全文获取类型
收费全文 | 2878篇 |
免费 | 95篇 |
国内免费 | 18篇 |
专业分类
测绘学 | 106篇 |
大气科学 | 339篇 |
地球物理 | 594篇 |
地质学 | 861篇 |
海洋学 | 322篇 |
天文学 | 542篇 |
综合类 | 4篇 |
自然地理 | 223篇 |
出版年
2023年 | 9篇 |
2022年 | 10篇 |
2021年 | 33篇 |
2020年 | 44篇 |
2019年 | 44篇 |
2018年 | 82篇 |
2017年 | 62篇 |
2016年 | 112篇 |
2015年 | 64篇 |
2014年 | 87篇 |
2013年 | 140篇 |
2012年 | 123篇 |
2011年 | 159篇 |
2010年 | 131篇 |
2009年 | 190篇 |
2008年 | 174篇 |
2007年 | 155篇 |
2006年 | 132篇 |
2005年 | 108篇 |
2004年 | 116篇 |
2003年 | 104篇 |
2002年 | 97篇 |
2001年 | 76篇 |
2000年 | 85篇 |
1999年 | 68篇 |
1998年 | 84篇 |
1997年 | 45篇 |
1996年 | 44篇 |
1995年 | 32篇 |
1994年 | 23篇 |
1993年 | 32篇 |
1992年 | 22篇 |
1991年 | 32篇 |
1990年 | 13篇 |
1989年 | 16篇 |
1987年 | 14篇 |
1986年 | 10篇 |
1985年 | 18篇 |
1984年 | 25篇 |
1983年 | 19篇 |
1982年 | 11篇 |
1981年 | 6篇 |
1980年 | 9篇 |
1979年 | 8篇 |
1978年 | 9篇 |
1977年 | 10篇 |
1976年 | 13篇 |
1975年 | 13篇 |
1973年 | 8篇 |
1971年 | 7篇 |
排序方式: 共有2991条查询结果,搜索用时 15 毫秒
41.
The fragmentation of SO2 following dissociative electron impact excitation has been studied under single collision conditions for incident electron energies up to 500 eV. The emission spectrum in the far v.u.v. spectral range (450–1100Å) shows many features arising from excited neutral oxygen and ionized oxygen and sulphur fragments. Absolute emission cross sections have been measured for the most intense lines and the maximum values were found to range from 1–12 × 10?19 cm2 with an uncertainty of approx. ± 35%. Dissociation mechanisms are discussed and in some cases the dissociation path could be uniquely identified. The striking differences between the v.u.v. emission spectrum produced by single step dissociation of SO2 and the spectra emitted by the plasma torus around Jupiter are discussed. 相似文献
42.
The late‐stage evolution of the southern central Pyrenees has been well documented but controversies remain concerning potential Neogene acceleration of exhumation rates and the influence of tectonic and/or climatic processes. A popular model suggests that the Pyrenees and their southern foreland were buried below a thick succession of conglomerates during the Oligocene, when the basin was endorheic. However, both the amount of post‐orogenic fill and the timing of re‐excavation remain controversial. We address this question by revisiting extensive thermochronological datasets of the Axial Zone. We use an inverse approach that couples the thermo‐kinematic model Pecube and the Neighbourhood inversion algorithm to constrain the history of exhumation and topographic changes since 40 Ma. By comparison with independent geological data, we identified a most probable scenario involving rapid exhumation (>2.5 km Myr?1) between 37 and 30 Ma followed by a strong decrease to very slow rates (0.02 km Myr?1) that remain constant until the present. Therefore, the inversion does not require a previously inferred Pliocene acceleration in regional exhumation rates. A clear topographic signal emerges, however: the topography has to be infilled by conglomerates to an elevation of 2.6 km between 40 and 29 Ma and then to remain stable until ca. 9 Ma. We interpret the last stage of the topographic history as recording major incision of the southern Pyrenean wedge, due to the Ebro basin connection to the Mediterranean, well before previously suggested Messinian ages. These results thus demonstrate temporally varying controls of different processes on exhumation: rapid rock uplift in an active orogen during late Eocene, whereas base‐level changes in the foreland basin control the post‐orogenic evolution of topography and exhumation in the central Pyrenees. In contrast, climate changes appear to play a lesser role in the post‐orogenic topographic and erosional evolution of this mountain belt. 相似文献
43.
Journal of Geographical Systems - In this paper, we investigate the relationship between adverse economic circumstances and the desire of Dutch households to move up or down the urban hierarchy. We... 相似文献
44.
