全文获取类型
收费全文 | 113456篇 |
免费 | 1535篇 |
国内免费 | 749篇 |
专业分类
测绘学 | 2745篇 |
大气科学 | 7357篇 |
地球物理 | 21541篇 |
地质学 | 42247篇 |
海洋学 | 9741篇 |
天文学 | 25897篇 |
综合类 | 409篇 |
自然地理 | 5803篇 |
出版年
2022年 | 757篇 |
2021年 | 1282篇 |
2020年 | 1362篇 |
2019年 | 1506篇 |
2018年 | 4563篇 |
2017年 | 4109篇 |
2016年 | 4093篇 |
2015年 | 1737篇 |
2014年 | 3256篇 |
2013年 | 5395篇 |
2012年 | 4001篇 |
2011年 | 5813篇 |
2010年 | 5136篇 |
2009年 | 6352篇 |
2008年 | 5471篇 |
2007年 | 5887篇 |
2006年 | 4311篇 |
2005年 | 3157篇 |
2004年 | 3072篇 |
2003年 | 2938篇 |
2002年 | 2847篇 |
2001年 | 2420篇 |
2000年 | 2319篇 |
1999年 | 1799篇 |
1998年 | 1851篇 |
1997年 | 1747篇 |
1996年 | 1469篇 |
1995年 | 1466篇 |
1994年 | 1253篇 |
1993年 | 1187篇 |
1992年 | 1125篇 |
1991年 | 1129篇 |
1990年 | 1136篇 |
1989年 | 995篇 |
1988年 | 908篇 |
1987年 | 1071篇 |
1986年 | 910篇 |
1985年 | 1148篇 |
1984年 | 1290篇 |
1983年 | 1237篇 |
1982年 | 1140篇 |
1981年 | 1074篇 |
1980年 | 1007篇 |
1979年 | 880篇 |
1978年 | 898篇 |
1977年 | 763篇 |
1976年 | 745篇 |
1975年 | 741篇 |
1974年 | 718篇 |
1973年 | 791篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
251.
The study of the evolution of planetary systems, primarily of the Solar System, is one of the basic problems of celestial mechanics. The stability of motion of giant planets on cosmogonic time scales was established by numerical and analytical methods, but the question about the evolution of orbits of terrestrial planets and arbitrary solar-type planetary systems remained open. This work initiates a series of papers allowing one to advance in solving the problem of the evolution of the solar-type planetary systems on cosmogonic time scales by using powerful analytical tools. In the first paper of this series, we choose the optimum reference system and obtain the Poisson series expansion of the Hamiltonian of the problem in all Keplerian elements. We propose to use the integral representation of the corresponding coefficients or the Poisson processor means instead of conventionally addressing any possible special functions. This approach extremely simplifies the algorithm. The next paper of this series deals with the calculation of the expansion coefficients. 相似文献
252.
The reflectance coefficient of the regolith layer of celestial bodies has been studied in relation to the physical properties of regolith particles (size, refractive index, and packing density) on the basis of an accurate numerical radiative-transfer algorithm for a semi-infinite flat layer. Using the geometric-optics approximation, we have found that a shape mixture of randomly oriented spheroids can successfully model the single-scattering phase function of independent soil grains. In order to take into account the effect of packing density in a regolith layer, the concept of the so-called static structure factor was used. The main effect of increasing packing density is to suppress the forward-scattering peak of the phase function and to increase the albedo of the reflecting surface. We also investigated the influence of fine dust on the reflected light. An addition of small particles not only increases the surface albedo, but also changes the brightness profile and enhances the backscattering. Although the problem of unique solution, which is inherent in the retrieval of the properties of a medium from the measurements of the intensity of light scattered by this media, cannot be removed in the proposed model, the procedure used here, in contrast to widely used approximations, allows us to fit observational data with a set of real characteristics of the regolith. Semiempirical approaches are able to fit the measurements well with a small number of free parameters, but they do not explicitly contain crucial physical characteristics of the regolith such as grain sizes or the refractive index. We compared the numerical solution of the radiative-transfer equation with the Hapke approximation, which is most often used by investigators. The errors introduced by the Hapke model are small only for near-isotropic scattering by isolated particles. However, independent regolith grains are known to scatter light mainly in the forward direction. 相似文献
253.
Universal cokriging is used to obtain predictions when dealing with multivariate random functions. An important type of nonstationarity is defined in terms of multivariate random functions with increments which are stationary of orderk. The covariance between increments of different variables is modeled by means of the pseudo-cross-covariance function. Criteria are formulated to which the parameters of pseudo-cross-covariance functions must comply so as to ensure positive-definiteness. Cokriging equations and the induced cokriging equations are given. The study is illustrated by an example from soil science. 相似文献
254.
