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排序方式: 共有653条查询结果,搜索用时 109 毫秒
651.
Frank Postberg Eberhard Grün Mihaly Horanyi Sascha Kempf Harald Krüger Jürgen Schmidt Frank Spahn Ralf Srama Zoltan Sternovsky Mario Trieloff 《Planetary and Space Science》2011,59(14):1815-1825
Classical methods to analyze the surface composition of atmosphereless planetary objects from an orbiter are IR and gamma ray spectroscopy and neutron backscatter measurements. The idea to analyze surface properties with an in-situ instrument has been proposed by Johnson et al. (1998). There, it was suggested to analyze Europa's thin atmosphere with an ion and neutral gas spectrometer. Since the atmospheric components are released by sputtering of the moon's surface, they provide a link to surface composition. Here we present an improved, complementary method to analyze rocky or icy dust particles as samples of planetary objects from which they were ejected. Such particles, generated by the ambient meteoroid bombardment that erodes the surface, are naturally present on all atmosphereless moons and planets. The planetary bodies are enshrouded in clouds of ballistic dust particles, which are characteristic samples of their surfaces. In situ mass spectroscopic analysis of these dust particles impacting onto a detector of an orbiting spacecraft reveals their composition. Recent instrumental developments and tests allow the chemical characterization of ice and dust particles encountered at speeds as low as 1 km/s and an accurate reconstruction of their trajectories. Depending on the sampling altitude, a dust trajectory sensor can trace back the origin of each analyzed grain with about 10 km accuracy at the surface. Since the detection rates are of the order of thousand per orbit, a spatially resolved mapping of the surface composition can be achieved. Certain bodies (e.g., Europa) with particularly dense dust clouds, could provide impact statistics that allow for compositional mapping even on single flybys. Dust impact velocities are in general sufficiently high at orbiters about planetary objects with a radius >1000 km and with only a thin or no atmosphere. In this work we focus on the scientific benefit of a dust spectrometer on a spacecraft orbiting Earth's Moon as well as Jupiter's Galilean satellites. This ‘dust spectrometer' approach provides key chemical and isotopic constraints for varying provinces or geological formations on the surfaces, leading to better understanding of the body's geological evolution. 相似文献
652.
Jérôme Carrayrou Joachim Hoffmann Peter Knabner Serge Kräutle Caroline de Dieuleveult Jocelyne Erhel Jan Van der Lee V. Lagneau K. Ulrich Mayer Kerry T. B. MacQuarrie 《Computational Geosciences》2010,14(3):483-502
Although multicomponent reactive transport modeling is gaining wider application in various geoscience fields, it continues
to present significant mathematical and computational challenges. There is a need to solve and compare the solutions to complex
benchmark problems, using a variety of codes, because such intercomparisons can reveal promising numerical solution approaches
and increase confidence in the application of reactive transport codes. In this contribution, the results and performance
of five current reactive transport codes are compared for the 1D and 2D subproblems of the so-called easy test case of the
MoMaS benchmark (Carrayrou et al., Comput Geosci, 2009, this issue). This benchmark presents a simple fictitious reactive transport problem that highlights the main numerical difficulties
encountered in real reactive transport problems. As a group, the codes include iterative and noniterative operator splitting
and global implicit solution approaches. The 1D easy advective and 1D easy diffusive scenarios were solved using all codes,
and, in general, there was a good agreement, with solution discrepancies limited to regions with rapid concentration changes.
Computational demands were typically consistent with what was expected for the various solution approaches. The differences
between solutions given by the three codes solving the 2D problem are more important. The very high computing effort required
by the 2D problem illustrates the importance of parallel computations. The most important outcome of the benchmark exercise
is that all codes are able to generate comparable results for problems of significant complexity and computational difficulty. 相似文献
653.
Gert Finger Frank Eisenhauer Reinhard Genzel Christopher Mandla Ian Baker Domingo Alvarez Antonio Amorim Wolfgang Brandner Christophe Dupuy Casey Deen Derek Ives Leander Mehrgan Manfred Meyer Karin Perraut Guy Perrin Jörg Stegmeier Christian Straubmeier Harald J. Weller Vincent Isgar 《Astronomische Nachrichten》2023,344(8-9):e20230069
Near-infrared adaptive optics as well as fringe tracking for coherent beam combination in optical interferometry require the development of high-speed sensors. Because of the high speed, a large analog bandwidth is required. The short exposure times result in small signal levels which require noiseless detection. Both requirements cannot be met by state-of-the-art conventional CMOS technology of near-infrared arrays as has been attempted previously. A total of five near-infrared SAPHIRA 320 × 256 pixel HgCdTe e−APD arrays have been deployed in the wavefront sensors and in the fringe tracker of the VLTI instrument GRAVITY. The current limiting magnitude for coherent exposures with GRAVITY is mk = 19, which is made possible with ADP technology. New avalanche photo-diode array (APD) developments since GRAVITY include the extension of the spectral sensitivity to the wavelength range from 0.8 to 2.5 μm. After GRAVITY a larger format array with 512 × 512 pixels has been developed for both AO applications at the ELT and for long integration times. Since dark currents of <10−3 e−/s have been demonstrated with 1Kx1K e−APD arrays and 2Kx2K e−APD arrays have already been developed, the possibilities and adaptations of e−APD technology to provide noiseless large-format science-grade arrays for long integration times are also discussed. 相似文献