全文获取类型
收费全文 | 61993篇 |
免费 | 1467篇 |
国内免费 | 1111篇 |
专业分类
测绘学 | 1593篇 |
大气科学 | 4782篇 |
地球物理 | 12529篇 |
地质学 | 22778篇 |
海洋学 | 5470篇 |
天文学 | 13297篇 |
综合类 | 298篇 |
自然地理 | 3824篇 |
出版年
2022年 | 396篇 |
2021年 | 662篇 |
2020年 | 676篇 |
2019年 | 736篇 |
2018年 | 1497篇 |
2017年 | 1470篇 |
2016年 | 1726篇 |
2015年 | 1080篇 |
2014年 | 1688篇 |
2013年 | 3208篇 |
2012年 | 2107篇 |
2011年 | 2804篇 |
2010年 | 2431篇 |
2009年 | 3106篇 |
2008年 | 2659篇 |
2007年 | 2727篇 |
2006年 | 2640篇 |
2005年 | 1897篇 |
2004年 | 1818篇 |
2003年 | 1662篇 |
2002年 | 1631篇 |
2001年 | 1445篇 |
2000年 | 1411篇 |
1999年 | 1118篇 |
1998年 | 1144篇 |
1997年 | 1181篇 |
1996年 | 965篇 |
1995年 | 964篇 |
1994年 | 882篇 |
1993年 | 749篇 |
1992年 | 763篇 |
1991年 | 701篇 |
1990年 | 773篇 |
1989年 | 684篇 |
1988年 | 655篇 |
1987年 | 754篇 |
1986年 | 622篇 |
1985年 | 792篇 |
1984年 | 887篇 |
1983年 | 846篇 |
1982年 | 818篇 |
1981年 | 723篇 |
1980年 | 761篇 |
1979年 | 638篇 |
1978年 | 610篇 |
1977年 | 603篇 |
1976年 | 546篇 |
1975年 | 531篇 |
1974年 | 546篇 |
1973年 | 561篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
A comparative analysis of spotless and spot group flares recorded at Hvar and Kanzelhöhe Observatories during the 21st cycle of solar activity is presented. The rate of occurrence of two-ribbon flares was found to be significantly higher for the spotless flares. In comparison with spot group flares of corresponding H importance, the soft X-ray peak values have been systematically lower for the spotless flares. The highest peak values and the energy released in soft X-rays was found for flares with a H ribbon protruding over a major spot umbra. It was found that the effective plasma temperatures in spotless flares have been considerably lower than the temperatures in spot group flares. 相似文献
992.
C. L. Goudas M. leftaki E. G. Petsagourakis 《Celestial Mechanics and Dynamical Astronomy》1989,47(1):1-14
The zero-velocity curves and surfaces created by two parallel or antiparallel magnetic dipoles moving on concentric circles are computed. It is found that for all combinations of values of the mass parameter µ and the ratio of the magnitudes of the magnetic moments of the two dipoles, there exist closed regions inside of which charged particles are permanently trapped. 相似文献
993.
K. T. Brazier A. Carramiñana P. M. Chadwick T. R. Currell N. A. Dipper E. W. Lincoln V. G. Mannings T. J. L. McComb K. J. Orford S. M. Rayner K. E. Turver 《Experimental Astronomy》1989,1(2):77-99
The design of very high energy (VHE) gamma ray telescopes is discussed and it is shown that a triple-mirror telescope operating in threefold fast coincidence provides an effective instrument. The design, construction and operation of a large VHE gamma ray telescope with an energy threshold of 300 GeV (at the zenith) and incorporating several novel features is described. The use of a coincidence guard ring technique is shown to result in the rejection of about 50% of the off-source (proton) events which form a high background and limit the sensitivity of such telescopes. 相似文献
994.
Time sequences of He i and He ii resonance line intensities at several sites within the flare of 15 June, 1973 are derived from observations obtained with the Naval Research Laboratory's Slitless Spectroheliograph on Skylab. The data are compared with predictions in six model flare atmospheres based on two values for the heating rate and three for the flux of photoionizing coronal X-rays and EUV. A peak ionizing flux more than 103 times that in the quiet Sun is indicated. For most conditions in flare kernels the He ii L and L lines are found to be formed by collisional excitation, thereby contributing to the local cooling of the plasma at temperatures above 6 × 104 K. Emission in the higher Lyman lines is generally the result of a mixture of collisional excitation at these temperatures and photoionization and recombination at temperatures near 2.5 × 104 K. We discuss implications for the common practice of deriving stellar coronal fluxes from He ii 1640 Å fluxes assuming dominance of the recombination mechanism.Chief, Quantum Physics Division, National Institute of Standards and Technology.Operated jointly by the National Institute of Standards and Technology and the University of Colorado.Operated by the National Optical Astronomy Observatories of the Association of Universities for Research in Astronomy, Inc. under contract with the National Science Foundation. 相似文献
995.
