全文获取类型
收费全文 | 43314篇 |
免费 | 2384篇 |
国内免费 | 3226篇 |
专业分类
测绘学 | 1492篇 |
大气科学 | 4169篇 |
地球物理 | 8192篇 |
地质学 | 18008篇 |
海洋学 | 4994篇 |
天文学 | 8277篇 |
综合类 | 1064篇 |
自然地理 | 2728篇 |
出版年
2023年 | 279篇 |
2022年 | 926篇 |
2021年 | 1256篇 |
2020年 | 1230篇 |
2019年 | 1318篇 |
2018年 | 1986篇 |
2017年 | 1860篇 |
2016年 | 1992篇 |
2015年 | 1245篇 |
2014年 | 1886篇 |
2013年 | 2578篇 |
2012年 | 2013篇 |
2011年 | 2365篇 |
2010年 | 2115篇 |
2009年 | 2343篇 |
2008年 | 2096篇 |
2007年 | 2149篇 |
2006年 | 1977篇 |
2005年 | 1152篇 |
2004年 | 1052篇 |
2003年 | 1059篇 |
2002年 | 1141篇 |
2001年 | 1041篇 |
2000年 | 888篇 |
1999年 | 819篇 |
1998年 | 754篇 |
1997年 | 741篇 |
1996年 | 638篇 |
1995年 | 599篇 |
1994年 | 598篇 |
1993年 | 467篇 |
1992年 | 445篇 |
1991年 | 383篇 |
1990年 | 402篇 |
1989年 | 322篇 |
1988年 | 296篇 |
1987年 | 322篇 |
1986年 | 259篇 |
1985年 | 351篇 |
1984年 | 315篇 |
1983年 | 294篇 |
1982年 | 305篇 |
1981年 | 220篇 |
1980年 | 262篇 |
1979年 | 217篇 |
1978年 | 226篇 |
1977年 | 177篇 |
1975年 | 190篇 |
1974年 | 174篇 |
1973年 | 179篇 |
排序方式: 共有10000条查询结果,搜索用时 234 毫秒
191.
A numerical study of gas flow through a porous cometary mantle is presented. A kinetic model based on the well-known Test Particle Monte Carlo Method for the solution of rarefied gas dynamics problems is proposed. The physical model consists of two spatial plane regions: the condensed ice phase and a porous dust mantle. The structure of the porous dust layer is described as a bundle of cylindrical inclined channels not crossing each other. A vertical temperature gradient may exist across the dust mantle. The aim is to investigate how the characteristics of molecular flow depend on the capillary length, inclination angle, and temperature gradient. Examples illustrating a significant deviation of some results from equilibrium values are shown. In particular, the gas velocity distribution at both ends of the pore is strongly non-Maxwellian if there is an important temperature contrast across the pore. The emergent gas flow rate is found to vary with the pore length/radius ratio in excellent agreement with Clausing's empirical formula. The degree of collimation of the flow is quantitatively studied as a function of the length/radius ratio, and consequences for the jet force of outgassing through a dust mantle or, indeed, a rough surface are estimated. 相似文献
192.
Yu. A. Rylov 《Astrophysics and Space Science》1987,132(2):353-386
In the spherically-symmetric case, a computer simulation of the electron acceleration inside the outflow channel of the pulsar magnetosphere is produced. The stationary motion of electrons is shown to be unstable in the case of > c, where is a parameter describing inhomogeneity of the background charge, and c is its critical value. The arising non-stationary motion of electrons leads to a formation of electron bunches, which move chaotically. The mean electron energy appears to be much greater at the non-stationary motion, than at the stationary one. The time-averaged parameters of the non-stationary electron flow and their dependence upon have been investigated. Distributions of the mean values of parameters (charge density, electron velocity, electric field energy density, pressure, and internal energy of the gas composed of the electron bunches) over the magnetosphere altitude have been investigated. The mean spectra of the charge density have been obtained. The results of numerical investigation of the spherically-symmetric model are used for estimation of the electron energy and of the electron flux in the case of the more realistic model. The radioemission loss is estimated, and is shown to be large enough for explaining the radiopulsar phenomenon as a thermal radioemission of the pulsar magnetosphere. In particular, such common properties of the pulsar radioemission as the high bright temperature, the sharp radioemission directivity, and the characteristic turn-over of the radioemission spectrum at the frequency of the order 108 Hz are found a natural explanation in frames of this model. 相似文献
193.
