全文获取类型
收费全文 | 37841篇 |
免费 | 2870篇 |
国内免费 | 4660篇 |
专业分类
测绘学 | 2729篇 |
大气科学 | 5040篇 |
地球物理 | 7720篇 |
地质学 | 18644篇 |
海洋学 | 2884篇 |
天文学 | 2166篇 |
综合类 | 3374篇 |
自然地理 | 2814篇 |
出版年
2024年 | 96篇 |
2023年 | 326篇 |
2022年 | 686篇 |
2021年 | 802篇 |
2020年 | 688篇 |
2019年 | 761篇 |
2018年 | 5387篇 |
2017年 | 4674篇 |
2016年 | 3304篇 |
2015年 | 1027篇 |
2014年 | 972篇 |
2013年 | 991篇 |
2012年 | 1846篇 |
2011年 | 3526篇 |
2010年 | 2857篇 |
2009年 | 3159篇 |
2008年 | 2689篇 |
2007年 | 3111篇 |
2006年 | 786篇 |
2005年 | 807篇 |
2004年 | 820篇 |
2003年 | 829篇 |
2002年 | 694篇 |
2001年 | 471篇 |
2000年 | 494篇 |
1999年 | 531篇 |
1998年 | 459篇 |
1997年 | 446篇 |
1996年 | 386篇 |
1995年 | 319篇 |
1994年 | 284篇 |
1993年 | 210篇 |
1992年 | 191篇 |
1991年 | 141篇 |
1990年 | 92篇 |
1989年 | 103篇 |
1988年 | 90篇 |
1987年 | 60篇 |
1986年 | 45篇 |
1985年 | 36篇 |
1984年 | 22篇 |
1983年 | 25篇 |
1982年 | 13篇 |
1981年 | 35篇 |
1980年 | 41篇 |
1979年 | 11篇 |
1978年 | 5篇 |
1977年 | 5篇 |
1976年 | 6篇 |
1958年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A four-component dusty plasma consisting of electrons, ions, and negative as well as positive dust particles has been considered. The basic features of shock waves that may exist in such a four-component dusty plasma have been theoretically investigated by the reductive perturbation method. The implications of our results in different regions of space (viz. cometary tails, mesosphere, Jupiter’s magnetosphere, etc.) are briefly discussed. 相似文献
992.
Exact Bianchi type-II, VIII and IX cosmological models are obtained in a scalar tensor theory proposed by Saez and Ballester (Phys. Lett. A 113:467, 1986) with perfect fluid as a source. Some physical and geometrical properties of the models are studied. It is observed that the models are free from initial singularities and they are expanding with time. 相似文献
993.
Adolfo L. Méndez Berhondo Ramón E. Rodríguez Taboada Paolo Zlobec 《Astrophysics and Space Science》2008,318(1-2):73-78
Pulsating structures recorded at 237 MHz that are associated to decimetric continuum enhancement during the September 9, 2001 solar radio burst are described. We analyzed the radiopolarimetric data recorded at the Trieste Solar Radio System (INAF—Trieste Astronomical Observatory—Basovizza Observing Station) with very high time resolution (1 ms) at metric frequencies. Two different types of pulsations that occur in about 4 minutes at the same frequency are described. The possible mechanisms are analyzed and some parameters of the associated magnetic structure are estimated. 相似文献
994.
This paper studies the existence and stability of equilibrium points under the influence of small perturbations in the Coriolis and the centrifugal forces, together with the non-sphericity of the primaries. The problem is generalized in the sense that the bigger and smaller primaries are respectively triaxial and oblate spheroidal bodies. It is found that the locations of equilibrium points are affected by the non-sphericity of the bodies and the change in the centrifugal force. It is also seen that the triangular points are stable for 0<μ<μ c and unstable for \(\mu_{c}\le\mu <\frac{1}{2}\), where μ c is the critical mass parameter depending on the above perturbations, triaxiality and oblateness. It is further observed that collinear points remain unstable. 相似文献
995.
In the present article a model of well behaved charged superdense star with surface density 2×1014 gm/cm3 is constructed by considering a static spherically symmetric metric with t=const hypersurfaces as hyperboloid. So far well behaved model described by such metric could not be obtained. Maximum mass
of the star is found to be 0.343457M
⊙ and the corresponding radius is 9.57459 km. The red shift at the centre and on the surface are given as 0.068887 and 0.031726
respectively. 相似文献
996.
This paper describes a new 2D model for the photospheric evolution of the magnetic carpet. It is the first in a series of
papers working towards constructing a realistic 3D non-potential model for the interaction of small-scale solar magnetic fields.
