全文获取类型
收费全文 | 95篇 |
免费 | 3篇 |
专业分类
测绘学 | 1篇 |
大气科学 | 3篇 |
地球物理 | 22篇 |
地质学 | 32篇 |
海洋学 | 6篇 |
天文学 | 26篇 |
自然地理 | 8篇 |
出版年
2022年 | 1篇 |
2020年 | 3篇 |
2018年 | 1篇 |
2017年 | 1篇 |
2016年 | 2篇 |
2015年 | 8篇 |
2014年 | 3篇 |
2013年 | 5篇 |
2012年 | 3篇 |
2011年 | 3篇 |
2010年 | 4篇 |
2009年 | 8篇 |
2008年 | 6篇 |
2007年 | 6篇 |
2006年 | 4篇 |
2005年 | 5篇 |
2004年 | 5篇 |
2003年 | 4篇 |
2002年 | 3篇 |
2001年 | 3篇 |
2000年 | 3篇 |
1999年 | 1篇 |
1998年 | 2篇 |
1997年 | 1篇 |
1995年 | 4篇 |
1994年 | 2篇 |
1992年 | 1篇 |
1989年 | 1篇 |
1980年 | 1篇 |
1974年 | 2篇 |
1970年 | 1篇 |
1964年 | 1篇 |
排序方式: 共有98条查询结果,搜索用时 15 毫秒
41.
Juan Argüelles Ricardo Tafur Anatolio Taipe Piero Villegas Friedeman Keyl Noel Dominguez Martín Salazar 《Progress in Oceanography》2008,79(2-4):308
Changes in population structure of the jumbo flying squid Dosidicus gigas in Peruvian waters were studied based on size-at-maturity from 1989 to 2004. From 1989 to 1999, mature squid belonging to the medium-sized group prevailed, but from 2001 on, mature squids were larger. This change is not related to the changes in sea surface temperature and we hypothesized that it was caused by the population increase of mesopelagic fishes as prey. 相似文献
42.
Hypervelocity stars (HVSs) ejected by the massive black hole at the Galactic Centre have unique kinematic properties compared to other halo stars. Their trajectories will deviate from being exactly radial because of the asymmetry of the Milky Way potential produced by the flattened disc and the triaxial dark matter halo, causing a change of angular momentum that can be much larger than the initial small value at injection. We study the kinematics of HVSs and propose an estimator of dark halo triaxiality that is determined only by instantaneous position and velocity vectors of HVSs at large Galactocentric distances ( r ≳ 50 kpc). We show that, in the case of a substantially triaxial halo, the distribution of deflection angles (the angle between the stellar position and velocity vector) for HVSs on bound orbits is spread uniformly over the range 10°–180°. Future astrometric and deep wide-field surveys should measure the positions and velocities of a significant number of HVSs, and provide useful constraints on the shape of the Galactic dark matter halo. 相似文献
43.
44.
An effective Microcanonical Thermodynamics of self gravitating systems(SGS) is proposed, analyzing the well known obstacles thought to prevent the formulation of a rigorous Statistical Mechanics (SM), as those due to the formal unboundedness of available phase space and to the unscreened, long range, nature of the interaction. The latter feature entails the well
known inequivalence of statistical ensembles, puts clearly into question the meaning, for these systems, of the Thermodynamic Limit, and rules out the use of canonical and grand-canonical ensembles. As to the first obstacle, we argue nevertheless that a hierarchy of timescales exist such that, at any finite time, the volume of the effectively available region of phase space is indeed finite, and that the dynamics satisfies a strong chaos criterion, leading to a fast, increasingly uniform, spreading of orbits over an effectively invariant subset of the constant (N,V,E) surface; thus leading to the definition of a secularly evolving, generalized microcanonical ensemble, which allows to define an (almost extensive)
effective entropy and to derive self-consistent definitions for other thermodynamic variables, giving thus an orthode for SGS. Moreover, a Second Law-like criterion allows to single out the hierarchy of secular equilibria describing, for any finite time, the macroscopic behaviour of SGS.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
45.
