全文获取类型
收费全文 | 1017篇 |
免费 | 64篇 |
国内免费 | 13篇 |
专业分类
测绘学 | 50篇 |
大气科学 | 157篇 |
地球物理 | 231篇 |
地质学 | 421篇 |
海洋学 | 59篇 |
天文学 | 101篇 |
综合类 | 2篇 |
自然地理 | 73篇 |
出版年
2023年 | 6篇 |
2022年 | 7篇 |
2021年 | 23篇 |
2020年 | 29篇 |
2019年 | 25篇 |
2018年 | 54篇 |
2017年 | 30篇 |
2016年 | 54篇 |
2015年 | 36篇 |
2014年 | 52篇 |
2013年 | 66篇 |
2012年 | 54篇 |
2011年 | 68篇 |
2010年 | 55篇 |
2009年 | 79篇 |
2008年 | 61篇 |
2007年 | 44篇 |
2006年 | 50篇 |
2005年 | 39篇 |
2004年 | 38篇 |
2003年 | 32篇 |
2002年 | 18篇 |
2001年 | 18篇 |
2000年 | 16篇 |
1999年 | 13篇 |
1998年 | 18篇 |
1997年 | 11篇 |
1996年 | 8篇 |
1995年 | 8篇 |
1994年 | 5篇 |
1993年 | 8篇 |
1992年 | 5篇 |
1991年 | 7篇 |
1990年 | 2篇 |
1989年 | 5篇 |
1988年 | 4篇 |
1987年 | 6篇 |
1985年 | 2篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1978年 | 2篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1974年 | 2篇 |
1969年 | 5篇 |
1968年 | 3篇 |
1967年 | 4篇 |
1964年 | 1篇 |
排序方式: 共有1094条查询结果,搜索用时 31 毫秒
981.
Ewald Puchwein Stefan Hilbert 《Monthly notices of the Royal Astronomical Society》2009,398(3):1298-1308
We use ray-tracing through the Millennium simulation to study how secondary matter structures along the line-of-sight and the stellar mass in galaxies affect strong cluster lensing, in particular the cross-section for giant arcs. Furthermore, we investigate the distribution of the cluster Einstein radii and the radial distribution of giant arcs. We find that additional structures along the line-of-sight increase the strong-lensing optical depth by ∼10–25 per cent , while strong-lensing cross-sections of individual clusters are frequently boosted by as much as ∼50 per cent . The enhancement is mainly due to structures that are not correlated with the lens. Cluster galaxies increase the strong-lensing optical depth by up to a factor of 2, while interloping galaxies are not significant. We conclude that these effects need to be taken into account for predictions of the giant arc abundance, but they are not large enough to fully account for the reported discrepancy between predicted and observed abundances.
Furthermore, we find that Einstein radii defined via the area enclosed by the critical curve are 10–30 per cent larger than those defined via radial surface mass density profiles. The contributions of radial and tangential arcs to the radial distribution of arcs can be clearly distinguished. The radial distribution of tangential arcs is very broad and extends out to several Einstein radii. Thus, individual arcs are not well suited for constraining Einstein radii. 相似文献
Furthermore, we find that Einstein radii defined via the area enclosed by the critical curve are 10–30 per cent larger than those defined via radial surface mass density profiles. The contributions of radial and tangential arcs to the radial distribution of arcs can be clearly distinguished. The radial distribution of tangential arcs is very broad and extends out to several Einstein radii. Thus, individual arcs are not well suited for constraining Einstein radii. 相似文献
982.
A three-point differencing scheme for the diffusion–convection equation is presented that offers all the advantages of both the central and the one-sided ('upwind') differencing scheme without suffering from their drawbacks. Specifically, the scheme is conservative, unconditionally stable, and second-order-accurate in space. It is free of oscillations and over- or undershoots, simple to code, and requires essentially no more computing time than the one-sided scheme. Although known for a relatively long time in numerical mathematics, the scheme apparently has not received sufficient attention from modellers of hydrothermal systems or contaminant transport in the geosciences. In order to fill this gap a comparison is made between this scheme and the widely used one-sided scheme for the transient diffusion–convection equation in different time discretizations. The results are discussed taking into account other approaches towards minimizing numerical diffusion. 相似文献
983.
