全文获取类型
收费全文 | 2266篇 |
免费 | 43篇 |
国内免费 | 26篇 |
专业分类
测绘学 | 48篇 |
大气科学 | 107篇 |
地球物理 | 577篇 |
地质学 | 770篇 |
海洋学 | 238篇 |
天文学 | 372篇 |
综合类 | 10篇 |
自然地理 | 213篇 |
出版年
2021年 | 21篇 |
2020年 | 31篇 |
2019年 | 28篇 |
2018年 | 42篇 |
2017年 | 32篇 |
2016年 | 63篇 |
2015年 | 44篇 |
2014年 | 55篇 |
2013年 | 120篇 |
2012年 | 64篇 |
2011年 | 114篇 |
2010年 | 78篇 |
2009年 | 95篇 |
2008年 | 82篇 |
2007年 | 92篇 |
2006年 | 85篇 |
2005年 | 90篇 |
2004年 | 76篇 |
2003年 | 54篇 |
2002年 | 74篇 |
2001年 | 40篇 |
2000年 | 38篇 |
1999年 | 43篇 |
1998年 | 28篇 |
1997年 | 30篇 |
1996年 | 32篇 |
1995年 | 41篇 |
1994年 | 33篇 |
1993年 | 26篇 |
1992年 | 25篇 |
1991年 | 32篇 |
1990年 | 23篇 |
1989年 | 24篇 |
1988年 | 34篇 |
1987年 | 27篇 |
1986年 | 24篇 |
1985年 | 35篇 |
1984年 | 38篇 |
1983年 | 41篇 |
1982年 | 32篇 |
1981年 | 30篇 |
1980年 | 27篇 |
1979年 | 32篇 |
1978年 | 24篇 |
1977年 | 33篇 |
1976年 | 31篇 |
1975年 | 25篇 |
1973年 | 19篇 |
1972年 | 22篇 |
1971年 | 18篇 |
排序方式: 共有2335条查询结果,搜索用时 11 毫秒
41.
Neal S. Gupta George D. Cody O. Erik Tetlie Derek E.G. Briggs Roger E. Summons 《Organic Geochemistry》2009,40(5):589-594
Diagenetic alteration is critical for the preservation of fossil cuticles of plant and animal origin and to the formation of kerogen. The process takes place over millions of years, but the stage at which it is initiated is not known. Laboratory decay experiments were carried out on shrimps, scorpions and cockroaches to monitor changes in the chitin–protein of the arthropod cuticle and associated lipids. The cockroach and scorpion exoskeleton remained largely unaltered morphologically, but the shrimp experienced rapid decomposition within a month, which progressed through the 44 week duration of the experiment as revealed using electron microscopy. Mass spectrometry and 13C NMR (nuclear magnetic resonance) spectroscopy revealed the association of an n-alkyl component with labile lipids, such as fatty acids with up to 24 carbon atoms, which were incorporated into the decaying macromolecule. The scorpion and cockroach cuticle did not reveal the incorporation of additional lipids, indicating that decay is important in initiating in situ lipid association. This experiment provides evidence that lipids can become associated with carbohydrate and proteinaceous macromolecules during the very early stages of decay, representing the first stage in the transformation process that contributes to the aliphatic rich composition ubiquitous in organic fossils and kerogen. 相似文献
42.
Non-LTE synthetic spectra derived from a detailed analysis of the formation of the CN (0, 0) λ13883 Å spectrum are compared with center-limb photoelectric spectra taken at Kitt Peak National Observatory. Kitt Peak National Observatory is operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation. Significant non-LTE effects are found and the Kurucz, Altrock-Cannon, Mount-Linsky II, and HSRA models are compared. We derive a solar carbon abundance of A c =8.30±0.10 for the Mount-Linsky model and A c =8.40±0.10 for the Altrock-Cannon model, compared to the HSRA value of A c =8.55±0.10, assuming a nitrogen abundance of logA N=7.93. In addition we specify the regions of formation for the CN(0, 0) 3883.35 Å bandhead at disc center and limb. 相似文献
43.
A sequence of extreme ultraviolet (EUV) spectroheliograms of McMath region No. 10283 were obtained by the Harvard College Observatory experiment on OSO-6. The lines Ovi λ1032 Mg × λ625, Si xii λ499 and Fe xvi λ 335 were used to determine coronal temperatures and densities above the active region. A comparison of theoretical and observed line ratios yielded coronal temperatures of 2.2 to 2.3 × 106K above the active region and 2.0 to 2.1 × 106K in the surrounding area. The temperatures derived from ratios involving the O vi intensities are systematically higher than the others. This is attributed to an error in the theoretical O vi intensities. The intensities observed above the limb are compared with intensities predicted with a simple model based on cylindrical geometry. The overall agreement shows that the assumption of an iso-thermal corona in hydrostatic equilibrium above the active region is a reasonable working hypothesis and that the adopted geometrical model for the electron density distribution is adequate. 相似文献
44.