By use of the H observations of the Astrophysical Observatory in Catania, Italy and the Purple Mountain Observatory in Nanking, China as well as hard X-ray and gamma-ray burst data from the Solar Maximum Mission (SMM) Gamma-Ray Spectrometer (GRS), a major eruptive loop prominence was studied during the limb solar flare event of 1981 April 27.Our preliminary analysis shows that there seems to exist a second abrupt energy release for this event, almost 20 min after the end of the impulsive phase of the flare. This energy release is probably associated with the rapidity in upward motion or activation of the loop prominence.A possible candidate for such a process could be the reconnection of the old magnetic field with a newly emerging magnetic field.A theoretical gross estimate for the energy release and particle acceleration has also been made in this work. It appears that the proposed model for charged particle acceleration is very efficient. 相似文献
996.
L. D. Barr 《Astrophysics and Space Science》1989,160(1-2):71-85
The National Optical Astronomy Observatories (NOAO) are planning to buld two 8 m telescopes, one for Mauna Kea, Hawaii, the other for a site in Chile. Optical configurations, primary mirror systems, and the telescope mounting are discussed. A new optical testing method is outlined. The system imaging goal is 0.25 FWHM. Construction could begin in the early 1990's.Paper presented at the Symposium on the JNLT and Related Engineering Developments, Tokyo, November 29–December 2, 1988.Operated by the Association of Universities for Research in Astronomy Inc., under contract with the National Science Foundation. 相似文献
997.
We observed near-Main-Sequence B-stars in the Magellanic Clouds with the 3.6 m telescope and CASPEC at La Silla. We obtained spectra of high resolution and high S/N-ratio. The stars are members of the blue globular clusters NGC 1818 (LMC) and NGC 330 (SMC). The spectra are used for differential abundance analyses using HER as galactic reference star. Apart from CNO the metals are underabundant by about a factor of three and four for the LMC and SMC star, respectively. The CNO pattern is interesting since in both stars oxygen is considerably more abundant than carbon.Based on observations collected at ESO, La Silla and on Calar Alto, Spain and on observations made with the International Ultraviolet Explorer. 相似文献
998.
Several interstellar molecules have been detected toward the highly perturbed B and G clouds associated with the supernova remnant IC 443 via their 3 mm transitions, including N2H+, SiO, SO, CN, HNC, and H13CO+. The (J, K) = (1, 1) and (2, 2) inversion lines of metastable ammonia have also been observed, as well as the J = 3-2 transition of HCO+ at 1.2 mm. Analysis of the (1, 1) and (2, 2) inversion lines of NH3 indicates minimum gas kinetic temperatures of TK = 70 K toward cloud B, and TK = 33 K in cloud G. Modeling of the J = 1-0 and J = 3-2 transitions of HCO+ implies densities greater than 10(5) cm-3 toward both positions. These data clearly show that hot and dense material is present in IC 443, and they suggest the presence of shocks in both regions. A careful analysis of the HCO+ lines indicates that the HCO+ abundance is at most enhanced by factors of a few over that found in cold, quiescent gas. This conclusion contradicts past claims of HCO+ abundance enhancements of several orders of magnitude in the perturbed regions. The N2H+ abundance was also found to be similar to that in cold gas, suggesting that there is no increase in ionization in the clouds. The abundances of SO and CS, as well as CN and NH3, do not appear to differ significantly from those found in cold dark clouds, although chemistry models predict sulfur-containing species to undergo high-temperature enhancements. SiO, however, is found to have an abundance in the perturbed gas 100 times larger than the upper limits observed in the dark cloud TMC 1, a result in agreement with high temperature chemistry models. In addition, the HNC/HCN ratio in both IC 443 B and G was found to be approximately 0.1--far from the ratio of 1 predicted by low-temperature ion-molecule chemistry, but similar to the values observed in clouds where elevated temperatures are present. 相似文献
999.
Veeder GJ Hanner MS Matson DL Tedesco EF Lebofsky LA Tokunaga AT 《The Astronomical journal》1989,97(4):1211-1219
We report 10 micrometers infrared photometry for 22 Aten, Apollo, and Amor asteroids. Thermal models are used to derive the corresponding radiometric albedos and diameters. Several of these asteroids appear to have surfaces of relatively high thermal inertia due to the exposure of bare rock or a coarse regolith. The Apollo asteroid 3103, 1982 BB, is recognized as class E. The Jupiter-crossing Amor asteroid 3552, 1983 SA, is confirmed as class D, but low albedos remain rare for near-Earth asteroids. 相似文献
1000.
In the late phase of some complex flare events which produce type IV radio emission, a narrow-band decimetric component with generally a high abundance of spectral fine structures is observed. We identify this late dm continuum as a hypothetical switch-off signature of the equivalent two-ribbon flare current system. The quasi-periodic and highly dynamic fiber-burst pattern studied in the February 5, 1986 example is understood as radio evidence for a final reactivation of a cyclic sequence of instabilities in the driven reconnecting current sheet below the rising prominence.Paper presented at the 4th CESRA Workshop in Ouranopolis (Greece) 1991. 相似文献