全文获取类型
收费全文 | 254184篇 |
免费 | 5180篇 |
国内免费 | 3352篇 |
专业分类
测绘学 | 6814篇 |
大气科学 | 18889篇 |
地球物理 | 53298篇 |
地质学 | 88054篇 |
海洋学 | 21065篇 |
天文学 | 55800篇 |
综合类 | 992篇 |
自然地理 | 17804篇 |
出版年
2021年 | 2214篇 |
2020年 | 2584篇 |
2019年 | 2846篇 |
2018年 | 3157篇 |
2017年 | 2828篇 |
2016年 | 5507篇 |
2015年 | 4186篇 |
2014年 | 6890篇 |
2013年 | 14199篇 |
2012年 | 6314篇 |
2011年 | 7500篇 |
2010年 | 6531篇 |
2009年 | 9222篇 |
2008年 | 8067篇 |
2007年 | 7417篇 |
2006年 | 9598篇 |
2005年 | 7630篇 |
2004年 | 7516篇 |
2003年 | 7015篇 |
2002年 | 6663篇 |
2001年 | 5953篇 |
2000年 | 5915篇 |
1999年 | 5189篇 |
1998年 | 5218篇 |
1997年 | 5015篇 |
1996年 | 4667篇 |
1995年 | 4413篇 |
1994年 | 4094篇 |
1993年 | 3831篇 |
1992年 | 3618篇 |
1991年 | 3589篇 |
1990年 | 3754篇 |
1989年 | 3509篇 |
1988年 | 3299篇 |
1987年 | 3840篇 |
1986年 | 3404篇 |
1985年 | 4219篇 |
1984年 | 4729篇 |
1983年 | 4406篇 |
1982年 | 4314篇 |
1981年 | 3920篇 |
1980年 | 3645篇 |
1979年 | 3509篇 |
1978年 | 3483篇 |
1977年 | 3280篇 |
1976年 | 3043篇 |
1975年 | 2956篇 |
1974年 | 2926篇 |
1973年 | 3079篇 |
1972年 | 2024篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
981.
The following physical parameters have been computed for the atmosphere of Venus between 65 and 90 km, by intervals of 1 km. (1) Pressure, (2) Density, (3) Speed of sound, (4) Number density, (5) Density scale, (6) Pressure scale, (7) Collisional frequency, (8) Mean particle velocity (9) Mean free path, (10) Columnar mass, (11) Viscosity. For these calculations we have used the temperature altitude measurements of Venera 15 and 16 at 52 °N and 72 °N latitudes, the night and 70 °N and 72 °N latitudes the day. 相似文献
982.
We have determined frequency distributions of flare parameters from over 12000 solar flares recorded with the Hard X-Ray Burst Spectrometer (HXRBS) on the Solar Maximum Mission (SMM) satellite. These parameters include the flare duration, the peak counting rate, the peak hard X-ray flux, the total energy in electrons, and the peak energy flux in electrons (the latter two computed assuming a thick-target flare model). The energies were computed above a threshold energy between 25 and 50 keV. All of the distributions can be represented by power laws above the HXRBS sensitivity threshold. Correlations among these parameters are determined from linear regression fits as well as from the slopes of the frequency distributions. Variations of the frequency distributions were investigated with respect to the solar activity cycle.Theoretical models for the frequency distribution of flare parameters depend on the probability of flaring and the temporal evolution of the flare energy build-up. Our results are consistent with stochastic flaring and exponential energy build-up, with an average build-up time constant that is 0.5 times the mean time between flares. The measured distributions of flares are also consistent with predicted distributions of flares from computer simulations of avalanche models that are governed by the principle of self-organized criticality. 相似文献
983.
Current carrying magnetic fields which penetrate sunspots can be unstable to current convective modes caused by the large gradient of electrical conductivity. The linear growth rates and wavelengths of the unstable modes are found. The unstable modes produce fine-scale vortices perpendicular to the magnetic field, which overshoot well into the solar corona. The modes provide a turbulent vorticity source at the photospheric footpoints of the field. This can cause braiding and reconnection of the coronal magnetic field. The modes twist the coronal magnetic field into loops with a typical radius of 200 km, consistent with recent X-ray observations. 相似文献
984.
S. Torres 《Earth, Moon, and Planets》1993,63(2):163-170
Observations and recommendations of the second un/esa workshop on basic space science 相似文献
985.
986.
987.
S. W. Allen R. W. Schmidt A. C. Fabian 《Monthly notices of the Royal Astronomical Society》2002,334(2):L11-L15
We present precise measurements of the X-ray gas mass fraction for a sample of luminous, relatively relaxed clusters of galaxies observed with the Chandra observatory, for which independent confirmation of the mass results is available from gravitational lensing studies. Parametrizing the total (luminous plus dark matter) mass profiles using the model of Navarro, Frenk & White, we show that the X-ray gas mass fractions in the clusters asymptote towards an approximately constant value at a radius r 2500 , where the mean interior density is 2500 times the critical density of the Universe at the redshifts of the clusters. Combining the Chandra results on the X-ray gas mass fraction and its apparent redshift dependence with recent measurements of the mean baryonic matter density in the Universe and the Hubble constant determined from the Hubble Key Project, we obtain a tight constraint on the mean total matter density of the Universe, , and measure a positive cosmological constant, . Our results are in good agreement with recent, independent findings based on analyses of anisotropies in the cosmic microwave background radiation, the properties of distant supernovae, and the large-scale distribution of galaxies. 相似文献
988.
989.
990.
J. Klačka 《Earth, Moon, and Planets》1994,64(2):125-132
The problem of the action of the solar radiation on the motion of interplanetary dust particle is discussed. Differences between the action of electromagnetic solar radiation and that of the solar wind are explained not only from the point of view of the physical nature of these phenomena but also from the point of view of dust particle's orbital evolution. As for the electromagnetic solar radiation, general equation of motion for the particle is written and the most important consequences are: (i) the process of inspiralling toward the Sun is not the only possible motion - even spiralling from the Sun is also possible, and, (ii) the orbital plane of the particle (its inclination) may change in time. As for the solar wind, the effect corresponding to the fact that solar wind particles spread out from the Sun in nonradial direction causes that the process of inspiralling toward the Sun is in more than 50% less effective than for radial spread out; in the region of the asteroid belt (long period orbits) the process of inspiralling is changed into offspiralling. Also shift in the perihelion of dust particle's orbit exists. 相似文献