全文获取类型
收费全文 | 53828篇 |
免费 | 436篇 |
国内免费 | 428篇 |
专业分类
测绘学 | 1735篇 |
大气科学 | 4020篇 |
地球物理 | 9757篇 |
地质学 | 21913篇 |
海洋学 | 3912篇 |
天文学 | 11048篇 |
综合类 | 273篇 |
自然地理 | 2034篇 |
出版年
2021年 | 279篇 |
2020年 | 279篇 |
2019年 | 326篇 |
2018年 | 3688篇 |
2017年 | 3428篇 |
2016年 | 2533篇 |
2015年 | 705篇 |
2014年 | 991篇 |
2013年 | 1693篇 |
2012年 | 2069篇 |
2011年 | 3853篇 |
2010年 | 3511篇 |
2009年 | 3846篇 |
2008年 | 3139篇 |
2007年 | 3753篇 |
2006年 | 1218篇 |
2005年 | 1451篇 |
2004年 | 1210篇 |
2003年 | 1280篇 |
2002年 | 1102篇 |
2001年 | 801篇 |
2000年 | 778篇 |
1999年 | 673篇 |
1998年 | 618篇 |
1997年 | 630篇 |
1996年 | 569篇 |
1995年 | 534篇 |
1994年 | 506篇 |
1993年 | 428篇 |
1992年 | 384篇 |
1991年 | 411篇 |
1990年 | 412篇 |
1989年 | 388篇 |
1988年 | 359篇 |
1987年 | 433篇 |
1986年 | 361篇 |
1985年 | 444篇 |
1984年 | 479篇 |
1983年 | 465篇 |
1982年 | 445篇 |
1981年 | 371篇 |
1980年 | 380篇 |
1979年 | 311篇 |
1978年 | 306篇 |
1977年 | 295篇 |
1976年 | 262篇 |
1975年 | 254篇 |
1974年 | 282篇 |
1973年 | 305篇 |
1972年 | 194篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
G. K. Rangarajan 《Planetary and Space Science》1977,25(12):1183-1185
The mean daily range in horizontal intensity at low latitudes shows a significant departure on the day of a sector boundary passage in relation to its magnitude on adjacent days with a measure of dependence on phase of the solar activity. It is shown that this arises because of a substantial difference, in the nature of the response to sector boundary passage, between the instantaneous maximum field and minimum field. From the fact that the responses at three stations spanning the latitudes near dip equator to that near the focus of Sq currents are almost identical, it is suggested that the cause of the observed feature is primarily disturbance and is essentially non-ionospheric. Differences in the nature of responses between pre-1957 and post-1957 periods reported earlier in the planetary indices or low latitude disturbance indices are shown to be true for the daily range, maximum and minimum fields at low latitudes. 相似文献
982.
K. L. Vithal 《Astrophysics and Space Science》1977,49(2):293-316
A form of general dispersion relation for electromagnetic waves in a fully ionized anisotropic plasma with loss-cone that explicates the contribution of the loss-cone to the dispersion relation is developed. By initially ignoring effects due to anisotropy, it is shown by means of Nyquist diagram technique that an isotropic loss-cone distribution can be unstable to EM waves corresponding to the whistler mode (0<<
e
). The growth rate is then determined analytically for this distribution, assuming cyclotron resonance between the waves in the whistler mode and particles in the high energy tail of the velocity distribution. By including the effects of anisotropy, a general growth rate is obtained which is found to depend on the anisotropy, the size of the loss-cone, the softness of the energy spectrum, and the fraction of the particles which are resonant with the wave. For particular distributions the relative contributions of the anisotropy and of the loss-cone to the growth rate have been determined. It is seen that loss-cone effects, which depend on the size of the loss-cone as well as the softness of the energy spectrum, can be a significant factor in the determination of the growth rate. For the Lorentzian distribution, the half-width of unstable waves is considerably broadened and the growth rates are somewhat more severe as compared to a two-temperature Maxwellian. The threshold frequency is
which confirms the presence of unstable EM waves in the magnetospheric plasma leading to turbulence. 相似文献
983.
On three nights in February 1976 we carried out polarimetric measurements, in V, of the short periodic eclipsing binary XY UMa, covering a complete cycle. The results are as follows:
- Within all phase intervals the linear polarization does not exceed 0.1%.
- In the phase range 0 p .95–1 p .35 the scatter of the Stokes parametersQ andU is about twice that within the phase interval 0 p .35–0 p .95.
- A periodogram analysis of these data revealed a period of 21000 s, which is equal to half the orbital periodP o=0d.47899 within 1.5%.
984.
985.
The paper considers a homogeneous Bianchi type II universe. Under each of the conditionsC
hijk
C
hijk
=0 and*
C
hijk
C
hijk
=0 different types of models have been studied and their physical and kinematical properties have been discussed. 相似文献
986.
The general equation for radiative transfer in the Milne-Eddington model is considered here. The scattering function is assumed to be quadratically anisotropic in the cosine of the scattering angle and Planck's intensity function is assumed for thermal emission. Here we have taken Planck's function as a nonlinear function of optical depth, viz.,B
v(T)=b
o+b
1
e
–. The exact solution for emergent intensity from the bounding face is obtained by the method of the Laplace transform in combination with the Wiener-Hopf technique. 相似文献
987.
The overall dynamical evolution and radiation mechanism of γ-ray burst (GRB) jets are briefly introduced. Various interesting
topics concerning beaming in GRBs are discussed, including jet structures, orphan afterglows and cylindrical jets. The possible
connection between GRBs and neutron stars is also addressed. 相似文献
988.
We consider two-layer (Fe-FeS core+silicate mantle) and three-layer (Fe-FeS core+silicate mantle+crust) models of the Galilean satellite Io. Two parameters are known from observations for the equilibrium figure of the satellite, the mean density ρ0 and the Love number k2. Previously, the Radau-Darwin formula was used to determine the mean moment of inertia. Using formulas of the Figure Theory, we calculated the principal moments of inertia A, B, and C and the mean moment of inertia I for the two-and three-layer models of Io using ρ0 and k2 as the boundary conditions. We concluded that when modeling the internal structure of Io, it is better to use the observed value of k2 than the moment of inertia I derived from k2 using the Radau-Darwin formula. For the models under consideration, we calculated the Chandlerian wobble periods of Io. For the three-layer model, this period is approximately 460 days. 相似文献
989.
The elements of dielectric tensor and dispersion relation for obliquely propagating whistler waves with finite in an infinite magnetoplasma are obtained for a kappa distribution in the presence of perpendicular a.c. electric field.
Integrals and modified plasma dispersion functions are reduced in power series form. Numerical calculations have been performed
to obtain temporal growth rate and real frequencies of the plasma waves for magnetospheric plasma, using linear theory of
dispersion relation. The effect and modification introduced by the perpendicular a.c. electric field on the temporal growth
rates, real frequencies and resonance condition are discussed for kappa and Maxwellian distributions. Our results and their
interpretation are compared with known whistler observations obtained by ground-based techniques and satellite observations. 相似文献
990.