全文获取类型
收费全文 | 149902篇 |
免费 | 2943篇 |
国内免费 | 1431篇 |
专业分类
测绘学 | 4015篇 |
大气科学 | 10925篇 |
地球物理 | 30909篇 |
地质学 | 52785篇 |
海洋学 | 12987篇 |
天文学 | 32815篇 |
综合类 | 471篇 |
自然地理 | 9369篇 |
出版年
2021年 | 1356篇 |
2020年 | 1569篇 |
2019年 | 1633篇 |
2018年 | 3536篇 |
2017年 | 3366篇 |
2016年 | 4367篇 |
2015年 | 2651篇 |
2014年 | 4201篇 |
2013年 | 7982篇 |
2012年 | 4456篇 |
2011年 | 6014篇 |
2010年 | 5217篇 |
2009年 | 6872篇 |
2008年 | 6167篇 |
2007年 | 5829篇 |
2006年 | 5674篇 |
2005年 | 4472篇 |
2004年 | 4521篇 |
2003年 | 4288篇 |
2002年 | 4032篇 |
2001年 | 3597篇 |
2000年 | 3512篇 |
1999年 | 2905篇 |
1998年 | 2983篇 |
1997年 | 2831篇 |
1996年 | 2517篇 |
1995年 | 2417篇 |
1994年 | 2187篇 |
1993年 | 1988篇 |
1992年 | 1910篇 |
1991年 | 1765篇 |
1990年 | 1971篇 |
1989年 | 1698篇 |
1988年 | 1576篇 |
1987年 | 1861篇 |
1986年 | 1641篇 |
1985年 | 2027篇 |
1984年 | 2289篇 |
1983年 | 2116篇 |
1982年 | 2003篇 |
1981年 | 1866篇 |
1980年 | 1672篇 |
1979年 | 1570篇 |
1978年 | 1649篇 |
1977年 | 1456篇 |
1976年 | 1386篇 |
1975年 | 1342篇 |
1974年 | 1297篇 |
1973年 | 1323篇 |
1972年 | 832篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
A space charge sheath is found to be formed whenever a high-velocity magnetized plasma stream penetrates a gas cloud. The sheath is always located at the head of the plasma stream, and its thickness is very small compared to the length of the plasma stream. Soon after the sheath is formed it quickly slows down to the Alfvén critical velocity. The plasma behind the sheath continues to move at higher velocity until the whole plasma stream is retarded to the critical velocity. In the interaction at gas density 1019 m–3 the sheaths are observed to be accompanied by a single loop of current with current density of 105 Å m–2. Maximum potential in the sheath ranges between 50 and 200 V.Presently available models for the sheath may explain the initiation of the sheath formation. Physical processes like heating of the electrons and ionization of the gas cloud which come into play at a later stage of the interaction are not included in these models. These processes considerably alter the potential structure in the sheath region. A schematic model of the observed sheath is presented here.Experiments reveal a threshold value of the magnetic field for plasma retardation to occur. This seems to correspond to the threshold condition for excitation of the modified two-stream instability which can lead to the electron heating. The observed current are found sufficient to account for the plasma retardation at a gas density of 1017 m–3. 相似文献
992.
S. Yabushita K. Wada T. Takai T. Inagaki D. Young E. T. Arakawa 《Astrophysics and Space Science》1986,124(2):377-388
The microorganism model of interstellar grains is investigated by spectroscopy from the infrared (IR), visible to the ultraviolet (UV) wave regions.E. coli, yeast and spores ofBacillus subtilis exhibit absorption bands at =3.1 and 9.7 m; they also exhibit several absorptions at 68 m which are in agreement with the observed IS extinction curves.To obtain the extinction curves in the visible and UV regions, dry films of microorganism are prepared on a MgF2 plate or synthesized quartz plate and their spectra measured. In the wavelength region 190400 nm, conventional spectrophotometers are adopted for the measurement. The extinction curve of the film ofE. coli is similar to the observed IS curve.For the wave-range 100<<400 nm, a vacuum UV spectrometer is adopted to avoid absorptions due to O2 in the atmosphere. The extinction spectra by this method are in agreement with the result obtained by the conventional method where comparison is possible. The extinction curves ofE. coli and yeast are such that they incrase towards the short wavelength and exhibit a peak at -190 nm, which is different from the well-known IS peak at =220 nm. It remains to be seen whether interstellar low temperatures (1040 K) can shift the peak position in the extinction curve of biochemical materials. 相似文献
993.
