首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   54410篇
  免费   712篇
  国内免费   562篇
测绘学   1480篇
大气科学   3865篇
地球物理   10060篇
地质学   19446篇
海洋学   5004篇
天文学   13035篇
综合类   188篇
自然地理   2606篇
  2022年   370篇
  2021年   640篇
  2020年   668篇
  2019年   720篇
  2018年   1588篇
  2017年   1515篇
  2016年   1893篇
  2015年   1003篇
  2014年   1770篇
  2013年   2910篇
  2012年   1876篇
  2011年   2417篇
  2010年   2092篇
  2009年   2713篇
  2008年   2325篇
  2007年   2381篇
  2006年   2233篇
  2005年   1641篇
  2004年   1660篇
  2003年   1568篇
  2002年   1499篇
  2001年   1323篇
  2000年   1235篇
  1999年   1011篇
  1998年   1053篇
  1997年   960篇
  1996年   829篇
  1995年   793篇
  1994年   700篇
  1993年   615篇
  1992年   599篇
  1991年   604篇
  1990年   632篇
  1989年   500篇
  1988年   515篇
  1987年   539篇
  1986年   493篇
  1985年   629篇
  1984年   689篇
  1983年   601篇
  1982年   568篇
  1981年   507篇
  1980年   472篇
  1979年   489篇
  1978年   464篇
  1977年   374篇
  1976年   350篇
  1975年   361篇
  1974年   313篇
  1973年   346篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Photoelectric observations of the WR binary CQ Cephei (WN6+O9) are presented. the depths of the eclipses in the light curves are best represented by an inclination of the orbit i = (68°.8±0.6) and the width of the very asymmetric eclipse curves can be represented by only an overcontact configuration (Ω1 = Ω2 = 3.65 ± 0.05, and f = 27%). Simultaneous solution of the light and radial velocity curves strongly supports CQ Cep's membership of the Cep OB1 association. By considering this membership we obtained absolute dimensions of the system, which lead to a consistent physical model for CQ Cephei. The more luminous WR primary turns out to be the hotter but slightly less massive component: MWR = 20.8 M⊙, RWR = 8.2R⊙, Teff(WR) = 43600 K, and Mo = 21.4 M⊙, Ro = 8.3 R⊙, Teff(O) = 37000 K.  相似文献   
992.
Suzuki  S. 《Solar physics》1974,38(1):3-7
Equipment has been developed to display in colour any circular polarization in the dynamic spectra of solar radio emissions. The potential and limitations of this simple equipment are discussed on the basis of a sample record. Modifications, now under development, to improve the system are described.  相似文献   
993.
Dyer  C. S.  Engel  A. R.  Quenby  J. J.  Webb  S. 《Solar physics》1974,34(1):243-245
Solar Physics - Quiet-time anisotropy data have been obtained by the HEOS-1 spacecraft between December 1968 and April 1970 using an 8-sector analysis of a ?erenkov telescope which has a...  相似文献   
994.
In contrast to earth, the atmosphere of the moon is exceedingly tenuous and appears to consist mainly of noble gases. The solar wind impinges on the lunar surface, supplying detectable amounts of helium, neon and 36Ar. Influxes of solar wind protons and carbon and nitrogen ions are significant, but atmospheric gases containing these elements have not been positively identified. Radiogenic 40Ar and 222Rn produced within the moon have been detected. The present rate of effusion of argon from the moon accounts for about 0.4% of the total production of 40Ar due to decay of 40K if the average abundance of potassium in the moon is 1000 ppm. Lack of weathering processes in the regolith suggests that most of the atmospheric 40Ar originates deep in the lunar interior, perhaps in a partially molten core. If so, other gases may be vented along with the argon.  相似文献   
995.