Bipolar active regions (ARs) are thought to be formed by twisted flux tubes, as the presence of such twist is theoretically
required for a cohesive rise through the whole convective zone. We use longitudinal magnetograms to demonstrate that a clear
signature of a global magnetic twist is present, particularly, during the emergence phase when the AR is forming in a much
weaker pre-existing magnetic field environment. The twist is characterised by the presence of elongated polarities, called
“magnetic tongues”, which originate from the azimuthal magnetic field component. The tongues first extend in size before retracting
when the maximum magnetic flux is reached. This implies an apparent rotation of the magnetic bipole. Using a simple half-torus
model of an emerging twisted flux tube having a uniform twist profile, we derive how the direction of the polarity inversion
line and the elongation of the tongues depend on the global twist in the flux rope. Using a sample of 40 ARs, we verify that
the helicity sign, determined from the magnetic polarity distribution pattern, is consistent with the sign derived from the
photospheric helicity flux computed from magnetogram time series, as well as from other proxies such as sheared coronal loops,
sigmoids, flare ribbons and/or the associated magnetic cloud observed in situ at 1 AU. The evolution of the tongues observed in emerging ARs is also closely similar to the evolution found in recent MHD
numerical simulations. We also found that the elongation of the tongue formed by the leading magnetic polarity is significantly
larger than that of the following polarity. This newly discovered asymmetry is consistent with an asymmetric Ω-loop emergence,
trailing the solar rotation, which was proposed earlier to explain other asymmetries in bipolar ARs. 相似文献
45.
A technique for modeling the Earth’s gravity field on the basis of satellite accelerations 总被引:1,自引:2,他引:1
A technique is proposed for Earths gravity field modeling on the basis of satellite accelerations that are derived from precise orbit data. The functional model rests on Newtons second law. The computational procedure is based on the pre-conditioned conjugate-gradient (PCCG) method. The data are treated as weighted average accelerations rather than as point-wise ones. As a result, a simple three-point numerical differentiation scheme can be used to derive them. Noise in the orbit-derived accelerations is strongly dependent on frequency. Therefore, the key element of the proposed technique is frequency-dependent data weighting. Fast convergence of the PCCG procedure is ensured by a block-diagonal pre-conditioner (approximation of the normal matrix), which is derived under the so-called Colombo assumptions. Both uninterrupted data sets and data with gaps can be handled. The developed technique is compared with other approaches: (1) the energy balance approach (based on the energy conservation law) and (2) the traditional approach (based on the integration of variational equations). Theoretical considerations, supported by a numerical study, show that the proposed technique is more accurate than the energy balance approach and leads to approximately the same results as the traditional one. The former finding is explained by the fact that the energy balance approach is only sensitive to the along-track force component. Information about the cross-track and the radial component of the gravitational potential gradient is lost because the corresponding force components do no work and do not contribute to the energy balance. Furthermore, it is shown that the proposed technique is much (possibly, orders of magnitude) faster than the traditional one because it does not require the computation of the normal matrix. Hints are given on how the proposed technique can be adapted to the explicit assembling of the normal matrix if the latter is needed for the computation of the model covariance matrix.Acknowledgments. Professor R. Klees is thanked for support of the project and for numerous fruitful discussions. The authors are also thankful to Dr. J. Kusche for useful remarks and to Dr. E. Schrama, his solid background in satellite geodesy proved to be very helpful. A large number of valuable comments were made by Dr. S.-C. Han, Dr. P. Schwintzer, and an anonymous reviewer; their contribution is greatly acknowledged. The satellite orbits used in the numerical study were kindly provided by Dr. P. Visser (Aerospace Department, Delft University of Technology). Access to the SGI Origin 3800 computer was provided by Stichting Nationale Computerfaciliteiten (NCF), grant SG-027. 相似文献
46.
Zohreh Masoumi John L. van Genderen Mohammad Sadi Mesgari 《International Journal of Digital Earth》2020,13(6):661-682
ABSTRACTAlmost all causative factors of diseases depend on location. The Digital Earth approach is suitable for studying diseases globally. Geospatial information systems integrated with statistical models can be used to model the relationship between a disease and its causative factors. Through modelling, the most important causative factors can be extracted and the epidemiology of the disease can be observed. In this paper, skin cancer (the most common type of cancer) has been modelled based on its causative factors, including climate factors, people's occupations, nutrition habits, socio-economic factors, and usage of chemical fertiliser. To fit the model, a data framework was first designed, and then data were gathered and processed. Finally, the disease was modelled using Generalised Linear Models (GLM), a statistical model based on the location of the factors. The results of this study identify the most important causative factors together with their relative priority. Furthermore, a model was used to predict the change in skin cancer occurrences caused by a change in one of its causative factors. This work illustrates the ability of the model to predict disease occurrence. Thus, by using this Digital Earth approach, skincancer can be studied in all the key countries around the world. 相似文献
47.
48.