D. Vandev K. Danov P. Mateev P. Petrov M. Kartalev N. Trendafilov Z. K. Smith M. Dryer 《Astrophysics and Space Science》1986,120(2):211-221
A study is presented of an algorithm, based on the statistical analysis of interplanetary magnetic field data, for the real-time detection of discontinuities in the solar wind. The analysis is based on the application of a sliding or searching algorithm together with a minimum variance treatment. A demonstration is given for a Pioneer data set from 29 August, 1966. This particular data set was chosen because it has been extensively studied (in terms of the fast forward MHD shock wave) by previous groups who utilized the three-dimensional Rankine-Hugoniot shock equations after identifying the shock on a subjective basis. Our procedure differs in that it is perfectly objective because of its searching technique. For this particular data set we identify the original shock with excellent agreement with the earlier studies of its normal vector, as well as an additional shock and tangential discontinuity, neither of which had been identified previously. 相似文献
994.
We discuss the contribution of cosmic-ray protons at all energies above 1 MeV to the absorbed doses of the surface layers of a comet. Since there exists no calculation which takes into account proton energy losses by means of losses to electrons and nuclear collisions (in a cascade process), and losses due to the low energy end-products of the cascade, we have made a rough estimate of all of these contributions. An analytical formula is proposed that allows a rapid estimate of ranges and the dose absorbed at any depth. We give dose-depth curves for two extreme values of the energy at which nuclear collisions begin to dominate the slowing-down process, and for an intermediate value we display the dose-depth curve down to 20 m from the surface. The relevance of these findings to dosimetry in comets and some alterations of cometary material are considered. The need for improving the analytical expression proposed is stressed. 相似文献
995.
H. J. Staude 《Astrophysics and Space Science》1986,128(1):179-197
The topic is reviewed with emphasis on observations in the optical and near infrared spectral range.Paper presented at a Workshop on The Role of Dust in Dense Regions of Interstellar Matter, held at Georgenthal, G.D.R., in March 1986. 相似文献
996.
A self-similar solution to the problem of the implosion of a cylindrical shock wave in the presence of a magnetic field has been investigated. A strong shock wave in a cylindrically-symmetric flow travels to the axis of symmetry through a gas of uniform initial density 0 and zero-pressure. A comparative study has been made between the results obtained in ordinary gasdynamics and magnetogasdynamics with transverse and axial components of the magnetic field. The value of similarity exponent has been assigned from that found in the paper of Whitham (1958). 相似文献
997.
We present a high-resolution Bragg spectrometer designed for the observation of the soft X-ray cosmic diffuse background. The instrument concept is derived from the de Broglie geometry for the study of extended sources. It consists in a mosaïc of spherical TlAP crystals associated with position sensitive detectors located on the focussing surface. The spectral resolution and its variation with the field of view is estimated by Monte-Carlo simulations for different X-ray energies chosen among the most intense lines emitted by an astrophysical plasma in the temperature range 1–4×106K. The estimated sensitivity and the simulations of actual space observations show that the instrument is capable to separate the strongest lines emitted by the most abundant ions (OVII,OVIII, FeXVII, NeIX, etc.) and to map the whole sky during a six month mission. 相似文献
998.
We have used observations of the rare isotopes of HCN and HNC to determine the relative abundance of these two chemical isomers along the central ridge of the Orion molecular cloud. The abundance ratio [HCN]/[HNC] decreases by more than an order of magnitude from the relatively warm plateau and hot core sources toward the KL nebula to the colder, more quiescent clouds to the north and south. Even in the cooler regions, however, the ratio is an order of magnitude larger than that found in previous investigations of cold dark clouds. We determine the kinetic temperature in the regions we have studied from new observations of methylacetylene (CH3CCH), together with other recent estimates of the gas temperature near KL. The results suggest that the warmer portions of the cloud are dominated by different chemical pathways than those in the general interstellar cloud material. 相似文献
999.
The assumption that the very low albedo determined for Halley's comet is typical of all short period comets, taken together with the assumption that the average sizes of long and short period comets are approximately equal, leads to an increase in the total mass of comets in the solar system by almost two orders of magnitude. If gravitational ejection from the Uranus - Neptune zone during the later phases of planet formation is indeed responsible for the classical Oort cloud between 104–1015 AU, then the mass of comets in this transplanetary region during cosmogonie times has to exceed the combined masses of Uranus and Neptune by over an order of magnitude. Furthermore, if the recent arguments for as many as 1014 comets in an inner Oort cloud between ~40– 104AU are valid, then the total mass of comets in the solar system approaches 2% of a solar mass. 相似文献
1000.
Evidence is presented to show that during epochs of high sunspot activity, the duration of manifestation of equatorial spread-F (ESF) irregularities in the Earth's equatorial ionosphere undergoes a systematic modulation around the times of crossing of the heliospheric current sheet by the Earth. The modulation which is assessed as an indirect and geomagnetic activity-associated effect, is characterised by an enhancement in the duration of ESF conditions prior to the current sheet crossing and a reduction thereafter. It is suggested that the observed response of the equatorial ionosphere to the current sheet passage is primarily a manifestation of the geomagnetic activity related modifications in the equatorial east-west electric field in the post-sunset period. 相似文献