全文获取类型
收费全文 | 79375篇 |
免费 | 1491篇 |
国内免费 | 710篇 |
专业分类
测绘学 | 1894篇 |
大气科学 | 5540篇 |
地球物理 | 15810篇 |
地质学 | 28558篇 |
海洋学 | 7014篇 |
天文学 | 18031篇 |
综合类 | 230篇 |
自然地理 | 4499篇 |
出版年
2022年 | 483篇 |
2021年 | 833篇 |
2020年 | 905篇 |
2019年 | 968篇 |
2018年 | 2212篇 |
2017年 | 2035篇 |
2016年 | 2561篇 |
2015年 | 1442篇 |
2014年 | 2457篇 |
2013年 | 4212篇 |
2012年 | 2586篇 |
2011年 | 3482篇 |
2010年 | 2980篇 |
2009年 | 3919篇 |
2008年 | 3636篇 |
2007年 | 3435篇 |
2006年 | 3198篇 |
2005年 | 2571篇 |
2004年 | 2454篇 |
2003年 | 2304篇 |
2002年 | 2118篇 |
2001年 | 1955篇 |
2000年 | 1886篇 |
1999年 | 1502篇 |
1998年 | 1601篇 |
1997年 | 1510篇 |
1996年 | 1186篇 |
1995年 | 1244篇 |
1994年 | 1057篇 |
1993年 | 947篇 |
1992年 | 916篇 |
1991年 | 837篇 |
1990年 | 937篇 |
1989年 | 769篇 |
1988年 | 722篇 |
1987年 | 896篇 |
1986年 | 735篇 |
1985年 | 957篇 |
1984年 | 1012篇 |
1983年 | 950篇 |
1982年 | 916篇 |
1981年 | 782篇 |
1980年 | 746篇 |
1979年 | 694篇 |
1978年 | 689篇 |
1977年 | 626篇 |
1976年 | 598篇 |
1975年 | 565篇 |
1974年 | 576篇 |
1973年 | 530篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
371.
372.
373.
We found the equilibrium conditions for a self-gravitating toroidal vortex by taking thermal pressure into account. These conditions are shown to significantly differ from those for a disk or a sphere. The evolution of a thin vortex turns it into a compact vortex that loses mechanical stability for low masses at a polytropic index γ<4/3 but retains stability for sufficiently high masses and densities determined by the velocity circulation in the vortex. 相似文献
374.
O. J. Bennett A. S. Dickinson T. Leininger F. X. Gadéa 《Monthly notices of the Royal Astronomical Society》2003,341(1):361-368
The role in radiative association of narrow resonances arising from quasi-bound levels is examined. For the rate coefficient the contribution from such levels is readily calculated in terms of the Einstein A -coefficient from bound rovibrational levels generated using a suitable alternative boundary condition. By locating systematically all the quasi-bound levels, an enhancement by more than an order of magnitude of the rate coefficient for Li(2s) + H(1s) association above about 600 K is obtained. The effect on the LiH abundance in the early Universe is no more than an increase of a factor of 3 for limited ranges of values of the redshift. For the Li(2p) state the effect of narrow resonances appears to have been taken into account in previous calculations. 相似文献
375.
本文首先对Gamma暴的观测特性和物理过程作了简要的介绍 ,而后 ,对火球模型的相对论流体动力学机制和同步加速辐射机制作了论述。主要工作是 :具体研究火球所抛出壳层的相对论流体动力学演化 ,应用同步加速辐射机制 ,通过由共动坐标系到实验室坐标系的相对论变换 ,得到Gamma暴余辉的光变曲线。对于火球壳层的不同的动力学演化规律 ,各向同性或各向异性的壳层抛出形式 ,以及不同的外部介质环境 ,所得到的光变曲线都各不相同。通过对这些不同的光变曲线的比较 ,明确了Gamma暴余辉的整体的物理演化图象以及各种物理过程在Gamma暴余辉演化过程中所起的作用 ,并从余辉演化的方面进一步理解了Gamma暴的物理本质 相似文献
376.
M. Marengo M. Karovska D.D. Sasselov C. Papaliolios J.T. Armstrong T.E. Nordgren 《Astrophysics and Space Science》2003,286(1-2):185-190
We present a new method to compute wavelength- and phase-dependent limb darkening corrections for classical Cepheids. These
corrections are derived from hydrodynamic simulations and radiative transfer modeling with a full set of atomic and molecular
opacities. Comparison with hydrostatic models having the same stellar parameters show a larger limb darkening for most phases
in our models, and temporal variations related to the hydrodynamics of the stellar pulsation. Weassess the implications of
our results with respect to the geometric Baade-Wesselink method, which uses interferometric measurements of Cepheid angular
diameters to determine their distances and radii. The relevance of the hydrodynamic effects predicted by our model on the
limb darkening of pulsating Cepheids is finally discussed in terms of the predicted capabilities of the VLTI.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
377.
