全文获取类型
收费全文 | 84497篇 |
免费 | 1360篇 |
国内免费 | 841篇 |
专业分类
测绘学 | 2390篇 |
大气科学 | 6084篇 |
地球物理 | 16618篇 |
地质学 | 29350篇 |
海洋学 | 7590篇 |
天文学 | 20047篇 |
综合类 | 280篇 |
自然地理 | 4339篇 |
出版年
2022年 | 479篇 |
2021年 | 836篇 |
2020年 | 910篇 |
2019年 | 1014篇 |
2018年 | 2212篇 |
2017年 | 2084篇 |
2016年 | 2653篇 |
2015年 | 1453篇 |
2014年 | 2502篇 |
2013年 | 4321篇 |
2012年 | 2671篇 |
2011年 | 3481篇 |
2010年 | 3100篇 |
2009年 | 4102篇 |
2008年 | 3530篇 |
2007年 | 3609篇 |
2006年 | 3374篇 |
2005年 | 2509篇 |
2004年 | 2562篇 |
2003年 | 2401篇 |
2002年 | 2360篇 |
2001年 | 2079篇 |
2000年 | 1976篇 |
1999年 | 1680篇 |
1998年 | 1716篇 |
1997年 | 1637篇 |
1996年 | 1369篇 |
1995年 | 1316篇 |
1994年 | 1164篇 |
1993年 | 1047篇 |
1992年 | 977篇 |
1991年 | 980篇 |
1990年 | 996篇 |
1989年 | 891篇 |
1988年 | 853篇 |
1987年 | 945篇 |
1986年 | 869篇 |
1985年 | 1046篇 |
1984年 | 1236篇 |
1983年 | 1061篇 |
1982年 | 1038篇 |
1981年 | 935篇 |
1980年 | 846篇 |
1979年 | 825篇 |
1978年 | 839篇 |
1977年 | 695篇 |
1976年 | 648篇 |
1975年 | 656篇 |
1974年 | 596篇 |
1973年 | 663篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
401.
The solar causes of geomagnetic disturbances 总被引:1,自引:0,他引:1
Geomagnetic disturbances have been identified with respect to their sources for 1977–1983. The disturbance level was found using the daily planetary index A
p. High-amplitude ( 50), mean-amplitude (24) and low-amplitude ( 12) disturbances are caused by solar flares of importance 1, coronal holes, and filament cavities, respectively. The ranges of probable amplitudes of disturbances of different nature and their relative number are found from Poisson random distributions of amplitudes. 相似文献
402.
P. L. Bornmann 《Solar physics》1985,102(1-2):111-130
The light curves of soft X-ray lines, observed by the Flat Crystal Spectrometer on Solar Maximum Mission during eight solar flares are modeled to determine the plasma temperature and emission measure as functions of time using the method first presented by Bornmann (1985, Paper I), but modified to include a 2 search routine. With this modification the technique becomes more general, more accurate, and applicable throughout the gradual phase of the flare. The model reproduces the light curves of the soft X-ray lines throughout these flares. Model fits were repeated for each flare using five different sets of published line emissivity calculations. The emissivities of Mewe and Gronenschild (1981) consistenly gave the best fits to the observed light curves for each flare. 相似文献
403.
Foggin PM 《Singapore journal of tropical geography》1983,4(1):11-24
This paper focuses on whether there is an organized structural pattern in the spatial distribution of oral contraceptive usage in rural Thailand and, if so, how this spatial pattern is linked to other related socioeconomic variables. The data concern the year 1975. The results suggest that the location of the health services infrastructure seems to have the most significant effect on the differences in level of oral contraceptive use observed. 相似文献
404.
Exact solutions have been obtained for a massive fluid sphere under the extreme causality condition (dP/dρ)=1. Radial pulsational stability of these structures has been discussed. It is found that for pulsationally stable configurations the surface to central density ratio is greater than 0.30, the maximum values for surface and central redshifts are 0.85 and 3.40 respectively in the extreme case, and the maximum mass and size are respectively 4.8M ⊙ and 20.1 km. It has also been shown that these structures are gravitationally bound, with a maximum binding energy per unit rest mass equal to 0.25 for a surface to central density ratio ?0.40. Slow rotation of these configurations has also been considered, and the relative drag and moment of inertia have been calculated. These results have been applied to the Crab pulsar and the mass of the pulsar has also been calculated based upon this model. 相似文献
405.