Giovanni?La?Mura Graziano?Chiaro Stefano?Ciroi Piero?Rafanelli David?Salvetti Marco?Berton Valentina?Cracco 《Journal of Astrophysics and Astronomy》2015,36(4)
During its first four years of scientific observations, the Fermi Large Area Telescope (Fermi-LAT) detected 3033 γ-ray sources above a 4 σ significance level. Although most of the extra-galactic sources are active galactic nuclei (AGN) of the blazar class, other families of AGNs are observed too, while a still high fraction of detections (~30%) remains with uncertain association or classification. According to the currently accepted interpretation, the AGN γ-ray emission arises from inverse Compton (IC) scattering of low energy photons by relativistic particles confined in a jet, which, in the case of blazars, is oriented very close to our line-of-sight. Taking advantage of data from radio and X-ray wavelengths, which we expect to be produced together with γ-rays, providing a much better source localization potential, we focused our attention on a sample of γ-ray Blazar Candidates of Undetermined type (BCUs), starting a campaign of optical spectroscopic observations. The main aims of our investigation include a census of the AGN families that contribute to γ-ray emission and a study of their redshift distribution, with the subsequent implications on the intrinsic source power. We furthermore analyze which γ-ray properties can better constrain the nature of the source, thus helping in the study of objects not yet associated with a reliable low frequency counterpart. Here we report on the instruments and techniques used to identify the optical counterparts of γ-ray sources, we give an overview on the status of our work, and we discuss the implications of a large scale study of γ-ray emitting AGNs. 相似文献
46.
Marco Bonini Giacomo Corti Chiara Del Ventisette Piero Manetti Genene Mulugeta Dimitrios Sokoutis 《地学学报》2007,19(5):360-366
Small‐scale analogue models were used to investigate the process of Cretaceous orthogonal extension in the West Antarctic Rift System. The models considered the transition from the East Antarctic Craton to a weaker lithosphere, and the results support previous hypotheses about the strong control exerted by lateral variations in lithospheric structures on the process of extension. Strain was mostly accommodated at the boundary between the two types of lithosphere, with a relative uplift of the cratonic block which remained essentially undeformed. Conversely, the weaker lithosphere showed wide‐rifting style geometry, locally associated with core complex‐like structures. In agreement with the natural prototype, this tectonic scenario led to a long‐lasting extension without continental break‐up, and to the absence of relevant surface magmatism. 相似文献
47.
Cesare Barbieri Carlo Blanco Beatrice Bucciarelli Regina Coluzzi Andrea Di Paola Luciano Lanteri Gian Luca Li Causi Ettore Marilli Piero Massimino Vincenzo Mezzalira Stefano Mottola Roberto Nesci Alessandro Omizzolo Fernando Pedichini Francesca Rampazzi Corinne Rossi Ruggero Stagni Milcho Tsvetkov Roberto Viotti 《Experimental Astronomy》2003,15(1):29-43
There is a widespread interest to digitize the precious information contained in the astronomical plate archives, both for
the preservation of their content and for its fast distribution to all interested researchers in order to achieve their better
scientific exploitation. This paper presents the first results of our large-scale project to digitize the archive of plates
of the Italian Astronomical Observatories and of the Specola Vaticana. Similar systems, composed by commercial flat-bed retro-illuminated
scanners plus dedicated personal computers and acquisition and analysis software, have been installed in all participating
Institutes. Ad-hoc codes have been developed to acquire the data, to test the suitability of the machines to our scientific needs, and to reduce
the digital data in order to extract the astrometric, photometric and spectroscopic content. Two more elements complete the
overall project: the provision of high quality BVRI CCD sequences in selected fields with the Campo Imperatore telescopes,
and the distribution of the digitized information to all interested researchers via the Web. The methods we have derived in
the course of this project have been already applied successfully to plates taken by other Observatories, for instance at
Byurakan and at Hamburg. 相似文献
48.
