全文获取类型
收费全文 | 64766篇 |
免费 | 1292篇 |
国内免费 | 1322篇 |
专业分类
测绘学 | 1787篇 |
大气科学 | 4895篇 |
地球物理 | 12373篇 |
地质学 | 26309篇 |
海洋学 | 5311篇 |
天文学 | 12884篇 |
综合类 | 437篇 |
自然地理 | 3384篇 |
出版年
2022年 | 324篇 |
2021年 | 504篇 |
2020年 | 499篇 |
2019年 | 569篇 |
2018年 | 3860篇 |
2017年 | 3663篇 |
2016年 | 2746篇 |
2015年 | 957篇 |
2014年 | 1291篇 |
2013年 | 2233篇 |
2012年 | 2526篇 |
2011年 | 4598篇 |
2010年 | 4154篇 |
2009年 | 4722篇 |
2008年 | 3812篇 |
2007年 | 4483篇 |
2006年 | 2002篇 |
2005年 | 1932篇 |
2004年 | 1728篇 |
2003年 | 1704篇 |
2002年 | 1463篇 |
2001年 | 1073篇 |
2000年 | 1051篇 |
1999年 | 834篇 |
1998年 | 884篇 |
1997年 | 829篇 |
1996年 | 706篇 |
1995年 | 689篇 |
1994年 | 589篇 |
1993年 | 500篇 |
1992年 | 477篇 |
1991年 | 464篇 |
1990年 | 521篇 |
1989年 | 454篇 |
1988年 | 407篇 |
1987年 | 522篇 |
1986年 | 401篇 |
1985年 | 487篇 |
1984年 | 591篇 |
1983年 | 518篇 |
1982年 | 518篇 |
1981年 | 473篇 |
1980年 | 486篇 |
1979年 | 405篇 |
1978年 | 369篇 |
1977年 | 387篇 |
1976年 | 353篇 |
1975年 | 327篇 |
1974年 | 340篇 |
1973年 | 377篇 |
排序方式: 共有10000条查询结果,搜索用时 765 毫秒
131.
R. T. Schilizzi W. Aldrich B. Anderson A. Bos R. M. Campbell J. Canaris R. Cappallo J. L. Casse A. Cattani J. Goodman H. J. van Langevelde A. Maccafferri R. Millenaar R. G. Noble F. Olnon S. M. Parsley C. Phillips S. V. Pogrebenko D. Smythe A. Szomoru H. Verkouter A. R. Whitney 《Experimental Astronomy》2001,12(1):49-67
132.
W. J. Chaplin G. Houdek Y. Elsworth D. O. Gough G. R. Isaak R. New 《Monthly notices of the Royal Astronomical Society》2005,360(3):859-868
We investigate the frequency dependence of the power spectral density of low-degree solar p modes by comparing measurements with the results of a stochastic-excitation model. In the past it was common practice to use the total power in such investigations. Using the maximum of the power spectral density instead provides a direct comparison with the measured mode heights in the observed power spectrum. This method permits a more careful calibration of the adjustable parameters in the excitation model, a model which we present here, for the first time, in a format that precisely and unambiguously relates the amplitudes of the modes of oscillation to the Reynolds stress in the equilibrium model. We find that errors in the theory of the linear mode damping rates, particularly at low frequency, have a dramatic impact on the predictions of the mode heights in the spectral density, whereas parameter changes in the stochastic excitation model, within a plausible domain of parameter space, have a comparatively small effect. 相似文献
133.
134.
Profiles of spectral lines emitted from an accretion ring around an object with strong gravitational field should be affected by Doppler shift, gravitational redshift, and deflection of light. Taking these effects into account, precise line profile of a Keplerian ring around a Kerr black hole for a distant observer is obtained by solving the kinetic equation of photons. 相似文献
135.
K.A. MAASCH P.A. MAYEWSKI E.J. ROHLING J.C. STAGER W. KARLEN L.D. MEEKER E.A. MEYERSON 《Geografiska Annaler: Series A, Physical Geography》2005,87(1):7-15
ABSTRACT. Although considerable attention has been paid to the record of temperature change over the last few centuries, the range and rate of change of atmospheric circulation and hydrology remain elusive. Here, eight latitudinally well-distributed (pole-equator-pole), highly resolved (annual to decadal) climate proxy records are presented that demonstrate major changes in these variables over the last 2000 years. A comparison between atmospheric 14 C and these changes in climate demonstrates a first-order relationship between a variable Sun and climate. The relationship is seen on a global scale. 相似文献
136.
