全文获取类型
收费全文 | 99785篇 |
免费 | 2171篇 |
国内免费 | 1144篇 |
专业分类
测绘学 | 2895篇 |
大气科学 | 7337篇 |
地球物理 | 20461篇 |
地质学 | 35857篇 |
海洋学 | 8389篇 |
天文学 | 22191篇 |
综合类 | 413篇 |
自然地理 | 5557篇 |
出版年
2022年 | 605篇 |
2021年 | 1050篇 |
2020年 | 1143篇 |
2019年 | 1210篇 |
2018年 | 2772篇 |
2017年 | 2600篇 |
2016年 | 3313篇 |
2015年 | 2018篇 |
2014年 | 3157篇 |
2013年 | 5475篇 |
2012年 | 3320篇 |
2011年 | 4336篇 |
2010年 | 3699篇 |
2009年 | 4738篇 |
2008年 | 4278篇 |
2007年 | 3980篇 |
2006年 | 3829篇 |
2005年 | 3104篇 |
2004年 | 2977篇 |
2003年 | 2798篇 |
2002年 | 2646篇 |
2001年 | 2399篇 |
2000年 | 2304篇 |
1999年 | 1883篇 |
1998年 | 1937篇 |
1997年 | 1907篇 |
1996年 | 1594篇 |
1995年 | 1593篇 |
1994年 | 1382篇 |
1993年 | 1248篇 |
1992年 | 1183篇 |
1991年 | 1109篇 |
1990年 | 1187篇 |
1989年 | 1062篇 |
1988年 | 946篇 |
1987年 | 1204篇 |
1986年 | 985篇 |
1985年 | 1223篇 |
1984年 | 1347篇 |
1983年 | 1267篇 |
1982年 | 1244篇 |
1981年 | 1045篇 |
1980年 | 995篇 |
1979年 | 894篇 |
1978年 | 876篇 |
1977年 | 814篇 |
1976年 | 772篇 |
1975年 | 725篇 |
1974年 | 747篇 |
1973年 | 755篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
The long-term modulation of galactic cosmic rays is investigated from 1957 up to 1992 analysing the dynamic and the quasi-stationary components, separately. It has been found that the dynamic component is characterized by the presence of two peaks at the maximum phase of each solar activity cycle. We infer that the time interval between the two peaks corresponds to a period (well-related to the polar heliomagnetic reversal) in which somewhat decreased activity occurs for intense and long-lasting solar events. In fact, a contemporary dip in the magnetic energy released from the Sun was observed, in agreement with the suggested double maximum displayed by the basic features of the 11-year solar-activity cycle (Gnevyshev, 1977, and references therein). Moreover, the dynamic component of cosmic-ray modulation often shows a multi-structured profile in both peaks of activity, fairly well-connected with the pattern of the green corona brightness. On the other hand, analysing the quasi-stationary long-term trend of cosmic-ray intensity we pick out a good relationship between periods of enhanced cosmic-ray modulation and the area expansion of coronal intensity levels. The relevance of our results for solar-terrestrial forecasting is underlined. 相似文献
92.
U. C. Joshi A. K. Sen M. R. Deshpande J. S. Chauhan 《Journal of Astrophysics and Astronomy》1992,13(3):267-277
Photopolarimetric observations of comet Austin with the IAU/IHW filter system were obtained on the 2.34 m Vainu Bappu Telescope
(VBT) of the Indian Institute of Astrophysics, at Kavalur, India, during pre-perihelion phase on February 20,1990 and on the
1.2 m telescope of the Physical Research Laboratory at Gurusikhar, Mount Abu during postperihelion phase on May 2 and 4, 1990.
The comet appeared bluer than a solar analog during post-perihelion phase on May 2 and 4. The percent polarization shows a
sharp increase towards the red on May 2 and 4. The dominant sizes of the dust particles appear to lie in a narrow range of
0.1 to 0.5 Μm. Regarding the molecular band emission, CN and C2 bands are quite strong; C3 emission was also found to be strong though the observations on May 2 and 4 show significant variation as compared to C2 emission. Molecular band polarization for CN, C3, C2 and H2 O+ have been calculated. It has been found that emission polarization in CN, C2 and C3 is between 1–7% (phase angle between 107.4–109 degrees). For CN and C2 the polarization values are close to the theoretically predicted values, but for C3 the polarization value falls much below the theoretically predicted value. A similar result was found for comet Halley. 相似文献
93.
