全文获取类型
收费全文 | 72459篇 |
免费 | 1664篇 |
国内免费 | 1194篇 |
专业分类
测绘学 | 1811篇 |
大气科学 | 5373篇 |
地球物理 | 14538篇 |
地质学 | 25817篇 |
海洋学 | 6648篇 |
天文学 | 16074篇 |
综合类 | 308篇 |
自然地理 | 4748篇 |
出版年
2022年 | 427篇 |
2021年 | 786篇 |
2020年 | 760篇 |
2019年 | 868篇 |
2018年 | 1693篇 |
2017年 | 1575篇 |
2016年 | 1951篇 |
2015年 | 1210篇 |
2014年 | 1856篇 |
2013年 | 3665篇 |
2012年 | 2278篇 |
2011年 | 3131篇 |
2010年 | 2803篇 |
2009年 | 3628篇 |
2008年 | 3051篇 |
2007年 | 3171篇 |
2006年 | 3025篇 |
2005年 | 2309篇 |
2004年 | 2222篇 |
2003年 | 2096篇 |
2002年 | 2009篇 |
2001年 | 1731篇 |
2000年 | 1652篇 |
1999年 | 1353篇 |
1998年 | 1403篇 |
1997年 | 1372篇 |
1996年 | 1136篇 |
1995年 | 1128篇 |
1994年 | 996篇 |
1993年 | 927篇 |
1992年 | 882篇 |
1991年 | 829篇 |
1990年 | 888篇 |
1989年 | 807篇 |
1988年 | 777篇 |
1987年 | 915篇 |
1986年 | 788篇 |
1985年 | 982篇 |
1984年 | 1119篇 |
1983年 | 997篇 |
1982年 | 932篇 |
1981年 | 924篇 |
1980年 | 829篇 |
1979年 | 763篇 |
1978年 | 738篇 |
1977年 | 730篇 |
1976年 | 656篇 |
1975年 | 631篇 |
1974年 | 659篇 |
1973年 | 690篇 |
排序方式: 共有10000条查询结果,搜索用时 18 毫秒
291.
The height of the lower red border of type-B aurora has been determined by triangulation using TV cameras at two ground stations. A mean height of 91.4 ± 1.1 km was determined from a set of 12 measurements made under ideal conditions. A TV spectrograph was used simultaneously to seek possible spectral changes between 6400 and 6900 Å which would be indicative of changes in the vibrational distribution in the N2 First Positive bands. No significant difference was found in this distribution between the spectra from 93 and 122 km. The height distribution of contributions to the OI 5577 Å emission relative to the N+2 First Negative emission was modelled from 80 to 160 km. Contributions from electron impact on atomic O, O+2 dissociative recombination and N2(A)O energy transfer were included. Account was taken of recent laboratory data on O(1S) quenching. It was concluded that these processes could explain the excitation of O(1S) in normal aurora and the height distribution of OI 5577 Å in type-B red aurora. It was confirmed that the lifetime ofO(1S) in type-B red auroral rapid time variations is about 0.5 s and it was found from the model that the observed time variation can be reproduced by the mechanisms considered, provided the concentration of NO in the auroral atmosphere is about 1 × 109 at 95 km. Before reasonable certainty can be attained in the correctness of the interpretation it will however be necessary to have reliable simultaneous observations of neutral atmospheric composition particularly for O and NO as well as unchallengeable measurements of the yields of O(1S) for the processes considered and for several other processes which have been suggested recently. 相似文献
292.
C. Caroubalos D. Maroulis N. Patavalis J.-L. Bougeret G. Dumas C. Perche C. Alissandrakis A. Hillaris X. Moussas P. Preka-Papadema A. Kontogeorgos P. Tsitsipis G. Kanelakis 《Experimental Astronomy》2001,11(1):23-32
We present the new solar radiospectrograph of the University of Athensoperating at the Thermopylae Station since 1996. Observations cover thefrequency range from 110 to 688 MHz. The radiospectrograph has a 7-meterparabolic antenna and two receivers operating in parallel. One is a sweepfrequency receiver and the other a multichannel acousto-optical receiver.The data acquisition system consists of a front-end VME based subsystem anda Sun Sparc-5 workstation connected through Ethernet. The two subsystems areoperated using the VxWorks real-time package. The daily operation is fullyautomated: pointing of the antenna to the sun, starting and stopping theobservations at pre-set times, data acquisition, data compression by`silence suppression', and archiving on DAT tapes. The instrument can beused either by itself to study the onset and evolution of solar radio bursts or in conjunction with other instruments including theNançay Decametric Array and the WIND/WAVES RAD1 and RAD2 low frequencyreceivers to study associated interplanetary phenomena. 相似文献
293.
