn 《ICES Journal of Marine Science》1998,55(1):58-66

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Microwave signatures of the sea ice in the east Greenland current     

Skou  N. Pedersen  L. 《Oceanic Engineering, IEEE Journal of》1984,9(5):339-343

Airborne microwave radiometer measurements at 5, 17, and 34 GHz have been carried out over the East Greenland Current. Sea ice signatures have been established for some of the basic ice types like first-year ice and multiyear ice. Other signatures have been experienced like that of presumably very old arctic ice and signatures associated more with the snow cover on the ice than with the ice itself. During MIZEX-83 measurements of total ice concentration were carried out.  相似文献   

Relative sea level changes during the Holocene in the Sisimiut area,south‐western Greenland     

Ole Bennike  Bernd Wagner  Andreas Richter 《第四纪科学杂志》2011,26(4):353-361

Holocene relative sea level (RSL) changes have been investigated by analysing and dating isolation sequences from five lakes near Sisimiut in south‐western Greenland. The transitions between marine and lacustrine sediments were determined from elemental analyses and analyses of macroscopic plant and animal remains. Radiocarbon dating was used to provide minimum ages for the transitions and to construct a RSL curve. Dating of a shell of the marine bivalve Macoma balthica indicates that deglaciation of the lowlands occurred in the early Holocene, at around 10 900 cal a BP. The RSL curve shows initial rapid regression from the marine limit at around 140 m, implying strong glacio‐isostatic rebound. We suggest that the margin of the Greenland Ice Sheet was located at the shelf break during the Last Glacial Maximum. Frequent remains of the ostracode Sarcypridopsis aculeata, which is a thermophilous brackish water species that is unknown from the extant fauna of Greenland, in one of the basins around 8500 cal a BP may mark the beginning of the Holocene thermal maximum in the region. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Chronology of the last recession of the Greenland Ice Sheet   总被引：1，自引：0，他引：1  

Ole Bennike  Svante Bjrck 《第四纪科学杂志》2002,17(3):211-219

A new deglaciation chronology for the ice‐free parts of Greenland, the continental shelf and eastern Ellesmere Island (Canada) is proposed. The chronology is based on a new compilation of all published radiocarbon dates from Greenland, and includes crucial new material from southern, northeastern and northwestern Greenland. Although each date provides only a minimum age for the local deglaciation, some of the dates come from species that indicate ice‐proximal glaciomarine conditions, and thus may be connected with the actual ice recession. In addition to shell dates, dates from marine algae, lake sediments, peat, terrestrial plants and driftwood also are included. Only offshore and in the far south have secure late‐glacial sediments been found. Other previous reports of late‐glacial sediments (older than 11.5 cal. kyr BP) from onshore parts of Greenland need to be confirmed. Most of the present ice‐free parts of Greenland and Nares Strait between Greenland and Ellesmere Island were not deglaciated until the early Holocene. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

1D inversion of DC resistivity data using a quality-based truncated SVD   总被引：1，自引：0，他引：1  

Elonio A. Muiuane  Laust B. Pedersen 《Geophysical Prospecting》2001,49(4):387-394

Many DC resistivity inversion schemes use a combination of standard iterative least-squares and truncated singular value decomposition (SVD) to optimize the solution to the inverse problem. However, until quite recently, the truncation was done arbitrarily or by a trial-and-error procedure, due to the lack of workable guidance criteria for discarding small singular values. In this paper we present an inversion scheme which adopts a truncation criterion based on the optimization of the total model variance. This consists of two terms: (i) the term associated with the variance of statistically significant principal components, i.e. the standard model estimate variance, and (ii) the term associated with statistically insignificant principal components of the solution, i.e. the variance of the bias term. As an initial model for the start of iterations, we use a multilayered homogeneous half-space whose layer thicknesses increase logarithmically with depth to take into account the decrease of the resolution of the DC resistivity technique with depth. The present inversion scheme has been tested on synthetic and field data. The results of the tests show that the procedure works well and the convergence process is stable even in the most complicated cases. The fact that the truncation level in the SVD is determined intrinsically in the course of inversion proves to be a major advantage over other inversion schemes where it is set by the user.  相似文献   