We establish limits on the total radiant energy of solar flares during the period 1980 February – November, using the solar-constant monitor (ACRIM) on board the Solar Maximum Mission. Typical limits amount to 6 × 1029 erg/s for a 32-second integration time, with 5σ statistical significance, for an impulsive emission; for a gradual component, about 4 × 1032 ergs total radiant energy. The limits lie about an order of magnitude higher than the total radiant energy estimated from the various known emission components, suggesting that no heretofore unknown dominant component of flare radiation exists. 相似文献
406.
It is proposed that the solar flare phenomenon can be understood as a manifestation of the electrodynamic coupling process of the photosphere-chromosphere-corona system as a whole. The system is coupled by electric currents, flowing along (both upward and downward) and across the magnetic field lines, powered by the dynamo process driven by the neutral wind in the photosphere and the lower chromosphere. A self-consistent formulation of the proposed coupling system is given. It is shown in particular that the coupling system can generate and dissipate the power of 1029 erg s#X2212;1 and the total energy of 1032 erg during a typical life time (103 s) of solar flares. The energy consumptions include Joule heat production, acceleration of current-carrying particles along field lines, magnetic energy storage and kinetic energy of plasma convection. The particle acceleration arises from the development of field-aligned potential drops of 10–150 kV due to the loss-cone constriction effect along the upward field-aligned currents, causing optical, X-ray and radio emissions. The total number of precipitating electrons during a flare is shown to be of order 1037–1038. 相似文献
407.
The data such as the H-spectrum-spectroheliographic (SSHG) observations, the H-chromospheric observations, etc., of a flare loop prominence which occurred on the western solar limb on 1981 April 27 have been obtained at Yunnan Observatory. The distribution of the internal motions and the macroscopical motion of the flare loop prominence with time and space in the course of its eruption and ascension is derived from the comprehensive analysis of the data. The possible physical pictures and the instability of the motions of the loop are inferred and discussed. 相似文献
408.
We have investigated numerically how a temperature difference between electrons and protons is produced in a flaring loop by adopting a one-fluid, two-temperature model instead of a single-temperature model. We have treated a case in which flare energy is released in the form of heating of electrons located in the top part of the loop.In this case, a large temperature difference (T
e/T
p 10) appears in the corona in the energy-input phase of the flare. When the material evaporated from the chromosphere fills the corona, the temperature difference in the loop begins to shrink rapidly from below. Eventually, in the loop apex, the proton temperature exceeds the electron temperature mainly due to cooling of the electrons by conduction down the loop and heating of the protons by compression of the ascending material. In the late phase of the flare (t 15 min from the flare onset), the temperature difference becomes less than 2% of the mean temperature of electrons and protons at every point in the loop. 相似文献
409.
P. Kaufmann F. M. Strauss J. E. R. Costa B. R. Dennis A. Kiplinger K. J. Frost L. E. Orwig 《Solar physics》1983,84(1-2):311-319
Simultaneous microwave and X-ray observations are presented for a solar flare detected on May 8, 1980 starting at 19:37 UT. The X-ray observations were made with the Hard X-Ray Burst Spectrometer on the Solar Maximum Mission and covered the energy range from 28–490 keV with a time resolution of 10 ms. The microwave observations were made with the 5 and 45 foot antennas at the Itapetinga Radio Observatory at frequencies of 7 and 22 GHz, with time resolutions of 100 ms and 1 ms, respectively. Detailed correlation analysis of the different time profiles of the event show that the major impulsive peaks in the X-ray flux preceded the corresponding microwave peaks at 22 GHz by about 240 ms. For this particular burst the 22 GHz peaks preceded the 7 GHz by about 1.5 s. Observed delays of the microwave peaks are too large for a simple electron beam model but they can be reconciled with the speeds of shock waves in a thermal model. 相似文献
410.
P. Farinella A. Milani A. M. Nobili P. Paolicchi V. Zappala' 《Earth, Moon, and Planets》1983,28(3):251-258
In the set of small satellites of Saturn recently imaged by the Voyager probes, we can observe the transition from irregularly-shaped, strength-dominated objects to larger, gravity-dominated bodies with shapes roughly fitting the theoretical equilibrium figures. The transition occurs for a radius of 100±50 km, corresponding to a typical material strength of the order of 107 dynes cm?2. We discuss briefly the cases of Mimas, Enceladus, Hyperion, Phoebe and the small coorbital and F-ring shepherding moons, showing that an analysis of the shape data can often provide interesting results on the physical properties, origin and collisional history of these objects. 相似文献