Skylab EUV observations of an active region near the solar limb were analyzed. Both cool (T < 106 K) and hot (T > 106 K) loops were observed in this region. For the hot loops the observed intensity variations were small, typically a few percent over a period of 30 min. The cool loops exhibited stronger variations, sometimes appearing and disappearing in 5 to 10 min. Most of the cool material observed in the loops appeared to be caused by the downward flow of coronal rain and by the upward ejection of chromospheric material in surges. The frequent EUV brightenings observed near the loop footpoints appear to have been produced by both in situ transient energy releases (e.g. subflares) and the infall/impact of coronal rain. The physical conditions in the loops (temperatures, densities, radiative and conducting cooling rates, cooling times) were determined. The mean energy required to balance the radiative and conductive cooling of the hot loops is approximately 3 × 10–3 erg cm–3 s–1. One coronal heating mechanism that can account for the observed behavior of the EUV emission from McMath region 12634 is heating by the dissipation of fast mode MHD waves. 相似文献
137.
Data from the Pulkovo spectrophotometric data base on the absolute quasimonochromatic fluxes from oCet in the 320–1080 nm range are used to determine the physical parameters of this star in different phases of its light curve. The continuum emission layer is found to expand between the phases of the cycle corresponding to the rising and falling branches of the light curve. The average expansion velocity is 32 km/s. By the time the star’s brightness has fallen by roughly three magnitudes, its radius has increased by almost a factor of three. Over this same time the temperature of the layer has fallen from 3000 K to 2200 K. For this expansion velocity, the calculated mass rate loss is ⊙ M /year.__________Translated from Astrofizika, Vol. 48, No. 2, pp. 175–189 (May 2005). 相似文献
138.
Magnetic stratigraphy observed in ferromanganese crust 总被引:1,自引:0,他引:1
Summary. A sample of ferromanganese crust deposit (SCHW-1D) was cut into two sets of 1 mm slices for palaeomagnetic study. Alternating field and thermal demagnetization, and isothermal remanent magnetization analyses were performed on each thin slice. The results reveal the presence of a stable magnetism and both normal and reversed polarity intervals in the specimens. The observed polarity intervals cannot be confidently correlated with the geomagnetic polarity time-scale of the last 10 Myr due to the polarity overlap inherent in the sampling technique. But the results confirm the slow accretion rate of ferromanganese deposit determined by 10 Be method and suggest potential use of ferromanganese deposits in palaeomagnetic and tectonic studies. 相似文献
139.
Summary. Exact spherical harmonic expansions are given for calculating the gravitational and magnetic fields associated with certain uniform solids of revolution. The figures are those made by rotating a conic section about one of its principal axes. The coefficients in the expansions can be computed accurately and efficiently and this approach leads to a very satisfactory method for calculating the fields of geological bodies with approximate circular symmetry about a vertical axis. A complete theory of convergence is given for the expansions. Somewhat unexpectedly, the sphere of convergence is determined by the location of a number of equivalent point or line sources that lie within the body or on its edges. 相似文献
140.
We present results of investigations into chromospheric velocity oscillations in sunspots, carried out at the Sayan Solar
Observatory. It is shown that the “chevron” structures in the space-time diagrams demonstrate wavetrain properties. Such structures
are indicators of a propagating wave process and they are typical of many sunspots. In the authors’ opinion, three-minute
umbral oscillations are not the source of running penumbral waves (RPW). It is very likely that umbral oscillations and RPW
initially propagate along different magnetic field lines. We explain the decrease in RPW propagation velocity and frequency
in the outer penumbra, as compared with the inner, by the combined action of different frequency modes. To better reveal the
properties of these modes, frequency filtering was used. Our measurements of the RPW (five-minute mode) wavelength and RPW
propagation velocity in different sunspots vary from 12″ to 30″ and from 28 to 60 – 70 km s−1 correspondingly. 相似文献