Martin Bange Stefan Jordan Michael Biermann Thomas Kämpke Ralf-Dieter Scholz 《Experimental Astronomy》2002,13(2):101-117
We propose an object detection algorithm which is efficient and fast enough to be used in (almost) real time with the limited
computer capacities onboard satellites. For stars below the saturation limit of the CCD detectors it is based on a four neighbourhood
local maximum criterion in order to find the centre of a stellar image. For saturated stars it is based on the assumption
that the image is increasing monotonically towards the centre in the unsaturated part of the image. The algorithm also calculates
approximate stellar magnitudes and efficiently rejects most of the cosmics which would otherwise lead to a large number of
false detections. The quality of the algorithm was evaluated with the help of a large set of simulated data for the DIVA satellite
mission; different assumptions were made for the noise level, and the presence of cosmics or for a variable sky background.We
could show that our algorithm fulfills the requirements for DIVA; only in the case of simulated images which included the
bright galaxy M31 some fainter stars could not be detected in the galaxy's vicinity. Since stellar images contain large areas
without any stars, we propose an additional block-skipping algorithm which can be coded on special-purpose hardware.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
984.
Aspects of open ocean deep convection variability are explored with a two-box model. In order to place the model in a region
of parameter space relevant to the real ocean, it is fitted to observational data from the Labrador Sea. A systematic fit
to OWS Bravo data allows us to determine the model parameters and to locate the position of the Labrador Sea on a stability
diagram. The model suggests that the Labrador Sea is in a bistable regime where winter convection can be either “on” or “off ”,
with both these possibilities being stable climate states. When shifting the surface buoyancy forcing slightly to warmer or
fresher conditions, the only steady solution is one without winter convection. We then introduce short-term variability by
adding a noise term to the surface temperature forcing, turning the box model into a stochastic climate model. The surface
forcing anomalies generated in this way induce jumps between the two model states. These state transitions occur on the interannual
to decadal time scale. Changing the average surface forcing towards more buoyant conditions lowers the frequency of convection.
However, convection becomes more frequent with stronger variability in the surface forcing. As part of the natural variability,
there is a non-negligible probability for decadal interruptions of convection. The results highlight the role of surface forcing
variability for the persistence of convection in the ocean. 相似文献
985.
Stefan W. Grab 《Geografiska Annaler: Series A, Physical Geography》2002,84(1):39-50
Limited research attention has focussed on turf exfoliation as a denudation process in mountain environments. This paper examines some characteristics of turf exfoliation forms identified within particular valley zones in the Drakensberg alpine belt. Morphological and sedimentological data are presented for turf exfoliated sites investigated in the Mashai Valley of eastern Lesotho. It is found that a variety of processes, including needle ice action, biological activity, fluvial processes and deflation, operating synergistically, are responsible for contemporary turf exfoliation in the high Drakensberg. It is apparent that the strong seasonality from mild, wet summers to cold, dry winters has helped induce the annual cycle of dominating processes. 相似文献
986.
Effects of cadmium exposure on plasma levels of calcitonin and free and protein-bound calcium were studied in vitellogenic female rainbow trout kept in brackish water (7%(.)). Fish were exposed to 100 μg cadmium litre−1 for four weeks. Exposure of female rainbow trout in the stage of vitellogenesis, with increased total plasma levels of calcium, resulted in a complex hypocalcemic response. Thus, hypocalcemia was found to be due to three different processes: (1) a decrease in the free plasma calcium, and a reduction in protein-bound calcium; due both to (2) decreased plasma levels of vitellogenin; and (3) a reduced binding of calcium to vitellogenin. These findings support the concept of an interference of cadmium with ionregulating tissues as a mechanism for hypocalcemia in rainbow trout. A direct effect on the vitellogenin-binding of calcium was also observed and reproductive function in the females was affected by decreased plasma levels of vitellogenin. In spite of the marked changes of plasma calcium in exposed fish, no significant effects on plasma calcitonin were observed, indicating a lack of a direct relationship between plasma calcium and calcitonin levels in rainbow trout. 相似文献
987.
