全文获取类型
收费全文 | 1569篇 |
免费 | 57篇 |
国内免费 | 11篇 |
专业分类
测绘学 | 72篇 |
大气科学 | 132篇 |
地球物理 | 355篇 |
地质学 | 504篇 |
海洋学 | 129篇 |
天文学 | 277篇 |
综合类 | 6篇 |
自然地理 | 162篇 |
出版年
2021年 | 11篇 |
2020年 | 16篇 |
2019年 | 21篇 |
2018年 | 37篇 |
2017年 | 39篇 |
2016年 | 42篇 |
2015年 | 31篇 |
2014年 | 44篇 |
2013年 | 86篇 |
2012年 | 59篇 |
2011年 | 65篇 |
2010年 | 68篇 |
2009年 | 81篇 |
2008年 | 72篇 |
2007年 | 69篇 |
2006年 | 55篇 |
2005年 | 56篇 |
2004年 | 44篇 |
2003年 | 44篇 |
2002年 | 45篇 |
2001年 | 27篇 |
2000年 | 46篇 |
1999年 | 26篇 |
1998年 | 27篇 |
1997年 | 21篇 |
1996年 | 25篇 |
1995年 | 21篇 |
1994年 | 13篇 |
1993年 | 23篇 |
1992年 | 25篇 |
1991年 | 14篇 |
1990年 | 24篇 |
1989年 | 19篇 |
1988年 | 18篇 |
1987年 | 18篇 |
1986年 | 15篇 |
1985年 | 18篇 |
1984年 | 23篇 |
1983年 | 27篇 |
1982年 | 21篇 |
1981年 | 16篇 |
1980年 | 16篇 |
1977年 | 10篇 |
1976年 | 10篇 |
1975年 | 13篇 |
1974年 | 10篇 |
1973年 | 10篇 |
1972年 | 20篇 |
1971年 | 13篇 |
1969年 | 9篇 |
排序方式: 共有1637条查询结果,搜索用时 11 毫秒
211.
We present a study at HST-like spatial resolution of the gas kinematics in the innermost part of NGC 1068. This work is based
on observations with the integral field spectrograph TIGER mounted on the CFH Telescope. We first derived the final spatial
PSF of our data and checked its photometric integrity by comparison with two independent [OIII] HST images (FOC and WFPC2).
We then performed a spatial deconvolution of the data-cube constraining the sum of the deconvolved velocity-slices to be as
close as possible to the HST image. The resulting deconvolved data-cube has then a resolution similar to the HST image. It
shows that the velocity distribution of the inner clouds detected with the HST differs strongly from a simple NE-SW ejection
flow. We thus show that the previous interpretations of the double-peaked lines NE of the nucleus must be revisited.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
212.
The generalized Eddington approximation, obtained by the three-stream division of the radiation field, is used to compute the intensity falling on the nucleus head of a spherically-symmetric comet illuminated by parallel solar radiation. 相似文献
213.
J.B. Wilson 《Estuarine, Coastal and Shelf Science》1983,17(5):592-593
214.
Solar limb brightening measurements at a wavelength of 3.3 mm were made during the 30 June 1973 total solar eclipse from a site at Lake Rudolf, Kenya. The results show that at this wavelength there is a limb brightening of about 20%, occurring within one half arc min of the limb. 相似文献
215.
216.
Eliane Guedes Monica Heilbron Paulo M. Vasconcelos Cludio de Morisson Valeriano Júlio Csar Horta de Almeida Wilson Teixeira Antonio Thomaz Filho 《Journal of South American Earth Sciences》2005,18(3-4):371-382
New K–Ar and 40Ar/39Ar data of tholeiitic and alkaline dike swarms from the onshore basement of the Santos Basin (SE Brazil) reveal Mesozoic and Tertiary magmatic pulses. The tholeiitic rocks (basalt, dolerite, and microgabbro) display high TiO2 contents (average 3.65 wt%) and comprise two magmatic groups. The NW-oriented samples of Group A have (La/Yb)N ratios between 15 and 32.3 and range in age from 192.9±2.2 to 160.9±1.9 Ma. The NNW-NNE Group B samples, with (La/Yb)N ratios between 7 and 16, range from 148.3±3 to 133.9±0.5 Ma. The alkaline rocks (syenite, trachyte, phonolite, alkaline basalts, and lamprophyre) display intermediate–K contents and comprise dikes, plugs, and stocks. Ages of approximately 82 Ma were obtained for the lamprophyre dikes, 70 Ma for the syenite plutons, and 64–59 Ma for felsic dikes. Because Jurassic–Early Cretaceous basic dikes have not been reported in SE Brazil, we might speculate that, during the emplacement of Group A dikes, extensional stresses were active in the region before the opening of the south Atlantic Ocean and coeval with the Karoo magmatism described in South Africa. Group B dikes yield ages compatible with those obtained for Serra Geral and Ponta Grossa magmatism in the Paraná Basin and are directly related to the breakup of western Gondwana. Alkaline magmatism is associated with several tectonic episodes that postdate the opening of the Atlantic Ocean and related to the upwelling of the Trindade plume and the generation of Tertiary basins southeast of Brazil. In the studied region, alkaline magmatism can be subdivided in two episodes: the first one represented by lamprophyre dykes of approximately 82 Ma and the second comprised of felsic alkaline stocks of approximately 70 Ma and associated dikes ranging from 64 to 59 Ma.