We present the two-dimensional imaging observations of radio bursts in the frequency range 25–50 MHz made with the Clark Lake multifrequency radioheliograph during a coronal mass ejection event (CME) observed on 1984, June 27 by the SMM Coronagraph/Polarimeter and Mauna Loa K-coronameter. The event was spatially and temporally associated with precursors in the form of meter-decameter type III bursts, soft X-ray emission and a H flare spray. The observed type IV emission in association with the CME (and the H spray) could be interpreted as gyrosynchrotron emission from a plasmoid containing a magnetic field of 2.5 G and nonthermal electrons with a number density of 105 cm–3 and energy 350 keV.On leave from Indian Institute of Astrophysics, Kodaikanal, India. 相似文献
194.
Observations made by the differential method in the H line have revealed longperiod (on a timescale of 40 to 80 min) line-of-sight velocity oscillations which increase in amplitude with distance from the centre to the solar limb and, as we believe, give rise to prominence oscillations. As a test, we present some results of simultaneous observations at the photospheric level where such periods are absent.Oscillatory processes in the solar chromosphere have been studied by many authors. Previous efforts in this vein led to the detection of shortperiod oscillations in both the mass velocities and radiation intensity (Deubner, 1981). The oscillation periods obtained do not, normally, exceed 10–20 min (Dubov, 1978). More recently, Merkulenko and Mishina (1985), using filter observations in the H line, found intensity fluctuations with periods not exceeding 78 min. However, the observing technique they used does not exclude the possibility that those fluctuations were due to the influence of the Earth's atmosphere. It is also interesting to note that in spectra obtained by Merkulenko and Mishina (1985), the amplitude of the 3 min oscillations is anomalously small and the 5 min period is altogether absent, while the majority of other papers treating the brightness oscillations in the chromosphere, do not report such periods in the first place. So far, we are not aware of any other evidence concerning the longperiod velocity oscillations in the chromosphere on a timescale of 40–80 min.Longperiod oscillations in prominences (filaments) in the range from 40 to 80 min, as found by Bashkirtsev et al. (1983) and Bashkirtsev and Mashnich (1984, 1985), indicate that such oscillations can exist in both the chromosphere and the corona (Hollweg et al., 1982).In this note we report on experimental evidence for the existence of longperiod oscillations of mass velocity in the solar chromosphere. 相似文献
195.
196.
E. N. Parker 《Solar physics》1987,110(1):11-21
The recent determination that the angular velocity of the Sun declines downward through the convective zone raises serious questions about the nature of the solar dynamo. The principal qualitative features of the Sun are the azimuthal fields that migrate toward the equator in association with an oscillating poloidal field which reverses at about the time of maximum appearance of bipolar magnetic regions. If decreases downward, or is negligible, the horizontal gradient in produces a dynamo with some of these essential characteristics. There is reason to think that the dynamo is confined to the lower half of the convective zone where has the opposite sign from the usual ( > 0 in the northern hemisphere) producing equatorward migration but reversing the sign of the associated poloidal field. Meridional circulation may play an essential role in shaping the dynamo. At the present time it is essential to measure accurately and determine the nature of the meridional circulation.Solar Cycle Workshop Paper. 相似文献
197.
198.
N. Mihalopoulos J. P. Putaud B. C. Nguyen S. Belviso 《Journal of Atmospheric Chemistry》1991,13(1):73-82
Measurements of the concentrations of carbonyl sulfide (COS) in the marine atmosphere were made over a period of two years in the southern Indian Ocean (Amsterdam Island, 37°50 S–77°31 E; March 1987–February 1988 and April 1989–February 1990). The mean atmospheric COS concentration for the whole period was 475±48 pptv (n=544). Atmospheric COS concentrations show no significant seasonal variation with a summer to winter ratio of 1.05. Taking into account the observed variability of the atmospheric COS concentration (10%), a value of 1.4 yr is estimated as a lower limit for the atmospheric COS lifetime. A comparison of the COS data at Amsterdam Island with those obtained in the Southern Hemisphere in the past 12 yr does not reveal any significant trend in the tropospheric background COS mixing ratio. 相似文献
199.
200.
L. N. Kogarko D. A. Plant C. M. B. Henderson B. A. Kjarsgaard 《Contributions to Mineralogy and Petrology》1991,109(1):124-129
Carbonate phases, some rich in Na2O and comparatively rich in SrO and BaO, occur as inclusions in perovskite and calzirtite (Ca2Zr5Ti2O16) in the carbonatite of the Guli complex, Siberia. This is the first record of alkali carbonates, akin to nyerereite [Na2Ca(CO3)2], in plutonic igneous rocks. The inclusion populations suggest that the parental magma of the complex was Ca-rich but developed Na-rich differentiates during the latest stages. This points to the dominant calcic carbonatites of the complex not being derivatives of alkali-rich parental carbonatites. These alkali-rich carbonate inclusions (and rare inclusions of djerfisherite) have been preserved due to the resistance of perovskite and calzirtite to processes of leaching, hydrothermal alteration and weathering. 相似文献