全文获取类型
收费全文 | 2234篇 |
免费 | 193篇 |
国内免费 | 377篇 |
专业分类
测绘学 | 85篇 |
大气科学 | 302篇 |
地球物理 | 242篇 |
地质学 | 514篇 |
海洋学 | 123篇 |
天文学 | 1311篇 |
综合类 | 67篇 |
自然地理 | 160篇 |
出版年
2024年 | 4篇 |
2023年 | 31篇 |
2022年 | 39篇 |
2021年 | 42篇 |
2020年 | 43篇 |
2019年 | 47篇 |
2018年 | 45篇 |
2017年 | 42篇 |
2016年 | 54篇 |
2015年 | 67篇 |
2014年 | 92篇 |
2013年 | 96篇 |
2012年 | 99篇 |
2011年 | 162篇 |
2010年 | 123篇 |
2009年 | 200篇 |
2008年 | 209篇 |
2007年 | 212篇 |
2006年 | 176篇 |
2005年 | 171篇 |
2004年 | 148篇 |
2003年 | 152篇 |
2002年 | 93篇 |
2001年 | 102篇 |
2000年 | 82篇 |
1999年 | 61篇 |
1998年 | 70篇 |
1997年 | 29篇 |
1996年 | 17篇 |
1995年 | 25篇 |
1994年 | 9篇 |
1993年 | 9篇 |
1992年 | 8篇 |
1991年 | 4篇 |
1990年 | 10篇 |
1989年 | 2篇 |
1988年 | 4篇 |
1987年 | 4篇 |
1986年 | 6篇 |
1985年 | 7篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1980年 | 2篇 |
1978年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有2804条查询结果,搜索用时 15 毫秒
351.
Michael K. Shepard Beth Ellen Clark Lance A.M. Benner Jon D. Giorgini Kandy Jarvis Lucy F. Lim James Bell Andrew Rivkin Daniel Scheeres 《Icarus》2008,193(1):20-38
We observed near-Earth asteroid (NEA) 2100 Ra-Shalom over a six-year period, obtaining rotationally resolved spectra in the visible, near-infrared, thermal-infrared, and radar wavelengths. We find that Ra-Shalom has an effective diameter of Deff=2.3±0.2 km, rotation period P=19.793±0.001 h, visual albedo pv=0.13±0.03, radar albedo , and polarization ratio μc=0.25±0.04. We used our radar observations to generate a three-dimensional shape model which shows several structural features of interest. Based on our thermal observations, Ra-Shalom has a high thermal inertia of ∼103 J m−2 s−0.5 K−1, consistent with a coarse or rocky surface and the inferences of others [Harris, A.W., Davies, J.K., Green, S.F., 1998. Icarus 135, 441-450; Delbo, M., Harris, A.W., Binzel, R.P., Pravec, P., Davies, J.K., 2003. Icarus 166, 116-130]. Our spectral data indicate that Ra-Shalom is a K-class asteroid and we find excellent agreement between our spectra and laboratory spectra of the CV3 meteorite Grosnaja. Our spectra show rotation-dependent variations consistent with global variations in grain size. Our radar observations show rotation-dependent variations in radar albedo consistent with global variations in the thickness of a relatively thin regolith. 相似文献
352.
Cassini UVIS star occultations by the F ring detect 13 events ranging from 27 m to 9 km in width. We interpret these structures as likely temporary aggregations of multiple smaller objects, which result from the balance between fragmentation and accretion processes. One of these features was simultaneously observed by VIMS. There is evidence that this feature is elongated in azimuth. Some features show sharp edges. At least one F ring object is opaque and may be a “moonlet.” This possible moonlet provides evidence for larger objects embedded in Saturn's F ring, which were predicted as the sources of the F ring material by Cuzzi and Burns [Cuzzi, J.N., Burns, J.A., 1988. Icarus 74, 284-324], and as an outcome of tidally modified accretion by Barbara and Esposito [Barbara, J.M., Esposito, L.W., 2002. Icarus 160, 161-171]. We see too few events to confirm the bi-modal distribution which Barbara and Esposito [Barbara, J.M., Esposito, L.W., 2002. Icarus 160, 161-171] predict. These F ring structures and other youthful features detected by Cassini may result from ongoing destruction of small parent bodies in the rings and subsequent aggregation of the fragments. If so, the temporary aggregates are 10 times more abundant than the solid objects. If recycling by re-accretion is significant, the rings could be quite ancient, and likely to persist far into the future. 相似文献
353.