Resumo
Novos dados K–Ar e 40Ar/39Ar de enxames de diques toleíticos e alcalinos localizados no embasamento onshore da Bacia de Santos (SE Brasil) apontam para diferentes pulsos magmáticos ocorridos entre o Jurássico e o Terciário. Os diques de rochas toleíticas (basalto, diabásio e microgabro), são mais velhos, exibem altos teores de TiO2 (3,65% peso na média) e podem ser subdivididos em dois grupos magmáticos. O Grupo A aflora a norte da Bacia de Resende, compreende diques orientados na direção NW, com razões (La/Yb)N entre 15 e 32,3, e idades entre 192.9±2.2 e 160.9±1.9 Ma. O Grupo B aflora a sul e a leste da Bacia de Resende, engloba diques orientados na direção NNW e NNE, com razões (La/Yb)N entre 7 e 16, e idades obtidas entre 148.3±3 e 133.9±0.5 Ma. As rochas alcalinas (sienito, traquito, fonolito, basalto alcalino e lamprófiro) possuem teores médios de K, e afloram como diques, plugs e stocks.. As idades obtidas são de ca. 82 Ma para os diques lamprofíricos, de ca. 70 Ma para os plugs sieníticos, e entre 64 e 59 Ma para os diques félsicos. Como estas idades Jurássicas para diques toleíticos ainda não foram descritas para a região sudeste do Brasil, pode-se especular que durante o emplacement dos diques do GrupoA o cenário tectônico indicaria esforços extensionais anteriores à abertura do Oceano atlântico Sul, e contemporânea ao derrame basáltico do Karoo na África do Sul. Já os diques do Grupo B são contemporâneos ao magmatismo Serra Geral e ao enxame de diques de Ponta Grossa, e portanto este episódio está diretamente relacionado à separação entre o Brasil e África no Cretáceo. O magmatismo alcalino está associado a diversos episódios tectônicos que sucedem à abertura do Oceano Atlântico Sul e que resultaram no desenvolvimento das Bacias terciárias do sudeste brasileiro. Está provavelmente relacionado à chegada da Pluma de Trindade e as idades obtidas para a região em estudo indicam que o magmatismo lamprofírico é mais antigo (ca. 82 Ma), seguido pelos plútons sieníticos (ca. 70 Ma) e diques associados (64 a 59 Ma.) 相似文献217.
Time variations in the Earths gravity field at periods longer than 1 year, for degree-two spherical harmonics, C21, S21, and C20, are estimated from accurately measured Earth rotational variations. These are compared with predictions of atmospheric, oceanic, and hydrologic models, and with independent satellite laser ranging (SLR) results. There is remarkably good agreement between Earth rotation and model predictions of C21 and S21 over a 22-year period. After decadal signals are removed, Earth-rotation-derived interannual C20 variations are dominated by a strong oscillation of period about 5.6 years, probably due to uncertainties in wind and ocean current estimates. The model-predicted C20 agrees reasonably well with SLR observations during the 22-year period, with the exception of the recent anomaly since 1997/1998. 相似文献
218.
P. R. Wilson 《Solar physics》1977,55(1):35-45
The configuration of a magnetic flux element in a static, compressible, gravitationally stratified plasma is considered. Under isothermal conditions an exact force-free solution is given for a two-dimensional cartesian flux sheath but for an axi-symmetric element, i.e. a flux tube, approximate solutions, applicable only to thin flux tubes, are obtained.When the isothermal condition is relaxed within the flux tube, non-force-free solutions are obtained exhibiting either a temperature excess or deficit on the axis of the flux tube. Using these solutions, the total potential energy of such a tube and its surroundings may be calculated. By comparing this with the total energy of an equivalent system ofn flux tubes the temperature excess or deficit in these tubes corresponding to the adiabatic subdivision of the single tube is determined.It is shown that under non-adiabatic conditions the subdivision process must be endothermic (i.e., external energy is required) if the temperature within the original tube is significantly less than its surroundings but exothermic if the temperatures are comparable. Thus it is conjectured that magnetic structures are less susceptible to subdivision if they are significantly cooler than their surroundings. 相似文献
219.
Two independent sets of high resolution time series spectra of the CaII H and K emission obtained at the Solar Tower and at the Big Dome of the Sacramento Peak Observatory on September 11th, 1971 are reported. The evolutionary behaviour of the emission first reported by Wilson and Evans is confirmed but the detail of the evolution is found to be more complex. In one case, a doubly peaked feature showing some K3 emission evolves into a single K2 (red) peak with no K3 emission. Coincidentally, a neighbouring doubly peaked feature evolves to a very strong blue peak. In an entirely independent sequence a doubly peaked feature evolves into a single red peak. The K2 emission then fades completely although the continuum threads are still strong. Finally a strong K2 blue peak appears. These developments are confirmed by intensity profiles obtained from the spectra.Image motion during the sequences is measured using slit-jaw photographs and changes in the overall pattern of the spectra. It is found to be less than the size of the individual features, i.e. 1–2.While considering that the evolution can be explained by the relative motion of one feature with respect to another during the sequence, it is shown that it is possible to account for all these examples in this way only by invoking coincidence of a very high order.It is concluded that in these cases the observed evolution of the K2 emission is due to temporal variations in the physical conditions which give rise to them. 相似文献
220.