全文获取类型
收费全文 | 29293篇 |
免费 | 423篇 |
国内免费 | 319篇 |
专业分类
测绘学 | 616篇 |
大气科学 | 1817篇 |
地球物理 | 5246篇 |
地质学 | 11268篇 |
海洋学 | 2963篇 |
天文学 | 6859篇 |
综合类 | 68篇 |
自然地理 | 1198篇 |
出版年
2022年 | 297篇 |
2021年 | 455篇 |
2020年 | 509篇 |
2019年 | 556篇 |
2018年 | 1091篇 |
2017年 | 1040篇 |
2016年 | 1146篇 |
2015年 | 545篇 |
2014年 | 1041篇 |
2013年 | 1686篇 |
2012年 | 1152篇 |
2011年 | 1434篇 |
2010年 | 1282篇 |
2009年 | 1557篇 |
2008年 | 1329篇 |
2007年 | 1395篇 |
2006年 | 1301篇 |
2005年 | 756篇 |
2004年 | 705篇 |
2003年 | 660篇 |
2002年 | 687篇 |
2001年 | 619篇 |
2000年 | 568篇 |
1999年 | 461篇 |
1998年 | 479篇 |
1997年 | 453篇 |
1996年 | 391篇 |
1995年 | 350篇 |
1994年 | 378篇 |
1993年 | 294篇 |
1992年 | 297篇 |
1991年 | 286篇 |
1990年 | 322篇 |
1989年 | 220篇 |
1988年 | 225篇 |
1987年 | 268篇 |
1986年 | 209篇 |
1985年 | 314篇 |
1984年 | 276篇 |
1983年 | 258篇 |
1982年 | 275篇 |
1981年 | 203篇 |
1980年 | 243篇 |
1979年 | 195篇 |
1978年 | 211篇 |
1977年 | 165篇 |
1976年 | 163篇 |
1975年 | 172篇 |
1974年 | 167篇 |
1973年 | 166篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
111.
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
112.
The rigorous Kerker-Matijevi formulae for light scattering by co-axial double cylinders are used to calculate the extinction properties of hollow organic grains. A size distribution of such particles together with iron whiskers of radii 0.01 m, silica spheres of radius 0.03 m and free aromatic molecular clusters comprised of 50–100 atoms yield excellent agreement with data on the extinction of starlight. The mass ratios of silica to organics and of iron to organics are in good accord with cosmic abundance constraints. 相似文献
113.
A static spherically-symmetric model, based on an exact solution of Einstein's equation, gives the permissible matter density 2×1014 g cm–3. If we use the change in the ratio of central density to the radiusr=a (i.e., central density per unit radius (0/a), we call it radius density) minimum, we have estimated the upper limit of the density variation parameter () and minimum mass limit of a superdense star like a neutron star. This limit gives an idea of the domain where the neutron abundance with negligible number of electrons and protons (may be treated as pure neutrons) and equilibrium in neutrons begins. 相似文献
114.
Radio astronomy experiments designed to probe the interplanetary plasma close to the Sun have been carried out at the wavelength = 2.92 m using the occultation method. The experiments are based on a modification of the occultation method by which the sources, in this case quasars, are used to measure the radial dependence of the apparent source size, the scattering angle (R). The radial dependence of this source size (R) reveals that an anomalous enhancement in the scattering appears at radial distances from about 16 to 30 R
, which is associated with the solar wind transsonic region. It is shown by analysis of the theoretical equations that the radial profiles of both the source size (R) and the scintillation index m(R) are consistent in the near solar interplanetary medium (R 40 R
). Combining these two independent applications of the occultation method extends the range of the investigation and provides a powerful new diagnostic of the solar wind transsonic region. 相似文献
115.
In this paper we present our most recent results on the sub-iron (Sc to Cr) to Fe nuclei abundance ratios in the low-energy cosmic rays of 50 to 250 MeV nucl.–1 and their implications as observed in theSkylab experiment. In view of the importance of this ratio in determining the cosmic-ray pathlength in interstellar medium, we have obtained additional data in the same detector module and the results of final analysis are reported. Charge determinations in the Lexan detector were made from an average of about four independent measurements ofZ for each of the cosmic-ray events and the mean charge resolution is obtained asZ/Z0.2. From about 100 events of calcium to nickel in low-energy cosmic rays, sub-iron (Sc to Cr) to Fe–Co ratio is determined as 1.43±0.40 in 50–250 MeV nucl.–1. This shows a large energy dependence of the ratio as compared to the value of 0.4–0.8 in 200–1000 MeV nucl.–1 as measured by many investigators. The origin of this large enhancement of the ratios in low-energy cosmic rays is not known at present. Some possible suggestions are briefly mentioned. 相似文献
116.
