全文获取类型
收费全文 | 10073篇 |
免费 | 946篇 |
国内免费 | 1643篇 |
专业分类
测绘学 | 342篇 |
大气科学 | 313篇 |
地球物理 | 814篇 |
地质学 | 3916篇 |
海洋学 | 957篇 |
天文学 | 5104篇 |
综合类 | 365篇 |
自然地理 | 851篇 |
出版年
2024年 | 32篇 |
2023年 | 74篇 |
2022年 | 216篇 |
2021年 | 210篇 |
2020年 | 235篇 |
2019年 | 326篇 |
2018年 | 237篇 |
2017年 | 209篇 |
2016年 | 243篇 |
2015年 | 274篇 |
2014年 | 506篇 |
2013年 | 536篇 |
2012年 | 553篇 |
2011年 | 677篇 |
2010年 | 737篇 |
2009年 | 1019篇 |
2008年 | 947篇 |
2007年 | 865篇 |
2006年 | 792篇 |
2005年 | 701篇 |
2004年 | 574篇 |
2003年 | 481篇 |
2002年 | 395篇 |
2001年 | 357篇 |
2000年 | 312篇 |
1999年 | 281篇 |
1998年 | 195篇 |
1997年 | 76篇 |
1996年 | 79篇 |
1995年 | 71篇 |
1994年 | 70篇 |
1993年 | 91篇 |
1992年 | 29篇 |
1991年 | 27篇 |
1990年 | 33篇 |
1989年 | 27篇 |
1988年 | 25篇 |
1987年 | 8篇 |
1986年 | 18篇 |
1985年 | 25篇 |
1984年 | 22篇 |
1983年 | 17篇 |
1982年 | 15篇 |
1981年 | 6篇 |
1980年 | 11篇 |
1979年 | 2篇 |
1978年 | 5篇 |
1977年 | 16篇 |
1877年 | 1篇 |
1875年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
991.
S.P. Littlefair V.S. Dhillon T.R. Marsh 《Monthly notices of the Royal Astronomical Society》2001,327(2):669-672
We present the K -band infrared spectrum of the intermediate polar XY Ari. The spectrum confirms the cataclysmic binary nature of XY Ari, showing emission lines of He i ( λ 2.0587 μm) and the Brackett and Paschen series of H i . The broad nature of these lines suggests an origin in an accretion disc. The spectrum is strongly reddened by absorption within the molecular cloud Lynds 1457 and shows prominent absorption features from the secondary star, from which we determine a spectral type for the secondary of M0V. The secondary contributes per cent of the K -band light. We derive a visual extinction to XY Ari of and a distance of , placing XY Ari behind the molecular cloud. 相似文献
992.
R. Gil-Merino L. J. Goicoechea M. Serra-Ricart A. Oscoz D. Alcalde E. Mediavilla 《Monthly notices of the Royal Astronomical Society》2001,322(2):397-410
From optical R -band data of the double quasar QSO 0957+561A,B, we made two new difference light curves (with an overlap of about 330 d between the time-shifted light curve for the A image and the magnitude-shifted light curve for the B image). We observed noisy behaviours around the zero line and no short time-scale events (with a duration of months), where the term 'event' refers to a prominent feature that may be a result of microlensing or another source of variability. Only one event lasting two weeks and rising −33 mmag was found . Measured constraints on the possible microlensing variability can be used to obtain information on the granularity of the dark matter in the main lensing galaxy and the size of the source. In addition, one can also test the ability of the observational noise to cause the rms averages and the local features of the difference signals. We focused on this last issue. The combined photometries were related to a process consisting of an intrinsic signal plus a Gaussian observational noise. The intrinsic signal has been assumed to be either a smooth function (polynomial), a smooth function plus a stationary noise process, or a correlated stationary process . Using these three pictures without microlensing, we derived some models totally consistent with the observations. We finally discussed the sensitivity of our telescope (at Teide Observatory) to several classes of microlensing variability. 相似文献
993.
G.I. Ogilvie 《Monthly notices of the Royal Astronomical Society》2001,325(1):231-248
A new theory of eccentric accretion discs is presented. Starting from the basic fluid-dynamical equations in three dimensions, I derive the fundamental set of one-dimensional equations that describe how the mass, angular momentum and eccentricity vector of a thin disc evolve as a result of internal stresses and external forcing. The analysis is asymptotically exact in the limit of a thin disc, and allows for slowly varying eccentricities of arbitrary magnitude. The theory is worked out in detail for a Maxwellian viscoelastic model of the turbulent stress in an accretion disc. This generalizes the conventional alpha viscosity model to account for the non-zero relaxation time of the turbulence, and is physically motivated by a consideration of the nature of magnetohydrodynamic turbulence. It is confirmed that circular discs are typically viscously unstable to eccentric perturbations, as found by Lyubarskij, Postnov & Prokhorov, if the conventional alpha viscosity model is adopted. However, the instability can usually be suppressed by introducing a sufficient relaxation time and/or bulk viscosity. It is then shown that an initially uniformly eccentric disc does not retain its eccentricity as had been suggested by previous analyses. The evolutionary equations should be useful in many applications, including understanding the origin of planetary eccentricities and testing theories of quasi-periodic oscillations in X-ray binaries. 相似文献
994.
