全文获取类型
收费全文 | 420篇 |
免费 | 11篇 |
国内免费 | 2篇 |
专业分类
测绘学 | 5篇 |
大气科学 | 21篇 |
地球物理 | 65篇 |
地质学 | 121篇 |
海洋学 | 88篇 |
天文学 | 62篇 |
自然地理 | 71篇 |
出版年
2023年 | 2篇 |
2020年 | 3篇 |
2019年 | 12篇 |
2018年 | 4篇 |
2017年 | 5篇 |
2016年 | 14篇 |
2015年 | 13篇 |
2014年 | 17篇 |
2013年 | 32篇 |
2012年 | 20篇 |
2011年 | 16篇 |
2010年 | 18篇 |
2009年 | 28篇 |
2008年 | 15篇 |
2007年 | 21篇 |
2006年 | 21篇 |
2005年 | 20篇 |
2004年 | 9篇 |
2003年 | 21篇 |
2002年 | 17篇 |
2001年 | 11篇 |
2000年 | 11篇 |
1999年 | 8篇 |
1998年 | 8篇 |
1997年 | 6篇 |
1996年 | 6篇 |
1995年 | 4篇 |
1994年 | 3篇 |
1993年 | 4篇 |
1992年 | 4篇 |
1991年 | 5篇 |
1990年 | 2篇 |
1989年 | 4篇 |
1986年 | 1篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1983年 | 7篇 |
1982年 | 6篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1976年 | 2篇 |
1975年 | 4篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1969年 | 2篇 |
1966年 | 1篇 |
1965年 | 2篇 |
1960年 | 2篇 |
排序方式: 共有433条查询结果,搜索用时 31 毫秒
61.
Between September 1981 and March 1982 many Pc5 micropulsation events have been observed by the Wick component of the new SABRE auroral radar. A feature of these observations is that in addition to the characteristic poleward drifting regions of radar backscatter, equatorward moving regions are frequently observed. A consistent diurnal pattern in the direction of drift emerges from a statistical analysis of the data. 相似文献
62.
63.
Jesper Sommer-Larsen Sune Toft Jesper Rasmussen Kristian Pedersen Martin Götz Laura Portinari 《Astrophysics and Space Science》2003,284(2):693-696
The X-ray properties of the haloes of disk galaxies formed in fully cosmological, hydro/gravity simulations are discussed.
The results are found to be consistent with observational X-ray detections and upper limits. Disk galaxy haloes are predicted
to be about an order of magnitude brighter at z ∼ 1 than at z=0. The Lyα and Hα surface brightness of an edge-on, Milky Way like model galaxy has been determined. The emission is found
to be quite extended, with a scale height of about 600 pc, neglecting dust corrections.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
64.
W. Baumjohann R.J. Pelunen H.J. Opgenoorth E. Nielsen 《Planetary and Space Science》1981,29(4):431-447
On 15 February, 1977, ground magnetic, ionospheric electric and auroral signatures of a multiple onset substorm were observed simultaneously by the Scandinavian Magnetometer Array (SMA), the Scandinavian Twin Auroral Radar Experiment (STARE) and the Finnish all-sky camera chain. Between 21:00 and 21:30 U.T., i.e. around local magnetic midnight, three consecutive local auroral break-ups were observed over Scandinavia. Each of these break-ups was preceded by a clear fading of the aurora and magnetic fields (while the electric fields remained unaffected), and occurred slightly south of the Harang discontinuity in the region of north-westward-directed electric fields. They were associated with a sudden change in direction of the electric field from north-west to south-west and the appearance of a westward equivalent current in the localized active region (about 1200 × 300 km2). These observations matched the features to be expected during the generation of a Cowling channel by a strong increase of the ionospheric conductivities due to precipitating auroral electrons. Numerical model calculations, based on the observations during the initial brightening and peak development of the second, most conspicuous break-up, show that the field-aligned currents at the northern and southern border of the active region are indeed very weak. However, highly localized and intense upward field-aligned currents at the western edge of the active region and more widespread and less intense downward currents in the eastern half preserve current continuity of the westward Cowling current and complete the substorm current wedge. 相似文献
65.