94.
MICHAEL K. WEISBERG MICHAEL E. ZOLENSKY MARTIN PRINZ 《Meteoritics & planetary science》1997,32(6):791-801
Abstract— Fayalitic olivine (Fa54–94) is a ubiquitous component in the matrix of Krymka (LL3.1) as well as in other highly unequilibrated chondrites (ordinary and carbonaceous). In Krymka, the fayalitic olivine has an unusual anisotropic platy morphology that occurs in at least five types of textural settings that can be characterized as: (1) isolated platelets, (2) clusters of platelets, (3) euhedral to subhedral crystals, (4) overgrowths of platelets on forsteritic olivine, and (5) fluffy (porous) aggregates. From transmission electron microscope (TEM) investigation, the direction of elongation of the platy olivine overgrowths on forsteritic olivine substrates is along the c axis and in most cases it corresponds with the c axis of the substrate olivine, which suggests that the fayalitic olivine grew in this unusual morphology and is not a replacement product of preexisting material. The fayalitic olivine in the matrix of Krymka is compositionally similar to olivine with platy morphology in the matrix of some CV3 chondrites and both have similar Fe/Mn ratios, but important morphological differences indicate that their relationship needs to be explored further. Textural and compositional data indicate that the fayalitic olivine in the matrix of Krymka, as well as in some other unequilibrated ordinary chondrites, formed prior to final lithification of the meteorite and probably prior to parent body accretion. We find that formation of the fayalitic olivine by vapor-solid growth provides the best explanation for our observations and data and is the only feasible mechanism for the formation of fayalitic olivine in the matrix of Krymka. We propose that the fayalitic olivine formed by vaporization and recondensation of olivine rich-dust, during a period of enhanced dust/gas ratio in the nebula. 相似文献
95.
96.
97.
We analyzed data from 23 boreholes at 19 sites in central and eastern Canada, for the purpose of estimating ground surface temperature (GST) histories. These boreholes were logged down to at least 550 m depth with thermistor probes. Thermal conductivity measurements had been previously made at small depth intervals for the entire depth ranges of most of the boreholes. The temperature profiles of these boreholes do not indicate water disturbance. We estimated terrain effects for each borehole using a time dependent solid-angle method. The thermal perturbations caused by lakes or deforestation near the borehole sites are insignificant in most cases. However, four of the holes were found to be severely influenced by terrain effects. GSTs estimated from the borehole data less influenced by the terraineffects form two groups. The first group, which are generally from data of better quality, show a cold period near the end of the last century before the recent warming trend; the second show it 80–100 years earlier. We consider the former typical of the climate of the Boreal climatic region of Canada. The difference between the two groups may reflect the spacial variability of the climate. Four GST estimates do not belong to either type, and the reasons are discussed. 相似文献
98.
The dynamics of the vortex lattice in the inner crust of a neutron star is considered. A general equation of motion is obtained
and solved under the assumption that there are regions of pinned and of free vortices. By comparing these solutions with observational
data for the Vela pulsar, the relative moments of inertia of regions of relaxation with the corresponding characteristic times
are calculated for two model stars with different equations of state. It is shown that the theory can be reconciled with observations
of the relaxation of pulsar angular velocity only for model stars with extremely stiff equations of state.
Translated from Astrofizika, Vol. 40, No. 1, pp. 67–76, January–March, 1997. 相似文献
99.
R. Sarmiento M. Gallardo A. Laquidara J. Reyna Almandos 《Astrophysics and Space Science》1997,256(1-2):359-363
This work presents preliminaries spectroscopic results about ionized xenon and xenon-helium mixture using a capillary pulsed discharge under several experimental results. 相似文献
100.
S. K. El-Labany 《Astrophysics and Space Science》1992,191(2):185-194
The propagation of an ion-acoustic soliton in a collisionless plasma with adiabatic positive and negative ions (with equal ion temperature) and hot non-isothermal electrons is studied by use of the renormalization method introduced by Kodama and Taniuti in the reductive perturbation method. The basic set of fluid equations describing the system is reduced to a Korteweg-de Vries (K-dV)-type equation for the first-order perturbed potential and to a linear inhomogeneous differential equation to the second-order of the perturbed potential. A stationary solution of the coupled equations is obtained. 相似文献