995.
We present profiles of the line-of-sight (l.o.s.) ionospheric wind velocities in the southern auroral/polar region of Saturn. Our velocities are derived from the measurement of Doppler shifting of the H3+ν2Q(1,0−) line at 3.953 microns. The data for this study were obtained using the facility high-resolution spectrometer CSHELL on the NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii, during the night of February 6, 2003 (UT). The l.o.s. velocity profiles finally derived are consistent with an extended region of the upper atmosphere sub-corotating with the planet: the ion velocities in the inertial reference are only 1/3 of those expected for full planetary corotation. We discuss the results in the light of recent proposals for the kronian magnetosphere, and suggest that, in this region, Saturn's ion winds may be under solar wind control. 相似文献
996.
One of the many significant findings of the Mars Global Surveyor mission is the presence of hundreds of quasi-circular depressions (QCDs) observed from high-resolution MOLA topography data. Their presence has recently been interpreted to reflect a northern lowlands that archive some of the earliest recorded rocks on Mars, mostly below a veneer of Hesperian and Amazonian materials. Here we analyze these data, coupled with a recent synthesis of geologic, geophysical, geomorphic, topographic, and magnetic information. Such analysis allows us to suggest a potential plate tectonic phase during the recorded Early into Middle Noachian martian history that transitioned into a monoplate world with episodic magmatic-driven activity persisting to present. This working hypothesis is based on: (1) the observation that the basement of the northern plains is younger than the basement of the southern highlands, but older than the material exposures of the cratered highlands, suggesting different formational ages for each one of the three geologic-time units; (2) the observation that parts of the very ancient highland's crust are highly magnetized, thus suggesting that most if not all of the formation of the lowlands basement postdates the shut off of the martian dynamo, some 4 Gyr ago, and so allowing hundreds of millions of years for the shaping of the buried lowlands. Consequently, the role of endogenic processes in the earliest geological evolution of Mars (Early perhaps into Middle Noachian) requires reconsideration, since MOLA topographic and MGS magnetic data afford a temporal window sufficient for very early, non-primordial shaping of the northern lowlands' basement. 相似文献
997.
998.
Sandip K. Chakrabarti † Santabrata Das 《Monthly notices of the Royal Astronomical Society》2001,327(3):808-812
We analytically study how the behaviour of accretion flows changes when the flow model is varied. We study the transonic properties of the conical flow, a flow of constant height and a flow in vertical equilibrium, and show that all these models are basically identical, provided that the polytropic constant is suitably changed from one model to another. We show that this behaviour is extendible even when standing shocks are produced in the flow. The parameter space where shocks are produced remains roughly identical in all these models when the same transformation among the polytropic indices is used. We present applications of these findings. 相似文献
999.
Douglas S. Hall Gregory W. Henry Dietmar Böhme Peter A. Brooks Sandy Chang Ales Dolzan George L. Fortier Robert E. Fried Russell M. Genet Bruce S. Grim James Hannon Darrel B. Hoff Kevin Krisciunas Howard J. Landis Howard P. Louth Larry P. Lovell Paul Nielsen Bobby E. Powell Harry D. Powell Don Pray Thomas R. Renner Charles W. Rogers Stephen Shervais Douglas M. Slauson Samuel Slote Harold J. Stelzer Arthur J. Stokes Jack C. Troeger Louis C. Turner Norman F. Wasson Kenneth W. Zeigler 《Journal of Astrophysics and Astronomy》1991,12(4):281-287
We present photoelectric photometry of λ And never before published, obtained between February 1982 and December 1990 at 29
different observatories. Then we combine it with all other photometry available to us (previously published, contained in
the I.A.U. Commission 27 Archives, and obtained with the Vanderbilt 16-inch automatic telescope but not yet published), to
yield a 14.8-year data base. Analysis reveals a long-term cycle in mean brightness, with a full range of 0m.15 and a period of 11.4 ± 0.4 years. Because most of our new photometry was concentrated in the 1983-84 observing season,
we analyze that one well-defined light curve with a two-spot model. Spot A keeps a 0m.04 amplitude throughout four rotation cycles whereas the amplitude of spot B diminishes from 0m.09 down almost to 0m.03. The spot rotation periods were 55d.9 ± 0d.6 and 520d.8 ± 1d.0, respectively. 相似文献
1000.
We estimate the contribution of millisecond pulsars to the diffuse gamma-ray background of the Galaxy, and show that a significant
fraction of the Galactic background may originate from them. A small number of the unidentified COS-B point sources may, in
fact, be millisecond pulsars. It is argued that several hundred millisecond pulsars may be detectable as point sources by
the GRO satellite
A preliminary version of these results was presented at the NATO Advanced Study Institute on Neutron stars, AGNs and Jets,
Erice, September 5–17, 1988.
On leave from Raman Research Institute, Bangalore, India. 相似文献