Atmospheric input of nitrogen into the North Sea: ANICE project overview     

Gerrit de Leeuw  Leo Cohen  Lise Marie Frohn  Gary Geernaert  Ole Hertel  Bjarne Jensen  Tim Jickells  Laura Klein  Gerard J. Kunz  Soren Lund  Marcel Moerman  Frank Müller  Britta Pedersen  Knut von Salzen  K. Heinke Schlünzen  Michael Schulz  Carsten A. Skjth  Lise-Lotte Sorensen  Lucinda Spokes  Susanne Tamm  Elisabetta Vignati 《Continental Shelf Research》2001,21(18-19)

The aim of the atmospheric nitrogen inputs into the coastal ecosystem (ANICE) project is to improve transport–chemistry models that estimate nitrogen deposition to the sea. To achieve this, experimental and modelling work is being conducted which aims to improve understanding of the processes involved in the chemical transformation, transport and deposition of atmospheric nitrogen compounds. Of particular emphasis within ANICE is the influence of coastal zone processes. Both short episodes with high deposition and chronic nitrogen inputs are considered in the project. The improved transport–chemistry models will be used to assess the atmospheric inputs of nitrogen compounds into the European regional seas (the North Sea is studied as a prototype) and evaluate the impact of various emission reduction strategies on the atmospheric nitrogen loads. Assessment of the impact of atmospheric nitrogen on coastal ecosystems will be based on comparisons of phytoplankton nitrogen requirements, other external nitrogen inputs to the ANICE area of interest and the direct nitrogen fluxes provided by ANICE. Selected results from both the experimental and modelling components are presented here. The experimental results show the large spatial and temporal variability in the concentrations of gaseous nitrogen compounds, and their influences on fluxes. Model calculations show the strong variation of both concentrations and gradients of nitric acid at fetches of up to 25 km. Aerosol concentrations also show high temporal variability and experimental evidence for the reaction between nitric acid and sea salt aerosol is provided by size-segregated aerosol composition measured at both sides of the North Sea. In several occasions throughout the experimental period, air mass back trajectory analysis showed connected flow between the two sampling sites (the Weybourne Atmospheric Observatory on the North Norfolk coast of the UK and Meetpost Noordwijk, a research tower at 9 km off the Dutch coast). Results from the METRAS/SEMA mesoscale chemistry transport model system for one of these cases are presented. Measurements of aerosol and rain chemical composition, using equipment mounted on a commercial ferry, show variations in composition across the North Sea. These measurements have been compared to results obtained with the transport–chemistry model ACDEP which calculates the atmospheric inputs into the whole North Sea area. Finally, the results will be made available for the assessment of the impact of atmospheric nitrogen on coastal ecosystems.  相似文献   

North Atlantic Water in the Barents Sea Opening, 1997 to 1999     

Jane O'Dwyer  Yoshie Kasajima  Ole Anders Nøst 《Polar research》2001,20(2):209-216

North Atlantic Water (NAW) is an important source of heat and salt to the Nordic seas and the Arctic Ocean. To measure the transport and variability of one branch of NAW entering the Arctic, a transect across the entrance to the Barents Sea was occupied 13 times between July 1997 and November 1999, and hydrography and currents were measured. There is large variability between the cruises, but the mean currents and the hydrography show that the main inflow takes place in Bjørnøyrenna, with a transport of 1.6 Sv of NAW into the Barents Sea. Combining the flow field with measurements of temperature and salinity, this results in mean heat and salt transports by NAW into the Barents Sea of 3.9×10¹³ W and 5.7×10⁷ kg s⁻¹, respectively. The NAW core increased in temperature and salinity by 0.7 °C yr⁻¹ and 0.04 yr⁻¹, respectively, over the observation period. Variations in the transports of heat and salt are, however, dominated by the flow field, which did not exhibit a significant change.  相似文献   

IAG Newsletter     

Ole Baltazar Andersen 《Journal of Geodesy》1996,70(6):374-382

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