全文获取类型
收费全文 | 265843篇 |
免费 | 5198篇 |
国内免费 | 3366篇 |
专业分类
测绘学 | 7007篇 |
大气科学 | 19594篇 |
地球物理 | 55540篇 |
地质学 | 93253篇 |
海洋学 | 22277篇 |
天文学 | 57553篇 |
综合类 | 1044篇 |
自然地理 | 18139篇 |
出版年
2021年 | 2245篇 |
2020年 | 2594篇 |
2019年 | 2860篇 |
2018年 | 4542篇 |
2017年 | 4261篇 |
2016年 | 6231篇 |
2015年 | 4226篇 |
2014年 | 6918篇 |
2013年 | 14197篇 |
2012年 | 6892篇 |
2011年 | 8749篇 |
2010年 | 7832篇 |
2009年 | 10356篇 |
2008年 | 9008篇 |
2007年 | 8742篇 |
2006年 | 9713篇 |
2005年 | 7939篇 |
2004年 | 7795篇 |
2003年 | 7282篇 |
2002年 | 6822篇 |
2001年 | 6034篇 |
2000年 | 5964篇 |
1999年 | 5223篇 |
1998年 | 5239篇 |
1997年 | 5038篇 |
1996年 | 4684篇 |
1995年 | 4434篇 |
1994年 | 4107篇 |
1993年 | 3858篇 |
1992年 | 3636篇 |
1991年 | 3592篇 |
1990年 | 3769篇 |
1989年 | 3521篇 |
1988年 | 3306篇 |
1987年 | 3850篇 |
1986年 | 3407篇 |
1985年 | 4228篇 |
1984年 | 4737篇 |
1983年 | 4412篇 |
1982年 | 4313篇 |
1981年 | 3929篇 |
1980年 | 3641篇 |
1979年 | 3508篇 |
1978年 | 3488篇 |
1977年 | 3282篇 |
1976年 | 3046篇 |
1975年 | 2961篇 |
1974年 | 2915篇 |
1973年 | 3078篇 |
1972年 | 2026篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
411.
412.
413.
A. G. Nikoghossian 《Astrophysics》2006,49(2):228-239
The problem of finding statistical averages which characterize the radiation field in a semi-infinite atmosphere has been
discussed by the author in a series of papers. In this paper some of the results obtained there are generalized to media of
finite optical thickness. The average number of scattering events undergone by a photon during its random walk in an atmosphere
and the average time required for this are determined. Three different types of averages are considered, depending on the
photon characteristics over which the averages are taken. Special attention is devoted to the asymptotic behavior of these
quantities for media with large optical depths taking the influence of continuum absorption into account.
__________
Translated from Astrofizika, Vol. 49, No. 2, pp. 263–275 (May 2006). 相似文献
414.
The large-scale structure associated with the 2′N HNCO peak in Sgr B2 [Minh, Y.C., Haikala, L., Hjalmarson, Å., Irvine, W.M., 1998. ApJ 498, 261 (Paper I)] has been investigated. A ring-like morphology of the HNCO emission has been found; this structure may be colliding with the Principal Cloud of Sgr B2. This “HNCO Ring” appears to be centered at (l,b) = (0.7°,−0.07°), with a radius of 5 pc and a total mass of 1.0 × 105 to 1.6 × 106 M. The expansion velocity of the Ring is estimated to be 30–40 km s−1, which gives an expansion time scale of 1.5 × 105 year. The morphology suggests that collision between the Ring and the Principal Cloud may be triggering the massive star formation in the Sgr B2 cloud sequentially, with the latest star formation taking place at the 2′N position. The chemistry related to HNCO is not certain yet, but if it forms mainly via reaction with the evaporated OCN− from icy grain mantles, the observed enhancement of the HNCO abundance can be understood as resulting from shocks associated with the collision between the Principal Cloud and the expanding HNCO Ring. 相似文献
415.
To derive a matched filter for detecting a weak target signal in a hyperspectral image, an estimate of the band-to-band covariance of the target-free background scene is required. We investigate the effects of including some of the target signal in the background scene. Although the covariance is contaminated by the presence of a target signal (there is increased variance in the direction of the target signature), we find that the matched filter is not necessarily affected. In fact, if the variation in plume strength is strictly uncorrelated with the variation in background spectra, the matched filter and its signal-to-clutter ratio (SCR) performance will not be impaired. While there is little a priori reason to expect significant correlation between the plume and the background, there usually is some residual correlation, and this correlation leads to a suppressing effect that limits the SCR obtainable even for strong plumes. These effects are described and quantified analytically, and the crucial role of this correlation is illustrated with some numerical examples using simulated plumes superimposed on real hyperspectral imagery. In one example, we observe an order-of-magnitude loss in SCR for a matched filter based on the contaminated covariance. 相似文献
416.
417.
This research paper analyses the grain-size characteristics of the Quaternary deposits at Xingshan near Siping, Jilin province in China by employing graphic measures to study the grain size distribution and its mode of transport and deposition. The Quaternary deposits at Xingshan lie unconformable on Cretaceous rocks made of siltstone, mudstone and sandstone. The average grain size is between 8.06 to 8.55Φ (0.002 6 ~0.003 7 mm). The Quaternary deposits at Xingshan mainly compose of very fine silt to clay. The compositions of the grade are clay 63% and silt 37%. The clay size components are weathered debris transported and deposited by flowing water from the SE highlands or hills to the low lying NW Xingshan plains whereas the silty components accumulated by aoelian process. The Quaternary deposits at Xingshan accumulated in the middle and late Pleistocene interglacial periods from (459.12~39.03) ka to (88.92~7.56) ka. The standard deviation ranged from 0.96 to 1.36Φ, indicating that the sediments are moderately to poorly sorted, Coefficient of skewness ranged from 0.16~0.31 with an average skewness of 0.218, (Positively skewed towards fine). Kurtosis values (0.84~1.05) from the grain size distribution and visual inspection of the frequency curves indicate platykurtic to mesokurtic curves and unimodal to bimodal grain-size distribution. The type of deposit formation is sand dune and the source is at a distal from its provenance. 相似文献
418.
419.
420.