全文获取类型
收费全文 | 76229篇 |
免费 | 640篇 |
国内免费 | 479篇 |
专业分类
测绘学 | 1937篇 |
大气科学 | 4855篇 |
地球物理 | 14180篇 |
地质学 | 30405篇 |
海洋学 | 6301篇 |
天文学 | 16169篇 |
综合类 | 298篇 |
自然地理 | 3203篇 |
出版年
2022年 | 573篇 |
2021年 | 879篇 |
2020年 | 942篇 |
2019年 | 996篇 |
2018年 | 4199篇 |
2017年 | 3761篇 |
2016年 | 3282篇 |
2015年 | 1104篇 |
2014年 | 2026篇 |
2013年 | 3315篇 |
2012年 | 2954篇 |
2011年 | 4650篇 |
2010年 | 4068篇 |
2009年 | 4697篇 |
2008年 | 3972篇 |
2007年 | 4557篇 |
2006年 | 2692篇 |
2005年 | 1847篇 |
2004年 | 1776篇 |
2003年 | 1768篇 |
2002年 | 1633篇 |
2001年 | 1412篇 |
2000年 | 1274篇 |
1999年 | 921篇 |
1998年 | 907篇 |
1997年 | 1002篇 |
1996年 | 757篇 |
1995年 | 789篇 |
1994年 | 759篇 |
1993年 | 637篇 |
1992年 | 635篇 |
1991年 | 617篇 |
1990年 | 665篇 |
1989年 | 590篇 |
1988年 | 568篇 |
1987年 | 596篇 |
1986年 | 491篇 |
1985年 | 689篇 |
1984年 | 687篇 |
1983年 | 702篇 |
1982年 | 667篇 |
1981年 | 591篇 |
1980年 | 650篇 |
1979年 | 504篇 |
1978年 | 519篇 |
1977年 | 474篇 |
1976年 | 433篇 |
1975年 | 431篇 |
1974年 | 429篇 |
1973年 | 431篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Based on our 1997 observations with a CCD camera and narrow-band filters attached to the 1-m Assy Obsev vatory telescope, we extensively study the latitudinal variations in limb darkening and normal albedo on Jupiter's disk at wavelengths of 387, 445, 502, 619, and 702 nm. In addition, we carried out observations in 1998 with broad-band red, green, and blue filters. Apart from a general regularity—the increase in darkening coefficient with normal albedo of Jupiter's cloud cover—there is an appreciable scatter of darkening coefficients for the same albedo, which is most pronounced in the ultraviolet. This scatter may result from differences in the optical depth of the aerosol haze above the clouds. The lack of any wavelength dependence of the darkening coefficients is confirmed for Jupiter's polar regions, while at other latitudes, they decrease with decreasing wavelength. 相似文献
92.
Bjerrum-Niese C. Bjorno L. Pinto M.A. Quellec B. 《Oceanic Engineering, IEEE Journal of》1996,21(2):143-149
The paper discusses the development of a simulation tool to model high data-rate acoustic communication in shallow water. The simulation tool is able to generate synthetic time series of signals received at a transducer array after transmission across a shallow-water communication channel. The simulation tool is suitable for testing advanced signal processing techniques for message recovery. A channel model has been developed based on the physical aspects of the acoustic channel. Special emphasis has been given to fluctuations of the signal transmission caused by time-varying multipath effects. At shorter ranges, the temporal variations are dominated by acoustic scattering from the moving sea surface. Therefore, the channel model produces a coherence function which may be interpreted as a time-varying reflection coefficient for the surface scattered acoustical path. A static, range-independent ray model identifies the significant multipaths, and the surface path is modulated with the time-varying reflection coefficient. The advantages and limitations of the channel model are discussed and assumptions necessary to overcome the limitations are emphasised. Based on the assumptions, an algorithm has been developed and implemented to model how a binary message will be modulated when transmitted by a transducer, is distorted in the channel and finally is received by a transducer array 相似文献
93.
94.
