全文获取类型
收费全文 | 72158篇 |
免费 | 1453篇 |
国内免费 | 672篇 |
专业分类
测绘学 | 1735篇 |
大气科学 | 5164篇 |
地球物理 | 14512篇 |
地质学 | 25601篇 |
海洋学 | 6266篇 |
天文学 | 16558篇 |
综合类 | 209篇 |
自然地理 | 4238篇 |
出版年
2022年 | 401篇 |
2021年 | 719篇 |
2020年 | 795篇 |
2019年 | 835篇 |
2018年 | 1923篇 |
2017年 | 1784篇 |
2016年 | 2269篇 |
2015年 | 1348篇 |
2014年 | 2222篇 |
2013年 | 3824篇 |
2012年 | 2303篇 |
2011年 | 3180篇 |
2010年 | 2658篇 |
2009年 | 3586篇 |
2008年 | 3317篇 |
2007年 | 3104篇 |
2006年 | 2933篇 |
2005年 | 2403篇 |
2004年 | 2287篇 |
2003年 | 2166篇 |
2002年 | 1952篇 |
2001年 | 1819篇 |
2000年 | 1743篇 |
1999年 | 1394篇 |
1998年 | 1489篇 |
1997年 | 1403篇 |
1996年 | 1119篇 |
1995年 | 1158篇 |
1994年 | 980篇 |
1993年 | 888篇 |
1992年 | 870篇 |
1991年 | 775篇 |
1990年 | 868篇 |
1989年 | 724篇 |
1988年 | 661篇 |
1987年 | 829篇 |
1986年 | 677篇 |
1985年 | 874篇 |
1984年 | 945篇 |
1983年 | 883篇 |
1982年 | 856篇 |
1981年 | 716篇 |
1980年 | 684篇 |
1979年 | 620篇 |
1978年 | 613篇 |
1977年 | 557篇 |
1976年 | 541篇 |
1975年 | 503篇 |
1974年 | 509篇 |
1973年 | 471篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
C. D'uston J. M. Bosqued F. Cambou V. V. Temny G. N. Zastenker O. L. Vaisberg E. G. Eroshenko 《Solar physics》1977,51(1):217-229
Solar wind and interplanetary magnetic field data were obtained by the PROGNOZ 1 and PROGNOZ 2 satellites during the period following the August 4, 1972 (0621 UT) solar flare. A thermalized plasma was recorded one hour after the shock followed two hours later by the plasma piston with a bulk velocity higher than 1700 km s-1. The comparison between the PROGNOZ and PIONEER 9 solar wind data shows an attenuation of the plasma properties with the deflection from the flare's meridian. 相似文献
992.
Photographs of the corona in the continuum spectrum (580–700 nm) have been obtained with a doublet camera (F=4m) through rotating sector polarizers with the vibrational directions oriented radially and tangentially to the solar limb. Isophotes of the total emission and its polarized component, as well as diagrams giving the degree of polarization for the (K+F)-corona and the electron corona proper (P
K), have been plotted up to the distance of 1R
from the limb. In some inner regions the polarizationP
K exceeds the highest value possible for Thomson scattering under the given conditions. The reality of this anomally is dubious; systematic photometrical errors may account for it. Three-dimensional forms of 15 different coronal rays have been ascertained by comparing the course of polarization along the rays to the family of Baumach curves. The rays are found to deflect substantially from the radial directions out of the plane of the sky. Mean values of the coronal brightness and polarization versus the distance from the Sun have been determined. Contrary to well-known models (Van de Hulst) the mean polarization of the electron corona (P
K) decreases with distance after reaching the maximum (50%) at (1.4–1.6)R
from the Sun's centre. This decrease can be explained by deflection of the streamers from radial directions. 相似文献
993.
A whistler study has been made of plasma convection within the plasmasphere during a transition from steady moderate geomagnetic activity to quiet conditions. Continuous whistler data recorded at Sanae, Antarctica (L= 3.98) for the period 0400 UT, 10 July to 0400 UT, 11 July 1973 have been analyzed in 15 min intervals.This study has revealed two distinct bulges in the plasmasphere centred on 1700 and 0100 UT. The bulges appear to result from the outward flow of plasma rather than the addition of new plasma. We tentatively interpret the late bulge at 0100 UT as being the duskside bulge of earlier studies rotated into the midnight region. In this bulge, plasma above L = 3.8 appears to convect outwards to form the bulge whereas plasma at lower L-values is relatively undisturbed. For the early bulge (1700 UT) the plasma convection pattern is similar over all observable L-values and closely reflects the shape of the estimated plasmapause in that region. Comparison of the bulges, with those obtained by Carpenter (1966) suggests that the onset of quiet conditions results in a general displacement of the bulges in an eastward direction by about 3 hr. 相似文献
994.
