全文获取类型
收费全文 | 1314篇 |
免费 | 30篇 |
国内免费 | 2篇 |
专业分类
测绘学 | 36篇 |
大气科学 | 121篇 |
地球物理 | 266篇 |
地质学 | 444篇 |
海洋学 | 124篇 |
天文学 | 258篇 |
综合类 | 2篇 |
自然地理 | 95篇 |
出版年
2022年 | 7篇 |
2021年 | 12篇 |
2020年 | 13篇 |
2019年 | 17篇 |
2018年 | 15篇 |
2017年 | 17篇 |
2016年 | 33篇 |
2015年 | 26篇 |
2014年 | 31篇 |
2013年 | 56篇 |
2012年 | 31篇 |
2011年 | 67篇 |
2010年 | 39篇 |
2009年 | 62篇 |
2008年 | 51篇 |
2007年 | 54篇 |
2006年 | 67篇 |
2005年 | 42篇 |
2004年 | 43篇 |
2003年 | 41篇 |
2002年 | 43篇 |
2001年 | 43篇 |
2000年 | 29篇 |
1999年 | 24篇 |
1998年 | 23篇 |
1997年 | 19篇 |
1996年 | 21篇 |
1995年 | 17篇 |
1994年 | 26篇 |
1993年 | 21篇 |
1992年 | 10篇 |
1991年 | 21篇 |
1990年 | 19篇 |
1989年 | 11篇 |
1988年 | 16篇 |
1987年 | 18篇 |
1986年 | 11篇 |
1985年 | 28篇 |
1984年 | 25篇 |
1983年 | 14篇 |
1982年 | 13篇 |
1981年 | 14篇 |
1979年 | 19篇 |
1978年 | 7篇 |
1977年 | 9篇 |
1974年 | 10篇 |
1973年 | 8篇 |
1972年 | 8篇 |
1971年 | 8篇 |
1970年 | 7篇 |
排序方式: 共有1346条查询结果,搜索用时 46 毫秒
1.
Sam J. Leuthold Stephanie A. Ewing Robert A. Payn Florence R. Miller Stephan G. Custer 《水文研究》2021,35(2):e14029
In snowmelt-driven mountain watersheds, the hydrologic connectivity between meteoric waters and stream flow generation varies strongly with the season, reflecting variable connection to soil and groundwater storage within the watershed. This variable connectivity regulates how streamflow generation mechanisms transform the seasonal and elevational variation in oxygen and hydrogen isotopic composition (δ18O and δD) of meteoric precipitation. Thus, water isotopes in stream flow can signal immediate connectivity or more prolonged mixing, especially in high-relief mountainous catchments. We characterized δ18O and δD values in stream water along an elevational gradient in a mountain headwater catchment in southwestern Montana. Stream water isotopic compositions related most strongly to elevation between February and March, exhibiting higher δ18O and δD values with decreasing elevation. These elevational isotopic lapse rates likely reflect increased connection between stream flow and proximal snow-derived water sources heavily subject to elevational isotopic effects. These patterns disappeared during summer sampling, when consistently lower δ18O and δD values of stream water reflected contributions from snowmelt or colder rainfall, despite much higher δ18O and δD values expected in warmer seasonal rainfall. The consistently low isotopic values and absence of a trend with elevation during summer suggest lower connectivity between summer precipitation and stream flow generation as a consequence of drier soils and greater transpiration. As further evidence of intermittent seasonal connectivity between the stream and adjacent groundwaters, we observed a late-winter flush of nitrate into the stream at higher elevations, consistent with increased connection to accumulating mineralized nitrogen in riparian wetlands. This pattern was distinct from mid-summer patterns of nitrate loading at lower elevations that suggested heightened human recreational activity along the stream corridor. These observations provide insights linking stream flow generation and seasonal water storage in high elevation mountainous watersheds. Greater understanding of the connections between surface water, soil water and groundwater in these environments will help predict how the quality and quantity of mountain runoff will respond to changing climate and allow better informed water management decisions. 相似文献
2.
3.
4.
Heinz Isliker 《Solar physics》1992,141(2):325-334
Solar flares have a fragmented structure. Dynamical systems theory, for instance in its form of dimensional analysis, can analyze such structures. It answers the question whether the underlying process is deterministic or stochastic. If the process is deterministic, it provides a measure of how complicated the process is (the fractal dimension). In order to be reliable, the analysis has to be combined with the investigation of stationarity.We apply this method to ms-spikes, observed in the decimetric range, which are possibly a manifestation of flare fragmentation. We compare the system-theoretical properties - such as stationarity, stochasticity or deterministic behaviour - of the ms-spikes to the properties of several classes of suggested scenarios. This permits us to discuss different scenarios from a general point of view and to derive general properties of the source.Paper presented at the 4th CESRA Workshop in Ouranopolis (Greece) 1991. 相似文献
5.