L. Hjalmarsdotter A. A. Zdziarski S. Larsson V. Beckmann M. McCollough D. C. Hannikainen O. Vilhu 《Monthly notices of the Royal Astronomical Society》2008,384(1):278-290
The X-ray binary Cygnus X-3 (Cyg X-3) is a highly variable X-ray source that displays a wide range of observed spectral states. One of the main states is significantly harder than the others, peaking at ∼20 keV, with only a weak low-energy component. Due to the enigmatic nature of this object, hidden inside the strong stellar wind of its Wolf–Rayet companion, it has remained unclear whether this state represents an intrinsic hard state, with truncation of the inner disc, or whether it is just a result of increased local absorption. We study the X-ray light curves from RXTE /ASM and CGRO /BATSE in terms of distributions and correlations of flux and hardness and find several signs of a bimodal behaviour of the accretion flow that are not likely to be the result of increased absorption in a surrounding medium. Using INTEGRAL observations, we model the broad-band spectrum of Cyg X-3 in its apparent hard state. We find that it can be well described by a model of a hard state with a truncated disc, despite the low cut-off energy, provided the accreted power is supplied to the electrons in the inner flow in the form of acceleration rather than thermal heating, resulting in a hybrid electron distribution and a spectrum with a significant contribution from non-thermal Comptonization, usually observed only in soft states. The high luminosity of this non-thermal hard state implies that either the transition takes place at significantly higher L / L E than in the usual advection models, or the mass of the compact object is ≳20 M⊙ , possibly making it the most-massive black hole observed in an X-ray binary in our Galaxy so far. We find that an absorption model as well as a model of almost pure Compton reflection also fit the data well, but both have difficulties explaining other results, in particular the radio/X-ray correlation. 相似文献
354.
The MPC database of the asteroid observations (each position from near 20 millions) was used in analysis of observational accuracy for more than 300 active world observatories both professional and amateur. The values of the “Mean error of a single observation” σ (for α,δ) were derived based on the Pulkovo method of accuracy estimation. These values may be used for observatory weight assignment in the orbital improvement procedures. The accuracy of the best amateur observations is proved to be comparable with professional one (σ=±0″.20). The detailed results in electronic format are accessible from the first author. 相似文献
355.
356.
Riccardo Campana Enrico Massaro Dario Gasparrini † Sara Cutini † Andrea Tramacere 《Monthly notices of the Royal Astronomical Society》2008,383(3):1166-1174
We developed a source detection algorithm based on the Minimal Spanning Tree (MST), that is a graph-theoretical method useful for finding clusters in a given set of points. This algorithm is applied to γ-ray bi-dimensional images where the points correspond to the arrival direction of photons, and the possible sources are associated with the regions where they clusterize. Some filters to select these clusters and to reduce the spurious detections are introduced. An empirical study of the statistical properties of MST on random fields is carried out in order to derive some criteria to estimate the best filter values. We also introduce two parameters useful to verify the goodness of candidate sources. To show how the MST algorithm works in practice, we present an application to an EGRET observation of the Virgo field, at high Galactic latitude and with a low and rather uniform background, in which several sources are detected. 相似文献
357.
358.
359.
360.
Assuming that the dark matter is entirely made up of neutralinos, we re-visit the role of their annihilation on the temperature of diffuse gas in the high-redshift universe ( z > 10) , before the formation of luminous structures. We consider neutralinos of particle mass 36 and 100 GeV. The former is able to produce ∼7 e − e + particles per annihilation through the fremionic channel, and the latter ∼53 particles assuming a purely bosonic channel. High-energy e − e + particles up-scatter the cosmic microwave background (CMB) photons into higher energies via the inverse-Compton scattering. The process produces a power-law e − e + energy spectrum of index −1 in the energy range of interest, independent of the initial energy distribution. The corresponding energy spectrum of the up-scattered photons is a power law of index −1/2, if absorption by the gas is not included. The scattered photons photoheat the gas by releasing electrons which deposit a fraction (14 per cent) of their energy as heat into the ambient medium. For uniformly distributed neutralinos, the heating is insignificant. The effect is greatly enhanced by the clumping of neutralinos into dense haloes. We use a time-dependent clumping model which takes into account the damping of density fluctuations on mass-scales smaller than ∼10−6 M⊙ . With this clumping model, the heating mechanism boosts the gas temperature above that of the CMB after a redshift of z ∼ 30 . By z ≈ 10 , the gas temperature is nearly 100 times its temperature when no heating is invoked. Similar increase is obtained for the two neutralino masses considered. 相似文献