全文获取类型
收费全文 | 37092篇 |
免费 | 596篇 |
国内免费 | 579篇 |
专业分类
测绘学 | 935篇 |
大气科学 | 3076篇 |
地球物理 | 7420篇 |
地质学 | 12223篇 |
海洋学 | 3364篇 |
天文学 | 8726篇 |
综合类 | 120篇 |
自然地理 | 2403篇 |
出版年
2021年 | 200篇 |
2020年 | 257篇 |
2019年 | 255篇 |
2018年 | 612篇 |
2017年 | 565篇 |
2016年 | 880篇 |
2015年 | 626篇 |
2014年 | 846篇 |
2013年 | 1887篇 |
2012年 | 1034篇 |
2011年 | 1453篇 |
2010年 | 1195篇 |
2009年 | 1776篇 |
2008年 | 1575篇 |
2007年 | 1499篇 |
2006年 | 1409篇 |
2005年 | 1288篇 |
2004年 | 1212篇 |
2003年 | 1169篇 |
2002年 | 1091篇 |
2001年 | 976篇 |
2000年 | 987篇 |
1999年 | 923篇 |
1998年 | 841篇 |
1997年 | 842篇 |
1996年 | 722篇 |
1995年 | 645篇 |
1994年 | 562篇 |
1993年 | 514篇 |
1992年 | 510篇 |
1991年 | 482篇 |
1990年 | 476篇 |
1989年 | 418篇 |
1988年 | 402篇 |
1987年 | 451篇 |
1986年 | 435篇 |
1985年 | 522篇 |
1984年 | 585篇 |
1983年 | 559篇 |
1982年 | 521篇 |
1981年 | 464篇 |
1980年 | 435篇 |
1979年 | 400篇 |
1978年 | 413篇 |
1977年 | 360篇 |
1976年 | 323篇 |
1975年 | 336篇 |
1974年 | 337篇 |
1973年 | 340篇 |
1972年 | 202篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
It is proposed that the solar flare phenomenon can be understood as a manifestation of the electrodynamic coupling process of the photosphere-chromosphere-corona system as a whole. The system is coupled by electric currents, flowing along (both upward and downward) and across the magnetic field lines, powered by the dynamo process driven by the neutral wind in the photosphere and the lower chromosphere. A self-consistent formulation of the proposed coupling system is given. It is shown in particular that the coupling system can generate and dissipate the power of 1029 erg s#X2212;1 and the total energy of 1032 erg during a typical life time (103 s) of solar flares. The energy consumptions include Joule heat production, acceleration of current-carrying particles along field lines, magnetic energy storage and kinetic energy of plasma convection. The particle acceleration arises from the development of field-aligned potential drops of 10–150 kV due to the loss-cone constriction effect along the upward field-aligned currents, causing optical, X-ray and radio emissions. The total number of precipitating electrons during a flare is shown to be of order 1037–1038. 相似文献
72.
73.
74.
75.
The Lagrangian diffusion equation appropriate for the dispersion of current followers (e. g., floats, drogues, drifters) is proposed. The analytical solution to the equation is obtained for a uniform deformation field, characterized by Lagrangian deformations and anisotropic eddy diffusivities both varying with time. Expressions are derived for the patch area and its elongation and rotation. For small values of elapsed time after the initial release the patch area can be accounted for by the exponential of the cumulative value of the horizontal divergence; the relative rate of change of the patch area can be accounted for by the horizontal divergence. 相似文献
76.
The wadic project: A comprehensive field evaluation of directional wave instrumentation 总被引:1,自引:0,他引:1
J. Allender T. Audunson S. F. Barstow S. Bjerken H. E. Krogstad P. Steinbakke L. Vartdal L. E. Borgman C. Graham 《Ocean Engineering》1989,16(5-6)
The results of a comprehensive field trial of nearly all commercially available directional wave measurement systems at the Edda field in the North Sea during winter 1985-86 are presented. The results summarize the accuracy of the principal engineering wave parameters from each system and the dependence on sea state. Limiting factors on system performance and operational problems are also included in the assessment. Overall experience has been good with systems utilizing widely different measurement principles returning consistent results. 相似文献
77.
The impact of wave loads and pore-water pressure generation on initiation of sediment transport 总被引:3,自引:0,他引:3
Edward C. Clukey Fred H. Kulhawy Philip L. -F. Liu George B. Tate 《Geo-Marine Letters》1985,5(3):177-183
The build-up of pore-water pressure by waves can lead to sediment liquefaction and subsequent transport by traction currents. This process was investigated by measuring pore-water pressures both in a field experiment and laboratory wave tank tests. Liquefaction was observed in the wave tank tests. The results suggest that sand is less susceptible than silts to wave-induced liquefaction because of the tendency to partially dissipate pore-water pressures. However, previous studies have determined that pore-water pressures must approach liquefaction before current velocities necessary to initiate transport are reduced. Once liquefaction has occurred more sediment can be transported. 相似文献
78.
The solution of equation of state corresponding to equality =3 gives non-terminating solutions for isothermal neutron star cores. Hence, for this equality, core-envelope models have been developed by taking another equation of state, corresponding to the condition 3, in the envelope. Various static, pulsational, and rotational parameters pertaining to neutron star models are calculated. These models are gravitationally bound and stable for radial perturbations and slow rotations. 相似文献
79.
We analyze the hypothesis of quantization in bands for the angular momenta of binary systems and for the maount of actionA
c in stable and pulsating stars. This parameter isA
c=Mv
eff
R
eff, where the effective velocity corresponds to the kinetic energy in the stellar interior and the effective radius corresponds to the potential energyGM
2/R
eff. Analogous parameters can be defined for a pulsating star withm=M where is the rate of the massm participating in the oscillation to the total massM andv
osc,R
osc the effective velocity and oscillation radius.From an elementary dimensional analysis one has thetA
c (energy x time) (period)1/3 independently ifA
c corresponds to the angular momentum in a binary system, or to the oscillation in a pulsating star or the inner energy and its time-scaleP
eff in a stable star.From evolving stellar models one has that P
effP
eff(solar)1.22 hr a near-invariant for the Main Sequence and for the range of masses 0.6M
<M<1.6M
.With this one can give scalesn
k=kn
1 withk integers andn
1=(P/P
1)1/3 withP
1=P
eff1.22 hr. In these scales proportional toA
c, one sees that the periods in binary and pulsating stars are clustered in discrete unitsn
1,n
2,n
3, etc.This can be seen in pulsating Scuti, Cephei, RR Lyrae, W Virginis, Cephei, semi-regular variables, and Miras and in binary stars as cataclysmic binaries, W Ursa Majoris, Algols, and Lyrae with the corresponding subgroups in all these materials. Phase functions (n
k) in RR Lyrae and Cephei are also associated with discrete levelsn
k.the suggested scenario is that the potential energies and the amounts of actionE
p(t), Ac(t) are indeed time-dependent, but the stars remain more time in determinated most proble states. The Main Sequence itself is an example of this. These most probable states in binary systems, or pulsating or stable stars, must be associated with velocities sub-multiplesc/
F
, given by the velocity of light and the fine structure constant.Additional tests for such a hypothesis are suggested when the sufficient amount of observational data are available. They can made with oscillation velocities in pulsating stars and velocity differences of pairs of galaxies. 相似文献
80.
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