Weak gravitational lensing by dark clusters     

Nevin N. Weinberg  Marc Kamionkowski 《Monthly notices of the Royal Astronomical Society》2002,337(4):1269-1281

  相似文献   

Thermal instability in slab geometry in the presence of anisotropical thermal conduction     

R. A. M. Van Der Linden  M. Goossens 《Solar physics》1991,131(1):79-105

Prominences and filaments are thought to arise as a consequence of a magnetized plasma undergoing thermal instability. Therefore, the thermal stability of a magnetized plasma is investigated under coronal conditions. The equilibrium structure of the plasma is approximated by a 1-D slab configuration. This is investigated in thermal instability taking into account optically thin plasma radiation and anisotropic thermal conduction. The thermal conduction perpendicular to the magnetic field is taken to be small but non-zero.The classic rigid wall boundary conditions which are often applied in the literature, either directly on the plasma or indirectly through some other medium, are replaced by a more physical situation in which the plasma column is placed in a low-density background stretching towards infinity. Results for a uniform equilibrium structure indicate the major effect of this change is on the eigenfunctions rather than on the growth rate. Essentially, perpendicular thermal conduction introduces field-aligned fine structure. It is also shown that in the presence of perpendicular thermal conduction, thermal instability in a slab model is only possible if the inner plasma has the shortest thermal instability time scale.Research Assistant of the National Fund for Scientific Research (Belgium).  相似文献   

A study of the palaeomagnetism of subglacial basalts, SW Iceland: a comparison with oceanic crust     

J. Helgason  N. A. Van Wagoner  P. J. C. Ryall 《Geophysical Journal International》1990,103(1):13-24

  相似文献   

Composition of the first bulk melt sample from a volcanic region of Mars: Queen Alexandra Range 94201     

David A. KRING  James D. GLEASON  Timothy D. SWINDLE  Kunihiko NISHIIZUMI  Marc W. CAFFEE  Dolores H. HILL  A. J. Timothy JULL  William V. BOYNTON 《Meteoritics & planetary science》2003,38(12):1833-1848

Abstract— Antarctic meteorite Queen Alexandra Range (QUE) 94201 is a 12 g basaltic achondrite dominated by plagioclase (now maskelynite) and zoned low‐ and high‐Ca pyroxene. Petrologic, geochemical, and isotopic analyses indicate that it is related to previously described basaltic and Iherzolitic shergottites, which are a group of igneous meteorites that are believed to be from Mars. Unlike previous shergottites, however, QUE 94201 represents a bulk melt rather than a cumulate fraction, meaning it can be used to infer magmatic source regions and the compositions of other melts on Mars. This melt has much more Fe and P than basaltic melts produced on Earth and formed at a much lower oxygen fugacity. This has altered the crystallization sequence of the melt, removing olivine from the liquidus to produce a plagioclase and 2‐pyroxene assemblage. If the high‐phosphorus and low‐oxygen fugacity conditions represented by QUE 94201 are common in magmatic regions of Mars, then olivine may be rare in marrian basalts. No solar cosmic ray effects were seen in the concentrations of ¹⁰Be, ²⁶A1, and ³⁶C1 with depth in the meteorite, implying at least 3 cm of ablation during entry to Earth. Significant excesses of neutron capture noble gas isotopes (^80,82Kr and ^128,131Xe) suggest that the QUE 94201 sample came from a depth >22 cm in a meteoroid of at least that radius. The meteorite also has very low ²¹Ne/²²Ne, which would often be interpreted to mean little ablation (contradicting above evidence) but, in this case, appears to reflect a very low abundance of Mg (the principal target element for Ne) in the meteorite, consistent with our bulk chemical analyses. The meteorite has a terrestrial ³⁶C1 age of 0.29 ± 0.05 Myr and a ¹⁰Be exposure age of 2.6 ± 0.5 Myr in a 47π geometry, implying an ejection age of 2.9 ± 0.5 Myr.  相似文献   

SOME PRELIMINARY RESULTS FROM STUDY OF CRUSTAL DEFORMATION IN VIETNAM     

Cao Dinh Trieu  Le Van Dung  Nguyen Huu Tuyen  Pham Nam Hung 《大地测量与地球动力学》2003,23(Z1)

1　IntroductionOneofthemaintasksinstudyinggeodynamicprocessisevaluatingtherateofearthcrustmove ments.Normally ,themethodsforstudyingearthcrustmovementareestablishedonthebasisoftheobjectofstudyandtherequirementsfortheresultstobeobtainedasfollows.1 )Tostudythemovingprocessofcrustplatesinalargeareaandwithinacertaintectonicperiod ,geologicmethodsareusuallyused .2 )Geodeticmeasurementsarecarriedoutinanareacoveringfromsomekm2 totensofkm2 andal lowtodeterminethedisplacementoftheearthcrustwithinaperio…  相似文献   

Tidally induced surface displacements, external potential variations, and gravity variations on Mars     

