全文获取类型
收费全文 | 53475篇 |
免费 | 302篇 |
国内免费 | 279篇 |
专业分类
测绘学 | 1358篇 |
大气科学 | 2993篇 |
地球物理 | 9473篇 |
地质学 | 22830篇 |
海洋学 | 4307篇 |
天文学 | 11225篇 |
综合类 | 246篇 |
自然地理 | 1624篇 |
出版年
2022年 | 488篇 |
2021年 | 707篇 |
2020年 | 755篇 |
2019年 | 827篇 |
2018年 | 3972篇 |
2017年 | 3528篇 |
2016年 | 2835篇 |
2015年 | 726篇 |
2014年 | 1443篇 |
2013年 | 2037篇 |
2012年 | 2368篇 |
2011年 | 3924篇 |
2010年 | 3472篇 |
2009年 | 3827篇 |
2008年 | 3193篇 |
2007年 | 3864篇 |
2006年 | 1780篇 |
2005年 | 1150篇 |
2004年 | 1083篇 |
2003年 | 1129篇 |
2002年 | 971篇 |
2001年 | 792篇 |
2000年 | 669篇 |
1999年 | 418篇 |
1998年 | 437篇 |
1997年 | 474篇 |
1996年 | 337篇 |
1995年 | 358篇 |
1994年 | 335篇 |
1993年 | 281篇 |
1992年 | 284篇 |
1991年 | 290篇 |
1990年 | 337篇 |
1989年 | 272篇 |
1988年 | 258篇 |
1987年 | 260篇 |
1986年 | 191篇 |
1985年 | 301篇 |
1984年 | 302篇 |
1983年 | 295篇 |
1982年 | 274篇 |
1981年 | 262篇 |
1980年 | 283篇 |
1979年 | 203篇 |
1978年 | 242篇 |
1977年 | 204篇 |
1976年 | 181篇 |
1975年 | 184篇 |
1974年 | 171篇 |
1973年 | 206篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
Ju. V. Glagolevsky 《Astrophysics》2005,48(2):194-207
Some difficulties in explaining the slow rotation of CP stars are discussed. The most likely hypotheses are (1) a loss of angular momentum involving a magnetic field during “pre-main sequence” evolution and (2) the slow rotation existed from the very start of the creation of these stars. The braking hypothesis is supported by only one property of CP stars— the lower the mass of the star is, the greater the difference between its average rotation velocity vsini and that of normal stars. On the other hand, there is another property— the lower the rotation speeds of CP stars are, the greater their fraction among normal stars. The latter property supports the hypothesis that the lower the initial rotation speed of a star is when it is created, the greater the probability will become chemically peculiar. This property is inherent in chemically peculiar stars both with and without a magnetic field. It is proposed that the cause of the slow rotation of CP stars must be sought in the very earliest phases of their formation, as should the cause of the separation into chemically peculiar magnetic, chemically peculiar nonmagnetic, and normal stars.__________Translated from Astrofizika, Vol. 48, No. 2, pp. 229–245 (May 2005). 相似文献
112.
The process of non-linear ambipolar diffusion in the region overlying the solar surface can be an effective mechanism for producing sharp magnetic structures and current sheets. These may be the sites responsible for the occurrence of connectivity of magnetic field lines, and the subsequent explosive input of energy for heating of some of the features in the atmosphere of the Sun.. 相似文献
113.
V. Castellani S. Degl'Innocenti P. G. Prada Moroni 《Monthly notices of the Royal Astronomical Society》2001,320(1):66-72
We compare theoretical stellar models for main sequence (MS) stars with the Hipparcos data base for the Hyades cluster to give a warning against the uncritical use of available theoretical scenarios and to show how formal MS fittings can be fortuitous if not fictitious. Moreover, we find that none of the current theoretical scenarios appears able to account for an observed mismatch between theoretical predictions and observations of the coolest Hyades MS stars. Finally, we show that current theoretical models probably give too faint He burning luminosities unlike the case of less massive He burning models, with degenerate progenitors, which have been suggested to suffer the opposite discrepancy. 相似文献
114.
