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531.
P. Jakobsson D. Malesani J. Hjorth J.P.U. Fynbo B. Milvang-Jensen 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
We describe the current status and recent results from our Swift/VLT legacy survey, a VLT Large Programme aimed at characterizing the host galaxies of a homogeneously selected sub-sample of Swift GRBs. The immediate goals are to determine the host luminosity function, study the effects of reddening, determine the fraction of Lyα emitters in the hosts, and obtain redshifts for targets without a reported one. The main effort so far has been the definition of a very carefully selected sample, obeying strict and well-defined criteria: 68 targets in total. Among the preliminary results is a large optical detection rate, the lack of extremely red objects (only one possible case in the sample) and an update of the Swift GRB redshift distribution with 〈z〉∼2.0. 相似文献
532.
G. Nath 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
The propagation of a strong cylindrical shock wave in an ideal gas with azimuthal magnetic field, and with or without axisymmetric rotational effects, is investigated. The shock wave is driven out by a piston moving with time according to power law. The ambient medium is assumed to have radial, axial and azimuthal component of fluid velocities. The fluid velocities, the initial density and the initial magnetic field of the ambient medium are assumed to be varying and obey power laws. Solutions are obtained, when the flow between the shock and the piston is isothermal. The gas is assumed to have infinite electrical conductivity and the angular velocity of the ambient medium is assumed to be decreasing as the distance from the axis increases. It is expected that such an angular velocity may occur in the atmospheres of rotating planets and stars. The shock wave moves with variable velocity and the total energy of the wave is non-constant. The effects of variation of the initial density and the Alfven-Mach number on the flow-field are obtained. A comparison is also made between rotating and non-rotating cases. 相似文献
533.
Konrad Dennerl 《Space Science Reviews》2006,126(1-4):403-433
X-rays from Mars were first detected in July 2001 with the satellite Chandra. The main source of this radiation was fluorescent
scattering of solar X-rays in its upper atmosphere. In addition, the presence of an extended X-ray halo was indicated, probably
resulting from charge exchange interactions between highly charged heavy ions in the solar wind and neutrals in the Martian
exosphere. The statistical significance of the X-ray halo, however, was very low. In November 2003, Mars was observed again
in X-rays, this time with the satellite XMM-Newton. This observation, characterized by a considerably higher sensitivity,
confirmed the presence of the X-ray halo and proved that charge exchange is indeed the origin of the emission. This was the
first definite detection of charge exchange induced X-ray emission from the exosphere of another planet. Previously, this
kind of emission had been detected from comets (which are largely exospheres) and from the terrestrial exosphere. Because
charge exchange interactions between atmospheric constituents and solar wind ions are considered as an important nonthermal
escape mechanism, probably responsible for a significant loss of the Martian atmosphere, X-ray observations may lead to a
better understanding of the present state of the Martian atmosphere and its evolution. X-ray images of the Martian exosphere
in specific emission lines exhibited a highly anisotropic morphology, varying with individual ions and ionization states.
With its capability to trace the X-ray emission out to at least 8 Mars radii, XMM-Newton can explore exospheric regions far
beyond those that have been observationally explored to date. Thus, X-ray observations provide a novel method for studying
processes in the Martian exosphere on a global scale. 相似文献
534.
靳凤山 《华北航天工业学院学报》2001,(1)
债转股作为经济体制改革的一个重要探索,已经成为当今法学界和经济学界讨论的主题。作为一个新事物的出现,债转股必然会遇到一系列来自不同方向的阻碍,本文就这些困惑进行论述,阐明了债转股面临的主要问题,并提出相应的解决方案和建议。国民经济体制改革需要大胆的探索和创新,债转股就是一个很好的范例,尽管其实施过程中难免遇到一些难题,但必将取得骄人的成果。 相似文献
535.
A design study of wing tip devices at high and low speeds is described. The basis of the design study is an equivalent drag approach containing both aerodynamic drag gain and structural weight penalty. A comprehensive parameter study is carried out using a rapid aerodynamic prediction tool named Lift and Drag Component Analysis (LIDCA). Adding to an available lifting-line method a databased module for airfoil data is employed that uses results of two-dimensional flow simulations by multidimensional interpolation. Detailed validation studies of the method at high lift and high speed have demonstrated good accuracy. RANS computations of the selected wing tip designs confirm the predicted benefits at cruise condition. The results of the most effective wing tip designs are analysed at both flight conditions. Finally, options for improving the performance at take-off are suggested. 相似文献
536.
