全文获取类型
收费全文 | 1146篇 |
免费 | 33篇 |
国内免费 | 8篇 |
专业分类
测绘学 | 16篇 |
大气科学 | 121篇 |
地球物理 | 234篇 |
地质学 | 416篇 |
海洋学 | 87篇 |
天文学 | 202篇 |
综合类 | 3篇 |
自然地理 | 108篇 |
出版年
2021年 | 10篇 |
2020年 | 12篇 |
2019年 | 10篇 |
2018年 | 15篇 |
2017年 | 24篇 |
2016年 | 19篇 |
2015年 | 19篇 |
2014年 | 28篇 |
2013年 | 60篇 |
2012年 | 25篇 |
2011年 | 47篇 |
2010年 | 35篇 |
2009年 | 64篇 |
2008年 | 34篇 |
2007年 | 28篇 |
2006年 | 44篇 |
2005年 | 33篇 |
2004年 | 49篇 |
2003年 | 48篇 |
2002年 | 33篇 |
2001年 | 30篇 |
2000年 | 27篇 |
1999年 | 34篇 |
1998年 | 29篇 |
1997年 | 18篇 |
1996年 | 19篇 |
1995年 | 21篇 |
1994年 | 18篇 |
1993年 | 18篇 |
1992年 | 18篇 |
1991年 | 14篇 |
1990年 | 14篇 |
1989年 | 12篇 |
1988年 | 12篇 |
1987年 | 15篇 |
1986年 | 8篇 |
1985年 | 13篇 |
1984年 | 24篇 |
1983年 | 14篇 |
1982年 | 15篇 |
1981年 | 23篇 |
1980年 | 14篇 |
1979年 | 15篇 |
1978年 | 18篇 |
1977年 | 13篇 |
1976年 | 12篇 |
1975年 | 19篇 |
1974年 | 12篇 |
1973年 | 10篇 |
1972年 | 11篇 |
排序方式: 共有1187条查询结果,搜索用时 312 毫秒
991.
Roger M.Hutchings F.Santoro P. A.Thomas H. M. P.Couchman 《Monthly notices of the Royal Astronomical Society》2002,330(4):927-936
We calculate the cooling times at constant density for haloes with virial temperatures from 100 K to 1×105 K that originate from a 3 σ fluctuation of a CDM power spectrum in three different cosmologies. Our intention is to determine the first objects that can cool to low temperatures, but not to follow their dynamical evolution. We identify two generations of haloes: those with low virial temperatures, T vir ≲9000 K that remain largely neutral, and those with larger virial temperatures that become ionized. The lower temperature, lower mass haloes are the first to cool to 75 per cent of their virial temperature. The precise temperature and mass of the first objects are dependent upon the molecular hydrogen (H2 ) cooling function and the cosmological model. The higher mass haloes collapse later but, in this paradigm, cool much more efficiently once they have done so, first via electronic transitions and then via molecular cooling: in fact, a greater residual ionization once the haloes cool below 9000 K results in an enhanced H2 production and hence a higher cooling rate at low temperatures than for the lower mass haloes, so that within our constant-density model it is the former that are the first to cool to really low temperatures. We discuss the possible significance of this result in the context of CDM models in which the shallow slope of the initial fluctuation spectrum on small scales leads to a wide range of halo masses (of differing overdensities) collapsing over a small redshift interval. This 'crosstalk' is sufficiently important that both high- and low-mass haloes collapse during the lifetimes of the massive stars which may be formed at these epochs. Further investigation is thus required to determine which generation of haloes plays the dominant role in early structure formation. 相似文献
992.
William J. Chaplin Andrew M. Dumbill Yvonne P. Elsworth George R. Isaak Clive P. McLeod Brek A. Miller Roger New Balázs Pintér 《Solar physics》2004,220(2):307-316
The Birmingham Solar-Oscillations Network (BiSON) has acquired high-precision solar mean magnetic field (SMMF) data on a 40-s
cadence for a decade. We present attempts to compare such data from recent years with the occurrence of coronal mass ejections
(CMEs) as recorded by LASCO, using correlation techniques applied to measurements from different BiSON instruments to maximise
the sensitivity to CME-related SMMF responses. SMMF measurements were recorded at the time of occurrence of several hundred
CMEs. No CME event shows a convincing response in our SMMF data at short periods setting a threshold amplitude of 12 mG. By
averaging data sets we are able to set lower thresholds, which depend somewhat on the distribution of response strengths.
