全文获取类型
收费全文 | 257176篇 |
免费 | 5000篇 |
国内免费 | 3368篇 |
专业分类
测绘学 | 6957篇 |
大气科学 | 19073篇 |
地球物理 | 54184篇 |
地质学 | 88922篇 |
海洋学 | 21322篇 |
天文学 | 56091篇 |
综合类 | 1011篇 |
自然地理 | 17984篇 |
出版年
2021年 | 2255篇 |
2020年 | 2600篇 |
2019年 | 2851篇 |
2018年 | 3285篇 |
2017年 | 2975篇 |
2016年 | 5597篇 |
2015年 | 4223篇 |
2014年 | 6929篇 |
2013年 | 14237篇 |
2012年 | 6432篇 |
2011年 | 7749篇 |
2010年 | 6674篇 |
2009年 | 9386篇 |
2008年 | 8310篇 |
2007年 | 7680篇 |
2006年 | 9698篇 |
2005年 | 7708篇 |
2004年 | 7652篇 |
2003年 | 7152篇 |
2002年 | 6771篇 |
2001年 | 6041篇 |
2000年 | 5967篇 |
1999年 | 5211篇 |
1998年 | 5231篇 |
1997年 | 5046篇 |
1996年 | 4694篇 |
1995年 | 4443篇 |
1994年 | 4120篇 |
1993年 | 3869篇 |
1992年 | 3661篇 |
1991年 | 3597篇 |
1990年 | 3771篇 |
1989年 | 3523篇 |
1988年 | 3307篇 |
1987年 | 3859篇 |
1986年 | 3407篇 |
1985年 | 4241篇 |
1984年 | 4748篇 |
1983年 | 4418篇 |
1982年 | 4329篇 |
1981年 | 3937篇 |
1980年 | 3646篇 |
1979年 | 3511篇 |
1978年 | 3483篇 |
1977年 | 3282篇 |
1976年 | 3046篇 |
1975年 | 2960篇 |
1974年 | 2922篇 |
1973年 | 3078篇 |
1972年 | 2029篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
461.
462.
Summary. Exact spherical harmonic expansions are given for calculating the gravitational and magnetic fields associated with certain uniform solids of revolution. The figures are those made by rotating a conic section about one of its principal axes. The coefficients in the expansions can be computed accurately and efficiently and this approach leads to a very satisfactory method for calculating the fields of geological bodies with approximate circular symmetry about a vertical axis. A complete theory of convergence is given for the expansions. Somewhat unexpectedly, the sphere of convergence is determined by the location of a number of equivalent point or line sources that lie within the body or on its edges. 相似文献
463.
Alan M Bond Julie R Bradbury Henry A Hudson John S Garnham Peter J Hanna Stanley Strother 《Marine Chemistry》1985,16(1):1-9
Evidence was found that uptake of lead from seawater in both model and natural systems by the leaves of the seagrass Zostera muelleri does occur for live, dead and scraped leaves at all the lead concentrations tested. Positive uptake of lead was measured using the three analytical techniques of radio-tracer, differential pulse anodic stripping voltammetry and atomic absorption spectrometry; similar uptake profiles were obtained for each technique. Profiles always showed an initial rapid uptake phase followed by a gradual transition to a plateau phase at which an approximate equilibrium between lead attached to seagrass and free ionic lead in seawater was reached. The presence of active lead uptake processes was indicated in experiments which attempted to remove lead from leaves by a chelating agent. 相似文献
464.
465.
W.J. Boulton 《Planetary and Space Science》1985,33(6):735-756
The influence of aerodynamic drag and the geopotential on the motion of the satellite 1964-52B is considered. A model of the atmosphere is adopted that allows for oblateness, and in which the density behaviour approximates to the observed diurnal variation. A differential equation governing the variation of the eccentricity, e, combining the effects of air drag with those of the Earth's gravitational field is given. This is solved numerically using as initial conditions 310 computed orbits of 1964-52B.The observed values of eccentricity are modified by the removal of perturbations due to luni-solar attraction, solid Earth and ocean tides, solar radiation pressure and low-order long-periodic tesseral harmonic perturbations. The method of removal of these effects is given in some detail. The behaviour of the orbital eccentricity predicted by the numerical solution is compared with the modified observed eccentricity to obtain values of atmospheric parameters at heights between 310 and 430 km. The daytime maximum of air density is found to be at 14.5 hours local time. Analysis of the eccentricity near 15th order resonance with the geopotential yielded values of four lumped geopotential harmonics of order 15, namely: , , , at inclination 98.68°. 相似文献
466.
We present results of investigations into chromospheric velocity oscillations in sunspots, carried out at the Sayan Solar
Observatory. It is shown that the “chevron” structures in the space-time diagrams demonstrate wavetrain properties. Such structures
are indicators of a propagating wave process and they are typical of many sunspots. In the authors’ opinion, three-minute
umbral oscillations are not the source of running penumbral waves (RPW). It is very likely that umbral oscillations and RPW
initially propagate along different magnetic field lines. We explain the decrease in RPW propagation velocity and frequency
in the outer penumbra, as compared with the inner, by the combined action of different frequency modes. To better reveal the
properties of these modes, frequency filtering was used. Our measurements of the RPW (five-minute mode) wavelength and RPW
propagation velocity in different sunspots vary from 12″ to 30″ and from 28 to 60 – 70 km s−1 correspondingly. 相似文献
467.
The gravitational instability of an infinite homogeneous finitely conducting viscid fluid through porous medium is studied in the presence of a uniform vertical magnetic field and finite ion Larmor radius (FLR) effects. The medium is considered uniformly rotating along and perpendicular to the direction of the prevalent magnetic field. A general dispersion relation is obtained from the relevant linearized perturbation equations of the problem. Furthermore, the wave propagation along and perpendicular to the direction of existing magnetic field has been discussed for each direction of the rotation. It is found that the simultaneous presence of viscosity finite conductivity, rotation, medium porosity, and FLR corrections does not essentially change the Jeans's instability condition. The stabilizing influence of FLR in the case of transverse propagation is reasserted for a non-rotating and inviscid porous medium. It is shown that the finite conductivity has destabilizing influence on the transverse wave propagation whereas for longitudinal propagation finite conductivity does not affect the Jean's criterion. 相似文献
468.
Christopher P. McKay 《Planetary and Space Science》1985,33(7):761-771
To investigate the occurrence of low temperatures and the formation of noctilucent clouds in the summer mésosphere a one-dimensional time-dependent photochemical-thermal numerical model of the atmosphere between 50 and 120 km has been constructed. The model includes the important chemistry of the hydrogen and oxygen species and transport by eddy and molecular processes. The thermal balance incorporates: heating by solar ultraviolet radiation; transport of chemical potential energy; eddy diffusion and dissipation; molecular conduction; airglow emissions; and infrared cooling by carbon dioxide. A non- LTE parameterization is used to calculate 15 μm band cooling by carbon dioxide. The model self-consistently solves the coupled photochemical and thermal equations as perturbation equations from a reference state assumed to be in equilibrium and is used to consider the effect of variability in water vapor in the lower mesosphere on the temperature in the region of noctilucent cloud formation. It is found that change in water vapor from an equilibrium value of 5 ppm at 50 km to a value of 10 ppm, a variation consistent with observations, can produce a ~ 15 K drop in temperature at 82 km. It is suggested that this process may produce long periods (weeks) of cold temperatures and influence noctilucent cloud formation. 相似文献
469.
470.