全文获取类型
收费全文 | 90944篇 |
免费 | 1444篇 |
国内免费 | 823篇 |
专业分类
测绘学 | 2687篇 |
大气科学 | 7076篇 |
地球物理 | 18527篇 |
地质学 | 30555篇 |
海洋学 | 7646篇 |
天文学 | 20716篇 |
综合类 | 268篇 |
自然地理 | 5736篇 |
出版年
2021年 | 583篇 |
2020年 | 712篇 |
2019年 | 776篇 |
2018年 | 1660篇 |
2017年 | 1582篇 |
2016年 | 2242篇 |
2015年 | 1481篇 |
2014年 | 2203篇 |
2013年 | 4588篇 |
2012年 | 2322篇 |
2011年 | 3274篇 |
2010年 | 2785篇 |
2009年 | 3946篇 |
2008年 | 3512篇 |
2007年 | 3230篇 |
2006年 | 3298篇 |
2005年 | 2808篇 |
2004年 | 2843篇 |
2003年 | 2659篇 |
2002年 | 2553篇 |
2001年 | 2300篇 |
2000年 | 2278篇 |
1999年 | 2003篇 |
1998年 | 1919篇 |
1997年 | 1923篇 |
1996年 | 1684篇 |
1995年 | 1618篇 |
1994年 | 1464篇 |
1993年 | 1324篇 |
1992年 | 1225篇 |
1991年 | 1125篇 |
1990年 | 1331篇 |
1989年 | 1152篇 |
1988年 | 1050篇 |
1987年 | 1266篇 |
1986年 | 1121篇 |
1985年 | 1367篇 |
1984年 | 1562篇 |
1983年 | 1479篇 |
1982年 | 1383篇 |
1981年 | 1267篇 |
1980年 | 1122篇 |
1979年 | 1062篇 |
1978年 | 1123篇 |
1977年 | 1022篇 |
1976年 | 958篇 |
1975年 | 889篇 |
1974年 | 927篇 |
1973年 | 965篇 |
1972年 | 601篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
911.
In a previous paper we presented a low-resolution (2°×2°) survey of radio recombination lines (RRLs) at 327 MHz in the longitude
rangel=330° to 0° to 89°. In this paper, we present the results of a higher resolution (2°×6′) survey of RRLs from seven 2°-wide
fields and two 6°-wide fields in the same longitude range. Observations were made using the Ooty Radio Telescope (ORT). A
total of 252 spectra that were obtained are presented. RRLs were detected in almost all the individual positions within the
fields withl<35° and at several individual positions within the fields in the longitude rangel=35° to 85°. Detailed analysis of the data towards the field centered at G45.5+0.0, shows that the line emission consists
of discrete zones of ionized gas. The angular extent of these zones are likely to be one degree or more corresponding to a
linear size of >110 pc at the kinematic distance. 相似文献
912.
Some evidence of climate change in twentieth-century India 总被引:1,自引:0,他引:1
The study of climate changes in India and search for robust evidences are issues of concern specially when it is known that
poor people are very vulnerable to climate changes. Due to the vast size of India and its complex geography, climate in this
part of the globe has large spatial and temporal variations. Important weather events affecting India are floods and droughts,
monsoon depressions and cyclones, heat waves, cold waves, prolonged fog and snowfall. Results of this comprehensive study
based on observed data and model reanalyzed fields indicate that in the last century, the atmospheric surface temperature
in India has enhanced by about 1 and 1.1°C during winter and post-monsoon months respectively. Also decrease in the minimum
temperature during summer monsoon and its increase during post-monsoon months have created a large difference of about 0.8°C
in the seasonal temperature anomalies which may bring about seasonal asymmetry and hence changes in atmospheric circulation.
Opposite phases of increase and decrease in the minimum temperatures in the southern and northern regions of India respectively
have been noticed in the interannual variability. In north India, the minimum temperature shows sharp decrease of its magnitude
between 1955 and 1972 and then sharp increase till date. But in south India, the minimum temperature has a steady increase.
The sea surface temperatures (SST) of Arabian Sea and Bay of Bengal also show increasing trend. Observations indicate occurrence
of more extreme temperature events in the east coast of India in the recent past. During summer monsoon months, there is a
decreasing (increasing) trend in the frequency of depressions (low pressure areas). In the last century the frequency of occurrence
of cyclonic storms shows increasing trend in the month of November. In addition there is increase in the number of severe
cyclonic storms crossing Indian Coast. Analysis of rainfall amount during different seasons indicate decreasing tendency in
the summer monsoon rainfall over Indian landmass and increasing trend in the rainfall during pre-monsoon and post-monsoon
months. 相似文献
913.