A brief summary of the very first results of this study is also given in Chaplin et al. 相似文献
993.
Cudnik Brian M. Dunham David W. Palmer David M. Cook Anthony Venable Roger Gural Peter S. 《Earth, Moon, and Planets》2003,93(3):145-161
The occurrence and visibility of meteoroid impacts on the moon as seen from the earth were little more than speculation prior to November 1999. The best evidence of present-day impact activity came from the seismic experiments left on the Moon during the Apollo era. Past systematic attempts at earth-based observations to document lunar impacts revealed nothing conclusive. However, during the Leonid storms of 1999 and 2001, lunar impact events were for the first time confirmed by multiple independent observers. A total of 15 meteoritic impact flash events have been verified during these storms, with an additional 12 unconfirmed but likely events awaiting confirmation. Estimates of the mass of these meteoroids range from less than one gram for the faintest flashes to more than 10 kg for the brightest observed flash. The fraction of visible light to total energy produced by these events, a quantity known as luminous efficiency, averages about 0.001 for the established events. The confirmation of lunar meteoritic events on the Moon opens a new avenue in lunar and planetary research, one which could help bridge the gap between atmospheric sampling of the smallest components of meteoroid streams and interplanetary debris to the larger scale objects accessible to ground-based telescopes. 相似文献
994.
Cudnik Brian M. Palmer David W. Palmer David M. Cook Anthony Venable Roger Gural Peter S. 《Earth, Moon, and Planets》2003,93(2):97-106
Confirmed observations of meteoroids from the Leonid stream impacting the Moon in 1999 and 2001 have opened up new opportunities in observational and theoretical astronomy. These opportunities could help bridge the gap between the ground-based (atmospheric) sampling of the smallest meteoroids and the larger objects observable with ground-based telescopes. The Moon provides a laboratory for the study of hypervelocity impacts, with collision velocities not yet possible in ground-based laboratories. Development of automatic detection software removes the time-intensive activity of laboriously reviewing data for impact event signatures, freeing the observer to engage in other activities. The dynamics of professional-amateur astronomer collaboration have the promise of advancing the study of lunar meteoritic phenomenon considerably. These three factors will assist greatly in the development of a systematic, comprehensive program for monitoring the Moon for meteoroid impacts and determining the physical nature of these impacts. 相似文献
995.
This paper presents the first detailed analysis of acetylene absorption features observed longward of 190.0 nm in a jovian spectrum by the Faint Object Spectrograph on board the Hubble Space Telescope. The presence of two features located near 207.0 nm can only be explained by a substantial abundance of acetylene in the upper troposphere. Using a Rayleigh-Raman radiative transfer model, it was determined that the acetylene vertical profile is characterized by a decrease in the mole fraction with increasing pressure in the upper stratosphere, a minimum around 14 to 29 mbar, followed by an increase to about 1.5 × 10−7 in the upper troposphere. Longward of 220 nm, the relatively high contrast of Raman features to the continuum precludes the existence of an optically significant amount of clouds from 150 to 500 mbar unless they are highly reflective. Instead, the reflectivity at these long wavelengths is determined by stratospheric, not tropospheric, scatterers and absorbers. Analysis of the data also suggests that ammonia is extremely undersaturated at pressures below 700 mbar. However, no firm conclusions can be reached because of the uncertainties surrounding its cross section longward of 217.0 nm, which are due to vibrationally excited states. 相似文献
996.
