全文获取类型
收费全文 | 36331篇 |
免费 | 6608篇 |
国内免费 | 8461篇 |
专业分类
测绘学 | 3275篇 |
大气科学 | 6000篇 |
地球物理 | 6325篇 |
地质学 | 20064篇 |
海洋学 | 4070篇 |
天文学 | 3610篇 |
综合类 | 2389篇 |
自然地理 | 5667篇 |
出版年
2024年 | 127篇 |
2023年 | 474篇 |
2022年 | 1235篇 |
2021年 | 1435篇 |
2020年 | 1325篇 |
2019年 | 1576篇 |
2018年 | 1220篇 |
2017年 | 1440篇 |
2016年 | 1513篇 |
2015年 | 1643篇 |
2014年 | 2098篇 |
2013年 | 2101篇 |
2012年 | 2232篇 |
2011年 | 2389篇 |
2010年 | 2032篇 |
2009年 | 2583篇 |
2008年 | 2524篇 |
2007年 | 2636篇 |
2006年 | 2579篇 |
2005年 | 2444篇 |
2004年 | 2100篇 |
2003年 | 1992篇 |
2002年 | 1709篇 |
2001年 | 1541篇 |
2000年 | 1525篇 |
1999年 | 1343篇 |
1998年 | 1161篇 |
1997年 | 822篇 |
1996年 | 643篇 |
1995年 | 577篇 |
1994年 | 557篇 |
1993年 | 471篇 |
1992年 | 325篇 |
1991年 | 295篇 |
1990年 | 188篇 |
1989年 | 152篇 |
1988年 | 128篇 |
1987年 | 69篇 |
1986年 | 39篇 |
1985年 | 35篇 |
1984年 | 18篇 |
1983年 | 13篇 |
1982年 | 11篇 |
1981年 | 9篇 |
1980年 | 9篇 |
1979年 | 6篇 |
1978年 | 14篇 |
1977年 | 5篇 |
1976年 | 4篇 |
1954年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 515 毫秒
121.
L. I. Matveenko 《Astronomy Letters》2001,27(11):686-690
The core (injector) and the jet (relativistic plasma outflow) of AGN objects are surrounded by an ionized medium, an H II region observed in emission lines. The synchrotron radiation from the core and the jet is observed through a thin screen that cocoons the structure under consideration. The screen transparency depends on wavelength and distance from the injector. We consider the objects 3C 345 and 1803+784 whose core emission at decimeter wavelengths is absorption by more than 25 dB. The visible bright compact component is the bright nearby portion of the jet that extends outside the dense part of the screen. We explore the possibility of measuring the screen transparency from absorption in Hα recombination lines with different quantum numbers at centimeter wavelengths. 相似文献
122.
123.
124.
The sand–loess transition zone in north China is sensitive to climate change, and is an ideal place to investigate past environmental changes. However, past climate change at millennial–centennial timescales in this region has not been well reconstructed because of limited numerical dating. Alternations of sandy loam soils with aeolian sand layers in the Mu Us and Otindag sand fields, which lie along the sand–loess transition zone, indicate multiple intervals of dune activity and stability. This change is probably a response to variations of the East Asian monsoon climate during the late Quaternary. The single aliquot regeneration (SAR) optically stimulated luminescence (OSL) dating protocol, which has been successfully applied to aeolian deposits worldwide, is applied to these two sand fields in this study. The OSL ages provide reliable constraints for reconstruction of past climate changes at suborbital timescale. Sections in both sand fields contain aeolian sand beds recording millennial‐scale episodes of dry climate and widespread dune activation, including episodes at about the same time as Heinrich Event 5 and the Younger Dryas in the North Atlantic region. These results demonstrate the potential of aeolian sediments in semi‐arid north China to record millennial‐scale climatic events, and also suggest that dry–wet climate variation at the desert margin in China may be linked to climatic change elsewhere in the Northern Hemisphere, through atmospheric circulation. This article was published online on 27 November 2008. An error was subsequently identified. This notice is included in the online and print versions to indicate that both have been corrected (16 December 2008). Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
125.
M. A. Smith 《第四纪科学杂志》2009,24(7):747-760
Puritjarra rock shelter provides a long record of late Quaternary vegetation in the Australian arid zone. Analysis of the sedimentary history of this rock shelter is combined with reanalysis of charcoal and phytolith records to provide a first‐order picture of changing landscapes in western Central Australia. These show a landscape responding to increasing aridity from 45 ka with deflation of clay‐rich red palaeosols (<45 ka) and sharp declines in grassland and other vegetation at 40–36 ka, and at the beginning of the Last Glacial Maximum (LGM) (24 ka). Vegetation in the catchment of the rock shelter recovered after 15 ka with expansion of both acacia woodland and spinifex grasslands, registering stronger summer rainfall in the interior of the continent. By 8.3 ka re‐vegetation of local palaeosols and dunes had choked off sediment supply to the rock shelter and the character of the sediments changed abruptly. Poaceae values peaked at 5.8 ka, suggesting the early–mid Holocene climatic optimum in Central Australia is bracketed between 8.3 and 5.8 ka. Local vegetation was disrupted in the late Holocene with a sharp decline in Poaceae at 3.8 ka, coinciding with an abrupt intensification of ENSO. Local grasslands recovered over the next two millennia and by 1.5 ka the modern vegetation appears to have become established. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
126.
127.
128.
129.
130.