The Northern Nordeste of Brazil has its short rainy season narrowly concentrated around March–April, when the interhemispheric southward gradient of sea surface temperature (SST) is weakest and the Intertropical Convergence Zone (ITCZ), which is the main rainbearing system for the Nordeste, reaches its southernmost position in the course of the year. The recurrent Secas (droughts) have a severe socio-economic impact in this semi-arid region. In drought years, the pre-season (October–January) rainfall is scarce, the interhemispheric SST gradient weakened and the basin-wide southerly (northerly) wind component enhanced (reduced), all manifestations of an anomalously far northward ITCZ position. Apart from this ensemble of Atlantic indicators, the Secas also tend to be preceded by anomalously warm equatorial Pacific waters in January. During El Niño years, an upper-tropospheric wave train extends from the equatorial eastern Pacific to the northern tropical Atlantic, affecting the patterns of upper-tropospheric topography and divergence, and hence of vertical motion over the Atlantic. The altered vertical motion leads to a weaker meridional pressure gradient on the equatorward flank of the North Atlantic subtropical high, and thus weaker North Atlantic tradewinds. The concomitant reduction of evaporation and wind stirring allows for warmer surface waters in the tropical North Atlantic and thus steeper interhemispheric meridional thermal gradient. Consequently, the ITCZ stays anomalously far North and the Nordeste rainy season becomes deficient. 相似文献
988.
989.
Experiments have been conducted to investigate the effects of temperature on the interfacial surface area and on the rate of oxygen transfer from air bubbles dispersed in aqueous NaCl solutions. Tests were also conducted to estimate the effects of salt concentration on the size of the bubbles. In addition to NaCl solutions, seawater was used in some tests. The temperature effects were investigated at 5, 10, 15, 20, 25, and 30°C. The results showed a pronounced effect of the salt on the size of the bubbles, which first decreased sharply with increasing concentration, but showed no further drop when the concentration was increased beyond 0.6 M. Both in seawater and in the 0.6 M solution, the mass transfer rate, KLA, increased almost linearly when temperature was increased within the range from 5 to 25°C. The salt solution, as well as the seawater, showed an increase of KLA of 60–70% over that in pure water at the same temperatures. This effect was the result of increased surface area of bubbles because of decreased coalescence. The increase in surface area was strongly temperature dependent, especially between 15 and 20°C. Contrary to this behavior the surface area in pure water showed, practically, no temperature dependence. The results are explained and discussed on the basis of ion-water interactions. 相似文献
990.
Henrik Lundstedt Anders Johannesson Göran Scharmer Jan Olof Stenflo Ulf Kusoffsky Birgitta Larsson 《Solar physics》1991,132(2):233-245
A high-resolution videomagnetograph that records the images of opposite circular polarization simultaneously has been constructed for the Swedish vacuum solar telescope at La Palma. Magnetograms are obtained by off-line integration of bursts consisting of typically 50 frames of 20 ms exposures, with bad frames rejected, and the frame-to-frame image motion of the remaining frames compensated for by cross-correlation techniques. The short exposures combined with frame selection and elimination of image motion optimizes the resolution and thereby also the S/N, allowing good magnetograms to be obtained with an effective exposure time of less than 1 s at an image scale of 0.1 pixel–1. The advantages and limitations of the system are discussed and compared with other techniques of making filter magnetograms are discussed. 相似文献