全文获取类型
收费全文 | 25401篇 |
免费 | 352篇 |
国内免费 | 312篇 |
专业分类
测绘学 | 716篇 |
大气科学 | 1790篇 |
地球物理 | 4873篇 |
地质学 | 8745篇 |
海洋学 | 2289篇 |
天文学 | 6295篇 |
综合类 | 52篇 |
自然地理 | 1305篇 |
出版年
2021年 | 200篇 |
2020年 | 237篇 |
2019年 | 298篇 |
2018年 | 607篇 |
2017年 | 587篇 |
2016年 | 724篇 |
2015年 | 408篇 |
2014年 | 696篇 |
2013年 | 1300篇 |
2012年 | 795篇 |
2011年 | 1037篇 |
2010年 | 954篇 |
2009年 | 1253篇 |
2008年 | 1126篇 |
2007年 | 1151篇 |
2006年 | 1124篇 |
2005年 | 839篇 |
2004年 | 835篇 |
2003年 | 760篇 |
2002年 | 718篇 |
2001年 | 616篇 |
2000年 | 638篇 |
1999年 | 562篇 |
1998年 | 555篇 |
1997年 | 526篇 |
1996年 | 396篇 |
1995年 | 396篇 |
1994年 | 408篇 |
1993年 | 313篇 |
1992年 | 309篇 |
1991年 | 258篇 |
1990年 | 311篇 |
1989年 | 271篇 |
1988年 | 254篇 |
1987年 | 279篇 |
1986年 | 237篇 |
1985年 | 317篇 |
1984年 | 339篇 |
1983年 | 329篇 |
1982年 | 313篇 |
1981年 | 249篇 |
1980年 | 267篇 |
1979年 | 216篇 |
1978年 | 206篇 |
1977年 | 215篇 |
1976年 | 179篇 |
1975年 | 190篇 |
1974年 | 177篇 |
1973年 | 167篇 |
1972年 | 114篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Tōnu Viik 《Astrophysics and Space Science》1992,198(2):265-280
The external field of radiation in Rayleigh-Cabannes atmospheres with constant and linear sources is found using the resolvent matrix approach. If the internal sources are constant the external field may be described by theX-, Y-, andH-matrices. For the case with linear sources we need the derivatives of these matrices with respect to angular variable. The respective scheme for their determination is given.A set of integro-differential equations for theX- andY-matrices is derived and solved numerically. Some relations between the moments of theH-matrix are given and a sample of results for external fields are provided. 相似文献
52.
We study the center-limb (CL) variation of the average profiles of four Ca i lines near 6500 and compare these observations with synthetic data obtained from several line formation models having different thermal structures, line parameters, LTE and non-LTE conditions, and micro and macroturbulence values, to assess the formation characteristics of our Ca i lines in the solar photosphere.Comparison of numerical results with observations indicates that non-LTE is indispensable to fit the CL variation of the central residual intensity for the line 6493, and anisotropic microturbulence is indispensable to improve the CL behavior of the equivalent widths for all lines. The Ca i line analysis favors a cool photospheric model, but this cannot be disentangled clearly from the effects of non-LTE and small-scale velocity fields on the grounds of the present line formation models. 相似文献
53.
Bianchi type-II, VIII, and IX models have been investigated in scalar-tensor theories developed by Saez (1985), Saez and Ballester (1985), and Lau and Prokhovnik (1986) (under the assumption of a certain relationship between the cosmological term and the scalar field ). The dynamical behaviour of these models has been studied. 相似文献
54.
Propagation of MHD body and surface waves in magnetically structured regions of the solar atmosphere
The fact that magnetically structured regions exist in the solar atmosphere has been known for a number of years. It has been suggested that different kinds of magnetohydrodynamic (MHD) waves can be efficiently damped in these regions and that the dissipated wave energy may be responsible for the observed enhancement in radiative losses. From a theoretical point of view, an important task would be to investigate the propagation and dissipation of MHD waves in these highly structured regions of the solar atmosphere. In this paper, we study the behavior of MHD body and surface waves in a medium with either a single or double (slab) magnetic interface by use of a nonlinear, two-dimensional, time-dependent, ideal MHD numerical model constructed on the basis of a Lagrangean grid and semi-implicit scheme. The processes of wave confinement and wave energy leakage are discussed in detail. It is shown that the obtained results depend strongly on the type of perturbations imposed on the interface or slab and on the plasma parameter, . The relevance of the obtained results to the heating problem of the upper parts of the solar atmosphere is also discussed. 相似文献
55.
J. J. Quenby T. J Sumner S. D. Sidher S. Immler 《Astrophysics and Space Science》1994,216(1-2):213-217
The shadowing effects of the molecular clouds in the nearby interstellar medium on the soft x-ray background has been investigated, using ROSAT WFC data in conjunction with previous rocket B and C band surveys. Shadowing over a 5° extent occurs only for a few percent of the sky, but the mixed model of the ISM is supported.Detailed modelling of the Draco shadowing region shows little evidence for a multi-temperature, hot ISM component. 相似文献
56.
