全文获取类型
收费全文 | 60340篇 |
免费 | 361篇 |
国内免费 | 338篇 |
专业分类
测绘学 | 1556篇 |
大气科学 | 3532篇 |
地球物理 | 10800篇 |
地质学 | 25160篇 |
海洋学 | 4997篇 |
天文学 | 12721篇 |
综合类 | 286篇 |
自然地理 | 1987篇 |
出版年
2022年 | 531篇 |
2021年 | 791篇 |
2020年 | 827篇 |
2019年 | 915篇 |
2018年 | 4064篇 |
2017年 | 3606篇 |
2016年 | 3014篇 |
2015年 | 847篇 |
2014年 | 1678篇 |
2013年 | 2438篇 |
2012年 | 2576篇 |
2011年 | 4140篇 |
2010年 | 3667篇 |
2009年 | 4120篇 |
2008年 | 3461篇 |
2007年 | 4083篇 |
2006年 | 2049篇 |
2005年 | 1382篇 |
2004年 | 1319篇 |
2003年 | 1342篇 |
2002年 | 1194篇 |
2001年 | 965篇 |
2000年 | 869篇 |
1999年 | 562篇 |
1998年 | 595篇 |
1997年 | 619篇 |
1996年 | 438篇 |
1995年 | 468篇 |
1994年 | 436篇 |
1993年 | 366篇 |
1992年 | 342篇 |
1991年 | 368篇 |
1990年 | 415篇 |
1989年 | 335篇 |
1988年 | 323篇 |
1987年 | 352篇 |
1986年 | 266篇 |
1985年 | 395篇 |
1984年 | 389篇 |
1983年 | 391篇 |
1982年 | 354篇 |
1981年 | 339篇 |
1980年 | 368篇 |
1979年 | 283篇 |
1978年 | 314篇 |
1977年 | 270篇 |
1976年 | 231篇 |
1975年 | 242篇 |
1974年 | 233篇 |
1973年 | 264篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
The interrelations of the latitudinal distribution of the coronal green emission maxima, maximal numbers and areas of prominences, magnetic fields, sunspots, and polar faculae in the 20th and 21st sunspot cycles have been investigated. It is again demonstrated how the behaviour of all studied data depends on their heliographic latitude. In the polar zone, well separated from the equatorial we observe following polarity magnetic fields transported only polewards, while the equatorial zone is occupied mostly by leading polarity fields, developed there, moving equatorwards, and crossing the equator to the other hemisphere with the new cycle during the minimum of sunspot activity.This magnetic field distribution is well emphasized by the places of maximal occurrence of prominences and by the distribution of coronal green emission maxima which also differ in dependence on latitude.The question of identifying the first and last evolutionary stages of an extended cycle of activity is discussed and the existence of a magnetic activity cycle lasting 15–17 years is suggested. 相似文献
952.
The subject of relativistic reference frames in astronomy is discussed with respect to the problems and needs of the various user groups. For didactical reasons the discussion is presented in form of a sequence of questions and answers. 相似文献
953.
A new moving slit star micrometer has been developed for the 18 cm Carlsberg Automatic Meridian Circle on La Palma. A zenith mean error of 0
inf.
sup
15 for one transit and a limiting magnitude of 14
inf.
supm
8 is obtained. It has a high efficiency and is well suited for improving the FK5 system and extending it to much fainter stars. A new collimator alignment micrometer and the necessary calibrations are also described. 相似文献
954.
The 5-dimensional Jordan-Brans-Dicke cosmologies in vacuum are found for the Bianchi type I metric, their relation with general relativity cosmologies is studied. Two solutions are possible, both produce effective pressure and energy density in the 4-dimensional G.R.-universes. One is a power-law relation, with two cases, the first one is forp
eff=eff and the other forp
eff=eff(– 1 < < 1) has a behaviour as the open flat universe. The second solution is an exponential only valid forp
eff=–eff. In all cases the three-space expansion reaches infinity ast and the fifth dimension can be made to decrease approaching zero. The scalar field can increase or decrease with time. 相似文献
955.
956.
Mininni Pablo D. López Fuentes Marcelo Mandrini Cristina H. Gómez Daniel O. 《Solar physics》2004,219(2):367-378
We present a bi-orthogonal decomposition of the temporal and latitudinal distribution of solar magnetic fields from synoptic
magnetograms. Results are compared with a similar decomposition of the distribution of sunspots since 1874. We show that the
butterfly diagrams can be interpreted as the result of approximately constant amplitudes and phases of two oscillations with
periods close to 22 years. A clear periodicity of 7 years can also be identified in the most energetic modes of both spatio-temporal
series. These results can be used to obtain relevant information concerning the physics of the solar dynamo. 相似文献
957.
V. A. Ruban 《Astrophysics and Space Science》1977,46(2):L23-L28
It is shown that, for the scalar-tensor cosmology (STC) by Jordan-Brans-Dicke (JBD), in general anisotropic solution the oscillatory mixmaster regime near the singularity will be destroyed by the scalar source-free field and replaced by monotonousV
3-collapse into the point or into the line and plane (only in caseG0) even in the presence of the primordial source-free electromagnetic (EM) field. 相似文献
958.
The height structure of a thick-target solar hard X-ray source is predicted for a beam injected vertically downward with a power-law spectrum and dominated by Coulomb collisional energy losses. This structure is characterised by the ratio of hard X-ray flux from an upper part of the source to that from the entire source, and is essentially a function only of the atmospheric column density ΔN (cm?2) in the upper region. These predictions are compared with the flux ratios at 150 keV and 350 keV observed by two spacecraft for five events in which the solar limb occults part of the source for one spacecraft. In three events the occulting levels h ranged from 0 to 2500 km. For these the theoretical and observed ratios are found to be comparable for values of ΔN in reasonable accord with those found at these altitudes by optical and UV spectroscopic modelling of flare chromospheres. In one event the occultation ratio was observed to rise after the burst peak and it is found that this rise is consistent with an increase in ΔN due to conductively driven chromospheric evaporation. However the energy dependence of the occultation ratio is not consistent with that predicted by the model and it is concluded that non-collisional losses must be significant in beam dynamics. In the other two events, the occultation level h was ? 25 000 km. For these the energy dependence of the occultation ratio is comparable with the model predictions. However the values of ΔN required demand extremely high coronal densities and/or acceleration altitudes. Furthermore, the one observed evolution of the occultation ratio is entirely inconsistent with the model. It is concluded that in these, bremsstrahlung emissions other than that from a beam must be important. 相似文献
959.
It is shown that emission lines, first of all the ultraviolet resonance doublet 2800 Mgii, can be used to decipher the internal dynamic structure of QSOs and active galactic nuclei. The electron temperatures of the emission regions of these objects are determined. A hypothesis is proposed explaining the observed broad profiles of the 2800 Mgii line as a summary effect of emission clouds with a Maxwellian velocity distribution. The main physical parameters of emission clouds in QSOs are obtained. A mechanism of formation of emission clouds is suggested: namely, tidal disruption of stars near to a massive compact object. 相似文献
960.
We have identified 20 of the 75 unidentified Equatorial Infrared Catalogue Number-1 sources with stars in the BD Catalogue. Two of these are also found in the HD Catalogue. 相似文献