We discuss the difficulties encountered when the Heisenberg-Kolmogoroff model for turbulence is applied to the large-scale turbulence in: (A) molecular clouds (specifically the velocity vs size relationship) and (B) stars (specifically, the estimate of convective fluxes).A new model for large-scale turbulence is, therefore, needed. 相似文献
115.
Seismic reflection profiles and long- and medium-range sidescan sonar were used to investigate a salt diapir complex and area of slope instability near the base of the Continental Slope off North Carolina. Within the area of investigation three diapirs are bounded on their upslope side by a scarp 60 m high and 50 km long. The slope above the scarp is characterized by a series of shallow rotational normal faults. The bottom below the scarp is furrowed by slide tracks, which were probably carved by large blocks that broke off the scarp face and slid downslope leaving rubble and scree lobes.Extensive slumping in this area appears to be a result of uplift and faulting associated with salt intrusion, which has fractured and oversteepened the slope leading to instability and failure. Sharply defined slide tracks suggest that slope failure above the breached diapir complex is a continuing process, in contrast to much of the surrounding slope area where few instability features were observed. 相似文献
116.
The first year of sodium nightglow observations from Natal (6°S, 35°W) are examined. Time variations appear to follow a pattern of their own, different from low latitude results. The major seasonal peak occurs in September-October and the average variation during the night decreases from dusk to dawn. Statistics on cloud coverage show that Natal has roughly only about 3 clear hours per night. The best observing period is April with an average of 5 clear hours per night. 相似文献
117.
V. Yu. Terebizh 《Astrophysics and Space Science》1992,193(2):269-288
Image restoration, computerized tomography, and other similar problems are considered as a unified class of stochastic inverse problems. The conventional approach to these problems that proceeds from some integral or functional equations suffers from three main shortcomings: (i) subjectivity, (ii) inability to account for the inner (radiational) noise, and (iii) inability to include the fundamental concept of the natural limit of solution accuracy. A general approach is developed, the Statistical Parameterization of Inverse Problems (SPIPR), that takes into account both the inner and external random noise and gives an explicit form of the above-mentioned natural limit. Applications of the SPIPR to various problems show that the maximum likelihood method as the concrete way to obtain an object estimate has practically limiting efficiency.Two new fields of applications of the SPIPR are outlined along with the image restoration problem: the elimination of blurring due to atmosphere turbulence and reconstruction of an object structure in the computerized tomography. The expressions for the main distribution function in all these problems are found. The corresponding real examples and model cases are considered as well. 相似文献
118.
119.
Resonant absorption of MHD waves on a nonuniform flux tube is investigated as a driven problem for a 1D cylindrical equilibrium. The variation of the fractional absorption is studied as a function of the frequency and its relation to the eigenvalue problem of the MHD radiating eigenmodes of the nonuniform flux tube is established. The optimal frequencies producing maximal fractional absorption are determined and the condition for total absorption is obtained. This condition defines an impedance matching and is fulfilled for an equilibrium that is fine tuned with respect to the incoming wave. The variation of the spatial wave solutions with respect to the frequency is explained as due to the variation of the real and imaginary parts of the dispersion relation of the MHD radiating eigenmodes with respect to the real driving frequency. 相似文献
120.
Current carrying magnetic fields which penetrate sunspots can be unstable to current convective modes caused by the large gradient of electrical conductivity. The linear growth rates and wavelengths of the unstable modes are found. The unstable modes produce fine-scale vortices perpendicular to the magnetic field, which overshoot well into the solar corona. The modes provide a turbulent vorticity source at the photospheric footpoints of the field. This can cause braiding and reconnection of the coronal magnetic field. The modes twist the coronal magnetic field into loops with a typical radius of 200 km, consistent with recent X-ray observations. 相似文献