We report the results of an experiment that produced a residue which closely matches the hydrocarbon component of the Murchison carbonaceous chondrite. This experiment suggests that the parent material of the meteoritic component originated as polycyclic aromatic hydrocarbon species in carbon stars during their later stages of evolution. The experiments also indicate that the pathway from those formation sites to eventual incorporation into the meteorite parent body involved hydrogenation in a plasma in the solar nebula or in H II regions prior to the solar nebula. This model is consistent with what is known about the meteoritic hydrocarbon component including deuterium abundance, the observation of cosmic infrared emission bands best attributed to polycyclic aromatic hydrocarbon molecules, and the inherent stability of these molecules that allows their formation in stars and subsequent survival in the interstellar medium. 相似文献
57.
Earlier models of compressible, rotating, and homogeneous ellipsoids with gas pressure are generalized to include the presence of radiation pressure. Under the assumptions of a linear velocity field of the fluid and a bounded ellipsoidal surface, the dynamical behaviour of these models can be described by ordinary differential equations. These equations are used to study the finite oscillations of massive radiative models with masses 10M
and 30M
in which the effects of radiation pressure are expected to be important.Models with two different degrees of equilibrium are chosen: an equilibrium (i.e., dynamically stable) model with an initial asymmetric inward velocity, and a nonequilibrium model with a nonequilibrium central temperature and which falls inwards from rest. For each of these two degrees of equilibrium, two initial configurations are considered: rotating spheroidal and nonrotating spherical models.From the numerical integration of the differential equations for these models, we obtain the time evolution of their principal semi-diametersa
1 anda
3, and of their central temperatures, which are graphically displayed by making plots of the trajectories in the (a
1,a
3) phase space, and of botha
1 and the total central pressureP
c against time.It is found that in all the equilibrium radiative models (in which radiation pressure is taken into account), the periods of the oscillations of botha
1 andP
c are longer than those of the corresponding nonradiative models, while the reverse is true for the nonequilibrium radiative models. The envelopes of thea
1 oscillations of the equilibrium radiative models also have much longer periods; this result also holds for the nonequilibrium models whenever the envelope is well defined. Further, as compared to the nonradiative models, almost all the radiative models collapse to smaller volumes before rebouncing, with the more massive model undergoing a larger collapse and attaining a correspondingly larger peakP
c.When the mass is increased, the dynamical behavior of the radiative model generally becomes more nonperiodic. The ratio of the central radiation pressure to the central gas pressure, which is small for low mass models, increases with mass, and at the center of the more massive model, the radiation pressure can be comparable in magnitude to the gas pressure. In all the radiative models, the average periods as well as the average amplitudes of both thea
1 andP
c oscillations also increase with mass.When either rotation or radiation pressure effects or both are included in the equilibrium nonradiative model, the period of the envelope of thea
1 oscillations is increased. The presence of rotation in the equilibrium radiative model, however, decreases this period.Some astrophysical implications of this work are briefly discussed. 相似文献
58.
M. Aglietta P. Antonioli G. Badino G. Bologna C. Castagnoli A. Castellina V. L. Dadykin W. Fulgione P. Galeotti F. F. Khalchukov E. V. Korolkova P. V. Kortchaguin V. B. Kortchaguin V. A. Kudryavtsev A. S. Malguin L. Periale V. G. Ryassny O. G. Ryazhskaya O. Saavedra G. Trinchero S. Vernetto C. Vigorito V. F. Yakushev G. T. Zatsepin 《Astrophysics and Space Science》1995,231(1-2):355-358
A search for low energy neutrinos of all flavours in correlation with 553 ray bursts detected by BATSE aboard the Compton Observatory has been performed by the LSD (Liquid Scintillator Detector) neutrino telescope. No excess ofe,, orv
e,, candidate has been detected by LSD during the time interval in which BATSE detected the 90% of the photon flux for any of the GRBs analyzed. Upper limits on the neutrino fluxes are given in the paper. 相似文献
59.
Summary Cosmic dust grains play an important role for the thermal, dynamical, and chemical structure of the interstellar medium. This is especially true for the star formation process and the late stages of stellar evolution. Dust grains determine the spectral appearance of protostars, very young stellar objects with disk-like structures as well as of evolved stars with circumstellar envelopes.In this review, we will demonstrate that solid particles in interstellar space are both agent and subject of galactic evolution. We will especially discuss the different dust populations in circumstellar envelopes, the diffuse interstellar medium, and the molecular clouds with strong emphasis on the evolutionary aspects and the metamorphosis of these populations. 相似文献
60.
Pluto and the chaotic satellite system of Neptune may have originated from a single encounter of Neptune with a massive solar system body. A series of numerical experiments has been carried out to try to set limits on the circumstances of such an encounter. These experiments show that orbits very much like those of Pluto, Triton, and Nereid can result from a single close encounter of such a body with Neptune. The implied mass range and encounter velocities limit the source of the encountering body to a former trans-Neptunian planet in the 2- to 5-Earth-mass range. 相似文献