全文获取类型
收费全文 | 58659篇 |
免费 | 1431篇 |
国内免费 | 1101篇 |
专业分类
测绘学 | 1398篇 |
大气科学 | 4499篇 |
地球物理 | 11933篇 |
地质学 | 21200篇 |
海洋学 | 5076篇 |
天文学 | 12954篇 |
综合类 | 281篇 |
自然地理 | 3850篇 |
出版年
2022年 | 347篇 |
2021年 | 570篇 |
2020年 | 589篇 |
2019年 | 657篇 |
2018年 | 1295篇 |
2017年 | 1222篇 |
2016年 | 1485篇 |
2015年 | 1053篇 |
2014年 | 1506篇 |
2013年 | 2873篇 |
2012年 | 1915篇 |
2011年 | 2580篇 |
2010年 | 2270篇 |
2009年 | 2971篇 |
2008年 | 2551篇 |
2007年 | 2590篇 |
2006年 | 2461篇 |
2005年 | 1870篇 |
2004年 | 1811篇 |
2003年 | 1687篇 |
2002年 | 1600篇 |
2001年 | 1405篇 |
2000年 | 1334篇 |
1999年 | 1152篇 |
1998年 | 1199篇 |
1997年 | 1150篇 |
1996年 | 953篇 |
1995年 | 895篇 |
1994年 | 809篇 |
1993年 | 716篇 |
1992年 | 682篇 |
1991年 | 619篇 |
1990年 | 710篇 |
1989年 | 606篇 |
1988年 | 565篇 |
1987年 | 708篇 |
1986年 | 573篇 |
1985年 | 734篇 |
1984年 | 866篇 |
1983年 | 785篇 |
1982年 | 744篇 |
1981年 | 685篇 |
1980年 | 696篇 |
1979年 | 613篇 |
1978年 | 629篇 |
1977年 | 572篇 |
1976年 | 554篇 |
1975年 | 547篇 |
1974年 | 526篇 |
1973年 | 546篇 |
排序方式: 共有10000条查询结果,搜索用时 125 毫秒
181.
182.
Report of the International Astronomical Union Division I Working Group on Precession and the Ecliptic 总被引:1,自引:0,他引:1
J. L. Hilton N. Capitaine J. Chapront J. M. Ferrandiz A. Fienga T. Fukushima J. Getino P. Mathews J.-L. Simon M. Soffel J. Vondrak P. Wallace J. Williams 《Celestial Mechanics and Dynamical Astronomy》2006,94(3):351-367
The IAU Working Group on Precession and the Equinox looked at several solutions for replacing the precession part of the IAU
2000A precession–nutation model, which is not consistent with dynamical theory. These comparisons show that the (Capitaine
et al., Astron. Astrophys., 412, 2003a) precession theory, P03, is both consistent with dynamical theory and the solution most compatible with the IAU 2000A
nutation model. Thus, the working group recommends the adoption of the P03 precession theory for use with the IAU 2000A nutation.
The two greatest sources of uncertainty in the precession theory are the rate of change of the Earth’s dynamical flattening,
ΔJ2, and the precession rates (i.e. the constants of integration used in deriving the precession). The combined uncertainties
limit the accuracy in the precession theory to approximately 2 mas cent−2.
Given that there are difficulties with the traditional angles used to parameterize the precession, zA, ζA, and θA, the working group has decided that the choice of parameters should be left to the user. We provide a consistent set of parameters
that may be used with either the traditional rotation matrix, or those rotation matrices described in (Capitaine et al., Astron.
Astrophys., 412, 2003a) and (Fukushima Astron. J., 126, 2003).
We recommend that the ecliptic pole be explicitly defined by the mean orbital angular momentum vector of the Earth–Moon barycenter
in the Barycentric Celestial Reference System (BCRS), and explicitly state that this definition is being used to avoid confusion
with previous definitions of the ecliptic.
Finally, we recommend that the terms precession of the equator and precession of the ecliptic replace the terms lunisolar precession and planetary precession, respectively. 相似文献
183.
Diurnal variation of the dayside, ionospheric, mid-latitude trough in the southern hemisphere at 800 km: Model and measurement comparison 总被引:1,自引:0,他引:1
J.J. Sojka W.J. Raitt R.W. Schunk J.L. Parish F.J. Rich 《Planetary and Space Science》1985,33(12):1375-1382
Our high latitude ionospheric model predicts the existence of a pronounced “dayside” trough in plasma concentration equatorward of the auroral oval in both the Northern and Southern Hemispheres for solar maximum, winter, and low geomagnetic activity conditions. The trough in the Southern Hemisphere is much deeper than that in the Northern Hemisphere, with the minimum trough density at 800 km being 2 × 103 cm−3 in the Southern Hemisphere and 104 cm−3 in the Northern Hemisphere. The dayside trough has a strong longitudinal (diurnal) dependence and appears between 11:00 and 19:00 U.T. in the Southern Hemisphere and between 02:00 and 08:00 U.T. in the Northern Hemisphere. This dayside trough is a result of the auroral oval moving to larger solar zenith angles at those universal times when the magnetic pole is on the antisunward side of the geographic pole. As the auroral ionization source moves to higher geographic latitudes, it leaves a region of declining photoionization on the dayside. For low convection speeds, the ionosphere decays and a dayside trough forms. The trough is deeper in the Southern Hemisphere than in the Northern Hemisphere because of the greater offset between the geomagnetic and geographic poles. Satellite data taken in both the Northern and Southern Hemispheres confirm the gross features of the dayside trough, including its strong longitudinal dependence, its depth, and the asymmetry between the Northern and Southern Hemisphere troughs. 相似文献
184.