The previously proposed (Rodin, 2002) method for calculating the microphysical properties of spatially inhomogeneous rarefied aerosol media with mixing using the lowest-order moments of the size distribution is generalized to particle coagulation. We show that when the problem is formulated in terms of moments, all of the solutions admitted by the stochastic coagulation equation lie within a narrow range whose boundaries can be determined by means of quadratic programming. We discuss the choice of an optimal solution within this range and compare the moment method with the results of our computations by the classical finite-difference method using a model of photochemical aerosols in Titan's atmosphere as an example. The moment method allows the efficiency of microphysical computations to be significantly increased by using precomputed low-dimension interpolation tables. It can be used to construct self-consistent models for the globular circulation of planetary atmospheres. 相似文献
95.
It was found recently that fast electrons travel through the plasma of the solar corona in the form of beam-plasma structure
(BPS), which consists of electrons and Langmuir waves. In this paper the influence of scattering BPS Langmuir waves off plasma
ions (l+i=l+i) on BPS velocity is studied. We show that the maximum BPS velocity equals 0.35c, which is close to the velocity of Type III bursts sources. 相似文献
96.
M. G. Revnivtsev 《Astronomy Letters》2003,29(6):383-386
Analysis of the RXTE slew data in October 1996 revealed a weak X-ray burst from the millisecond pulsar SAX J 1808.4-3658. The 3–20-keV energy spectrum of the source can be described by a power law with an index of 2.0 and a(3-to 20 keV) luminosity of ~1.4×1035 erg s?1 (the distance to the source was taken to be 2.5 kpc). Because of the short exposure time, we failed to detect weak pulsations at a frequency of 401 Hz in the source. The (2σ) upper limit of the pulse fraction is ~13%. 相似文献
97.
The dynamical evolution of meteoroid streams associated with cornets Encke, Halley, Machholz 1986 VIII and asteroid Phaethon is discussed. It is shown that the planetary perturbations can greatly increase the streams thickness and each stream may produce several couples of meteor showers active in different seasons of the year. The theoretical and observed data are in a satisfactory accordance. 相似文献
98.
B. R. Johnson M. E. Abroe P. Ade J. Bock J. Borrill J. S. Collins P. Ferreira S. Hanany A. H. Jaffe T. Jones A. T. Lee L. Levinson T. Matsumura B. Rabii T. Renbarger P. L. Richards G. F. Smoot R. Stompor H. T. Tran C. D. Winant 《New Astronomy Reviews》2003,47(11-12):1067
We discuss MAXIPOL, a bolometric balloon-borne experiment designed to measure the E-mode polarization anisotropy of the cosmic microwave background radiation (CMB) on angular scales of 10′ to 2°. MAXIPOL is the first CMB experiment to collect data with a polarimeter that utilizes a rotating half-wave plate and fixed wire-grid polarizer. We present the instrument design, elaborate on the polarimeter strategy and show the instrument performance during flight with some time domain data. Our primary dataset was collected during a 26 h turnaround flight that was launched from the National Scientific Ballooning Facility in Ft. Sumner, New Mexico in May 2003. During this flight five regions of the sky were mapped. Data analysis is in progress. 相似文献
99.
The effect of particle-particle interaction on the adiabatic index γ for an electron-positron plasma is considered. An improved method for numerically calculating the Hartree-Fock exchange integral is presented and its relativistic asymptotics is determined. An approximation formula is derived for the correlation part of the interaction in the low-density limit. This formula includes degeneracy and the positron component. 相似文献
100.
Lance B. Morrissey Philippe Janvier Simon J. Braddy John P. Bennett Susan B. Marriott Peter R. Tarrant 《Geology Today》2006,22(2):66-67
Armoured jawless fish, or 'ostracoderms', lived 450–360 million years ago, and display unusual morphologies, unlike any modern fish group. Since they left no living descendants, their mode of swimming has, until recently, remained speculative, although this is a crucial question as the first true pectoral fins evolved within the 'ostracoderms'. The discovery of the oldest-known fish trails, from the Early Devonian (400 million year old) 'Lower Old Red Sandstone' of south-east Wales offers new insights into the swimming behaviour of these early fish, notably the osteostracan 'ostracoderms' (or cephalaspids), whose horseshoe-shaped head and paddle-shaped pectoral fins have remained a functional riddle. 相似文献