Miller B.S. Zosuls A.L. Ketten D.R. Mountain D.C. 《Oceanic Engineering, IEEE Journal of》2006,31(1):87-94
Previous research on the cetacean auditory system has consisted mostly of behavioral studies on a limited number of species. Little quantitative physiologic data exists on cetacean hearing. The frequency range of hearing varies greatly across different mammalian species. Differences among species correlate with differences in the middle-ear transfer function. Middle-ear transfer functions depend on the mechanical stiffness of the middle ear and the cochlear input impedance. The purpose of this study was to measure the middle-ear stiffness for the bottlenose dolphin (Tursiops truncatus), a species specialized for underwater high-frequency hearing and echolocation. Middle-ear stiffness was measured with a force probe that applied a known displacement to the stapes and measured the restoring force. The average middle-ear stiffness in ten dolphin ears was 1.37 N//spl mu/m, which is considerably higher than that reported for most terrestrial mammals. The relationship between middle-ear stiffness and low-frequency hearing cutoff in Tursiops was shown to be comparable to that of terrestrial mammals. 相似文献
6.
7.
8.
Variable surface albedo features on Mars are likely caused by the entrainment and deposition of dust by the wind. Most discrete markings are associated with topographic forms or with regional slopes that serve to alter the effective wind shear stress on the surface. Some of the largest variable features, here termed mesoscale linear streaks, are up to 400 km in length and repeatedly occur in one of the smoothest regions of Mars: Amazonis Planitia. Their orientations and apparent season of variability as observed by Viking and Mars Orbiter cameras indicate linear streak formation by enhanced surface wind stresses during regional or local dust storms and during the initial stages of global dust storms. They provide an example of the ability of large-scale winds, without significant local enhancement, to initiate dust motion on Mars. The sizes and spacing of the linear streaks may be controlled by boundary layer rolls. The repetitive formation of these streaks, over a span of more than 11 Mars years, gives one measure of the stability of Mars’ eolian processes. 相似文献
9.
Kunde Yang Yuanliang Ma Chao Sun Miller J.H. Potty G.R. 《Oceanic Engineering, IEEE Journal of》2004,29(4):964-972
This paper discusses the results of geoacoustic inversion carried out using explosive charge data from the Asian Seas International Acoustic Experiment (ASIAEX) East China Sea (ECS) Experiment. A multifrequency incoherent matched-field inversion processor and a genetic algorithm (GA) are used for the inversion. A multistep matched field inversion approach is presented, which makes use of the varying sensitivities of wave fields at various frequencies to reduce the inversion problem into a sequence of smaller inversions with fewer unknowns to estimate at each stage. Different parameters are estimated using data at different frequencies according to their sensitivities. Inversion results for different areas in the ECS region are summarized and compared with core data. 相似文献
10.
We present archival Rossi X-ray Timing Explorer ( RXTE ) and simultaneous Advanced Satellite for Cosmology and Astrophysics ( ASCA ) data of the eclipsing low mass X-ray binary (LMXB) X 1822−371. Our spectral analysis shows that a variety of simple models can fit the spectra relatively well. Of these models, we explore two in detail through phase-resolved fits. These two models represent the case of a very optically thick and a very optically thin corona. While systematic residuals remain at high energies, the overall spectral shape is well approximated. The same two basic models are fitted to the X-ray light curve, which shows sinusoidal modulations interpreted as absorption by an opaque disc rim of varying height. The geometry we infer from these fits is consistent with previous studies: the disc rim reaches out to the tidal truncation radius, while the radius of the corona (approximated as spherical) is very close to the circularization radius. Timing analysis of the RXTE data shows a time-lag from hard to soft consistent with the coronal size inferred from the fits. Neither the spectra nor the light curve fits allow us to rule out either model, leaving a key ingredient of the X 1822−371 puzzle unsolved. Furthermore, while previous studies were consistent with the central object being a 1.4 M⊙ neutron star, which has been adopted as the best guess scenario for this system, our light curve fits show that a white dwarf or black hole primary can work just as well. Based on previously published estimates of the orbital evolution of X 1822−371, however, we suggest that this system contains either a neutron star or a low mass (≲2.5 M⊙ ) black hole and is in a transitional state of duration shortward of 107 yr. 相似文献