首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   167篇
  免费   1篇
测绘学   4篇
大气科学   3篇
地球物理   28篇
地质学   30篇
海洋学   37篇
天文学   60篇
综合类   1篇
自然地理   5篇
  2021年   3篇
  2020年   2篇
  2018年   5篇
  2017年   4篇
  2016年   3篇
  2015年   3篇
  2014年   3篇
  2013年   3篇
  2012年   2篇
  2011年   5篇
  2010年   7篇
  2009年   7篇
  2008年   3篇
  2007年   6篇
  2006年   4篇
  2005年   4篇
  2004年   6篇
  2003年   4篇
  2002年   4篇
  2001年   7篇
  2000年   6篇
  1999年   5篇
  1998年   2篇
  1997年   3篇
  1996年   6篇
  1995年   2篇
  1994年   2篇
  1993年   1篇
  1991年   3篇
  1990年   4篇
  1989年   1篇
  1988年   1篇
  1987年   1篇
  1985年   1篇
  1983年   4篇
  1982年   2篇
  1980年   3篇
  1979年   2篇
  1978年   3篇
  1977年   1篇
  1976年   3篇
  1975年   2篇
  1974年   3篇
  1973年   2篇
  1972年   2篇
  1971年   1篇
  1970年   2篇
  1969年   6篇
  1968年   9篇
排序方式: 共有168条查询结果,搜索用时 902 毫秒
161.
Electron paramagnetic resonance of Ir2+ in forsterite is studied at Q-band frequency and room temperature. There are four equivalent spectra superimposed along the three crystallographic axes. The individual spectrum consists of four hyperfine lines of approximately equal intensity separated from each other by 42 G; one axis of the g tensor is near the c axis. Ir2+ is certainly substituted for Mg2+. Because of the fourfold degeneracy of the EPR spectrum, it may be suggested that iridium occurs at M1. Taking into account that one eigenvector of the g tensor is nearly parallel to c, it seems also possible that the substitution takes place at the M2 position. In this case, the lattice relaxation of the crystal structure around Ir2+ at M2 must break the point symmetry m at M2.  相似文献   
162.
The objective of the 20 Nautile dives of the recent Kanaut cruise was to study the southern wall of the Kane Fracture Zone from its eastern intersection with the Mid-Atlantic Ridge (MAR) to 5 Myr in age. The geological mapping shows four successive massifs, wrench faulted and slightly tilted. The transform-facing walls of these massifs exhibit outcrops of fresh and serpentinized peridotites, gabbros and basalts. The entire crustal exposure is cataclased and metamorphosed to greenschist facies.  相似文献   
163.
A (15)N tracer study was performed during an experimental transplantation trial of natural Posidonia oceanica cuttings. The experiment was done in situ at 17 m depth in the Revellata Bay (Calvi, NW Corsica, France). Despite high survival rates of transplants (>90%) after one year, the weight and the N content of transplants are significantly lower than those of reference plants. In absence of roots, the transplants are not able to meet their N requirement, because leaf uptake is insufficient to replenish the N lost during the natural leaf decay. This could constitute a major cause of long-term failure for transplantation experiments or natural re-colonisation processes. The increase of the (15)N content in the roots shows that the plant re-allocates the nitrogen of one organ (i.e. leaves, rhizomes) to ensure the growth of another (i.e. roots).  相似文献   
164.
From simultaneous filtergrams taken in opposite wings of the 6569.2 solar absorption line we derive the velocity profile of an average solar granule. We definitely established the existence of the horizontal outflow in a granule in addition to the vertical upflow at the granule center. The magnitude of this outflow is strongly dependent on the influence of instrumental and atmospheric smoothing and on the effective height in the solar atmosphere at which the velocity is measured. The maximum upflow (at the granule center) equals 0.4 km/sec, the maximum outflow (500 km from the granule center) equals 0.25 km/sec.Part of the research reported here was carried out while the author was at the High Altitude Observatory, Boulder, Colorado.  相似文献   
165.