914.
Preservation of biological information in thermal spring deposits: developing a strategy for the search for fossil life on Mars 总被引:1,自引:0,他引:1
Current interpretations of the early history of Mars suggest many similarities with the early Earth and therefore raise the possibility that the Archean and Proterozoic history of life on Earth could have a counterpart on Mars. Terrestrial experience suggests that, with techniques that can be employed remotely, ancient springs, including thermal springs, could well yield important information. By delivering water and various dissolved species to the sunlit surface of Mars, springs very likely created an environment suitable for life, which could have been difficult, if not impossible, to attain elsewhere. The chemical and temperature gradients associated with thermal springs sort organisms into sharply delineated, distinctive and different communities, and so diverse organisms are concentrated into relatively small areas in a predictable and informative fashion. A wide range of metabolic strategies are concentrated into small areas, thus furnishing a useful and representative sampling of the existing biota. Mineral-charged springwaters frequently deposit chemical precipitates of silica and/or carbonate which incorporate microorganisms and preserve them as fossils. The juxtaposition of stream valley headwaters with volcanoes and impact craters on Mars strongly implies that subsurface heating of groundwater created thermal springs. On Earth, thermal springs create distinctive geomorphic features and chemical signatures which can be detected by remote sensing. Spring deposits can be quite different chemically from adjacent rocks. Individual springs can be hundreds of meters wide, and complexes of springs occupy areas up to several kilometers wide. Benthic microbial mats and the resultant stromatolites occupy a large fraction of the available area. The relatively high densities of fossils and microbial mat fabrics within these deposits make them highly prospective in any search for morphological evidence of life, and there are examples of microbial fossils in spring deposits as old as 300 Myr. 相似文献
915.
Unattenuated solar photo rate coefficients and excess energies for dissociation, ionization, and dissociative ionization are presented for atomic and molecular species that have been identified or are suspected to exist in the atmospheres of planets, satellites (moons), comets, or as pollutants in the Earth atmosphere. The branching ratios and cross sections with resonances have been tabulated to the greatest detail possible and the rate coefficients and excess energies have been calculated from them on a grid of small wavelength bins for the quiet and the active Sun at 1 AU heliocentric distance. 相似文献
916.
J. I. S. Zonneveld Dr. 《GeoJournal》1983,7(2):121-129
Modern geography is based on ecological thinking. In this paper some geoecological notions are discussed: the scales and levels regarding spatial magnitude and complexity, the “hierarchy” of landscape units and espcially the scale of time and the concept of the supper-imposing “stamps”. This last mentioned concept is very important in relation to the problem regarding regionalization and its possibilities. Three “stamps” are recognized: 1. the substrate, 2. the climatic conditions, 3. the human (noötic) influence. Holistic regionalization in which all elements (including the human influences) are telling to full advantage, is unsatisfactory. 相似文献
917.
Solar Physics - We study a quiet-Sun blowout jet which was observed on 2014 May 16 by the instruments on board the Solar Dynamics Observatory (SDO). We find the twin CME as jet-like and bubble-like... 相似文献
918.
919.
H. J. Peng Q. Gao Z. G. Wu W. X. Zhong 《Celestial Mechanics and Dynamical Astronomy》2011,110(4):319-342
In this paper, from a Hamiltonian point of view, the nonlinear optimal control problems are transformed into nonlinear two-point
boundary value problems, and a symplectic adaptive algorithm based on the dual variational principle is proposed for solving
the nonlinear two-point boundary value problem. The state and the costate variables within a time interval are approximated
by using the Lagrange polynomial and the costate variables at two ends of the time interval are taken as independent variables.
Then, based on the dual variational principle, the nonlinear two-point boundary value problems are replaced by a system of
nonlinear equations which can preserve the symplectic structure of the nonlinear optimal control problem. Furthermore, the
computational efficiency of the proposed symplectic algorithm is improved by using the adaptive multi-level iteration idea.
The performance of the proposed algorithm is tested by the problems of Astrodynamics, such as the optimal orbital rendezvous
problem and the optimal orbit transfer between halo orbits. 相似文献
920.