Autoclaves are closed vessels in which liquids can be heated above their ‘normal' boiling points. As a consequence the pressure will rise. This equipment has permitted a variety of processes like the syntheses of organics and the leaching of metal ores. Pressure technologies can be used for accelerating reactions, for specific syntheses or they offer an environmental or economical alternative for existing processes. Wet pressure oxidation of sewage sludge, an organic waste product, has become a proven technology. If less oxygen is added than stoichiometrically required for a complete oxidation, cellulose will break down by a different reaction mechanism, releasing electrons. Under these circumstances reduction of inorganic compounds is possible. 相似文献
49.
The Kerio valley lies between the Elgeyo escarpment and the Tugen hills which mark the western margin of the Kenya rift valley. The main fluorite deposits are located in the southern part of the valley at Kimwarer, Choff and Kamnaon.Three types of inclusion fillings were identified: Liquid+Vapour, Liquid+Daughter Minerals and Liquid. The L+V type is dominant. Inclusions occur as clusters, trails along the crystal growth zones and as isolated ones. Low salinities, apparently lower than the 5% wt. NaCl equivalent, were established. Homogenization temperatures suggest that fluorite mineralization took place at different stages and at temperatures between 120 and 180 °C. Isolated readings above 180°C may be referring to the original inclusions in limestone. These measurements and the absence of CO2 in the inclusions, as well as the occurrence of vugs and crustifications with fluorite, suggest that mineralization took place at relatively shallow depths.Emission spectrum lines representing Eu2+, Dy3+, Tb3+ and Sm3+ in fluorite were identified. Sm3+ was detected only in the pinkish luminescence of veined fluorite, whereas the pinkish zone in banded fluorite contains Tb3+. Eu2+ which gives the strongest emission lines in the blue part of the visible spectrum, apparently is responsible for the strong blue cathodoluminescence (CL) in fluorite. The dominance of Eu2+ peaks further points to the fact that fluorite mineralization in the Kerio valley took place in an environment that was enriched in Lanthanide Rare Earth Elements (LREE). The presence of rare earths and radioactive elements in fluorite points towards their enrichment in the environment of fluorite mineralization. A juvenile origin of mineral forming solutions is proposed.Two generations of fluorite were established: allotriomorphic fluorite, forming the matrix, and the idiomorphic variety, occurring either in barite or in druzes in early fluorite. Barite in turn forms idiomorphic crystals in allotriomorphic fluorite. Relics of calcite occur in both K-feldspars and in early fluorite. Oxides and hydroxides of Fe, Mn, Ti and Al commonly occur in open spaces in fluorite. Of significance is the presence of gold in fluorite. Fluorite mineralization is of hydrothermal origin in the post-Miocene era and was formed as a result of metasomatic replacement of marble and open space fillings. 相似文献
50.
Matthew J. Genge Luke Alesbrook Natasha V. Almeida Helena C. Bates Phil A. Bland Mark R. Boyd Mark J. Burchell Gareth S. Collins Luke T. Cornwell Luke Daly Hadrien A. R. Devillepoix Matthias van Ginneken Ansgar Greshake Daniel Hallatt Christopher Hamann Lutz Hecht Laura E. Jenkins Diane Johnson Rosie Jones Ashley J. King Haithem Mansour Sarah McMullan Jennifer T. Mitchell Gavyn Rollinson Sara S. Russell Christian Schröder Natasha R. Stephen Martin D. Suttle Jon D. Tandy Patrick Trimby Eleanor K. Sansom Vassilia Spathis Francesca M. Willcocks Penelope J. Wozniakiewicz 《Meteoritics & planetary science》2024,59(5):948-972
Fusion crusts form during the atmospheric entry heating of meteorites and preserve a record of the conditions that occurred during deceleration in the atmosphere. The fusion crust of the Winchcombe meteorite closely resembles that of other stony meteorites, and in particular CM2 chondrites, since it is dominated by olivine phenocrysts set in a glassy mesostasis with magnetite, and is highly vesicular. Dehydration cracks are unusually abundant in Winchcombe. Failure of this weak layer is an additional ablation mechanism to produce large numbers of particles during deceleration, consistent with the observation of pulses of plasma in videos of the Winchcombe fireball. Calving events might provide an observable phenomenon related to meteorites that are particularly susceptible to dehydration. Oscillatory zoning is observed within olivine phenocrysts in the fusion crust, in contrast to other meteorites, perhaps owing to temperature fluctuations resulting from calving events. Magnetite monolayers are found in the crust, and have also not been previously reported, and form discontinuous strata. These features grade into magnetite rims formed on the external surface of the crust and suggest the trapping of surface magnetite by collapse of melt. Magnetite monolayers may be a feature of meteorites that undergo significant degassing. Silicate warts with dendritic textures were observed and are suggested to be droplets ablated from another stone in the shower. They, therefore, represent the first evidence for intershower transfer of ablation materials and are consistent with the other evidence in the Winchcombe meteorite for unusually intense gas loss and ablation, despite its low entry velocity. 相似文献