In the model, the basic evolution of the magnetic elements is governed by a supergranular flow profile. In addition, magnetic
elements may evolve through the processes of emergence, cancellation, coalescence and fragmentation. Model parameters for
the emergence of bipoles are based upon the results of observational studies. Using this model, several simulations are considered,
where the range of flux with which bipoles may emerge is varied. In all cases the model quickly reaches a steady state where
the rates of emergence and cancellation balance. Analysis of the resulting magnetic field shows that we reproduce observed
quantities such as the flux distribution, mean field, cancellation rates, photospheric recycle time and a magnetic network.
As expected, the simulation matches observations more closely when a larger, and consequently more realistic, range of emerging
flux values is allowed (4×1016 – 1019 Mx). The model best reproduces the current observed properties of the magnetic carpet when we take the minimum absolute flux
for emerging bipoles to be 4×1016 Mx. In future, this 2D model will be used as an evolving photospheric boundary condition for 3D non-potential modeling. 相似文献
997.
P. Pappa Kalaivani S. Umapathy A. Shanmugaraju O. Prakash 《Astrophysics and Space Science》2010,330(2):237-242
We have studied the characteristics of coronal mass ejections (CMEs) associated with Deca-Hectometric (DH) type II radio bursts (1–14 MHz) in the interplanetary medium during the year 1997–2005. The DH CMEs are divided into two parts: (i) DH CMEs (All) and (ii) DH CMEs (Limb). We found that 65% (177/273) of all events have the speed >900 km?s?1 and the remaining 35% (96/273) events have the speed below 900 km?s?1. The average speed of all and limb DH CMEs are 1230 and 1288 km?s?1, respectively, which is nearly three times the average speed of general population of CMEs (473 km?s?1). The average widths of all and limb DH CMEs are 105° and 106°, respectively, which is twice the average width (52°) of the general population of CMEs. We found a better correlation between the speed and width of limb DH CMEs (R=?0.61) than all DH CMEs (R=?0.53). Only 28% (177/637) of fast >900 km?s?1 general population of CMEs are reported with DH type II bursts counterpart. The above results gives that the relation between the CME properties is better for limb events. 相似文献
998.
The orbital period variation of the oEA system Y Leo is revised by taking into account new times of minimum light covering
an extended time base of 101.8 yr. A multiperiodic ephemeris was finally established by carefully approaching the problem of periodicity detection for the considered periodic
components. A method relying on Monte Carlo simulations was applied. The problem of the long-term behaviour of the O–C curve was taken into account using parabolic, and parabolic + periodic ephemerides. The physical interpretation of the mathematical models describing both long- and short-term behaviour of the
O–C curve was performed by considering different mechanisms: the conservative mass transfer, the light-time effect, and the orbital
period modulation through the cyclic magnetic activity of the late spectral type secondary component in the system. The consequences
of these interpretations are rather intriguing and emphasize the need of new and detailed observational studies on Y Leo. 相似文献
999.
K. A. Firoz J. Hwang I. Dorotovič T. Pintér Subhash C. Kaushik 《Astrophysics and Space Science》2011,331(2):469-484
Cosmic rays registered by Neutron Monitor on the surface of the Earth are believed to originate from outer space, and sometimes
also from the exotic objects of the Sun. Whilst the intensities of the cosmic rays are observed to be enhanced with sudden,
sharp and short-lived increases, they are termed as ground level enhancements (GLEs). They are the occurrences in solar cosmic
ray intensity variations on short-term basis, so different solar factors erupted from the Sun can be responsible for causing
them. In this context, an attempt has been made to determine quantitative relationships of the GLEs having peak increase >5%
with simultaneous solar, interplanetary and geophysical factors from 1997 through 2006, thereby searching the responsible
factors which seem to cause the enhancements. Results suggest that GLE peaks might be caused by solar energetic particle fluxes
and solar flares. The proton fluxes which seemed to cause GLE peaks were also supported by their corresponding fluences. For
most of the flares, the time integrated rising portion of the flare emission refers to the strong portion of X-ray fluxes
which might be the concern to GLE peak. On an average, GLE peak associated X-ray flux (0.71×10−4 w/m2) is much stronger than GLE background associated X-ray flux (0.11×10−6 w/m2). It gives a general consent that the GLE peak is presumably caused by the solar flare. Coronal mass ejection alone does
not seem to cause GLE. Coronal mass ejection presumably causes geomagnetic disturbances characterized by geomagnetic indices
and polarities of interplanetary magnetic fields. 相似文献
1000.
Similarity model of feed support system for FAST 总被引:1,自引:0,他引:1
A new design of feed support system for Five hundred meter Aperture Spherical Telescope (FAST) is proposed in this paper.
According to the similarity theory, a 1:15 scale model of feed support system has been built to make systemic research on
the feasibility of the system. Then the control system and hardware structure of the feed support system are illustrated.
A complete astronomical observation track is run by the scale model and the experiments results demonstrate that the new feed
support system can satisfy the observation accuracy requirement of FAST. 相似文献