Jesper Bartholdy 《Estuarine, Coastal and Shelf Science》1984,18(5):527-541
The river Varde Å discharges into the bay of Ho Bugt on the western coast of Jutland forming a small, bar-built estuary. This paper deals with tidal fluxes of water and sediment in the Varde Å estuary.The inflowing water at flood tide is part of a turbidity maximum in the northern part of the bay. At high tide slack water the suspended material deposits inside the estuary. During ebb-tide it is resuspended, and the estuary bottom is washed clean coinciding with the influx of relatively pure freshwater from the drainage area.From one station in the estuary mouth, current velocities and concentrations of suspended material have been measured during 10 tidal periods covering all four seasons. It is shown how these data can be used in a quantitative calculation of the transport of water and suspended material through the cross-section of study.A model has been formulated which—based on half-tidal periods—quantifies the transport of water and suspended material through the estuary mouth.The model is calibrated on the basis of measurements made during the above-mentioned 10 tidal periods. The rather small number of measurements is to some extent compensated for by a carefully pre-arranged selection of tidal periods.The model is discussed in relation to the prediction of net suspended transport through the estuary mouth in different weather and tidal situations. 相似文献
66.
There is an emerging requirement for processing global navigation satellite system (GNSS) signals indoor where the signal
is very weak and subjected to spatial fading. Typically, longer coherent integration intervals provide the additional processing
gain required for the detection and processing of such weak signals. However, the arbitrary physical motion of the handset
imputed by the user limits the effectiveness of longer coherent integration intervals due to the spatial decorrelation of
the multipath-faded GNSS signal. In this paper, limits of coherent integration due to spatial decorrelation are derived and
corroborated with experimental verification. A general result is that the processing gain resulting from direct coherent integration
saturates after the antenna has moved through a certain distance, which for typical indoor propagation, is about half a carrier
wavelength. However, a refined Doppler search coupled with a prolonged coherent integration interval extends this limit, which
is effectively a manifestation of selective diversity. 相似文献
67.
Anders Wästfelt Tsegaye TegenuMichael M. Nielsen Bo Malmberg 《Applied geography (Sevenoaks, England)》2012,32(2):465-476
Satellite images have enormous potential for qualitative land use analysis. This paper presents empirical results that demonstrate how normally invisible dimensions produced by land use can be identified by enriching satellite data with qualitative information from field studies.Land use can be defined as the intentional use of a specific piece of land resulting in patterns of ecological responses that are visible in the land cover and landscape. Responses to land use often result in a heterogeneous combination of reflectance in satellite images. Statistical methods used in the classification of satellite imagery are limited in their capacity to handle categories consisting of heterogeneous combinations of spectral values. To overcome this limitation, a contextual post-classification method has been used to map land cover configurations as related to different agricultural practices in the district of Sodo, Ethiopia.The results show that it is possible to map socio-spatial distribution of different agricultural and socioeconomic practices on a regional level by combining field observations and spatial contextual information. The empirical findings show local agricultural activity variations in cash crop production and subsistence agriculture in the Sodo district of Ethiopia. 相似文献
68.
Peter Nørtoft Nielsen 《Planetary and Space Science》2009,57(4):491-497
Geochemical disequilibrium of Earth's atmosphere is a sign of life. The fact that Earth's atmosphere is just right for life led Lovelock to propose the Gaia hypothesis: life itself regulates the environment on planetary scale in order to maintain habitability. This hypothesis is supported by the so-called Daisyworld parable, which illustrates a possible mechanism for such a self regulation. Here we revisit Daisyworld and challenge some of its conclusions from a closer examination of the model. We find that even within this simple, conceptual model of a Gaian planet there are regimes where climate is less homeostatic than on a dead planet. Furthermore, in other regimes, bistability between two climate states is found to exist due to the presence of life. This indicates that even if the Gaian stability might describe life in some planetary conditions, it need not be generic to all inhabited planets. 相似文献
69.