Giovanni Pitari Eleonora Coppari Natalia De Luca Piero Di Carlo Loretta Pace 《Pure and Applied Geophysics》2014,171(9):2425-2441
Two year measurements of aerosol concentration and size distribution (0.25 μm < d < 30 μm) in the atmospheric surface layer, collected in L’Aquila (Italy) with an optical particle counter, are reported and analysed for the different modes of the particle size distribution. A different seasonal behaviour is shown for fine mode aerosols (largely produced by anthropogenic combustion), coarse mode and large-sized aerosols, whose abundance is regulated not only by anthropogenic local production, but also by remote natural sources (via large scale atmospheric transport) and by local sources of primary biogenic aerosols. The observed total abundance of large particles with diameter larger than 10 μm is compared with a statistical counting of primary biogenic particles, made with an independent technique. Results of these two observational approaches are analysed and compared to each other, with the help of a box model driven by observed meteorological parameters and validated with measurements of fine and coarse mode aerosols and of an atmospheric primary pollutant of anthropogenic origin (NOx). Except in winter months, primary biogenic particles in the L’Aquila measurement site are shown to dominate the atmospheric boundary layer population of large aerosol particles with diameter larger than 10 μm (about 80 % of the total during summer months), with a pronounced seasonal cycle, contrary to fine mode aerosols of anthropogenic origin. In order to explain these findings, the main mechanisms controlling the abundance and variability of particulate matter tracers in the atmospheric surface layer are analysed with the numerical box-model. 相似文献
49.
Rudy C. Gilmore Piero Madau Joel R. Primack Rachel S. Somerville Francesco Haardt 《Monthly notices of the Royal Astronomical Society》2009,399(4):1694-1708
We present new calculations of the evolving UV background out to the epoch of cosmological reionization and make predictions for the amount of GeV gamma-ray attenuation by electron–positron pair production. Our results are based on recent semi-analytic models of galaxy formation, which provide predictions of the dust-extinguished UV radiation field due to starlight, and empirical estimates of the contribution due to quasars. We account for the reprocessing of ionizing photons by the intergalactic medium. We test whether our models can reproduce estimates of the ionizing background at high redshift from flux decrement analysis and proximity effect measurements from quasar spectra, and identify a range of models that can satisfy these constraints. Pair production against soft diffuse photons leads to a spectral cut-off feature for gamma rays observed between 10 and 100 GeV. This cut-off varies with redshift and the assumed star formation and quasar evolution models. We find only negligible amounts of absorption for gamma rays observed below 10 GeV for any emission redshift. With observations of high-redshift sources in sufficient numbers by the Fermi Gamma-ray Space Telescope and new ground-based instruments, it should be possible to constrain the extragalactic background light in the UV and optical portion of the spectrum. 相似文献
50.
Piero Di Carlo Giovanni Pitari Natalia De Luca Domenico Battisti 《Environmental Geology》2009,58(2):431-436
Two years of in situ radon concentration measurements in the atmospheric surface layer have been collected in a central Italy
town (L’Aquila), located in the Aterno river valley. These data have been analyzed in order to study the controlling mechanisms
of surface radon abundance; observations of coincident meteorological parameters confirmed the role of dynamics on the local
removal rate of this tracer. The relatively high negative correlation of hourly data of surface wind speed and radon activity
concentration (R = −0.54, on annual scale) suggests that dynamical removal of radon is one of the most important controlling processes of
the tracer accumulation in the atmospheric surface layer. An attempt is made to quantify the precipitation impact on radon
soil fluxes. No anticorrelation of radon and precipitation comes out from the data (R = −0.15), as in previous studies. However, since the main physical parameter affecting the ground radon release is expected
to be the soil accumulation of water, snow or ice, the emission flux has also been correlated with soil moisture; in this
way a much clearer anticorrelation is found (R = −0.54). 相似文献