全文获取类型
收费全文 | 82439篇 |
免费 | 1334篇 |
国内免费 | 1174篇 |
专业分类
测绘学 | 2173篇 |
大气科学 | 5427篇 |
地球物理 | 15723篇 |
地质学 | 32801篇 |
海洋学 | 6988篇 |
天文学 | 17589篇 |
综合类 | 460篇 |
自然地理 | 3786篇 |
出版年
2022年 | 688篇 |
2021年 | 1040篇 |
2020年 | 1067篇 |
2019年 | 1167篇 |
2018年 | 4310篇 |
2017年 | 3900篇 |
2016年 | 3464篇 |
2015年 | 1325篇 |
2014年 | 2242篇 |
2013年 | 3560篇 |
2012年 | 3306篇 |
2011年 | 5096篇 |
2010年 | 4482篇 |
2009年 | 5204篇 |
2008年 | 4388篇 |
2007年 | 4993篇 |
2006年 | 3162篇 |
2005年 | 2197篇 |
2004年 | 2052篇 |
2003年 | 2007篇 |
2002年 | 1829篇 |
2001年 | 1531篇 |
2000年 | 1392篇 |
1999年 | 1027篇 |
1998年 | 1082篇 |
1997年 | 1094篇 |
1996年 | 867篇 |
1995年 | 861篇 |
1994年 | 775篇 |
1993年 | 671篇 |
1992年 | 657篇 |
1991年 | 629篇 |
1990年 | 728篇 |
1989年 | 621篇 |
1988年 | 579篇 |
1987年 | 665篇 |
1986年 | 516篇 |
1985年 | 686篇 |
1984年 | 789篇 |
1983年 | 706篇 |
1982年 | 689篇 |
1981年 | 629篇 |
1980年 | 647篇 |
1979年 | 525篇 |
1978年 | 566篇 |
1977年 | 517篇 |
1976年 | 468篇 |
1975年 | 457篇 |
1974年 | 457篇 |
1973年 | 526篇 |
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
231.
G.H.L. Read 《Estuarine, Coastal and Shelf Science》1985,21(3):313-324
A dry (1979–1980) and a wet (1980–1981) season had a marked effect on the freshwater inflow into the Keiskamma estuary. Under low inflow conditions, which results in elevated salinities in the upper reaches, an upstream migration of adult Macrobrachium petersi (Hilgendorf) to freshwater takes place. During periods of increased river inflow adult M. petersi move downstream to the more saline reaches of the estuary. These two migratory responses have been interpreted as (a) a breeding migration under high inflow conditions which ensures that larvae are in close proximity to salinities that favour growth and development, and (b) an adult upstream migration back to freshwater to escape elevated estuarine salinities as a result of the low freshwater inflow. 相似文献
232.
233.
The first year of sodium nightglow observations from Natal (6°S, 35°W) are examined. Time variations appear to follow a pattern of their own, different from low latitude results. The major seasonal peak occurs in September-October and the average variation during the night decreases from dusk to dawn. Statistics on cloud coverage show that Natal has roughly only about 3 clear hours per night. The best observing period is April with an average of 5 clear hours per night. 相似文献
234.
L. Trafton 《Icarus》1985,63(3):374-405
We report the results of monitoring Saturn's H2 quadrupole and CH4 band absorptions outside of the equatorial zone over one-half of Saturn's year. This interval covers most of the perihelion half of Saturn's elliptical orbit, which happens to be approximately bounded by the equinoxes. Marked long-term changes occur in the CH4 absorption accompanied by weakly opposite changes in the H2 absorption. Around the 1980 equinox, the H2 and CH4 absorptions in the northern hemisphere appear to be discontinuous with those in the southern hemisphere. This discontinuity and the temporal variation of the absorptions are evidence for seasonal changes. The absorption variations can be attributed to a variable haze in Saturn's troposphere, responding to changes in temperature and insolation through the processes of sublimation and freezing. Condensed or frozen CH4 is very unlikely to contribute any haze. The temporal variation of the absorption in the strong CH4 bands at south temperate latitudes is consistent with a theoretically expected phase lag of 60° between the tropopause temperature and the seasonally variable insolation. We model the vertical haze distribution of Saturn's south temperature latitudes during 1971–1977 in terms of a distribution having a particle scale height equal to a fraction of the atmospheric scale height. The results are a CH4/H2 mixing ratio of (4.2 ± 0.4) × 10?3, a haze particle albedo of , and a range of variation in the particle to gas scale-height ratio of 0.6 ± 0.2. The haze was lowest near the time of maximum temperature. We also report spatial measurements of the absorption in the 6450 Å NH3 band made annually since the 1980 equinox. A 20 ± 4% increase in the NH3 absorption at south temperate latitudes has occurred since 1973–1976 and the NH3 absorption at high northern latitudes has increased during spring. Increasing insolation, and the resulting net sublimation of NH3 crystals, is probably the cause. Significant long-term changes apparently extend to the deepest visible parts of Saturn's atmosphere. An apparently anomalous ortho-para H2 ratio in 1978 suggests that the southern temperate latitudes experienced an unusual upwelling during that time. This may have signaled a rise in the radiative-convective boundary from deep levels following maximum tropospheric temperature and the associated maximum radiative stability. This would be further evidence that the deep, visible atmosphere is governed by processes such as dynamics and the thermodynamics of phase changes, which have response times much shorter than the radiative time constant. 相似文献
235.
Sunetra Giridhar N. Kameswara Rao David L. Lambert 《Journal of Astrophysics and Astronomy》1996,17(1-2):23-40
Two high resolution spectra of the hot RCrB star DY Cen in the red region are compared. The photospheric absorption lines
show a radial velocity variation of 12 kms-1 between 1989 July and 1992 May. Emission components to some CII lines present
in 1989 are almost entirely absent in 1992. Nebular forbidden lines of [OI], [NII] and [SII] appear unchanged from 1989 to
1992 相似文献
236.
V. Yu. Terebizh 《Astrophysics and Space Science》1992,193(2):269-288
Image restoration, computerized tomography, and other similar problems are considered as a unified class of stochastic inverse problems. The conventional approach to these problems that proceeds from some integral or functional equations suffers from three main shortcomings: (i) subjectivity, (ii) inability to account for the inner (radiational) noise, and (iii) inability to include the fundamental concept of the natural limit of solution accuracy. A general approach is developed, the Statistical Parameterization of Inverse Problems (SPIPR), that takes into account both the inner and external random noise and gives an explicit form of the above-mentioned natural limit. Applications of the SPIPR to various problems show that the maximum likelihood method as the concrete way to obtain an object estimate has practically limiting efficiency.Two new fields of applications of the SPIPR are outlined along with the image restoration problem: the elimination of blurring due to atmosphere turbulence and reconstruction of an object structure in the computerized tomography. The expressions for the main distribution function in all these problems are found. The corresponding real examples and model cases are considered as well. 相似文献
237.
238.
239.
Resonant absorption of MHD waves on a nonuniform flux tube is investigated as a driven problem for a 1D cylindrical equilibrium. The variation of the fractional absorption is studied as a function of the frequency and its relation to the eigenvalue problem of the MHD radiating eigenmodes of the nonuniform flux tube is established. The optimal frequencies producing maximal fractional absorption are determined and the condition for total absorption is obtained. This condition defines an impedance matching and is fulfilled for an equilibrium that is fine tuned with respect to the incoming wave. The variation of the spatial wave solutions with respect to the frequency is explained as due to the variation of the real and imaginary parts of the dispersion relation of the MHD radiating eigenmodes with respect to the real driving frequency. 相似文献
240.
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