A time sequence of high-resolution sunspot photographs, exposed almost simultaneously in two continuum wavelengths (4680 Å and 6400 Å), was used to study some properties of umbral fine structures (umbral dots). The lifetime of the umbral dots is found to be 1500 sec. Photometry of some bright dots leads to an observed intensity excess of 0.129 I phot and 0.134 I phot in the blue and red respectively. The observed mean diameter of the dots is found to be 420 km. These values still include the action of image blurring. From the color index the true intensity and diameter of the dots are estimated. It appears that the umbral dots are in reality of photospheric brightness having true diameters of 150–200 km. The spatial distribution of the dots in sunspot umbrae is discussed. Some peculiarities in recent sunspot magnetic-field observations may be explained by magnetic inhomogeneities associated with umbral dots.Presently guest investigator at the Göttingen Observatory.Previously member of the High Altitude Observatory solar project at Sacramento Peak (Contract Nr. AF (628) - 4078).  相似文献   
166.
167.
An observational program at the Sacramento Peak Observatory in 1965 provided high-dispersion spectra of the solar chromosphere in several spectral regions simultaneously. These regions included various combinations of the spectral lines Hα, Hβ and H?, the D3-line of Hei, the infrared triplet of Oi, and the H- and K-lines and the infrared triplet of Caii. With the use of an image slicer the observations were made simultaneously at two heights in the solar chromosphere separated by several thousand kilometers. From these data we draw the following conclusions:
  1. Emission of different lines arises in the same chromospheric features. The intensity ratio of lines of different elements varies significantly from spicule to spicule. For the H- and K-lines of ionized calcium, this ratio remains constant, independent of wavelength throughout the line, overall intensity, and height in the chromosphere. Two rare-earth lines in the wing of the H-line show no spicular structure at all.
  2. The line-of-sight velocities of many features reverse as a function of time, although most spicules show velocities in only one direction. The simultaneous spectra at two heights show most spicules to have the same line-of-sight velocity at both. There may be an additional class of features, mostly rapidly moving, whose members have line-of-sight velocities that increase with height. These features comprise perhaps 10% of the total. Velocity changes occur simultaneously, to within 20 sec, at two heights separated by 1800 km, indicating velocities of propagation of hundreds of km/sec. The velocity field of individual features is often quite complicated; many spectral features are inclined to the direction of dispersion, implying that differential mass motions are present.
  3. The existence of anomalously broad H and K profiles is real. Even with high dispersion and the best seeing, such profiles are not resolved into smaller features. The central reversal in K, H and Hα appears to remain unshifted when the wings are displaced in wavelength, indicating that the reversal is non-spicular.
  相似文献   
168.
A rotating, acoustic gas bubble detector, BOB (Bubble OBservatory) module was deployed during two surveys, conducted in 2009 and 2011 respectively, to study the temporal variations of gas emissions from the Marmara seafloor, along the North Anatolian Fault zone. The echosounder mounted on the instrument insonifies an angular sector of 7° during a given duration (of about 1 h). Then it rotates to the next, near-by angular sector and so forth. When the full angular domain is insonified, the “pan and tilt system” rotates back to its initial position, in order to start a new cycle (of about 1 day). The acoustic data reveal that gas emission is not a steady process, with observed temporal variations ranging between a few minutes and 24 h (from one cycle to the other). Echo-integration and inversion performed on the acoustic data as described in the companion paper of Leblond et al. (Mar Geophys Res, 2014), also indicate important variations in, respectively, the target strength and the volumetric flow rates of individual sources. However, the observed temporal variations may not be related to the properties of the gas source only, but reflect possible variations in sea-bottom currents, which could deviate the bubble train towards the neighboring sector. During the 2011 survey, a 4-component ocean bottom seismometer (OBS) was co-located at the seafloor, 59 m away from the BOB module. The acoustic data from our rotating, monitoring system support, but do not provide undisputable evidence to confirm, the hypothesis formulated by Tary et al. (2012), that the short-duration, non-seismic micro-events recorded by the OBS are likely produced by gas-related processes within the near seabed sediments. Hence, the use of a multibeam echosounder, or of several split beam echosounders should be preferred to rotating systems, for future experiments.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号