Radiative recombination of N and O provides a significant source for auroral emission in the γ and δ bands of NO with selective population of vibrational levels in the A2Σ+ and C2Π states. This mechanism may account for emissions detected near 2150 Å. Models are derived for the auroral ionosphere and include estimates for the concentrations of N and NO. The concentration of NO is estimated to have a value of about 108 cm?1 near 140 km in an IBC III aurora. The corresponding density for N is about 5 × 107cm?3 and the concentration ratio has a value of about 5.5. 相似文献
995.
M. Hénon 《Celestial Mechanics and Dynamical Astronomy》1977,15(1):99-105
We show the existence of a general relation between the parameters of periodic solutions in dynamical systems with ignorable coordinates. In particular, for time-independent systems with an axis of symmetry, the relation takes the form T/A=–/E, whereT is the period,A is the angular momentum, is the angle through which the system has rotated after one period, andE is the energy. 相似文献
996.
We present digital pictures of an active region network cell in five quantities, measured simultaneously: continuum intensity, line-center intensity, equivalent width, magnetogram signal, and magnetic field strength. These maps are derived from computer analysis of circularly polarized line profiles of FeI 5250.2; spectral and spatial resolution are 1/40 Å and 1.5, respectively. Measured Zeeman splittings show the existence of strong magnetic fields (1000–1800 G) at nearly all points with a magnetogram signal exceeding 125 G. The mean and rms deviation of the field strengths change by less than 20% over a factor-of-four range of fluxes. From the significant disparity between measured fluxes and field strengths, we conclude that large flux patches (up to 4 across) consist of closely-packed unresolved filaments. The smallest filaments must be less than 0.7 in diameter. We also observe the dark component of the photospheric network, which appears to contain sizable transverse fields. 相似文献
997.
998.
The correlation between the polar cap geomagnetic variations (H-traces) and the changes of the azimuthal (YSE) and vertical (ZSE) components of the interplanetary magnetic field (IMF) during undisturbed periods is examined. It is shown that peak-to-peak correlation between YSE and geomagnetic horizontal component variations may be generally observed in the daytime cusp region, independently of the magnitude and polarity of the ZSE. The existence of the DP3 disturbances associated with the northward component ZSE > 0 is confirmed. It is shown that the disturbances due to the vertical component of the IMF dominate in the region near the pole. In so far as the southward component of the IMF generates both polar cap disturbances and geomagnetic substorms, the disturbances in the region near the pole, associated with ZSE < 0, may be regarded as a precursor of a substorm. On this basis a new index of the polar cap magnetic activity PCL, characterizing the changeability of the magnetic field is proposed. It is shown that the increase of the PCL index is followed in 1–2 hr by a substorm in 70% of events considered. 相似文献
999.
The tridiurnal wave in cosmic-ray intensity expected from a free space anisotropy is theoretically calculated for different cosmic-ray stations which are characterized by different shapes of asymptotic cones of acceptance. The amplitude A and the time of maximum Tmax are given for latitude dependence of the form cosn λ and rigidity dependence of the form , where λ and R are the latitude and rigidity respectively and n, β, R0 are constants. The values of A and Tmax, are calculated for different values of n, β and R0 for each station. The dependence of A and Tmax on the anisotropy parameters is studied for the proper selection of cosmic-ray stations whose data may be used in determining these parameters.Available experimental data were used to find the observed amplitudes of the tridiurnal variations at five stations using power spectrum analysis with hanning applied on the averaged trains. Minimum variance analysis of the theoretical and experimental amplitudes showed that β has a value between 1 and 2, R0 greater than 100 GV and n smaller than 3. 相似文献
1000.
The scattering and absorption properties of Axel dust were investigated by means of Mie theory. We find that a flat distribution of particle radii between 0 and 0.1 μm, and an imaginary part of the index of refraction which varies as λ?2.5 produce a good fit to the variation of Titan's geometric albedo with wavelength (λ) provided that τext, the extinction optical depth of Titan's atmosphere at 5000 Å, is about 10. The real part of the complex index is taken to be 2.0. The model assumes that the mixing ratio of Axel dust to gas is uniform above the surface of Titan. The same set of physical properties for Axel dust also produces a good fit to Saturn's albedo if τext = 0.7 at 5000 Å. To match the increase in albedo shortward of 3500 Å, a clear layer (containing about 7 km-am H2) is required above the Axel dust. Such a layer is also required to explain the limb brightening in the ultraviolet. These models can be used to analyze the observed equivalent widths of the visible methane bands. The analysis yields an abundance of the order of 1000 m-am CH4 in Titan's atmosphere. The derived CH4/H2 mixing ratio for Saturn is about 3.5 × 10?3 or an enhancement of about 5 over the solar ratio. 相似文献