Berry Head, a limestone headland in Torbay, southwest England, exhibits a series of subaerial marine erosion platforms and raised beaches spanning an altitude range of 97 m. Solution caves on the headland show preferred horizontal development at elevations that are correlated with the marine erosion platforms, and developed in a marine/freshwater mixing zone whose position was controlled by high sea-level still-stands. Corbridge Cave in Berry Head Quarry lies below the raised beaches in Torbay, and contains evidence of three marine transgressions in the form of fine-grained marine ponding deposits with a marine microfauna. Uranium-series dating of intercalated speleothems indicates that a transgression during oxygen isotope stage 5e reached an elevation of 5.8 m OD, while an earlier transgression (probably during stage 7) reached at least 7.2 m OD. These findings are used to constrain possible interpretations of the aminostratigraphy of raised beaches in Southwest Britain, and a correlation of the Unnamed Stage of Bowen, Sykes, Reeves, Miller, Andrews, Brew and Hare with oxygen isotope stage 5e is proposed. 相似文献
294.
The study of the evolution of planetary systems, primarily of the Solar System, is one of the basic problems of celestial mechanics. The stability of motion of giant planets on cosmogonic time scales was established by numerical and analytical methods, but the question about the evolution of orbits of terrestrial planets and arbitrary solar-type planetary systems remained open. This work initiates a series of papers allowing one to advance in solving the problem of the evolution of the solar-type planetary systems on cosmogonic time scales by using powerful analytical tools. In the first paper of this series, we choose the optimum reference system and obtain the Poisson series expansion of the Hamiltonian of the problem in all Keplerian elements. We propose to use the integral representation of the corresponding coefficients or the Poisson processor means instead of conventionally addressing any possible special functions. This approach extremely simplifies the algorithm. The next paper of this series deals with the calculation of the expansion coefficients. 相似文献
295.
Universal cokriging is used to obtain predictions when dealing with multivariate random functions. An important type of nonstationarity is defined in terms of multivariate random functions with increments which are stationary of orderk. The covariance between increments of different variables is modeled by means of the pseudo-cross-covariance function. Criteria are formulated to which the parameters of pseudo-cross-covariance functions must comply so as to ensure positive-definiteness. Cokriging equations and the induced cokriging equations are given. The study is illustrated by an example from soil science. 相似文献
296.
Results are presented for polarimetric observations of 17 red giants and supergiants, of which nine are long-period Mira variables, five are semiregular variables (SR), and three are slowly fluctuating variables (Lb). Light polarization is detected for eight stars, seven of them for the first time.Translated fromAstrofizika, Vol. 39, No. 3, pp. 385–391, July–September, 1996. 相似文献
297.
L. I. Matveenko 《Astronomy Letters》2001,27(11):686-690
The core (injector) and the jet (relativistic plasma outflow) of AGN objects are surrounded by an ionized medium, an H II region observed in emission lines. The synchrotron radiation from the core and the jet is observed through a thin screen that cocoons the structure under consideration. The screen transparency depends on wavelength and distance from the injector. We consider the objects 3C 345 and 1803+784 whose core emission at decimeter wavelengths is absorption by more than 25 dB. The visible bright compact component is the bright nearby portion of the jet that extends outside the dense part of the screen. We explore the possibility of measuring the screen transparency from absorption in Hα recombination lines with different quantum numbers at centimeter wavelengths. 相似文献
298.
299.
Randall D. Campbell 《Experimental Astronomy》2002,14(1):57-60
Keck's Long Wavelength Spectrometer (LWS), is the facility instrument used for imaging and spectroscopy in the wavelength
range of 3–28 μm at the Keck Observatory. LWS uses an 128 × 128 Si:As blocked impurity band (BIB) array manufactured by the
Boeing Corporation. This paper discusses the method used for optimizing the detector's operating parameters at a temperature
of 8.5 K and bias voltage of 1.2 V. A process for characterizing detective quantum efficiency of BIB detectors is also presented.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
300.
The quadrupole mass spectrometer flown by the Air Force Geophysics Laboratory on STS-4 in 1982 detected large intensities of several ions, primarily O+, H2O+ and H3O+, with energies less than 1.5 e V with respect to the Shuttle Orbiter. Ion-molecule reactions and non-reactive scattering between the outgassing neutral flux from the Orbiter surfaces and the ambient ionic species are identified as the primary source of these low energy ions. 相似文献