N. B. Ograpishvili 《Solar physics》1988,115(1):33-41
An attempt is made to study the relations between emergence of active regions and the solar background large-scale structures on the basis of Solar Geophysical Data, including Kitt-Peak magnetograms, H filtergrams, and Ca images.The emergence of 217 active regions (a.r.s) that have appeared on the solar disk not farther than ± 60° from the central meridian is studied. The a.r.s are divided into two classes A and B according to their birth location. Class A contains a.r.s emerged far (8–10°) from the background field boundaries, and class B- those emerged near to (55°) or just at the boundaries.It was found that a.r.s of class A differ appreciably from those of class B; in particular, the dimensions and the intensity (S, I) of class B a.r.s are nearly twice as large as those of class A. For class A a.r.s some alterations of the solar large-scale structure boundaries were found in 15% of all the cases, whereas for those of class B in 60%. 相似文献
117.
A space charge sheath is found to be formed whenever a high-velocity magnetized plasma stream penetrates a gas cloud. The sheath is always located at the head of the plasma stream, and its thickness is very small compared to the length of the plasma stream. Soon after the sheath is formed it quickly slows down to the Alfvén critical velocity. The plasma behind the sheath continues to move at higher velocity until the whole plasma stream is retarded to the critical velocity. In the interaction at gas density 1019 m–3 the sheaths are observed to be accompanied by a single loop of current with current density of 105 Å m–2. Maximum potential in the sheath ranges between 50 and 200 V.Presently available models for the sheath may explain the initiation of the sheath formation. Physical processes like heating of the electrons and ionization of the gas cloud which come into play at a later stage of the interaction are not included in these models. These processes considerably alter the potential structure in the sheath region. A schematic model of the observed sheath is presented here.Experiments reveal a threshold value of the magnetic field for plasma retardation to occur. This seems to correspond to the threshold condition for excitation of the modified two-stream instability which can lead to the electron heating. The observed current are found sufficient to account for the plasma retardation at a gas density of 1017 m–3. 相似文献
118.
D. Vandev K. Danov P. Mateev P. Petrov M. Kartalev N. Trendafilov Z. K. Smith M. Dryer 《Astrophysics and Space Science》1986,120(2):211-221
A study is presented of an algorithm, based on the statistical analysis of interplanetary magnetic field data, for the real-time detection of discontinuities in the solar wind. The analysis is based on the application of a sliding or searching algorithm together with a minimum variance treatment. A demonstration is given for a Pioneer data set from 29 August, 1966. This particular data set was chosen because it has been extensively studied (in terms of the fast forward MHD shock wave) by previous groups who utilized the three-dimensional Rankine-Hugoniot shock equations after identifying the shock on a subjective basis. Our procedure differs in that it is perfectly objective because of its searching technique. For this particular data set we identify the original shock with excellent agreement with the earlier studies of its normal vector, as well as an additional shock and tangential discontinuity, neither of which had been identified previously. 相似文献
119.
We have investigated the evolutionary behaviour of intermediate mass (2, 3, 4, 5, and 7M
) Population I stars, assuming two different rates of rotation at the threshold of stability.In the first part of the study, stars are assumed to start with a critical rotation (fast rotation model) and to progress to the point of rotational instability. The stars evolve by losing mass and become rotationally unstable before they reach the zero-age Main Sequence. It is argued that multiple star systems might be formed through the evolution of rapidly rotating stars. An expression for the rotational mass loss rate is derived as a function of the physical parameters of stars.In the second part of the study, stars are assumed to rotate at a rate below the critical value (slow rotation model). The evolution of slowly rotating stars is followed as far as zero-age Main Sequence on the theoretical Hertzsprung-Russell diagram and compared with that of normal stars. The evolutionary paths are found to be more or less similar to those of normal stars; but their positions on the Main Sequence are characterized by effective temperatures and luminosities lower than those of normal stars. The zero-age Main-Sequence times of these stars are longer than those of normal stars. The rotational rates obtained for the zero-age Main Sequence are in good agreement with observed values. 相似文献
120.
N. Seehafer 《Astrophysics and Space Science》1986,122(2):247-255
It is shown that, at neutral points of force-free magnetic fields, the electric current density must vanish. This property is independent of whether the neutral points are isolated or (e.g.) fill lines or surfaces. One implication is the fact that in a cold pressureless plasma the formation of neutral current sheets cannot be adiabatically slow. The field-line topology in the neighbourhood of neutral points is discussed. At neutral points of force-free magnetic fields in general three constant- surfaces, defined by the equation ×B=B, with the same value of intersect orthogonally. If, during a time-development, the magnetic field gradient matrix B
i/x
j becomes singular at a neutral point, the field topology can change qualitatively — in general connected with the merger of two or more neutral points into one and/or the splitting up of one neutral point into several others. This can be interpreted as implying the transition from a quasi-static evolution to a dynamical state in which magnetic energy is released. 相似文献