全文获取类型
收费全文 | 1545篇 |
免费 | 53篇 |
国内免费 | 10篇 |
专业分类
测绘学 | 38篇 |
大气科学 | 159篇 |
地球物理 | 278篇 |
地质学 | 487篇 |
海洋学 | 140篇 |
天文学 | 350篇 |
自然地理 | 156篇 |
出版年
2022年 | 12篇 |
2020年 | 12篇 |
2019年 | 27篇 |
2018年 | 41篇 |
2017年 | 27篇 |
2016年 | 35篇 |
2015年 | 38篇 |
2014年 | 48篇 |
2013年 | 102篇 |
2012年 | 60篇 |
2011年 | 68篇 |
2010年 | 58篇 |
2009年 | 57篇 |
2008年 | 66篇 |
2007年 | 57篇 |
2006年 | 50篇 |
2005年 | 52篇 |
2004年 | 69篇 |
2003年 | 48篇 |
2002年 | 41篇 |
2001年 | 27篇 |
2000年 | 30篇 |
1999年 | 24篇 |
1998年 | 21篇 |
1997年 | 14篇 |
1996年 | 16篇 |
1995年 | 15篇 |
1994年 | 16篇 |
1993年 | 14篇 |
1992年 | 18篇 |
1991年 | 13篇 |
1990年 | 13篇 |
1989年 | 12篇 |
1988年 | 19篇 |
1987年 | 18篇 |
1986年 | 22篇 |
1985年 | 32篇 |
1984年 | 26篇 |
1983年 | 26篇 |
1982年 | 19篇 |
1981年 | 23篇 |
1980年 | 23篇 |
1979年 | 25篇 |
1978年 | 28篇 |
1977年 | 24篇 |
1976年 | 12篇 |
1975年 | 10篇 |
1973年 | 17篇 |
1972年 | 17篇 |
1971年 | 18篇 |
排序方式: 共有1608条查询结果,搜索用时 15 毫秒
191.
The height structure of a thick-target solar hard X-ray source is predicted for a beam injected vertically downward with a power-law spectrum and dominated by Coulomb collisional energy losses. This structure is characterised by the ratio of hard X-ray flux from an upper part of the source to that from the entire source, and is essentially a function only of the atmospheric column density ΔN (cm?2) in the upper region. These predictions are compared with the flux ratios at 150 keV and 350 keV observed by two spacecraft for five events in which the solar limb occults part of the source for one spacecraft. In three events the occulting levels h ranged from 0 to 2500 km. For these the theoretical and observed ratios are found to be comparable for values of ΔN in reasonable accord with those found at these altitudes by optical and UV spectroscopic modelling of flare chromospheres. In one event the occultation ratio was observed to rise after the burst peak and it is found that this rise is consistent with an increase in ΔN due to conductively driven chromospheric evaporation. However the energy dependence of the occultation ratio is not consistent with that predicted by the model and it is concluded that non-collisional losses must be significant in beam dynamics. In the other two events, the occultation level h was ? 25 000 km. For these the energy dependence of the occultation ratio is comparable with the model predictions. However the values of ΔN required demand extremely high coronal densities and/or acceleration altitudes. Furthermore, the one observed evolution of the occultation ratio is entirely inconsistent with the model. It is concluded that in these, bremsstrahlung emissions other than that from a beam must be important. 相似文献
192.
193.
Reverse-phase thin layer chromatography was used to quantify algal carotenoid degradation resulting from grazing by Daphnia magna in suspensions of lyophilized Oscillatoria utermöhlii, O. limnetica, Anabaena flos-aquae and Synechococcus sp. Three samples were removed every 24 h from 0 h to 144 h and the carotenoid, carbon and nitrogen content of the remaining algal-fecal mixture was determined. Grazing by Daphnia reduced carotenoid concentrations by 30%–80% after 144 h, with myxoxanthophyll being the most labile pigment in experiments with filamentous cyanophytes. However, as a group, the carotenoids were more resistant to the combined effects of grazing by Daphnia and bacterial action than were particulate algal carbon (78% to 82% lost by 144 h) or nitrogen (86%–90%) during the period in which easily digestible material remained. The maximal extent of carotenoid enrichment relative to particulate carbon ranged from 150% to 300% of initial pigment concentrations but declined with repeated coprophagy. On the basis of these results and published pigment budgets, we conclude both that fecal transportation of pigments may be an important determinant of carotenoid accumulation rates and that carotenoid stratigraphies should record predator-mediated changes in zooplankton community structure, especially in oligotrophic conditions. 相似文献
194.
195.
196.
G. M. Brown 《Geophysical Journal International》1986,86(2):467-473
Summary. Analysis of geomagnetic data has shown that the superposed northward magnetic field, which reduces the S q ( H ) amplitude at northern mid-latitude stations on Abnormal Quiet Days, and increases the amplitude at stations on the equatorward side of the S q focus, builds up in amplitude over four to five days before the AQD occurs, and subsides over a similar period after the AQD. It is inferred indirectly that the azimuthal component By of the interplanetary magnetic field varies similarly. Data for the opposite meridian show that the imposed field reverses to a southward direction at lower latitudes. The inferred currents to account for these fields are believed to flow in the ionosphere, but to arise from magnetospheric electric fields induced by the solar wind-transported IMF. 相似文献
197.
198.
Udy J Gall M Longstaff B Moore K Roelfsema C Spooner DR Albert S 《Marine pollution bulletin》2005,51(1-4):224-238
For the managers of a region as large as the Great Barrier Reef, it is a challenge to develop a cost effective monitoring program, with appropriate temporal and spatial resolution to detect changes in water quality. The current study compares water quality data (phytoplankton abundance and water clarity) from remote sensing with field sampling (continuous underway profiles of water quality and fixed site sampling) at different spatial scales in the Great Barrier Reef north of Mackay (20 degrees S). Five transects (20-30 km long) were conducted from clean oceanic water to the turbid waters adjacent to the mainland. The different data sources demonstrated high correlations when compared on a similar spatial scale (18 fixed sites). However, each data source also contributed unique information that could not be obtained by the other techniques. A combination of remote sensing, underway sampling and fixed stations will deliver the best spatial and temporal monitoring of water quality in the Great Barrier Reef. 相似文献
199.
A number of models which would explain ocean wave imagery taken with a synthetic aperture imaging radar are analyzed analytically and numerically. Actual radar imagery is used to support some conclusions. The models considered correspond to three sources of radar backscatter cross section modulation: tilt modulation, roughness variation, and the wave orbital velocity. The effect of the temporal changes of the surface structure, parametric interactions, and the resulting distortions are discussed. 相似文献
200.
Bryan C. Brown 《Celestial Mechanics and Dynamical Astronomy》1981,23(3):203-221
This paper presents the results of an investigation into the secular behavior of the orbits of the Galilean satellites of Jupiter. Kamel's perturbation method is used to remove all the explicitly periodic variables from the differential equations that describe the long period behavior of the orbits to third order in the masses, and the resulting differential equations for the secular behavior are then solved. Several numerical examples are given to illustrate the sensitivity of the solution to variations in the masses of the satellites. 相似文献