Ying Wang Alex L. Sessions Robert J. Nielsen William A. Goddard III 《Geochimica et cosmochimica acta》2009,73(23):7060-7075
Carbon-bound hydrogen in sedimentary organic matter can undergo exchange over geologic timescales, altering its isotopic composition. Studies investigating the natural abundance distribution of 1H and 2H in such molecules must account for this exchange, which in turn requires quantitative knowledge regarding the endpoint of exchange, i.e., the equilibrium isotopic fractionation factor (αeq). To date, relevant data have been lacking for molecules larger than methane. Here we describe an experimental method to measure αeq for C-bound H positions adjacent to carbonyl group (Hα) in ketones. H at these positions equilibrates on a timescale of days as a result of keto-enol tautomerism, allowing equilibrium 2H/1H distributions to be indirectly measured. Molecular vibrations for the same ketone molecules are then computed using Density Functional Theory at the B3LYP/6-311G** level and used to calculate αeq values for Hα. Comparison of experimental and computational results for six different straight and branched ketones yields a temperature-dependent linear calibration curve with slope = 1.081−0.00376T and intercept = 8.404−0.387T, where T is temperature in degrees Celsius. Since the dominant systematic error in the calculation (omission of anharmonicity) is of the same size for ketones and C-bound H in most other linear compounds, we propose that this calibration can be applied to analogous calculations for a wide variety of organic molecules with linear carbon skeletons for temperatures below 100 °C. In a companion paper (Wang et al., 2009) we use this new calibration dataset to calculate the temperature-dependent equilibrium isotopic fractionation factors for a range of linear hydrocarbons, alcohols, ethers, ketones, esters and acids. 相似文献
70.
Sune G. Nielsen Helen M. Williams William L. Griffin Norman Pearson 《Geochimica et cosmochimica acta》2009,73(24):7387-6704
Cratonic eclogites are inferred to originate either from subducted ocean crust or mantle melts accreted onto the roots of continents. These models have different implications for the growth of continents, but it is currently difficult to determine the origin of individual eclogite suites.Upper ocean crust altered at low temperatures and marine sediments both display high thallium (Tl) concentrations and strongly fractionated Tl isotope signatures relative to the ambient upper mantle. In this study we carry out the first examination of the suitability of Tl isotopes as a tracer for an ocean-crust origin of cratonic eclogites. We have analysed the Tl isotope composition of clinopyroxene and garnet in six eclogites from the Kaalvallei and Bellsbank kimberlite pipes in South Africa. Minerals were pre-cleaned with an HCl leaching technique and the leachates display variably light Tl isotope ratios. These most likely reflect low-temperature hydrothermal alteration occurring after eruption of the kimberlite that carried the eclogites to the surface.The leached mineral pairs all display identical Tl isotope ratios, strongly suggesting that the source of the analysed Tl is identical for each mineral pair. It is, however, not possible to exclude the possibility that the analysed Tl originates from kimberlitic material that was not removed by the cleaning procedure.Only one of the six samples exhibits a Tl isotope composition different from ambient mantle. Assuming that the Tl isotope signatures indeed represent the eclogite minerals and not any form of contamination, the Tl isotope composition in this sample is consistent with containing a minor component (<3%) of ocean crust altered at low temperatures.Thallium isotopes may become one of the most sensitive indicators for the presence of low-T altered ocean crust because of the stark contrast in Tl concentration and isotopic composition between the mantle and altered ocean crust. In fact, no other chemical or isotopic tracer could have provided an indication that any of the samples studied here had a subduction origin. However, much work is still required before it becomes clear if Tl isotope measurements are a viable means to establish the origin of cratonic eclogites. 相似文献