T. Van Hoolst  V. Dehant  P. Lognonné 《Icarus》2003,161(2):281-296

We have used and extended Roosbeek’s tidal potential for Mars to calculate tidal displacements, gravity variations, and external gravitational potential variations. The tides on Mars are caused by the Sun, and to a lesser degree by the natural satellites Phobos (8%, relative to the Sun) and Deimos (0.08%, relative to the Sun). To determine the reaction of Mars to the tidal forcing, the Love numbers h, l, and k and the gravimetric factor δ were calculated for interior models of Mars with different state, density, and radius of the core and for models which include mantle anelasticity. The latitude dependence and frequency dependence of the Love numbers have been taken explicitly into account. The Love numbers are about three times smaller than those for the Earth and are very sensitive to core changes; e.g., a difference of about 30% is found between a model with a liquid core and an otherwise similar model with a solid core. Tidal displacements on Mars are much smaller than on Earth due to the smaller tidal potential, but also due to the smaller reaction of Mars (smaller Love numbers). For both the tidal diplacement and the tidal external potential perturbations, the tidal signal is at the limit of detection and is too small to permit properties of Mars’s interior to be inferred. On the other hand, the Phobos tidally induced gravity changes, which are subdiurnal with typical periods shorter than 12 h, can be measured very precisely by the very broad band seismometer with thermal control of the seismological experiment SEIS of the upcoming NetLander mission. It is shown that the Phobos-induced gravity tides could be used to study the Martian core.  相似文献   

Effective Sensitivity of A Non-uniform Phased Array of Short Dipoles     

W. A. Van Cappellen  J. D. Bregman  M. J. Arts 《Experimental Astronomy》2004,17(1-3):101-109

Short dipoles are a key element in new low frequency array antennas as proposed for LOFAR and other astronomical applications. Unfortunately standard texts on short dipole antennas are based on the effective area and do not lead to an astronomically useful sensitivity formulation in a straightforward manner. The concept of maximum effective area is applied to arrays of short dipoles and allows expressing the sensitivity as the ratio of this area over the effective sky brightness temperature as long as the output noise power is dominated by the antenna input radiation. For both quantities we only need to know the array directivity pattern that includes the mutual coupling effects when the actual loading conditions of the array elements are taken into account. Short dipole elements have a constant directivity pattern for frequencies below resonance, but they exhibit strong complex impedance variations that provide only narrow band performance when power matching is applied as required in transmit applications. However, in receive applications voltage or current sensing can be realized, for example with an active balun. Assisted by the steep increase of the sky brightness with wavelength for frequencies below 300 MHz, this can provide sky noise dominated performance over at least a three to one frequency range. Still the low frequency limit is determined by the amplifier noise contribution and the losses in the antenna and in the dielectric ground surrounding the elements. We show that for a sparse array with the elements non-uniformly distributed according to an exponential shell model, a constant sensitivity can be obtained over a frequency range of at least two octaves. In addition, such a configuration has a factor of six greater sensitivity than a rectangular array for a large part of the frequency band.  相似文献   

September 2004     

Thomas W. Merrill  Michael Platt  Marc D. Guerra  Ty Shaffer 《Environmental Geology》2004,46(5):683-684

  相似文献   

Cosmogenic radionuclide dating of glacial landforms in the Lahul Himalaya,northern India: defining the timing of Late Quaternary glaciation     

Lewis A. Owen  Lyn Gualtieri  Robert C. Finkel  Marc W. Caffee  Doug I. Benn  Milap C. Sharma 《第四纪科学杂志》2001,16(6):555-563

The timing of glaciation in the Lahul Himalaya of northern India was ascertained using the concentrations of cosmogenic ¹⁰Be and ²⁶Al from boulders on moraines and drumlins, and from glacially polished bedrock surfaces. Five glacial stages were identified: Sonapani I and II, Kulti, Batal and Chandra. Of these, cosmogenic exposure ages were obtained on samples representative of the Batal and Kulti glacial cycles. Stratigraphical relationships indicate that the Sonapani I and II are younger. No age was obtained for the Chandra glacial advance. Batal Glacial Stage deposits are found throughout the valley, indicating the presence of an extensive valley glacial system. During the Kulti Stage, glaciers advanced ca. 10 km beyond their current positions. Moraines produced during the Batal Stage, ca. 12–15.5 ka, are coeval with the Northern Hemisphere Late‐glacial Interstadial (Bølling/Allerød). Deglaciation of the Batal Glacial Stage was completed by ca. 12 ka and was followed by the Kulti Glacial Stage during the early Holocene, at ca. 10–11.4 ka. On millennial time‐scales, glacier oscillations in the Lahul Himalaya apparently reflect periods of positive mass‐balance coincident with times of increased insolation. During these periods the South Asian summer monsoon strengthened and/or extended its influence further north and west, thereby enhancing high‐altitude summer snowfall. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

Observations of beam propagation     

M. Pick  G. H. J. Van Den Oord 《Solar physics》1990,130(1-2):83-99

We discuss the observational evidence for propagating electron distributions in the solar corona as obtained from type III observations. The location of the beam acceleration region together with the effects of the ambient medium on beam propagation are discussed in the framework of recent observational and theoretical developments. The results of combined X-ray and radio observations obtained during and after the Solar Maximum Year are summarized. Special attention is paid to the effect of the small-scale structure of the corona on the observed radio emission. We outline various desirable developments which are necessary to improve the understanding of the behaviour of charged particle beams in magnetized plasmas.  相似文献