The paper considers the random phase errors in the phase shifters which are used in an antenna array to steer the beam in the look direction, and analyzes the effect of these errors on the performance of the optimal processor which maximizes the output SNR by deriving the expressions for the output signal power, residual interference power, output SNR, and the array gain as a function of the variance of these errors. The paper also considers the phase quantization error which arises in the digital phase shifters and shows how the performance of the optimal processor depends on the number of bits of the digital phase shifters. 相似文献
185.
The present work applies the method of characteristics to study the behaviour of planar and cylindrical wave-heads propagating through a perfectly electrically conducting and thermally radiating inviscid gas under the optically thin limit in the presence of a transverse magnetic field. The true nonlinear progress of the flow variable gradients at the wavefront is predicted and the critical distance at which the characteristics pile up at the wavefront to form a shock wave is obtained. It is investigated as to how the effects of radiative flux, the magnetic field strength and the specific heat ratio influence the process of steepening or flattening of the characteristic wavefront. 相似文献
186.
The paper models the suprathermal plasma as the concatenation of two different gravitating fluids each having its own separate density and pressure. One of the fluids has isotropic pressure while the other has an anisotropic pressure. The system is subjected to a uniform magnetic field which is frozen with the fluids. The gravitational instability of the model for low frequency plane waves has been investigated in a linear framework for plasma parameters relevant to the spiral arms of the galaxy and the cosmic gas clouds. The analysis shows that the wavelength of the instability is strongly dependent upon the anisotropy of the non-thermal plasma. 相似文献
187.
F.J. Davey 《Geophysical Journal International》1987,89(1):443-448
Summary. The active Australian-Pacific plate boundary passes through New Zealand. In the north, the Pacific plate subducts beneath the Australian plate with an accretionary wedge forming the eastern continental (Hikurangi) margin of the North Island. The structure of the region behind the Hikurangi margin changes from the extensional back-arc basin under central North Island to a postulated crustal downwarp under the southern North Island. A 100 km long multichannel seismic reflection profile was recorded across the region of crustal downwarp. The data show discontinuous coherent reflectors dipping westwards at the east end of the profile, and east dipping reflectors at the west end, from depths of 9 to 15 s two way time. Simple hand migration of these events indicate that the east dipping reflectors, interpreted as the base of the Australian plate crust, abut against the west dipping reflectors which are interpreted as marking the top of the subducted Pacific plate. Detailed earthquake hypocentre locations in the area show a dipping zone of high seismicity, the top of which coincides closely with the west dipping events, thus supporting this interpretation. 相似文献
188.
J. CAMPSIE M. H. RASMUSSEN L. C. KOVACS F. DITTMER J. C. BAILEY N. O. HANSEN J. LAURSEN L. JOHNSON 《Polar research》1990,8(2):237-243
New aeromagnetic data, K-Ar age determinations of dredged marine igneous rocks, as well as other geophysical evidence have shed light on the chronology, nature and evolution of the northern Iceland Plateau. Correspondence between seismic refraction profiles taken on the Jan Mayen Ridge and westward through Jan Mayen Island, suppressed aeromagnetic anomalies, earthquake surface wave studies, and ages of dredged igneous rocks suggest these strata may form an extended region of thickened crust, possibly of Caledonian age, extending westward toward the Kolbeinsey Ridge and northwest to the south wall of the Jan Mayen Fracture Zone. 相似文献
189.
P. A. M. Van Hoof M. E. Foord R. F. Heeter J. E. Bailey H.-K. Chung M. E. Cuneo W. H. Goldstein V. Jonauskas F. P. Keenan R. Kisielius D. A. Liedahl C. Ramsbottom S. J. Rose P. T. Springer R. S. Thoe 《Astrophysics and Space Science》2005,298(1-2):147-153
In experiments at the high-power Z-facility at Sandia National Laboratory in Albuquerque, New Mexico, we have been able to
produce a low density photoionized laboratory plasma of Fe mixed with NaF. The conditions in the experiment allow a meaningful
comparison with X-ray emission from astrophysical sources. The charge state distributions of Fe, Na and F are determined in
this plasma using high resolution X-ray spectroscopy. Independent measurements of the density and radiation flux indicate
unprecedented values for the ionization parameter ξ = 20–25 erg cm s−1 under nearly steady-state conditions. First comparisons of the measured charge state distributions with X-ray photoionization
models show reasonable agreement, although many questions remain. 相似文献
190.
The uranium deposits in the basin of Franceville (Gabon) host the only natural fission reactors known in the world. Unique
geological conditions favoured a natural fission reaction 2 Ga ago. This was detected by anomalous isotopic compositions of
uranium and rare earth elements (REE), which are produced by the fission reaction. In total, 16 reactor zones were found.
Most of them are mined out. The reactor zone of Bangombé, is only 10–11 m below the surface. This site has been influenced
by surface weathering processes. Six drill cores have been sampled at the site of the reactor zone of Bangombé during the
course of the study and only one drill core (BAX 08) hit the core of the reactor. From these data and previous drilling campaigns,
the reactor size is estimated to be 10 cm thick, 2–3 m wide and 4–6 m long. The migration of fission products can be traced
by the anomalous isotope ratios of REE because of the fission process. The 149Sm/147Sm ratio close to the reactor zone is only 0.28 (normal: 0.92) because of the intense neutron capture of 149Sm and subsequent transmutation, whereas 147Sm is enriched by the fission reaction. Similar changes in isotopic patterns are detectable on other REE. The isotope ratios
of Sm and Nd of whole rock and fracture samples surrounding the reactor indicate that fission-genic REE migrated only a few
decimetres above and mainly below the reactor zone. Organic matter (bitumen) seems to act as a trap for fission-genic REE.
Additional REE-patterns show less intense weathering with increasing depth in the log profile and support a simple weathering
model.
Received: 26 November 1999 · Accepted: 2 May 2000 相似文献