This paper presents a brief summary of an extensive correlative study of ATS-5 particle and magnetic field data with all-sky photographs from Great Whale River which is near the ‘foot’ of the field lines passing through the ATS-5 satellite. In particular, an effort is made to identify specific particle features with specific auroral displays during substorms, such as a westward travelling surge, poleward expansive motion and drifting patches. Some of the important findings are (i) in early evening hours, the first encounter of ATS-5 with hot plasma is associated with the equatorward shift of the diffuse aurora, but not necessarily with westward travelling surges (even when the satellite is embedded in the plasma sheet.) (ii) In the midnight sector, an injection corresponds very well to the initial brightening of an auroral arc. (iii) Specific features of morning sector auroras (for example, drifting patches) are difficult to correlate with specific particle features (gross features, but not specific).Comparing these results with particle data from low-latitude polar orbiting satellites, it is concluded that the plasma sheet near the earthward edge (consisting of plasmas injected during earlier substorms) is little affected during substorms.  相似文献   
996.
The Moon is represented as an inhomogeneous spherical body in a steady thermal state. Radioactive heat sources are supposed distributed in a manner which is consistent both with the total measured heat flux near the surface and with the broad seismic evidence. Surface concentrations of uranium and thorium are those suggested by the study of Apollo 11 samples. The resultant internal temperature profile allows the details of Sonett's electrical conductivity profile to be understood if it is accepted that the Moon was not cold 4.5 × 109 yr ago. It would appear further that at least one of the maria was formed by the impact of planetesimals.  相似文献   
997.
The diffusion of electrons through interstellar space, and the energy dependence of the diffusion coefficient are considered. Apart from the caseD=const the spectral index for electrons with spectral index γ0 changes according to γ0+μ→γ0+½μ+½→γ0+1 (D(E)=D0(E/E0)μ) for μ<1; for μ>1 to γ0+1→γ0+½μ+½→γ0+μ. We consider the radio emission spectrum in such a case. From a comparison with observations the limit μ≤0.4 is obtained.  相似文献   
998.
Chandrasekhar'sH-functionH(z) corresponding to the dispersion functionT(z)=| rs frs(z)|, where [f rs (z)] is of rank 1, is obtained in terms of a Cauchy integral whose density functionQ(x, 1, 2,...) can be approximated by approximating polynomials (uniformly converging toQ(x)) having their coefficients expressed as known functions of the parameters r 's. A closed form approximation ofH(z) to a sufficiently high degree of accuracy is then readily available by term by term integration.  相似文献   
999.
The results of detailed calculations on the production of H2 and He3 nuclei by cosmic ray protons and helium nuclei in interstellar medium are presented. The flux and energy spectra of these nuclei as well as those of cosmic ray H1 and He4 nuclei in the vicinity of the Earth are calculated. For this purpose the source spectra are assumed to be in the form of a power law in total energy per nucleon with an additional velocity dependent term. This spectrum denoted as Fermi Spectrum, is about midway between the power law spectrum in rigidity and in total energy per nucleon. The fluxes are calculated taking into account: (1) energy dependent cross-sections of thirteen nuclear reactions of cosmic ray protons and helium nuclei with interstellar H1 and He4 leading to the production of H2 and He3 nuclei, (2) angular distributions and kinematics of these reactions, (3) ionization loss of the primary and secondary nuclei in interstellar medium, (4) elastic collisions of cosmic ray protons and helium nuclei, (5) distributions of cosmic ray path-lengths in in terstellar space as in gaussian and exponential forms, and (6) interplanetary modulation of cosmic rays from the numerical solution of the complete Fokker-Planck equation describing the diffusion, convection and adiabatic deceleration of cosmic ray nuclei in the solar system. On comparing the calculated values of H2/He4 and He3/(He3+He4) as a function of energy with the observed data of several investigators, it is found that agreement between the calculated values and most of the observed data is obtained on the basis of: (a) source spectrum in the form of Fermi Spectrum, (b) distribution of path-lengths as in the gaussian form with a mean value of 4 g cm–2 of hydrogen or as in exponential form with leakage path length of 4 g cm–2.  相似文献   
1000.
Eleven large directional discontinuities in the solar wind, simultaneously observed by Explorers 33, 35 and Heos 1 in the period Dec. 1968 to Jan. 1969, have been studied by the triangulation method. Testing the discontinuity normals deduced from the wind speed and timing parameters against those given by the magnetometer observations on different models suggests that 8 were tangential, 2 were rotational while 1 could not be identified. 3 small tangential discontinuities arriving within a period 35 min were also studied and as a result it is suggested that the solar wind remained stratified in separation planes over a distance 0.01 AU during that time. The implication of these results is briefly discussed.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号