255.
A. G. Lyne R. S. Pritchard F. Graham-Smith 《Monthly notices of the Royal Astronomical Society》2001,321(1):67-70
Between 1997 August and October, the radio pulses from the Crab pulsar were followed by discrete moving echoes, which appear to be reflections from part of an ionized shell in the outer part of the Crab Nebula, crossing the line of sight to pulsar. Similar events have now been recognized in recordings from the past 30 yr, and it seems that the Nebula must contain a large number of ionized shell-like surfaces on a much finer scale than recognized hitherto. 相似文献
256.
G. A. Moiseev 《Physical Oceanography》1993,4(3):183-192
The idea of the weak variability of the fields is introduced, which means that when they are recorded by some measuring instrument (MI), the output signal obtains increments of the order of one/several quanta of sensitivity at the discreteness interval. At small scales (microstructure), this field is described in terms of random discrete values which reflect the properties of both the field measured and the MI. Theoretical relationships which define the probabilities of increments at the compound intervals in terms of the probabilities of increments at the sampling intervals, are derived for the case of independent random increments at the sampling intervals of discreteness. The validity of this model is illustrated by respective computations using data of microstructure measurements of the temperature profile in the main thermocline in the Sargasso Sea.Translated by Mikhail M. Trufanov. 相似文献
257.
An ensemble of twenty-three 14-year experiments conducted with the ECHAM-4 GCM has been examined to test the model's capability
to simulate the principal modes of interannual variability. The integrations were performed under specified monthly SST between
1979–1993. The analysis was focused on the Southern Hemisphere (SH) extratropics. Empirical orthogonal functions analysis
(EOF) using seasonal anomaly fields has been performed to isolate the principal modes that dominate the southern extratropical
variability at the interannual time scale. Leading patterns of 500 hPa geopotential height (z500) have been compared with
those estimated from the ECMWF re-analysis dataset. The model is able to adequately reproduce the spatial pattern of the annular
mode, but it represents the temporal variations of the oscillation less satisfactorily. The model simulation of the Pacific
South American (PSA) pattern is better, both in the shape of the pattern and in the temporal evolution. To verify if the capability
of the model to adequately simulate the temporal oscillation of the propagating patterns is related to the increased influence
of the tropical external forcing, covarying SST-atmospheric modes have been identified by singular value decomposition (SVD).
In winter (July-August-September, JAS) the tropical SST variability is highly correlated with the ENSO mode. In summer (January-February-March,
JFM) the strength of the teleconnections is related to strong westerly anomalies, disrupted by a meridional out of phase relation
near to South America. The large size of the ensemble was exploited by comparing the time-varying model spread and degrees
of freedom of the simulated extratropical circulation. Results show that when the extratropical circulation has a few degrees
of freedom, the reproducibility is relatively low and the ensemble is governed by a fairly robust zonally symmetric structure
of dispersion.
Received: 9 May 2000 / Accepted: 30 January 2001 相似文献
258.
We briefly describe an advanced 3D gas dynamical model developed for the simulation of theenvironment of active cometary nuclei.
The model canhandle realistic nucleus shapes and alternative physical models for the gas and dust production mechanism.The
inner gas coma structure is computed by solving self-consistently(a) near to the surface the Boltzman Equation(b) outside
of it, Euler or Navier-Stokes equations.The dust distribution is computed from multifluid ``zero-temperature' Euler equations,extrapolated
with the help of a Keplerian fountain model.The evolution of the coma during the nucleus orbital and spin motion,is computed
as a succession of quasi-steady solutions. Earlier versions of the model using simple,``paedagogic' nuclei have demonstrated
that the surface orographyand the surface inhomogeneity contribute similarly to structuring the near-nucleusgas and dust coma,casting
a shadow on the automatic attribution of such structures to ``active areas'.The model was recently applied to comet P/Halley,
for whichthe nucleus shape is available. In the companion paper of this volume,we show that most near-nucleus dust structuresobserved
during the 1986 Halley flybys are reproduced, assuming that the nucleus is strictly homogeneous. Here, we investigate the
effect of shape perturbations and homogeneityperturbations. We show that the near nucleus gas coma structure is robust vis-a-vissuch
effects. In particular, a random distribution of active and inactive areaswould not affect considerably this structure, suggesting that such areas,even if present, could not be easily identified on images
of the coma. 相似文献
259.
The propagation of axisymmetric magnetohydrodynamic waves near the equatorial plane of the crust of a neutron star in a transverse magnetic field is considered. The magnetic field is perpendicular to the equatorial plane. The magnetic fields and electric currents excited by this wave beam at the stellar surface are determined. 相似文献
260.