P. G. Jonker G. Nelemans Z. Wang A. K. H. Kong D. Chakrabarty M. Garcia P. J. Groot M. van der Klis T. Kerr B. Mobasher M. Sullivan T. Augusteijn B. W. Stappers P. Challis R. P. Kirshner J. Hjorth A. Delsanti 《Monthly notices of the Royal Astronomical Society》2003,344(1):201-206
We have obtained optical and near-infrared images of the field of the accreting millisecond X-ray pulsar XTE J1751−305. There are no stars in the 0.7-arcsec error circle (0.7 arcsec is the overall uncertainty arising from tying the optical and X-ray images and from the intrinsic uncertainty in the Chandra X-ray astrometric solution). We derive limiting magnitudes for the counterpart of R > 23.1, I > 21.6, Z > 20.6, J > 19.6 and K > 19.2 . We compare these upper limits with the magnitudes one would expect for simple models for the possible donor stars and the accretion disc subject to the reddening observed in X-rays for XTE J1751−305 and when put at the distance of the Galactic Centre (8.5 kpc). We conclude that our non-detection does not constrain any of the models for the accretion disc or possible donor stars. Deep, near-infrared images obtained during quiescence will, however, constrain possible models for the donor stars in this ultracompact system. 相似文献
378.
Scott M. MCLENNAN 《Meteoritics & planetary science》2003,38(6):895-904
Abstract— Basaltic shergottites display a systematic decrease in K/Th, K/U, and K/La ratios with increasing K content. These trends are interpreted as mixing lines between relatively young martian magmas derived from highly depleted mantle sources and an ancient large‐ion lithophile (LIL) element‐enriched crustal component. One implication of this is that a substantial fractionation of these ratios occurs during the early crustal differentiation on Mars. Isotopic evidence from SNC meteorites and compositional data from Pathfinder and orbital gamma ray spectroscopy suggest that in excess of 50% of the LIL element complement of Mars resides in the crustal reservoir. If so, the primitive mantle of Mars is significantly more volatile‐depleted (i.e., lower K/Th, K/U, K/La) than previously thought but probably (though not necessarily) still less volatile‐depleted than the primitive mantle of the Earth. The La/Th ratios of virtually all SNC meteorites are subchondritic, including those with the most severe LREE‐depletion. Extrapolation of the basaltic shergottite trend suggests that both the depleted mantle end member and the enriched crustal end member have subchondritic La/Th ratios. This is in contrast with the Earth where basalts from LIL element‐depleted sources such as MORB have superchondritic La/Th ratios, complementary to the subchondritic ratios of the continental crust. Accordingly, assuming that the refractory elements are in chondritic proportions for the Mars primitive mantle, an additional major geochemical reservoir must exist on Mars that may not yet have been sampled. 相似文献
379.
P. G. Jonker M. van der Klis C. Kouveliotou M. Méndez W. H. G. Lewin T. Belloni 《Monthly notices of the Royal Astronomical Society》2003,346(2):684-688
We have observed the eclipsing low-mass X-ray binary MS 1603.6+2600 with Chandra for 7 ks. The X-ray spectrum is well fit with a single absorbed power law with an index of ∼2. We find a clear sinusoidal modulation in the X-ray light curve with a period of 1.7 ± 0.2 h , consistent with the period of 1.85 h found before. However, no (partial) eclipses were found. We argue that if the X-ray flare observed in earlier X-ray observations was a type I X-ray burst, then the source can only be an accretion disc corona source at a distance of ∼11–24 kpc (implying a height above the Galactic disc of ∼8–17 kpc). It has also been proposed in the literature that MS 1603.76+2600 is a dipper at ∼75 kpc. We argue that, in this dipper scenario, the observed optical properties of MS 1603.6+2600 are difficult to reconcile with the optical properties one would expect on the basis of comparisons with other high-inclination, low-mass X-ray binaries, unless the X-ray flare was not a type I X-ray burst. In that case, the source can be a nearby soft X-ray transient accreting at a quiescent rate, as was proposed by Hakala et al., or a high-inclination source at ∼15–20 kpc. 相似文献
380.
Subramanian Prasad Ananthakrishnan S. Janardhan P. Kundu M.R. White S.M. Garaimov V.I. 《Solar physics》2003,218(1-2):247-259
We present the first observations of a solar flare with the GMRT. An M2.8 flare observed at 1060 MHz with the GMRT on 17 November 2001 was associated with a prominence eruption observed at 17 GHz by the Nobeyama radioheliograph and the initiation of a fast partial halo CME observed with the LASCO C2 coronagraph. Towards the start of the eruption, we find evidence for reconnection above the prominence. Subsequently, we find evidence for rapid growth of a vertical current sheet below the erupting arcade, which is accompanied by the flare and prominence eruption. 相似文献