Yang Su W.Q. GanY.P. Li 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,41(6):988-991
Based on the light curves and images of RHESSI flares, we tried to make a preliminary classification of solar flares. Three basic types of flares seem to be existed: accordantly gradual flares, accordantly impulsive flares, and early impulsive flares. The proportion for each type is given. The possible physical meaning related to different types is discussed. 相似文献
537.
len Balman 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2006,38(12):2840-2843
I discuss morphology and spectrum of the first resolved and detected classical nova shell in the X-rays – the remnant of GK Persei (1901). The existence of such a nebulosity brings about the possibility of other nova remnants emitting X-rays. I calculate that the X-ray luminosity should be about 1026–1033 ergs s−1 on the onset of cooling for nova remnants. I have done an archival search on 250 classical and recurrent nova candidates using Chandra, XMM-Newton, ROSAT and ASCA databases. There is no significant extended emission detected which places an upper limit of Fx < × 10−12 erg s−1 cm−2 (unabsorbed). Only exceptions are GK Per, RR Pic and DQ Her (all observed by Chandra ACIS-S and GK Per also by ROSAT HRI) where the latter two show marginal extended emission in the X-rays associated with emission knots (DQ Her) or an equatorial ring (RR Pic). 相似文献
538.
V. V. Izmodenov 《Space Science Reviews》2007,130(1-4):377-387
Interstellar atoms penetrate deep into the heliosphere after passing through the heliospheric interface—the region of the
interaction of the solar wind with the interstellar medium. The heliospheric interface serves as a filter for the interstellar
atoms of hydrogen and oxygen, and, to a lesser extent, nitrogen, due to their coupling with interstellar and heliospheric
plasmas by charge exchange and electron impact ionization. The filtration has great importance for the determination of local
interstellar abundances of these elements, which becomes now possible due to measurements of interstellar pickup by Ulysses
and ACE, and anomalous cosmic rays by Voyagers, Ulysses, ACE, SAMPEX and Wind. The filtration of the different elements depends
on the level of their coupling with the plasma in the interaction region. The recent studies of the filtration of the interstellar
atoms in the heliospheric interface region is reviewed in this paper. The dependence of the filtration on the local interstellar
proton and H atom number densities is discussed and the roles of the charge exchange and electron impact ionization on the
filtration are evaluated. The influence of electron temperature in the inner heliosheath on the filtration process is discussed
as well. Using the filtration coefficients obtained from the modeling and SWICS/Ulysses pickup ion measurements, the local
interstellar abundances of the considered elements are determined. 相似文献
539.
The ionic charge of solar energetic particles (SEP) as observed in interplanetary space is an important parameter for the
diagnostic of the plasma conditions at the source region and provides fundamental information about the acceleration and propagation
processes at the Sun and in interplanetary space. In this paper we review the new measurements of ionic charge states with
advanced instrumentation onboard the SAMPEX, SOHO, and ACE spacecraft that provide for the first time ionic charge measurements
over the wide energy range of ∼0.01 to 70 MeV/nuc (for Fe), and for many individual SEP events. These new measurements show
a strong energy dependence of the mean ionic charge of heavy ions, most pronounced for iron, indicating that the previous
interpretation of the mean ionic charge being solely related to the ambient plasma temperature was too simplistic. This energy
dependence, in combination with models on acceleration, charge stripping, and solar and interplanetary propagation, provides
constraints for the temperature, density, and acceleration time scales in the acceleration region. The comparison of the measurements
with model calculations shows that for impulsive events with a large increase of Q
Fe(E) at energies ≤1 MeV/nuc the acceleration occurs low in the corona, typically at altitudes ≤0.2 R
S
. 相似文献
540.
英语课堂提问策略调研 总被引:7,自引:0,他引:7
提问是课堂教学中最常见的教学活动,提问的质量将直接影响课堂教学活动的开展。提问的有效进行依赖于策略的使用,那么课堂提问中有哪些策略可以利用,目前大学英语课堂提问策略使用状况如何,本文结合在北京、河北两地的部分高校中所做的调研,揭示了目前大学英语课堂提问中存在的问题,也为更好地开展课堂教学提供的必要的启示。 相似文献