We derive the electron density distribution in the ecliptic plane, from the corona to 1 AU, using observations from 13.8 MHz to a few kHz by the radio experiment WAVES aboard the spacecraft Wind. We concentrate on type III bursts whose trajectories intersect the spacecraft, as determined by the presence of burst-associated Langmuir waves, or by energetic electrons observed by the 3-D Plasma experiment. For these bursts we are able to determine the mode of emission, fundamental or harmonic, the electron density at 1 AU, the distance of emission regions along the spiral, and the time spent by the beams as they proceed from the low corona to 1 AU. For all of the bursts considered, the emission mode at burst onset was the fundamental; by contrast, in deriving many previous models, harmonic emission was assumed.By measuring the onset time of the burst at each frequency we are able to derive an electron density model all along the trajectory of the burst. Our density model, after normalizing the density at 1 AU to be ne(215 R0)=7.2 cm–3 (the average value at the minimum of solar activity when our measurements were made), is ne=3.3×105 r–2+4.1×106 r–4+8.0×107 r–6 cm–3, with r in units of R0. For other densities at 1 AU our result implies that the coefficients in the equation need to be multiplied by n
e
(1 AU)/7.2.We compare this with existing models and those derived from direct, in-situ measurements (normalized to the same density at 1 AU) and find that it agrees very well with in-situ measurements and poorly with radio models based on apparent source positions or assumptions of the emission mode. One implication of our results is that isolated type III bursts do not usually propagate in dense regions of the corona and solar wind, as it is still sometimes assumed. 相似文献
45.
This paper presents a new Doppler spectral type of VHP (42 MHz) radio auroral backscatter. This spectrum, which has a double-peaked structure, was observed repeatedly during the morning sector of an exceptionally strong event (Ap = 48) and is due to irregularities moving northwards with quite different velocities. The stronger spectral component, which has a smaller Doppler shift, is centred in frequency at ~?130 Hz, corresponding to the ion-acoustic velocity range in the medium; the weak component, which has a greater frequency shift, usually is centred at about ?300 Hz (~ 1050 m s?1). Evidence based on spectral analysis of sequential short time sequences shows that the spectral power alternates in time between the two distinct frequency bands where the peaks are located, suggesting that the double-peaked spectrum may result from two competing processes which cannot operate simultaneously. The possibility exists that the theoretical model proposed by Sato (1977), which predicts two different quasi-linear stabilization mechanisms for the two-stream instability, could explain the observed double-peaked spectral type. 相似文献
46.
47.
北喀斯喀特边缘IODP 311航次断面的地震结构、天然气水合物含量和滑塌构造 总被引:1,自引:0,他引:1
北喀斯喀特活动边缘的增生楔沉积物中含有分布广泛的天然气水合物一直延伸到海底下几百米的深度。为了评估天然气水合物的长期能源潜力和它在全球气候变化及边坡稳定性中的作用,2005年9月IODP311航次建立了一个详细的地质模型来解释水合物的形成与分解以及它与穿过大陆边缘流体流的伴生关系。在考察前,先在大陆坡上进行地震试验以提供支持钻井的地球物理信息。 相似文献
48.
A model is presented which shows that large numbers of energetic electrons (0.3-> 10 MeV) and protons (1–30 MeV) can be stored in the solar corona at altitudes around 3 × 105 km for periods in excess of 5 days. Specific reference is made to the time period July 6–16 1968 as an excellent example of energetic solar particle storage. Time histories of interplanetary charged particle intensities observed by the IMP-4 and Pioneer 8 satellites are used to substantiate this contention. Detailed reference is also made to solar X-ray, optical and radio data obtained during the period in question, in addition to interplanetary magnetometer data. This model provides a unique solution to many hitherto unexplained solar particle events, and can also account for the lack of prompt particle emission from certain large solar flares recorded in the past. 相似文献
49.
George S. Greenstein 《Astrophysics and Space Science》1968,2(2):155-165
We discuss the implications of the Brans-Dicke scalar-tensor theory for cosmology with particular emphasis on the primordial element abundances that would obtain. Two general classes of models are found. Models of one class expand through the nuclear burning stage slightly more rapidly than the general relativistic case: models of the other class may expand at any rate whatsoever. The first class of models yeilds primordial abundances of D, He3 and He4 in agreement with their general relativistic values if the present mass density is low. High-density cosmologies, however, would produce too much He4. The second class of models yields element abundances which are far too high unless the expansion rate was quite large: in this case no He4 at all is produced. Finally, we determine the rate of change of the constant of gravitationG at the present epoch. For all but a very small class of models is negative at the present epoch. Models with positive values of at the present epoch produce no primordial He4 whatsoever, and have ages significantly lower than the corresponding general relativistic ages. 相似文献
50.
We have computed line profiles and curves of growth for both reflected and transmitted radiation for typical lines in CO2 bands (in the photographic infrared) which occur in the spectrum of Venus. In our model the pressure variation with altitude was considered and the base of the cloud deck was set at the 2 bar level. The temperature was held constant at 250K and a Voigt profile was used for the lineshape. We also assumed that the scale height of the cloud particles was equal to the scale height of the gas. The calculations were made for four values of the scattering optical thickness (τc = 0.1, 1.0, 10, and 100) using a continuum single scattering albedo (which gives a Bond albedo of 0.896 for τc = 100, the value observed for Venus at these wavelengths). Curves of growth are also presented for reflected radiation which has been averaged over the visible disk for three values of the Venus phase angle (0, 86, and 166°). 相似文献