Jeffrey Turner Hans‐Jrgen Albrechtsen Mike Bonell Jean‐Pierre Duguet Bob Harris Rainer Meckenstock Kevin McGuire Roger Moussa Norman Peters Hans H. Richnow Barbara Sherwood‐Lollar Stefan Uhlenbrook Henny van Lanen 《水文研究》2006,20(1):205-213
A summary is provided of the first of a series of proposed Integrated Science Initiative workshops supported by the UNESCO International Hydrological Programme. The workshop brought together hydrologists, environmental chemists, microbiologists, stable isotope specialists and natural resource managers with the purpose of communicating new ideas on ways to assess microbial degradation processes and reactive transport at catchment scales. The focus was on diffuse contamination at catchment scales and the application of compound‐specific isotope analysis (CSIA) in the assessment of biological degradation processes of agrochemicals. Major outcomes were identifying the linkage between water residence time distribution and rates of contaminant degradation, identifying the need for better information on compound specific microbial degradation isotope fractionation factors and the potential of CSIA in identifying key degradative processes. In the natural resource management context, a framework was developed where CSIA techniques were identified as practically unique in their capacity to serve as distributed integrating indicators of process across a range of scales (micro to diffuse) of relevance to the problem of diffuse pollution assessment. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
997.
The results of a comparison between chemical water quality determinants and river water fluorescence on the River Tyne, NE England, demonstrate that tryptophan‐like fluorescence intensity shows statistically significant relationships between nitrate, phosphate, ammonia, biochemical oxygen demand (BOD) and dissolved oxygen. Tryptophan‐like fluorescence intensity at the 280 nm excitation/350 nm emission wavelength fluorescence centre correlates with both phosphate (r = 0·80) and nitrate (r = 0·87), whereas tryptophan‐like fluorescence intensity at the 220 nm excitation/350 nm emission wavelength centre correlates with BOD (r = 0·85), ammonia (r = 0·70) and dissolved oxygen (r = ?0·65). The strongest correlations are between tryptophan‐like fluorescence intensity and nitrate and phosphate, which in the Tyne catchment derive predominantly from point and diffuse source sewage inputs. The correlation between BOD and the tryptophan‐like fluorescence intensity suggests that this fluorescence centre is related to the bioavailable or labile dissolved organic matter pool. The weakest correlations are observed between tryptophan‐like fluorescence intensity and ammonia concentration and dissolved oxygen. The weaker correlation with ammonia is due to removal of the ammonia signal by wastewater treatment, and that with dissolved oxygen due to the natural aeration of the river such that this is not a good indicator of water quality. The observed correlations only hold true when treated sewage, sewerage overflows or cross connections, or agricultural organic pollutants dominate the water quality—this is not true for two sites where airport deicer (propylene glycol, which is non‐fluorescent) or landfill leachate (which contains high concentrations of humic and fulvic‐like fluorescent DOM) dominate the dissolved organic matter in the river. Mean annual tryptophan‐like fluorescence intensity agrees well with the General Water Quality Assessment as determined by the England and Wales environmental regulators, the Environment Agency. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
998.
Yasser Rathore Roger D. Blandford Avery E. Broderick 《Monthly notices of the Royal Astronomical Society》2005,357(3):834-846
We consider the evolution of white dwarfs with compact object companions (specifically black holes with masses up to ∼106 M⊙ , neutron stars, and other white dwarfs). We suppose that the orbits are initially quite elliptical and then shrink and circularize under the action of gravitational radiation. During this evolution, the white dwarfs will pass through resonances when harmonics of the orbital frequency match the stellar oscillation eigenfrequencies. As a star passes through these resonances, the associated modes will be excited and can be driven to amplitudes that are so large that there is a back reaction on the orbit which, in turn, limits the growth of the modes. A formalism is presented for describing this dynamical interaction for a non-rotating star in the linear approximation when the orbit can be treated as non-relativistic. A semi-analytical expression is found for computing the resonant energy transfer as a function of stellar and orbital parameters for the regime where back reaction may be neglected. This is used to calculate the results of passage through a sequence of resonances for several hypothetical systems. It is found that the amplitude of the ℓ= m = 2 f -mode can be driven into the non-linear regime for appropriate initial conditions. We also discuss where the no back reaction approximation is expected to fail, and the qualitative effects of back reaction. 相似文献
999.
Several new times of minimum light recorded with photoelectric or CCD means have been gathered for the eccentric eclipsing
binary V889 Aql (P = 11.1 days, e = 0.37). Its O–C diagram is presented, and improved elements of the apsidal motion and the light-time effect (LITE) are given. We found a
long apsidal motion period of about 24 400 ± 2400 years and a period of the third body of about 52 ± 2 years. 相似文献